No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
On transient climate change at the Cretaceous−Paleogene boundary due to atmospheric soot injections
Abstract
Significance
A mass extinction occurred at the Cretaceous−Paleogene boundary coincident with the impact of a 10-km asteroid in the Yucatán peninsula. A worldwide layer of soot found at the boundary is consistent with global fires. Using a modern climate model, we explore the effects of this soot and find that it causes near-total darkness that shuts down photosynthesis, produces severe cooling at the surface and in the oceans, and leads to moistening and warming of the stratosphere that drives extreme ozone destruction. These conditions last for several years, would have caused a collapse of the global food chain, and would have contributed to the extinction of species that survived the immediate effects of the asteroid impact.
... Therefore, these model outcomes suggest that plants in northern latitudes would be most likely to suffer from insufficient light for growth and survival. Similarly, evidence from the K-Pg boundary shows that there would likely not be enough light to reach LCP after a large meteor impact like the K-Pg bolide impact discussed later (Bardeen et al., 2017). Additionally, the Community Earth System Model-Whole Atmosphere Community Climate Model version 4 (WACCM4), predicts a decrease of global net primary production (NPP) to near 0% after a 150 Tg atmospheric soot injection . ...
... Contrarily, recent models seem to suggest that the soot needed to reduce light below the photosynthetic threshold to cause plant death could only have occurred if there were widespread firestorms immediately after impact. These would have been caused by ejecta and a bolide heat wave (Bardeen et al., 2017;Tabor et al., 2020). One of these recent models published by Tabor et al. in 2020 estimated critical soot levels for widescale plant death based solely on the sun-blocking effect of soot and not the potential coincident changes in environmental conditions (drought, cold, increased UV-B/C radiation) and their cumulative effects on LCD and thus plant health as we have discussed in the previous section. ...
... consideration decreases the initial amount of soot needed to cause plant death from that predicted in the Tabor et al. (2020) model and may suggest earlier forest death and a later onset of forest fires as an indirect effect due to drought. Besides the timing of wildfires, both Bardeen et al. (2017) and Tabor et al. (2020) further support our predictions about forest health with very robust climate models. ...
A global, sun-blocking catastrophe like nuclear war, an asteroid strike, or super volcano eruption spells disaster for most aspects of life as we know it. There have been many studies on how differing magnitudes of sun-blocking catastrophes would affect the global climate, and many mention the effects of this cold, dark climate on forests and cropping systems. However, few studies have solely focused on the effects of nuclear winter on forests in terms of food, resources, and decomposition. Forests already provide over a billion people with food and fuel for their livelihoods. In this review we connect how prehistoric catastrophes affected the world’s forests to how a current day catastrophe may affect forest health, forest resource availability, and wood decomposition rates. We briefly discuss how forest resources may be used in this post-catastrophe climate for food and fuel in an energy and fuel depleted world. We use this information to make policy and education suggestions to prepare for future catastrophes, build resilience from smaller local disasters, prepare for the many effects of climate change, and discourage nuclear weapon stockpiling.
... which followed the global cooling process triggered by the impact of the Chicxulub asteroid 66 million years ago. According to the latest reconstructions, the impact led to an average drop of around 16 • C in global surface temperature within three years from the impact [27]. In our simulations, a decrease in temperature more than three times smaller than this was enough to doom planetary life through a combination of primary extinctions and co-extinctions ( Fig. 11.6). ...
... A simple explanation for this inconsistency is that we assumed a spatially homogeneous global change in temperature in our simulations. In contrast, the temperature changes following the asteroid impact in the late Cretaceous were highly heterogeneous across the planet [27] (Fig. 11.7). Thus, despite the dramatic global circumstances, sparse climatic refugia ensured the survival of the species responsible for the later recolonization of the Earth. ...
... The map results from climate simulations based on the atmospheric injection of 15000 megatonnes of soot, which is the amount estimated to be present at the Cretaceous-Paleogene boundary, and which most likely originated from global wildfires ignited by the asteroid impact. Adapted from [27], with kind permission from the National Academy of Sciences microorganisms) thrive in extreme environments. For example, species of the genus Sulfolobus have been isolated from several acidic thermal habitats. ...
The direct pressures exerted by global change and human activity on global biodiversity—alteration of local environmental conditions, habitat destruction, mass and selective killing of individuals—account for only one part of the current biodiversity crisis. Increasing theoretical and empirical evidence supports the idea that secondary (co-) extinctions triggered by primary ones and propagating through networks of ecological interactions provide a substantial contribution to global species loss. Assessing the actual magnitude of co-extinctions in the context of the ongoing mass extinction remains, however, a challenging problem due, in particular, to limited data availability. Although available information in specific resource–consumer networks makes it possible to simulate co-extinction processes locally, how to scale up models globally is unclear. In recent work, to tackle the issue, we generated a planetary system of virtual, interconnected ecological networks by combining real-world information on species ecology and functional traits to create simulated yet plausible species. We subjected those networks to extreme environmental change (either a monotonic increase or decrease in air temperature) until planetary life annihilation. Comparing the effects of primary extinctions triggered by climate change alone with those of co-extinctions suggested that not accounting for the latter might result in an underestimation of diversity loss of up to ten times. The scenario of planetary life annihilation explored in the model prompts essential questions about the actual chances for life on our planet to withstand catastrophic events and survive in the long term (i.e. in a geological time scale). The presence of life in extreme environments seems to suggest a positive answer (even if not necessarily relevant for humanity). However, despite their exceptional properties, extreme ecosystems (such as thermal vents) might not be immune to global change threats. Lessons from the fossil records and past cyclical extinction events teach us that the stability of natural systems might be only a parenthesis between major collapses. Although an increase in planetary life robustness following disruptive events is a possible scenario, we have to recognize that changes are irreversible when it comes to mass extinctions, and most losses are irreplaceable.
... Therefore, these model outcomes suggest that plants in northern latitudes would be most likely to suffer from insufficient light for growth and survival. Similarly, evidence from the K-Pg boundary shows that there would likely not be enough light to reach LCP after a large meteor impact like the K-Pg bolide impact discussed later (Bardeen et al., 2017). Additionally, the Community Earth System Model-Whole Atmosphere Community Climate Model version 4 (WACCM4), predicts a decrease of global net primary production (NPP) to near 0% after a 150 Tg atmospheric soot injection . ...
... Contrarily, recent models seem to suggest that the soot needed to reduce light below the photosynthetic threshold to cause plant death could only have occurred if there were widespread firestorms immediately after impact. These would have been caused by ejecta and a bolide heat wave (Bardeen et al., 2017;Tabor et al., 2020). One of these recent models published by Tabor et al. in 2020 estimated critical soot levels for widescale plant death based solely on the sun-blocking effect of soot and not the potential coincident changes in environmental conditions (drought, cold, increased UV-B/C radiation) and their cumulative effects on LCD and thus plant health as we have discussed in the previous section. ...
... consideration decreases the initial amount of soot needed to cause plant death from that predicted in the Tabor et al. (2020) model and may suggest earlier forest death and a later onset of forest fires as an indirect effect due to drought. Besides the timing of wildfires, both Bardeen et al. (2017) and Tabor et al. (2020) further support our predictions about forest health with very robust climate models. ...
... Local fires could have been triggered by the impactor while approaching the Earth or by the rising fireball close to the impact site, whereas ejecta reentering the atmosphere could have ignited fires globally (Toon et al., 1997). When reaching the stratosphere, the soot from fires had a light-blocking effect for several years which is suggested to act stronger on limiting photosynthesis compared to sulfate aerosols (Bardeen et al., 2017). Additionally, the atmospheric CO 2 levels were increased by the combustion of terrestrial carbon (Tyrrell et al., 2015). ...
... As it was shown that the radiative effects of impact dust were dominated by the effects of sulfate aerosols (Pierazzo et al., 2003;Pope, 2002) and possibly soot (Bardeen et al., 2017), we only consider the effects of the impact dust from the projectile on the ocean, in particular from the important nutrients iron and phosphorus. We explicitly model the atmospheric transport and deposition of impact dust from the projectile to derive the dust distribution in the ocean. ...
... The longer-term warming trend observed in proxy data and the comparison of surface δ 13 C data with our model results suggest an additional carbon release of ∼1,500 Gt from the terrestrial biosphere. If the carbon is the result of wildfires caused by the infrared radiation of the reentering ejecta (Robertson et al., 2013), the produced soot would have further reduced the incoming surface short wave radiation with significant effects on photosynthesis (Bardeen et al., 2017;Tabor et al., 2020). In addition, nutrients brought into the ocean by the wildfire ash could have intensified and prolonged the increase in marine primary productivity and the algal bloom (Abram et al., 2003). ...
The evolution of life on Earth has been driven by disturbances of different types and magnitudes over the 4.6 million years of Earth’s history (Raup, 1994, Alroy, 2008). One example for such disturbances are mass extinctions which are characterized by an exceptional increase in the extinction rate affecting a great number of taxa in a short interval of geologic time (Sepkoski, 1986). During the 541 million years of the Phanerozoic, life on Earth suffered five exceptionally severe mass extinctions named the “Big Five Extinctions”. Many mass extinctions are linked to changes in climate (Feulner, 2009). Hence, the study of past mass extinctions is not only intriguing, but can also provide insights into the complex nature of the Earth system. This thesis aims at deepening our understanding of the triggers of mass extinctions and how they affected life. To accomplish this, I investigate changes in climate during two of the Big Five extinctions using a coupled climate model. During the Devonian (419.2–358.9 million years ago) the first vascular plants and vertebrates evolved on land while extinction events occurred in the ocean (Algeo et al., 1995). The causes of these formative changes, their interactions and their links to changes in climate are still poorly understood. Therefore, we explore the sensitivity of the Devonian climate to various boundary conditions using an intermediate-complexity climate model (Brugger et al., 2019). In contrast to Le Hir et al. (2011), we find only a minor biogeophysical effect of changes in vegetation cover due to unrealistically high soil albedo values used in the earlier study. In addition, our results cannot support the strong influence of orbital parameters on the Devonian climate, as simulated with a climate model with a strongly simplified ocean model (De Vleeschouwer et al., 2013, 2014, 2017). We can only reproduce the changes in Devonian climate suggested by proxy data by decreasing atmospheric CO2. Still, finding agreement between the evolution of sea surface temperatures reconstructed from proxy data (Joachimski et al., 2009) and our simulations remains challenging and suggests a lower δ18O ratio of Devonian seawater. Furthermore, our study of the sensitivity of the Devonian climate reveals a prevailing mode of climate variability on a timescale of decades to centuries. The quasi-periodic ocean temperature fluctuations are linked to a physical mechanism of changing sea-ice cover, ocean convection and overturning in high northern latitudes. In the second study of this thesis (Dahl et al., under review) a new reconstruction of atmospheric CO2 for the Devonian, which is based on CO2-sensitive carbon isotope fractionation in the earliest vascular plant fossils, suggests a much earlier drop of atmo- spheric CO2 concentration than previously reconstructed, followed by nearly constant CO2 concentrations during the Middle and Late Devonian. Our simulations for the Early Devonian with identical boundary conditions as in our Devonian sensitivity study (Brugger et al., 2019), but with a low atmospheric CO2 concentration of 500 ppm, show no direct conflict with available proxy and paleobotanical data and confirm that under the simulated climatic conditions carbon isotope fractionation represents a robust proxy for atmospheric CO2. To explain the earlier CO2 drop we suggest that early forms of vascular land plants have already strongly influenced weathering. This new perspective on the Devonian questions previous ideas about the climatic conditions and earlier explanations for the Devonian mass extinctions. The second mass extinction investigated in this thesis is the end-Cretaceous mass extinction (66 million years ago) which differs from the Devonian mass extinctions in terms of the processes involved and the timescale on which the extinctions occurred. In the two studies presented here (Brugger et al., 2017, 2021), we model the climatic effects of the Chicxulub impact, one of the proposed causes of the end-Cretaceous extinction, for the first millennium after the impact. The light-dimming effect of stratospheric sulfate aerosols causes severe cooling, with a decrease of global annual mean surface air temperature of at least 26◦C and a recovery to pre-impact temperatures after more than 30 years. The sudden surface cooling of the ocean induces deep convection which brings nutrients from the deep ocean via upwelling to the surface ocean. Using an ocean biogeochemistry model we explore the combined effect of ocean mixing and iron-rich dust originating from the impactor on the marine biosphere. As soon as light levels have recovered, we find a short, but prominent peak in marine net primary productivity. This newly discovered mechanism could result in toxic effects for marine near-surface ecosystems. Comparison of our model results to proxy data (Vellekoop et al., 2014, 2016, Hull et al., 2020) suggests that carbon release from the terrestrial biosphere is required in addition to the carbon dioxide which can be attributed to the target material. Surface ocean acidification caused by the addition of carbon dioxide and sulfur is only moderate. Taken together, the results indicate a significant contribution of the Chicxulub impact to the end-Cretaceous mass extinction by triggering multiple stressors for the Earth system. Although the sixth extinction we face today is characterized by human intervention in nature, this thesis shows that we can gain many insights into future extinctions from studying past mass extinctions, such as the importance of the rate of change (Rothman, 2017), the interplay of multiple stressors (Gunderson et al., 2016), and changes in the carbon cycle (Rothman, 2017, Tierney et al., 2020).
... Aerosols play a crucial role in Earth's climate and environmental processes, but their formation is complex even under normal circumstances, influenced by atmospheric conditions such as temperature, humidity, and air motion combined with the availability of particulate matter in the troposphere (Yang et al., 2022). Atmospheric aerosols have a major cooling effect through light scattering, influencing the amount of radiation reaching Earth's surface (Proud et al., 2022) and the principal killing mechanism following the Chicxulub impact was reduced light levels due to silica dust, soot and other particles forming nuclei for aerosols (Bardeen et al., 2017;Bralower et al., 2020;Ferrow et al., 2011;Gulick et al., 2019;Kaiho et al., 2016;Lyons et al., 2020;Schulte et al., 2010;Senel et al., 2023;Tabor et al., 2020;Toon et al., 2016;Vajda et al., 2001Vajda et al., , 2015Vajda and McLoughlin, 2014;Vellekoop et al., 2014). This led to limited photosynthesis and drastic decline in primary productivity Ferrow et al., 2011;Gulick et al., 2019;Kaiho et al., 2016;Lyons et al., 2020;Schulte et al., 2010;Senel et al., 2023;Tabor et al., 2020;Toon et al., 2016;Vajda et al., 2001Vajda et al., , 2015 but the mechanism behind the long-term darkness is debated. ...
... This has a direct bearing on the post-impact events and the recovery time for life. Models have shown that soot particles from wildfires would have formed a soot aerosol layer following the impact, hindering full penetration of sunlight for about five years, followed by a year with increased UV-radiation (Bardeen et al., 2017). However, other elements are highly unreactive, such as gold, platinum, iridium and copper (e.g. ...
The Chicxulub asteroid that ended the Cretaceous Era ~66.05 million years ago caused a prolonged time of global darkness the impact winter leading to mass extinctions. Elements from the asteroid, including the platinum group elements (PGEs) osmium, iridium and platinum are known from the globally distributed boundary clay but their carrier elements have so far been unknown. We identify, for the first time in detail, the presence of these PGEs within Chicxulub impact spherules and importantly, we identify their carrier elements. We show through synchrotron Nano-XRF how these PGEs occur in nanostructures as un-ordered cube- and/or needle-like crystals co-localizing with both siderophile and chalcophile elements including Co, Ni, Cu, Zn, and Pb, derived from the asteroid. These crystals are set within a matrix of iron-rich calcium and silica glass revealing the mix of vaporized target rock and the asteroid. The results provide insights into the combination of elements present in the spherules, indicating formation of new minerals. We argue that the nano-shards of unreactive elements such as platinum, iridium and copper acted as nuclei for aerosol formation and significantly contributed to a prolonged impact winter with darkness and cooling leading to a profound and long-term climate change.
... Moreover, no combined palaeoclimate scenario emitting concurrently all fine-grained ejecta components has been considered so far for the Chicxulub case 10 . Quantifying relative and combined roles of these fine-grained ejecta on the global K-Pg climate crisis is paramount to better understand groups ( Fig. 1c): (1) sulfur-bearing particles produced by water and sulfur-bearing gases, resulting from shock-vaporized evaporites 8,16 , (2) soot particles, probably generated from burning of organic-rich target rocks and release from global wildfires [20][21][22][23] and (3) silicate dust particles, derived from pulverization of the Yucatán crystalline basement 24,25 . Besides these fine-grained ejecta, the collision also released other products from the Yucatán target area into the atmosphere, such as carbon dioxide, water vapour and methane 16 . ...
... Since the mid-1980s, a soot-driven K-Pg impact winter has also been postulated 20 . The impact-generated soot particles could also have played a dominant role in the global blockage of solar irradiance and the prolonged post-impact cooling 30 , as fine soot constitutes a strong sunlight absorber 18,22,23 . Soot and charcoal remains found in the K-Pg boundary intervals around the world suggest widespread wildfires in the aftermath of the Chicxulub impact 20 . ...
The Chicxulub impact is thought to have triggered a global winter at the Cretaceous-Palaeogene (K-Pg) boundary 66 million years ago. Yet the climatic consequences of the various debris injected into the atmosphere following the Chicxulub impact remain unclear, and the exact killing mechanisms of the K-Pg mass extinction remain poorly constrained. Here we present palaeoclimate simulations based on sedimentological constraints from an expanded terrestrial K-Pg boundary deposit in North Dakota, United States, to evaluate the relative and combined effects of impact-generated silicate dust and sulfur, as well as soot from wildfires, on the post-impact climate. The measured volumetric size distribution of silicate dust suggests a larger contribution of fine dust (~0.8–8.0 μm) than previously appreciated. Our simulations of the atmospheric injection of such a plume of micrometre-sized silicate dust suggest a long atmospheric lifetime of 15yr, contributing to a global-average surface temperature falling by as much as 15°C. Simulated changes in photosynthetic active solar radiation support a dust-induced photosynthetic shut-down for almost 2 yr post-impact. We suggest that, together with additional cooling contributions from soot and sulfur, this is consistent with the catastrophic collapse of primary productivity in the aftermath of the Chicxulub impact.
... Bolide impact models suggest an intense heat pulse in the first minutes to hours after impact caused by the return flux of larger ejecta and flash heating of the atmosphere (Lewis et al., 1982;Melosh et al., 1990); an "impact winter" lasting months to millennia due to atmospheric loading of dust, soot, and sulfate aerosols (Pope et al., 1994;Bardeen et al., 2017;Brugger et al., 2017); and global warming caused by CO 2 from impact-volatilized carbonates (and wildfires) beginning 10 3 yr after impact (O'Keefe and Ahrens, 1989). Establishing a relationship between Deccan volcanism and climate change at the K-Pg boundary is limited by difficulties in dating the lava flows (Schoene et al., 2019;Sprain et al., 2019) and constraining the amounts and rates of associated CO 2 and SO 2 release (Self et al., 2006;Schmidt et al., 2016). ...
... Latest Cretaceous warmth was weakly enhanced by 0.7 °C between the last ∼4 k.y. of the Cretaceous and first ∼10 k.y. of the Paleogene (Fig. 3). Climate models suggest that the impact winter lasted only years to decades (Pope et al., 1994;Tabor et al., 2016;Bardeen et al., 2017), which is below the resolution of our record. Our data therefore do not 1 Supplemental Material. ...
... Bolide impact models suggest an intense heat pulse in the first minutes to hours after impact caused by the return flux of larger ejecta and flash heating of the atmosphere (Lewis et al., 1982;Melosh et al., 1990); an "impact winter" lasting months to millennia due to atmospheric loading of dust, soot, and sulfate aerosols (Pope et al., 1994;Bardeen et al., 2017;Brugger et al., 2017); and global warming caused by CO 2 from impact-volatilized carbonates (and wildfires) beginning 10 3 yr after impact (O'Keefe and Ahrens, 1989). Establishing a relationship between Deccan volcanism and climate change at the K-Pg boundary is limited by difficulties in dating the lava flows (Schoene et al., 2019;Sprain et al., 2019) and constraining the amounts and rates of associated CO 2 and SO 2 release (Self et al., 2006;Schmidt et al., 2016). ...
... Latest Cretaceous warmth was weakly enhanced by 0.7 °C between the last ∼4 k.y. of the Cretaceous and first ∼10 k.y. of the Paleogene (Fig. 3). Climate models suggest that the impact winter lasted only years to decades (Pope et al., 1994;Tabor et al., 2016;Bardeen et al., 2017), which is below the resolution of our record. Our data therefore do not 1 Supplemental Material. ...
The Cretaceous-Paleogene (K-Pg) boundary marks one of the five major mass extinctions of the Phanerozoic. The ways in which the climate system responded to a bolide impact and extensive volcanism at this time over different time scales are highly debated. We used the distribution of branched tetraether lipids (brGDGT) from fossil peats at two sites in Saskatchewan, Canada (paleolatitude ~55°N), to generate a high-resolution (millennial) record of mean annual air temperature (MAAT) spanning the last ~4 k.y. of the Cretaceous and the first ~30 k.y. of the Paleogene. Our study shows that MAATs ranged from 16 to 29 °C, with the highest value in the first millennia of the Paleogene. The earliest Paleogene averaged ~25 °C—maintaining or enhancing warmth from the latest Cretaceous—followed by a general cooling to ~20 °C over the following ~30 k.y. No abrupt postboundary cooling (e.g., an “impact winter”) or abrupt warming is evident in our data, implying that if such phenomena occurred, their duration was relatively short-lived (i.e., sub-millennial-scale). Further, no long-term impact- or volcanism-driven warming is evident. The range of temperature change observed is considerably greater than that derived from marine proxy records over the same time interval. Our findings therefore more properly place bounds on the magnitude and duration of temperature change on land during this critical interval—the main setting for the demise of nonavian dinosaurs and the rise of mammals.
... Bolide impact models suggest an intense heat pulse in the first minutes to hours after impact caused by the return flux of larger ejecta and flash heating of the atmosphere (Lewis et al., 1982;Melosh et al., 1990); an "impact winter" lasting months to millennia due to atmospheric loading of dust, soot, and sulfate aerosols (Pope et al., 1994;Bardeen et al., 2017;Brugger et al., 2017); and global warming caused by CO 2 from impact-volatilized carbonates (and wildfires) beginning 10 3 yr after impact (O'Keefe and Ahrens, 1989). Establishing a relationship between Deccan volcanism and climate change at the K-Pg boundary is limited by difficulties in dating the lava flows (Schoene et al., 2019;Sprain et al., 2019) and constraining the amounts and rates of associated CO 2 and SO 2 release (Self et al., 2006;Schmidt et al., 2016). ...
... Latest Cretaceous warmth was weakly enhanced by 0.7 °C between the last ∼4 k.y. of the Cretaceous and first ∼10 k.y. of the Paleogene (Fig. 3). Climate models suggest that the impact winter lasted only years to decades (Pope et al., 1994;Tabor et al., 2016;Bardeen et al., 2017), which is below the resolution of our record. Our data therefore do not 1 Supplemental Material. ...
The Cretaceous-Paleogene (K-Pg) boundary marks one of the five major mass extinctions of the Phanerozoic. How the climate system responded to a bolide impact and extensive volcanism at this time over different timescales is highly debated. Here we use the distribution of branched tetraether lipids (brGDGT) from fossil peats at two sites in Saskatchewan, Canada (paleolatitude ~55°N), to generate a high-resolution (millennial) record of mean annual air temperature (MAAT) spanning the last ~4 ka of the Cretaceous and first ~30 ka of the Paleogene. Our study shows that MAATs ranged from 16–29°C, with the highest value in the first millennia of the Paleogene/ The earliest Paleogene averaged ~25°C—maintaining or enhancing warmth from the latest Cretaceous—followed by a general cooling to ~20°C over the following ~30 ka. No abrupt post-boundary cooling (e.g., an “impact winter”) or abrupt warming are evident in our data, implying that if such phenomena occurred, their duration was relatively short-lived (i.e., sub-millennial). Further, no long-term impact- or volcanism-driven warming is evident. The range of temperature change observed is considerably greater than that derived from marine proxy records over the same time interval. Our findings therefore more properly place bounds on the magnitude and duration of temperature change on land during this critical interval—the main setting for the demise of non-avian dinosaurs and the rise of mammals.
... While plumes provide dramatic images, they are not a significant hazard until the volume of material deposited in the atmosphere and the fallout from it have a noticeable effect on crops downwind. At the extreme end, effects on the climate are the dominant hazard from multi-kilometer asteroids (Bardeen et al. 2017). ...
... However, asteroid impacts generally do not release sulfur gases. Rather, global cooling following those impacts is assumed to be driven by fine (< 1 um) stratospheric dust particles kicked up by the impact (Covey et al. 1994), or by soot from fires (Bardeen et al. 2017). The importance of these two mechanisms depends on how much of these materials are injected, and how long they remain. ...
Modern civilization has no collective experience with possible wide-ranging effects from a medium-sized asteroid impactor. Currently, modeling efforts that predict initial effects from a meteor impact or airburst provide needed information for initial preparation and evacuation plans, but longer-term cascading hazards are not typically considered. However, more common natural disasters, such as volcanic eruptions, earthquakes, wildfires, dust storms, and hurricanes, are likely analogs that can provide the scope and scale of these potential effects. These events, especially the larger events with cascading effects, are key for understanding the scope and complexity of mitigation, relief, and recovery efforts for a medium-sized asteroid impact event. This paper reviews the initial and cascading effects of these natural hazards, describes the state of the art for modeling these hazards, and discusses the relevance of these hazards to expected long-term effects of an asteroid impact. Emergency managers, resource managers and planners, and research scientists involved in mitigation and recovery efforts would likely derive significant benefit from a framework linking multiple hazard models to provide a seamless sequence of related forecasts.
... While it is known that extreme global cooling events can trigger a rate-dependent hysteresis in the ocean physical state (Bardeen et al., 2017;Mills et al., 2014;Slawinska & Robock, 2018), here, we explore this in detail, and report for the first time, that the ocean also enters a new biogeochemical and ecosystem state. Our study aims to investigate what is driving the new state in global marine productivity, how long this new state is likely to persist, if there is something fundamentally distinct about a large war relative to the smaller cooling events, and if other aspects of the ocean are also affected differently in the more extreme cooling case. ...
... Atmospheric circulation and chemistry are simulated in this version of CESM1 using the Whole Atmosphere Community Climate Model (WACCM; Marsh et al., 2013) with nominal 2° resolution and 66 vertical levels, a model top at ∼145 km, and uses the Rapid Radiative Transfer Model for GCMs (Iacono et al., 2000). The Community Aerosol and Radiation Model for Atmospheres (CARMA; Toon et al., 1988;Bardeen et al., 2008) is coupled with WACCM to simulate the injection, lofting, advection, and removal of soot aerosols in the troposphere and stratosphere, and their subsequent impact on climate (Bardeen et al., 2017. CARMA has 21 different size bins for the aerosols with different optical properties, allowing particle growth to change the amount of absorption and scattering. ...
Nuclear war would produce dire global consequences for humans and our environment. We simulated climate impacts of US-Russia and India-Pakistan nuclear wars in an Earth System Model, here, we report on the ocean impacts. Like volcanic eruptions and large forest fires, firestorms from nuclear war would transport light-blocking aerosols to the stratosphere, resulting in global cooling. The ocean responds over two timescales: a rapid cooling event and a long recovery, indicating a hysteresis response of the ocean to global cooling. Surface cooling drives sea ice expansion, enhanced meridional overturning, and intensified ocean vertical mixing that is expanded, deeper, and longer lasting. Phytoplankton production and community structure are highly modified by perturbations to light, temperature, and nutrients, resulting in initial decimation of production, especially at high latitudes. A new physical and biogeochemical ocean state results, characterized by shallower pycnoclines, thermoclines, and nutriclines, ventilated deep water masses, and thicker Arctic sea ice. Persistent changes in nutrient limitation drive a shift in phytoplankton community structure, resulting in increased diatom populations, which in turn increase iron scavenging and iron limitation, especially at high latitudes. In the largest US-Russia scenario (150 Tg), ocean recovery is likely on the order of decades at the surface and hundreds of years at depth, while changes to Arctic sea-ice will likely last thousands of years, effectively a “Nuclear Little Ice Age.” Marine ecosystems would be highly disrupted by both the initial perturbation and in the new ocean state, resulting in long-term, global impacts to ecosystem services such as fisheries.
... To illustrate scale, 150 Tg is roughly eleven times heavier than all three pyramids of Giza combined (Hemeda and Sonbol 2020). As perspective, the K-Pg bolide impact that caused the mass extinction of most dinosaurs 65 million years ago lofted anywhere from 750 to 35 000 Tg of fine soot into the atmosphere (Bardeen et al. 2017). Although this is a large range, climate models have predicted that even the smaller 750 Tg estimate would have reduced sunlight to below 1% and caused extinctions, and this is only five times the current estimate for worldwide nuclear war (Bardeen et al. 2017). ...
... As perspective, the K-Pg bolide impact that caused the mass extinction of most dinosaurs 65 million years ago lofted anywhere from 750 to 35 000 Tg of fine soot into the atmosphere (Bardeen et al. 2017). Although this is a large range, climate models have predicted that even the smaller 750 Tg estimate would have reduced sunlight to below 1% and caused extinctions, and this is only five times the current estimate for worldwide nuclear war (Bardeen et al. 2017). ...
A global sun-blocking catastrophe is more plausible than anyone would like to think. Models have consistently shown the devastating effects these events could have to the world's agricultural systems for upwards of 15 years. New shade-, drought-, and cool-tolerant crops and more food stockpile sources must be found if there would be any hope of feeding the global population in such a scenario. Wild edible plants (WEPs) are important buffers of food security to indigenous peoples, impoverished peoples, and those in areas with erratic growing seasons across the globe. Here, we suggest WEP species that have the potential to be scaled up through cultivation in post-catastrophe conditions, and the use of foraged food stockpiles to function as stopgap foods until conventional agriculture returns. We also propose policy initiatives for habitat protection, education programs, and general preparedness.
... However, a supported configuration is also offered to use the spectral element (SE) dynamical core on a cubed sphere grid (Lauritzen et al. 2014). The FV dynamical core has been modified to improve numerical stability in more strongly forced atmospheres by incrementally applying physics tendencies for temperature and wind speed evenly throughout the dynamical substeps, rather than only at the beginning of the dynamics step (Bardeen et al. 2017). Several studies (Lebonnois et al. 2012;Lauritzen et al. 2014) found that NCAR's FV dynamical core does not properly conserve angular momentum for slow rotating planets and inadequately captures upper-level superrotation for Venus and Titan atmospheres (Larson et al. 2014). ...
... Source code is also provided that links ExoCAM and ExoRT with the Community Aerosol and Radiation Model for Atmospheres (CARMA) model (Turco et al. 1979;Toon et al. 1988; Bardeen et al. 2008). CARMA is a flexible cloud and aerosol model that resides on the publicly supported trunk of CESM and can be flexibly configured to treat any manner of cloud and aerosol problem, including photochemical hazes (Wolf & Toon 2010;Larson et al. 2014), Martian ice clouds and dust (Hartwick et al. 2019), historical volcanic eruptions (English et al. 2013), geoengineering (English et al. 2012), nuclear winter (Mills et al. 2008), extinction level asteroid impacts (Bardeen et al. 2017), forest fires (Yu et al. 2019) and a wide variety of other problems. At the time of the finalizing of this paper, we offer a photochemical haze CARMA configuration (Wolf & Toon 2010) linkable to our 28 spectral interval radiation (see n28archean.haze in ExoRT); however, developing a more formal ExoCAM-CARMA integration with n68equiv for multiple cloud and aerosol types is an active area of work in the coming years. ...
The TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI) project was initiated to compare 3D climate models that are commonly used for predicting theoretical climates of habitable zone extrasolar planets. One of the core models studied as part of THAI is ExoCAM, an independently curated exoplanet branch of the National Center for Atmospheric Research (NCAR) Community Earth System Model (CESM) version 1.2.1. ExoCAM has been used for studying atmospheres of terrestrial extrasolar planets around a variety of stars. To accompany the THAI project and provide a primary reference, here we describe ExoCAM and what makes it unique from standard configurations of CESM. Furthermore, we also conduct a series of intramodel sensitivity tests of relevant moist physical tuning parameters while using the THAI protocol as our starting point. A common criticism of 3D climate models used for exoplanet modeling is that cloud and convection routines often contain free parameters that are tuned to the modern Earth, and thus may be a source of uncertainty in evaluating exoplanet climates. Here, we explore sensitivities to numerous configuration and parameter selections, including a recently updated radiation scheme, a different cloud and convection physics package, different cloud and precipitation tuning parameters, and a different sea ice albedo. Improvements to our radiation scheme and the modification of cloud particle sizes have the largest effect on global mean temperatures, with variations up to ~10 K, highlighting the requirement for accurate radiative transfer and the importance of cloud microphysics for simulating exoplanetary climates. However for the vast majority of sensitivity tests, climate differences are small. For all cases studied, intramodel differences do not bias general conclusions regarding climate states and habitability.
... In addition, a massive release of finegrained ejecta reduced the amount of solar radiation reaching the Earth's surface, leading to a global impact winter state 13 . These fine-grained ejecta consist of silicate dust originating from the pulverization of the deep Yucatán granitic basement following the impact 5,13,14 , sulfate aerosols formed from the vaporized evaporites and seawater 15 , and soot from buried hydrocarbons 16,17 and possible impact-induced wildfires 16,[18][19][20] . The thus-induced impact winter triggered extremely cold conditions and a blockage of photosynthesis that affected Earth at the scale of years to decades 13,21-24 . ...
The Chicxulub asteroid impact event at the Cretaceous-Paleogene (K-Pg) boundary ~66 Myr ago is widely considered responsible for the mass extinction event leading to the demise of the non-avian dinosaurs. Short-term cooling due to massive release of climate-active agents is hypothesized to have been crucial, with S-bearing gases originating from the target rock vaporization considered an important driving force. Yet, the magnitude of the S release remains poorly constrained. Here we empirically estimate the amount of impact-released S relying on the concentration of S and its isotopic composition within the impact structure and a set of terrestrial K-Pg boundary ejecta sites. The average value of 67 ± 39 Gt obtained is ~5-fold lower than previous numerical estimates. The lower mass of S-released may indicate a less prominent role for S emission leading to a milder impact winter with key implications for species survival during the first years following the impact.
... The Chicxulub meteorite impact [Mexico (4)(5)(6)] and eruption of the Deccan Traps [India (7)(8)(9)(10)(11)(12)] have emerged as the primary-but fiercely contested-trigger mechanisms for the mass extinction and global climate change. Models of the climate response to the meteorite impact include an "impact winter" lasting months to millennia due to atmospheric loading of dust, soot, and sulfate aerosols (13)(14)(15)(16)(17), and longer-term warming caused by CO 2 released by wildfires and/or impact-volatilized carbonates (18). Two principal climate models are associated with Deccan volcanism: first, global warming, caused by eruption-, venting-, and contact-metamorphism-derived CO 2 (19) and sustained over thousands to hundreds of thousands of years [e.g., (20)]; and second, global cooling driven by the conversion of SO 2 into sulfate aerosols, but lasting only for the duration of the eruption (21)(22)(23). ...
Alongside the Chicxulub meteorite impact, Deccan volcanism is considered a primary trigger for the Cretaceous–Paleogene (K–Pg) mass extinction. Models suggest that volcanic outgassing of carbon and sulfur—potent environmental stressors—drove global temperature change, but the relative timing, duration, and magnitude of such change remains uncertain. Here, we use the organic paleothermometer MBT′ 5me and the carbon-isotope composition of two K–Pg-spanning lignites from the western Unites States, to test models of volcanogenic air temperature change in the ~100 kyr before the mass extinction. Our records show long-term warming of ~3°C, probably driven by Deccan CO 2 emissions, and reveal a transient (<10 kyr) ~5°C cooling event, coinciding with the peak of the Poladpur “pulse” of Deccan eruption ~30 kyr before the K–Pg boundary. This cooling was likely caused by the aerosolization of volcanogenic sulfur. Temperatures returned to pre-event values before the mass extinction, suggesting that, from the terrestrial perspective, volcanogenic climate change was not the primary cause of K–Pg extinction.
... Alternatively, others have considered the risk of abrupt cooling due to natural or human causes (e.g., [3,4]). At least three drivers of global cooling are global catastrophic risks [5]: nuclear warfare [4,6], asteroid impacts [7,8], and large volcanic eruptions [3,[9][10][11][12]. All involve injections of absorbing and/or reflective materials into the upper atmosphere, reducing tropospheric and surface insolation. ...
Large volcanic eruptions, such as the prehistoric Yellowstone eruption, induce abrupt global cooling—by some estimates at a rate of ~1 °C/year, lasting for more than a decade. An abrupt global cooling of several °C—even if only lasting a few years—would present immediate, drastic stress on biodiversity and food production. This cooling poses a global catastrophic risk to human society beyond the immediate and direct impact of eruptions. Using a simple climate model, this paper discusses the possibility of counteracting large volcanic cooling with the intentional release of greenhouse gases. Longer-lived compounds (e.g., CO2 and CH4) are unsuitable for this purpose, but selected fluorinated gases (F-gases), either individually or in combinations, could be released at gigaton scale to offset large volcanic cooling substantially. We identify candidate F-gases (e.g., C4F6 and CH3F) and derive radiative and chemical properties of ‘ideal’ compounds matching specific cooling events. Geophysical constraints on manufacturing and stockpiling due to mineral availability are considered, alongside technical and economic implications based on present-day market assumptions. The effects and uncertainty due to atmospheric chemistry related to aerosol injection, F-gases release, and solar dimming are discussed in the context of large volcanic perturbation. The caveats and future steps using more complex chemistry–climate models are discussed. Despite the speculative nature of the magnitude and composition of F-gases, our conceptual analysis has implications for testing the possibility of mitigating certain global catastrophic cooling risks (e.g., nuclear winter, asteroid impact, and glacier transition) via intentional intervention.
... The LGM was up to 7.4°C cooler than the pre-industrial by some measures. In addition to the LGM, this same model was used by Bardeen et al. (2017) and Tabor et al. (2020) to simulate a very large asteroid impact at the K-Pg boundary 66 million years ago with even more soot than what we used for nuclear winter. The model remained stable in their simulations. ...
Plain Language Summary
The topic of potential nuclear war has come to the forefront recently given the context of the Ukraine war. The smoke and soot released into the atmosphere from nuclear explosions and subsequent fires would dramatically affect both global and regional climates. By blocking solar radiation, these particles could trigger significant global cooling, disrupting the atmospheric circulation and altering oceanic conditions. This work shows a consistent decrease in global‐scale tropical cyclones (TCs) in the simulation of nuclear war between the United States and Russia with a release of 150 Tg black carbon. In addition to unfavorable thermodynamic conditions caused by large‐scale cooling, we examine the potential driver mechanisms of the global reduction in TCs in response to nuclear war. Due to the spatially uneven cooling, there would be an anomalous sea surface temperature gradient, as well as low‐level westerly anomalies. Near the tropopause, the reduced pole–equator temperature gradient induces strong upper‐level easterly anomalies. The anomalous zonal winds could further alter the zonal vertical circulation, reducing global TCs via decreases in upward motion and increases in vertical wind shear. This study improves our understanding of the impact of nuclear war on the TC environment as well as the large‐scale atmospheric and oceanic environment.
... In WACCM4, this includes the integration of TUV and modules used to simulate aerosol injections relevant for asteroid impacts (Bardeen et al., 2017(Bardeen et al., , 2021. New BWFs relevant for the phytoplankton species simulated in MARBL were added to TUV. ...
Ultraviolet (UV) radiation can damage DNA and kill cells. We use laboratory and observational studies of the harmful effect of UV radiation on marine photosynthesizers to inform the implementation of a UV radiation damage function for phytoplankton photosynthesis in a modified version of the Community Earth System Model version 2 (CESM2-UVphyto). CESM2-UVphyto is capable of simulating UV inhibition of photosynthesis among modelled phytoplankton and ocean column 5 penetration of UV-A and UV-B radiation. We conduct a series of simulations with CESM2-UVphyto using the Marine Biogeochemistry Library (MARBL) ecosystem model to calibrate estimates of the sensitivity of phytoplankton productivity to UV radiation. Results indicate that increased UV radiation shifts the vertical distribution of phytoplankton biomass and productivity deeper into the column, causes a moderate decline in total global productivity, and changes phytoplankton community structure to favor diatoms. Our new CESM2-UVphyto model configuration can be used to quantify the potential ocean biogeochemical 10 and ecosystem impacts resulting from events that disturb the stratospheric ozone layer, such as an asteroid impact, a volcanic eruption, a nuclear war, and stratospheric aerosol injection-based geoengineering.
... The maritime setting may have buffered the climate extremes of the impact winter sufficiently that land plants were not affected to the same degree as their counterparts in more continental areas. The possible mid-latitude coastal position at the time of deposition may have modulated the environmental consequences of the K/Pg event (Bardeen et al., 2017;Brugger et al., 2017;Morgan et al., 2022;Wilf et al., 2023). Palynofloras from the Southern Hemisphere, namely Patagonia and New Zealand, have been known for faster recoveries and reduced palynological extinctions relative to the Northern Hemisphere, which may reflect such a low latitude buffering effect (Wilf et al., 2023). ...
A well-preserved suite of ~163 spore-pollen taxa from a recently discovered K/Pg interval within the maritime Oyster Bay Formation, Vancouver Island, British Columbia, Canada, reveals a pattern of floral turnover across the boundary event with local extirpations of ~15% of Cretaceous taxa. Along the margin of the eastern North Pacific, a shift occurred in near-coastal vegetation composition from uppermost Cretaceous diverse fern and bryophyte-dominated communities to Danian conifer-dominated forests with a fern understory. The 'fern spore spike' common in other K/Pg records was not detected within the sandstone to mudstone sequence. Spore-pollen assemblages preserved herein align with those of the Continental Margin floristic province. Palm pollen is noteworthy in the studied sections including Arecipites spp. (aff. Arecaceae), Spinizonocolpites spp. (aff. Nypa) and Pandaniidites typicus (aff. Pandanus) suggesting a warm, frost-free, subtropical climate prevailed across the K/Pg interval. The presence of numerous endemic spore-pollen taxa is indicative of geographic isolation from the North American Western Interior. Maritime climate buffering along the west coast of North America contributed to microrefugia permitting greater stability in terrestrial plant communities than in continental regions.
... Thus, the first effect usually considered is the distance from ground zero and the proximal effects of shockwaves, tsunamis, and large ejecta. A second factor highly relevant to plants is maritime and latitudinal buffering of the impact winter ( Figure 1; Bardeen et al., 2017;Brugger et al., 2017;Morgan et al., 2022). Both distance and buffering gradients predict large extinctions in western North America and the Neotropics, especially if the tropics froze, and less severe extinctions in the maritime areas of temperate Gondwana, where there is a growing list of survivor taxa (e.g., McLoughlin et al., 2011;summarized in Wilf et al., 2013). ...
The Cretaceous-Paleogene (K-Pg) mass extinction was geologically instantaneous, causing the most drastic extinction rates in Earth's History. The rapid species losses and environmental destruction from the Chicxulub impact at 66.02 Ma made the K-Pg the most comparable past event to today's projected "sixth" mass extinction. The extinction famously eliminated major clades of animals and plankton. However, for land plants, losses primarily occurred among species observed in regional studies but left no global trace at the family or major-clade level, leading to questions about whether there was a significant K-Pg plant extinction. We review emerging paleobotanical data from the Americas and argue that the evidence strongly favors profound (generally >50%), geographically heterogeneous species losses and recovery consistent with mass extinction. The heterogeneity appears to reflect several factors, including distance from the impact site and marine and latitudinal buffering of the impact winter. The ensuing transformations have affected all land life, including true angiosperm dominance in the world's forests, the birth of the hyperdiverse Neotropical rainforest biome, and evolutionary radiations leading to many crown angiosperm clades. Although the worst outcomes are still preventable, the sixth mass extinction could mirror the K-Pg event by eliminating comparable numbers of plant species in a geologic instant, impoverishing and eventually transforming terrestrial ecosystems while having little effect on global plant-family diversity.
... Температура на континентах упала на 28 градусов 0С , в океанах на 11 0С. Примерно 16 лет температура была ниже +3 0С [12][13][14]. ...
Представлено объяснение наличия месторождений каменного угля в приполярных областях Северного полушария. Миллионы лет назад в ныне приполярных холодных областях климат соответствовал благоприятному произрастанию папоротниковидных, дендровидных и других теплолюбивых растений. Причиной изменения климата в Северном полушарии на основании существующих фактических материалов, представляется вследствии падения астероида или нескольких размером более 10 км с космической скоростью на земную поверхность, что вызвало поворот оси вращения Земли на 20-25 градусов. Это привело к значительному уменьшению солнечной инсоляции в северном полушарии и превращению его в холодную полупустыню.
... To simulate a nuclear war, soot (black carbon) is injected into the upper troposphere and lower stratosphere in a layer between 100 hPa and 300 hPa over a one week period starting on 15 May of what will be referred to as "Year 0." Soot is evenly distributed horizontally over the countries involved in each nuclear war scenario. We use the Community Aerosol and Radiation Model for Atmospheres (CARMA), a sectional aerosol model coupled to WACCM4, which treats black carbon aerosols as fractal particles (Bardeen et al., 2017). CARMA has 21 different size bins for the aerosols with different optical properties, allowing particle growth to change the amount of absorption and scattering. ...
A large‐scale nuclear war could inject massive amounts of soot into the stratosphere, triggering rapid global climate change. In climate model simulations of nuclear war, global cooling contributes to an expansion of sea ice in the Northern Hemisphere. However, in the Southern Hemisphere (SH), an initial expansion of sea ice shifts suddenly to a 30% loss of sea ice volume over the course of a single melting season in the largest nuclear war simulation. In smaller nuclear war simulations an expansion in sea ice is instead observed which lasts for approximately 15 years. In contrast, in the largest nuclear war simulation, Antarctic sea ice remains below the long term control mean for 15 years, indicating a threshold that must be crossed to cause the response. Declining sea ice in the SH following a global cooling event has been previously attributed to shifts in the zonal winds around Antarctica, which can reduce the strength of the Weddell Gyre. In climate model simulations of nuclear war, the primary mechanisms responsible for Antarctic sea ice loss are: (a) enhanced atmospheric poleward heat transport through teleconnections with a strong nuclear war‐driven El Niño, (b) increased upwelling of warm subsurface waters in the Weddell Sea due to changes in wind stress curl, and (c) decreased equatorward Ekman transport due to weakened Southern Ocean westerlies. The prospect of sudden Antarctic sea ice loss after an episode of global cooling may have implications for solar geoengineering and further motivates this study of the underlying mechanisms of change.
... In terms of rate, it appears that the current loss of species may be the fastest ever experienced by the biosphere (Barnosky et al., 2011), except for the end-Cretaceous. The end-Cretaceous is likely to have been even faster than today's losses, especially if the driver of the extinctions was rapid bolide-induced climate change (Chiarenza et al., 2020), including the blocking of sunlight due to the impact debris with a resulting decrease in photosynthesis for a few years (Bardeen et al., 2017;Tabor et al., 2020), and/or due to cooling of 26°C or more with 3-16 years of freezing temperatures (Brugger et al., 2017). The Cambrian biomere extinctions (Palmer, 1984(Palmer, , 1998Saltzman et al., 2015) may also have been very rapid, depending on the timescale over which anoxic waters flooded the shallow water shelves. ...
Over 40 years ago, Raup and Sepkoski identified five episodes of elevated extinction in the marine fossil record that were thought to be statistically distinct, thus warranting the term the “Big Five” mass extinctions. Since then, the term has become part of standard vocabulary, especially with the naming of the current biodiversity crisis as the “sixth mass extinction.” However, there is no general agreement on which time intervals should be viewed as mass extinctions, in part because the Big Five turn out not to be statistically distinct from background rates of extinction, and in part, because other intervals of time have even higher extinction rates, in the Cambrian and early Ordovician. Nonetheless, the Big Five represent the five largest events since the early Ordovician, including in analyses that attempt to compensate for the incompleteness of the fossil and rock records. In the last 40 years, we have learned a great deal about the causes of many of the major and minor extinction events and are beginning to unravel the mechanisms that translated the initial environmental disturbances into extinction. However, for many of the events, further understanding will require going back to the outcrop, where the patchy distribution of environments and pervasive temporal gaps in the rock record challenge our ability to establish true extinction patterns. As for the current biodiversity crisis, there is no doubt that the rate of extinction is among the highest ever experienced by the biosphere, perhaps the second highest after the end-Cretaceous bolide impact. However (and fortunately), the absolute number of extinctions is still relatively small – there is still time to prevent this becoming a genuine mass extinction. Given the arbitrariness of calling out the Big Five, perhaps the current crisis should be called the “incipient Anthropocene mass extinction” rather than the “sixth mass extinction.”
... While it is usually construed as a short-term event, mounting evidence suggests that extinctions began before the Chicxulub impact. Climate change caused directly by the impact, while severe, likely occurred on an extremely brief timescale of 4-5 years (Bardeena et al., 2017). Additionally, global climate was already changing in the Maas-trichtian (Bralower et al., 2002). ...
Faunal turnover is a pattern of diversification and extinction in taxa throughout the geologic record. Patterns of repeated faunal turnovers are referred to as faunal progression, demonstrated by Decapoda in clawed lobsters and podotrematous and heterotrematous crabs. The transition between podotrematous and heterotrematous crabs is the most recent. Among these, section Raninoida Ahyong, Lai, Sharkey, Colgan & Ng, 2007, commonly called “frog crabs,” constitutes a major monophyletic group with podotrematous body forms, and the focus of our study. Declines in raninoidan diversity were aligned with mass extinction events and major climate shifts, especially cooling. Likewise, diversification within Raninoida occurred in warm, greenhouse climates. Thus, a major factor in patterns of faunal turnover in Decapoda is shown to be environmental conditions. Raninoidan families exhibiting adaptations facilitating back-burrowing preferentially survived the end-Cretaceous mass extinction event, whereas raninoidans lacking such adaptions did not go completely extinct at the end-Cretaceous but failed to recover diversity. Given the diversification of heterotrematous crabs into a wide variety of ecological niches throughout the Cenozoic, competition may be a secondary, but still crucial, factor in this faunal turnover.
... At the end of the Cretaceous Period (66.0 Ma), the impact of an asteroid on the Yucatán carbonate platform in the southern Gulf of Mexico caused the extinction of ∼75% of marine species (Alvarez et al., 1980;Hildebrand et al., 1991;Jablonski, 1995;Schulte et al., 2010;Smit & Hertogen, 1980), including ∼90% of pelagic calcifiers such as planktic foraminifera and calcareous nannoplankton (Bown et al., 2004;Fraass et al., 2015;Lowery et al., 2020). Dust and sulfate aerosols ejected from the evaporite-rich carbonates of the target rock, soot from wildfires, and petrogenic carbon from the crater (Kaiho et al., 2016;Lyons et al., 2020) blocked the sun, resulting in severe short-term cooling (Artemieva & Morgan, 2020;Artemieva et al., 2017;Bardeen et al., 2017;Brugger et al., 2017;Vellekoop et al., 2014Vellekoop et al., , 2016Gulick et al., 2019;Pope et al., 1994;Wolbach et al., 1985) and collapse of the food chain due to a sharp decline in photosynthesis (D'Hondt et al., 1998;Kring, 2007;Gibbs et al., 2020;Zachos et al., 1989). These effects were short-lived, however, as most dust, soot, and aerosols were removed from the atmosphere on the order of years (Brugger et al., 2017;Tabor et al., 2020), and the oceans quickly became hospitable for life, even at ground zero in the Chicxulub crater (Lowery et al., 2018). ...
The Chicxulub impact caused a crash in productivity in the world's oceans which contributed to the extinction of ∼75% of marine species. In the immediate aftermath of the extinction, export productivity was locally highly variable, with some sites, including the Chicxulub crater, recording elevated export production. The long-term transition back to more stable export productivity regimes has been poorly documented. Here, we present elemental abundances, foraminifer and calcareous nannoplankton assemblage counts, total organic carbon, and bulk carbonate carbon isotope data from the Chicxulub crater to reconstruct changes in export productivity during the first 3 Myr of the Paleocene. We show that export production was elevated for the first 320 kyr of the Paleocene, declined from 320 kyr to 1.2 Myr, and then remained low thereafter. A key interval in this long decline occurred 900 kyr to 1.2 Myr post impact, as calcareous nannoplankton assemblages began to diversify. This interval is associated with fluctuations in water column stratification and terrigenous flux, but these variables are uncorrelated to export productivity. Instead, we postulate that the turnover in the phytoplankton community from a post-extinction assemblage dominated by picoplankton (which promoted nutrient recycling in the euphotic zone) to a Paleocene pelagic community dominated by relatively larger primary producers like calcareous nannoplankton (which more efficiently removed nutrients from surface waters, leading to oligotrophy) is responsible for the decline in export production in the southern Gulf of Mexico. Plain Language Summary The end Cretaceous mass extinction was caused by the impact of an asteroid in what is now the Yucatán Peninsula, México. The impact ejected aerosols and dust into the air that reduced sunlight transmission, causing a severe decline in photosynthesis and the collapse of marine food webs. However, the change in the amount of organic matter created by photosynthesizing plankton that was delivered to the seafloor (export productivity) was variable across the oceans. At some places, including the Chicxulub crater, export productivity was actually high immediately after the impact. We produced a ∼3-million-year record of export productivity in the crater to determine how long it remained elevated and why it eventually declined. Export production was very high for the first 320,000 years after the impact, declined from 320,000 to 1,200,000 years after the impact, and then remained low. We found that this production was not related to the input of nutrients nor the degree of stratification of the ocean, but instead was probably driven by the increase in the cell size of phytoplankton. Larger phytoplankton removed nutrients from the surface waters as they sank, prompting an increase in species which are better adapted to low-nutrient waters. LOWERY ET AL.
... Modifications have been brought to its dynamical core to improve numerical stability in more strongly forced atmospheres. They consist in incrementally applying physics tendencies for temperature and wind speed evenly throughout the dynamical time step, rather than only at the beginning of it (Bardeen et al. 2017). The number of points in the latitudinal and longitudinal directions is 72 and 46, respectively. ...
With the commissioning of powerful, new-generation telescopes such as the James Webb Space Telescope (JWST) and the ground-based Extremely Large Telescopes, the first characterization of a high molecular weight atmosphere around a temperate rocky exoplanet is imminent. Atmospheric simulations and synthetic observables of target exoplanets are essential to prepare and interpret these observations. Here we report the results of the first part of the TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI) project, which compares 3D numerical simulations performed with four state-of-the-art global climate models (ExoCAM, LMD-Generic, ROCKE-3D, Unified Model) for the potentially habitable target TRAPPIST-1e. In this first part, we present the results of dry atmospheric simulations. These simulations serve as a benchmark to test how radiative transfer, subgrid-scale mixing (dry turbulence and convection), and large-scale dynamics impact the climate of TRAPPIST-1e and consequently the transit spectroscopy signature as seen by JWST. To first order, the four models give results in good agreement. The intermodel spread in the global mean surface temperature amounts to 7 K (6 K) for the N 2 -dominated (CO 2 -dominated) atmosphere. The radiative fluxes are also remarkably similar (intermodel variations less than 5%), from the surface (1 bar) up to atmospheric pressures ∼5 mbar. Moderate differences between the models appear in the atmospheric circulation pattern (winds) and the (stratospheric) thermal structure. These differences arise between the models from (1) large-scale dynamics, because TRAPPIST-1e lies at the tipping point between two different circulation regimes (fast and Rhines rotators) in which the models can be alternatively trapped, and (2) parameterizations used in the upper atmosphere such as numerical damping.
... Asteroid impacts pose a credible threat to Earth. While impacts that cause global-scale devastation (such as the Chicxulub asteroid impact that preceded the end-Cretaceous mass extinction affecting the dinosaurs; Alvarez et al. 1980;Schulte et al. 2010; Bardeen et al. 2017) are unlikely; more recent examples, like the Tunguska (Robertson & Mathias 2019) and Chelyabinsk (Popova et al. 2013) events, attest that even asteroids tens of meters in size are a cause for concern. ...
Understanding how to deflect an incoming asteroid is of great importance and a focus of research undertaken internationally by the planetary defense community. Deflection of an asteroid by a kinetic impactor is one such mitigation method that has a high degree of technological maturity. In 2022, NASA’s planetary defense mission, the Double Asteroid Redirection Test (DART), will provide the first full-scale technology demonstration of a kinetic impactor. However, the DART mission is just a single test of one kinetic impactor design, prompting the question: Is it possible to optimize the design of a kinetic impactor to make it the most efficient deflector that it can be? In this paper, we use high-fidelity hydrodynamic simulations to examine the effect of five mission parameters (impactor mass, impact velocity, impactor composition, impactor geometry, and target strength) on three observables related to deflection efficiency (crater morphology, ejecta distribution, and momentum transfer) resulting from kinetic impacts. We find that the most significant mission parameters for determining postimpact factors are the impact velocity, impactor mass, and target strength. The impactor geometry and impactor composition emerge as statistically nonsignificant. Overall, we find that the ogive impactor geometry results in the highest variance in predicting the momentum enhancement factor ( β ), and that generally, impactors with smaller volumes and flat leading edges (plates and rings) produce smaller craters and less ejecta mass compared to impactors with larger volumes and sharper leading edges (spheres, ogives, and cones).
... Nevertheless, cells capable of photosynthesis had probably existed for over a billion years by 2.02 Ga (Blankenship, 2010). Thus, when the dust and aerosols from the impact filled the atmosphere and blocked the sunlight to the surface, they could have rendered photosynthesis impossible and thus may have caused drastic damage to organisms that relied on it (Bardeen et al., 2017). These materials in the atmosphere would have prevented sunlight from reaching the surface and caused global cooling. ...
The Vredefort impact structure, located in South Africa and formed 2.02 Ga, is the largest confirmed remnant impact crater on Earth. The widely accepted impactor diameter and velocity to form this crater are 15 km and 15 km/s, respectively, which produce a crater diameter of 172 km. This is much smaller than the most commonly cited estimates (250–280 km), and while previous results were able to match the geologic evidence known at that time, these impact parameters are not consistent with more recent geological constraints. Here, we conduct impact simulations to model the Vredefort crater formation with the shock physics code impact Simplified Arbitrary Lagrangian Eulerian (iSALE). Our numerical simulations show that combinations of the impactor diameter and impact velocity of 25 km and 15 km/s or 20 km and 25 km/s are able to recreate the larger crater size of ∼250 km. Moreover, these configurations can reproduce shock‐metamorphic features present in the impact structure today, including the distributions of breccia, shatter cones, planar deformation features in quartz and zircon, and melt. Our model also predicts that Vredefort and Karelia, Russia, where an ejecta layer from the impact was found, were approximately 2,000–2,500 km apart based on the layer thickness. Additionally, we use this model to predict the potential global effects of such a large impact by estimating the amount of climatically important gases released to the atmosphere at the time. Our work demonstrates the need to revisit previously estimated impactor parameters for large terrestrial craters in order to better characterize impact events on Earth and elsewhere.
... Bayesian networks have been developed, interpreted, and widely applied as models of causal relations between events (Pearl, 1988(Pearl, , 2009Spirtes, 2010;Spirtes et al., Fig. 1 A partial graph of the causal Alvarez theory regarding massive extinction at the Cretaceous-Paleogene boundary in terms of linked propositions. Sources include Jablonski and Chaloner (1994), Pope et al. (1997Pope et al. ( , 1998, Gale et al. (2001), Bardeen et al. (2017), and Henehan et al. (2019) 2000). But it is clear that they can equally well be taken as models of causal inference between propositions descriptive of those events: as theories. ...
Our scientific theories, like our cognitive structures in general, consist of propositions linked by evidential, explanatory, probabilistic, and logical connections. Those theoretical webs ‘impinge on the world at their edges,’ subject to a continuing barrage of incoming evidence (Quine 1951, 1953). Our credences in the various elements of those structures change in response to that continuing barrage of evidence, as do the perceived connections between them. Here we model scientific theories as Bayesian nets, with credences at nodes and conditional links between them modelled as conditional probabilities. We update those networks, in terms of both credences at nodes and conditional probabilities at links, through a temporal barrage of random incoming evidence. Robust patterns of punctuated equilibrium, suggestive of ‘normal science’ alternating with ‘paradigm shifts,’ emerge prominently in that change dynamics. The suggestion is that at least some of the phenomena at the core of the Kuhnian tradition are predictable in the typical dynamics of scientific theory change captured as Bayesian nets under even a random evidence barrage.
... Climate change is the most common explanation for all of them. In the case of dinosaurs, researchers have modelled that the amount of sulphur and carbon dioxide instantly exploded into the atmosphere by the Chicxulub asteroid would have started a nuclear winter lasting more than 30 years and characterized by a planetary drop of temperatures up to 20 degrees Celsius [3]. ...
Differently from previous mass extinct events identifying the main culprits of the ongoing diversity loss is relatively straightforward. We kill animals, we destroy habitats, we pollute the environment, we promote biological invasions. And, of course, we fuel global warming. These mechanisms are not the only ones behind the ongoing Sixth Mass Extinction event. It is becoming increasingly clear that cascading effects triggered by primary extinctions (e.g. extinctions caused directly by climate change, habitat destruction or over-harvesting) might play a lead role in the current biodiversity crisis. Yet, interpreting, modelling and possibly predicting how different kinds of perturbations propagate across the multitude of elusive links connecting species in networks of ecological interactions is challenging. However, tackling this challenge is fundamental not only to advance our understanding of how ecosystems respond to global change threats but also to strengthen the efficacy of conservation and restoration actions. Novel theoretical knowledge generated by physicists and mathematicians offers promising tools to bring classical ecological concepts such as that of keystone species into modern days. In particular, formal metrics of ecosystem-wide species importance—where species are considered as nodes in an ecological network—appear as valuable alternatives to identifying conservation priorities based on less objective, often purely emotional criteria. Unfortunately, due to their overarching complexity and dynamic nature, we are still a long way from a magic recipe for choosing where to allocate resources to save natural systems from collapse. Approaching that ideal target requires improving our knowledge of real-world ecological networks’ existing architecture and getting a deeper comprehension of how the quality of disturbances—and not just their magnitude—interacts with natural systems’ structure. The same communities might be, at the same time, extremely robust against some kinds of strong disturbance and very fragile against some other weak stressors. This “relative” vulnerability might be of crucial importance for current ecosystems, as evidence suggests that they might be substantially unprepared to face global change challenges.
... 至-34.7°C [44] . 这个黑暗而寒冷的阶段长达几年 [61,62] , 不仅超出了白垩纪末大部分生物的热生理极限 [42] , 还 抑制了初级生产者的光合作用, 通过生态级联效应, 导 致全球陆地和海洋食物网的崩溃 [43,63] . 同时, 小行星撞 击带来大量的金属元素进入地球生态系统 [64] , 超高金 属浓度在撞击地点附近 [64,65] [74] , 而且火山灰沉积到海水中会大量释 放金属元素, 改变海水pH, 导致生物地球化学循环发 生重大变化 [75] . ...
... While a major eruption could lead to periods of climate warmth for, for example, early Mars, it may have also accelerated planetary water loss by increasing the amount of water vapor at high altitudes that is more susceptible to photodissociation and escape of hydrogen. Interestingly, Bardeen et al. (2017) found similar stratospheric moistening and ozone destruction while simulating the climate impacts of a major asteroid impact and ejection of ∼15 Tg of soot into the atmosphere due to the subsequent global fires, as well as a comparable ∼15 years duration to return to pre-event climate conditions. Increases in stratospheric water vapor following volcanic eruptions has been measured after Pinatubo and modeled for larger eruptions (Joshi & Shine, 2003;Löffler et al., 2016;Robock et al., 2009). ...
Volcanic flood basalt eruptions have been linked to or are contemporaneous with major climate disruptions, ocean anoxic events, and mass extinctions throughout at least the last 400 M years of Earth's history. Previous studies and recent history have shown that volcanically‐driven climate cooling can occur through reflection of sunlight by H2SO4 aerosols, while longer‐term climate warming can occur via CO2 emissions. We use the Goddard Earth Observing System Chemistry‐Climate Model to simulate a 4‐year duration volcanic SO2 emission of the scale of the Wapshilla Ridge member of the Columbia River Basalt eruption. Brief cooling from H2SO4 aerosols is outweighed by dynamically and radiatively driven warming of the climate through a three orders of magnitude increase in stratospheric H2O vapor.
... However, a supported configuration is also offered to use the spectral element (SE) dynamical core on a cubed sphere grid (Lauritzen et al. 2014). The FV dynamical core has been modified to improve numerical stability in more strongly forced atmospheres by incrementally applying physics tendencies for temperature and wind speed evenly throughout the dynamical substeps, rather than only at the beginning of the dynamics step (Bardeen et al. 2017). Several studies (Lebonnois et al. 2012;Lauritzen et al. 2014) found that NCAR's FV dynamical core does not properly conserve angular momentum for slowrotating planets and inadequately captures upper-level superrotation for Venus and Titan atmospheres (Larson et al. 2014). ...
The TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI) project was initiated to compare 3D climate models that are commonly used for predicting theoretical climates of habitable zone extrasolar planets. One of the core models studied as part of THAI is ExoCAM, an independently curated exoplanet branch of the National Center for Atmospheric Research Community Earth System Model (CESM), version 1.2.1. ExoCAM has been used for studying atmospheres of terrestrial extrasolar planets around a variety of stars. To accompany the THAI project and provide a primary reference, here we describe ExoCAM and what makes it unique from standard configurations of CESM. Furthermore, we also conduct a series of intramodel sensitivity tests of relevant moist physical tuning parameters while using the THAI protocol as our starting point. A common criticism of 3D climate models used for exoplanet modeling is that cloud and convection routines often contain free parameters that are tuned to the modern Earth, and thus may be a source of uncertainty in evaluating exoplanet climates. Here, we explore sensitivities to numerous configuration and parameter selections, including a recently updated radiation scheme, a different cloud and convection physics package, different cloud and precipitation tuning parameters, and a different sea ice albedo. Improvements to our radiation scheme and the modification of cloud particle sizes have the largest effects on global mean temperatures, with variations up to ∼10 K, highlighting the requirement for accurate radiative transfer and the importance of cloud microphysics for simulating exoplanetary climates. However, for the vast majority of sensitivity tests, climate differences are small. For all cases studied, intramodel differences do not bias general conclusions regarding climate states and habitability.
... At the end of the Cretaceous Period (66.0 Ma), the impact of an asteroid on the Yucatán carbonate platform in the southern Gulf of Mexico caused the extinction of ∼75% of marine species (Alvarez et al., 1980;Hildebrand et al., 1991;Jablonski, 1995;Schulte et al., 2010;Smit & Hertogen, 1980), including ∼90% of pelagic calcifiers such as planktic foraminifera and calcareous nannoplankton (Bown et al., 2004;Fraass et al., 2015;Lowery et al., 2020). Dust and sulfate aerosols ejected from the evaporite-rich carbonates of the target rock, soot from wildfires, and petrogenic carbon from the crater (Kaiho et al., 2016;Lyons et al., 2020) blocked the sun, resulting in severe short-term cooling (Artemieva & Morgan, 2020;Artemieva et al., 2017;Bardeen et al., 2017;Brugger et al., 2017;Vellekoop et al., 2014Vellekoop et al., , 2016Gulick et al., 2019;Pope et al., 1994;Wolbach et al., 1985) and collapse of the food chain due to a sharp decline in photosynthesis (D'Hondt et al., 1998;Kring, 2007;Gibbs et al., 2020;Zachos et al., 1989). These effects were short-lived, however, as most dust, soot, and aerosols were removed from the atmosphere on the order of years (Brugger et al., 2017;Tabor et al., 2020), and the oceans quickly became hospitable for life, even at ground zero in the Chicxulub crater (Lowery et al., 2018). ...
The Chicxulub impact caused a crash in productivity in the world's oceans which contributed to the extinction of ∼75% of marine species. In the immediate aftermath of the extinction, export productivity was locally highly variable, with some sites, including the Chicxulub crater, recording elevated export production. The long‐term transition back to more stable export productivity regimes has been poorly documented. Here, we present elemental abundances, foraminifer and calcareous nannoplankton assemblage counts, total organic carbon, and bulk carbonate carbon isotope data from the Chicxulub crater to reconstruct changes in export productivity during the first 3 Myr of the Paleocene. We show that export production was elevated for the first 320 kyr of the Paleocene, declined from 320 kyr to 1.2 Myr, and then remained low thereafter. A key interval in this long decline occurred 900 kyr to 1.2 Myr post impact, as calcareous nannoplankton assemblages began to diversify. This interval is associated with fluctuations in water column stratification and terrigenous flux, but these variables are uncorrelated to export productivity. Instead, we postulate that the turnover in the phytoplankton community from a post‐extinction assemblage dominated by picoplankton (which promoted nutrient recycling in the euphotic zone) to a Paleocene pelagic community dominated by relatively larger primary producers like calcareous nannoplankton (which more efficiently removed nutrients from surface waters, leading to oligotrophy) is responsible for the decline in export production in the southern Gulf of Mexico.
... Modifications have been brought to its dynamical core to improve numerical stability in more strongly forced atmospheres. They consist in incrementally applying physics tendencies for temperature and wind speed evenly throughout the dynamical time step, rather than only at the beginning of it (Bardeen et al. 2017). The number of points in the latitudinal and longitudinal directions is 72 and 46, respectively. ...
With the commissioning of powerful, new-generation telescopes such as the JWST and the ELTs, the first characterization of a high molecular weight atmosphere around a temperate rocky exoplanet is imminent. The best target we have so far of a potentially habitable planet accessible to these telescopes is TRAPPIST-1e. Numerical atmospheric simulations and synthetic observables of such targets are essential to prepare and eventually interpret these observations. Here we report the results of the first part of the THAI (TRAPPIST-1 Habitable Atmosphere Intercomparison) project, which compares the results of 3D numerical simulations performed with four state-of-the-art Global Climate Models (ExoCAM, LMD-Generic, ROCKE-3D, Unified Model) for TRAPPIST-1e. In this first part, we present the results of dry atmospheric simulations. These simulations serve as a benchmark to test how radiative transfer, subgrid-scale mixing (dry turbulence and convection) and large-scale dynamics impact the climate of TRAPPIST-1e and consequently the transit spectroscopy signature as seen by JWST. To first order, the four models give results in fairly good agreement. The inter-model spread in the global mean surface temperature amounts to 7K (6K) for the N2-dominated (CO2-dominated, respectively) atmosphere. The radiative fluxes are also remarkably similar (inter-model variations < 5%), from the surface up to ~5 millibar. Moderate differences between the models appear in the atmospheric circulation pattern and the (stratospheric) thermal structure. These differences arise between the models from (1) large scale dynamics because TRAPPIST-1e lies at the tipping point between two different circulation regimes (fast and Rhines rotators) in which the models can be alternatively trapped ; and (2) parameterizations used in the upper atmosphere such as numerical damping (e.g., the presence and formulation of a sponge layer).
... However, the event was biologically selective as the extinction had greater intensity in the Northern Hemisphere (Keller et al. 1993;Nichols & Johnson 2002;Jiang et al. 2010;Donovan et al. 2016). The extinction event records paleooceanographic, paleobiological and climatic changes evolving from much warmer to cool greenhouse and are responsible for wiping out the planktonic foraminifera in majority of sections around the world (Molina et al. 1996;Hay & Floegel 2012;Robertson et al. 2013;Kaiho et al. 2016;Bardeen et al. 2017;Arenillas et al. 2018). Immediately after the event, the marine ecosystem recovered under the "Strangelove Ocean" conditions and the benthic foraminifera re-organized after their drop due to biologic productivity (Rhodes & Thayer 1991;Alegret et al. 2001Alegret et al. , 2002. ...
The sedimentary strata were sampled in the lesser Himalayas to probe paleoenvironmental changes across the Cretaceous-Paleogene (K-Pg) boundary in the eastern Tethys. The study provides an integrated lithologic and bio-sequence stratigraphic analysis, leading to paleoecology and paleoenvironmental interpretations. The planktic foraminiferal limestone of the late Cretaceous is overlain by lateritic sandstones and sandy foraminiferal limestones, the latter being of Paleocene age. Though the deposition of cretaceous strata mainly occurred in transgressive and high stand system tracts, the top of cretaceous is marked by type-I sequence boundary and low stand system tract, corresponding to the Paleocene Hangu Formation. Deposits below the K-Pg boundary zone interval have been correlated to the late Cenomanian Rotalipora reichel biozone to early Campanian Globotruncana ventricosa zone, with absence of Maastrichtian fauna. A marked change in fauna above the K-Pg boundary zone interval has been observed and manifested by presence of larger benthic foraminifera such as Lockhartia Davies, 1932 and Globanomalina Haque, 1956 genera. The boundary occurs at the contrasting inter-facial contact of the two rock units and advocates an early lowered sea-levels or dead ocean model. An organic bed of late Turonian-early Coniacian corresponds to the probable presence of the OAE3 and could represent a missing link in the late Cretaceous of lesser Himalayas in the Pakistani domain. Prior to the K-Pg event and Indo-Eurasian collision, an influx of siliciclastics suggests a major episode of uplift and shortening caused by ophiolite obduction or magmatic upwelling during the Campanian. The subsequent erosion and its re-deposition shaped the platform, evolving it from relatively steeper ramp geometry in the Campanian to gentler epeiric ramp in the Selandian and Thanetian, and triggered deposition of shallow ramp larger benthic foraminiferal facies. The boundary is similar in nature with erosional phase in the whole region but its duration was prolonged in the study section and its upper limit has some regional changes. As finding of this study, the late Cretaceous “Nara Sandstone Member” of the Kawagarh Formation in Hazara area of earlier workers could be revised as Paleocene Hangu Formation.
... Furthermore, Mössbauer analyses of boundary clay from proximal and distal sites have revealed that Fe nanoparticles, formed during the impact, served as nuclei for aerosols, causing prolonged darkness (51,52). Ejecta descending from high altitudes can radiate heat and potentially ignite wildfires (53)(54)(55)(56)(57). Soot within the K-Pg boundary layer indicates that extensive impact-induced fires occurred instantaneously or within months of the impact (58) and could have intensified global cooling (59). One of the major objectives of drilling at the peak ring was to explore evidence for the drivers of these profound environmental changes that were potentially responsible for the severity of the mass extinction at the K-Pg boundary. ...
Highly expanded Cretaceous–Paleogene (K-Pg) boundary section from the Chicxulub peak ring, recovered by International Ocean Discovery Program (IODP)–International Continental Scientific Drilling Program (ICDP) Expedition 364, provides an unprecedented window into the immediate aftermath of the impact. Site M0077 includes ∼130 m of impact melt rock and suevite deposited the first day of the Cenozoic covered by <1 m of micrite-rich carbonate deposited over subsequent weeks to years. We present an interpreted series of events based on analyses of these drill cores. Within minutes of the impact, centrally uplifted basement rock collapsed outward to form a peak ring capped in melt rock. Within tens of minutes, the peak ring was covered in ∼40 m of brecciated impact melt rock and coarse-grained suevite, including clasts possibly generated by melt–water interactions during ocean resurge. Within an hour, resurge crested the peak ring, depositing a 10-m-thick layer of suevite with increased particle roundness and sorting. Within hours, the full resurge deposit formed through settling and seiches, resulting in an 80-m-thick fining-upward, sorted suevite in the flooded crater. Within a day, the reflected rim-wave tsunami reached the crater, depositing a cross-bedded sand-to-fine gravel layer enriched in polycyclic aromatic hydrocarbons overlain by charcoal fragments. Generation of a deep crater open to the ocean allowed rapid flooding and sediment accumulation rates among the highest known in the geologic record. The high-resolution section provides insight into the impact environmental effects, including charcoal as evidence for impact-induced wildfires and a paucity of sulfur-rich evaporites from the target supporting rapid global cooling and darkness as extinction mechanisms.
Despite several, sometimes prominent propagators, meteorite impact research had a long period of peripheral status until the 1980s. Since then, there has been an intense search for impact-extinction pairs, driven by the rapid acceptance of Alvarez’s hypothesis of a catastrophic Chicxulub impact at the end of the Mesozoic era.
However, substantial errors have occurred for incompletely identified and/or indirectly dated impact craters in the context of purportedly coeval mass extinctions. For example, supposed giant craters based only on geophysical studies, such as those alleged as evidence of impact-driven end-Permian and Late Ordovician extinctions, are not supported by any real impact evidence (e.g., catastrophic sedimentation) in adjacent areas.
The updated three-step methodology presents an accurate approach to cause-effect inference in impact catastrophism. It begins with (1) conclusive recognition of impact craters and ejecta, followed by (2) their precise radiometric or biostratigraphic dating, and concludes with (3) assessing the impact’s “kill” potential. The impact contribution to widely defined mass extinctions has been falsified based on the latest crater information from the global database and the updated ages of stratigraphic boundaries. In the Phanerozoic, two contrasting collision phenomena occurred: the Chicxulub asteroid mega-impact and a prolonged asteroid shower from a shattered
chondritic body in the Middle to Late Ordovician. Accordingly, a distinction has been proposed between steady background conditions (impacts occurring singly and rarely in clusters) and perturbation (bombardment) intervals. Current evidence for an impact trigger has been reviewed in detail for the other four Big Five mass extinctions, but no confirmation has been found. The probability of a prolonged impact-enhanced Late Eocene to
Early Oligocene crisis, caused by an asteroid shower, is considered, as well as biotic changes accompanying other
major cratering events: the mid-Norian Manicouagan and the end-Jurassic Morokweng structures. In particular, for the Popigai asteroid swarm, implied from paired 100-km-sized craters, and the possible Morokweng-Mjølnir coincidence, the relationships between impact signatures and likely stepwise biotic events are far from conclusive. Even if medium-sized bolide impacts, recorded in ~40-km-diameter craters, may have initiated near-global climatic hazards, the killing effect is unpredictable due to the diversity of cataclysm severity controls. Also the Ordovician cosmic bombardment did not have any negative influence on the great biodiversification. However, the asteroid swarms may have (by unusual dustiness of the inner Solar System) ultimately triggered or accelerated ice ages in the Late Ordovician and Oligocene, respectively. Overall, this implies a continuum in the
biosphere’s response to extraterrestrial stimuli.
Furthermore, a first attempt was made to explain the hidden record of predicted additional Chicxulub-type mega-events. ‘Lost’ oceanic impacts in the Middle Ordovician, Late Devonian, and Late Triassic were traced in the context of previously suggested records of mega-tsunamis and seismicity. The Frasnian-Famennian transition seems to be the most likely case of such a cryptic cataclysm, manifested in the worldwide “top-Frasnian
reworking event.” In summary, of the 18 extinctions, one confirmed impact-induced mass extinction and 3–4 possible impact-enhanced biotic crises can now be considered in terms of extraterrestrial forcing. This tentative conclusion is only superficially consistent with the simplistic assertion in recent literature of four ‘mass extinctions’ associated with the four largest impacts, as much substantial evidence is still needed. In fact, well-documented volcanic cataclysms currently shape the mainstream neocatastrophic geology.
Many proposals, mostly by non-geologists, of periodic causal connections between extraterrestrial factors and biosphere turnovers are shown once more to be totally inconclusive. In this context, the future of actualistic impact catastrophism and Alvarez’s ‘bolide theory’ remains open to many fascinating topics. In contrast, ‘nonbolide’ catastrophic concepts, such as the triggering role of intergalactic dark matter, are too questionable to
provide real evidence in the fossil record for these ‘invisible’ phenomena.
A palynological study was carried out based on 157 samples collected from four representative stratigraphic sections of the Maastrichtian-Danian deposits of the La Colonia Formation outcropping in northern Chubut Province, Patagonia, Argentina. About 240 palynomorphs were recognized. Plant communities were dominated in terms of richness by ferns and angiosperms, but algae and gymnosperms are also well-represented. In this contribution, we present the systematic study of bryophyte, lycophyte, and fern spores. Bryophytes comprise eight species (10% of spore diversity), including representatives of Marchantiophyta, Bryophyta, and Antho- cerotophyta. Lycophytes encompass 15 species (20% of spore diversity) and are represented by the families Lycopodiaceae and Selaginellaceae. Ferns comprise 53 species (70% of spore diversity), including members of Anemiaceae, Dicksoniaceae, Dipteridaceae, Gleicheniaceae, Lygodiaceae, Marsileaceae, Matoniaceae, Osmun- daceae, Polypodiaceae, Salviniaceae, and Schizaeaceae, among others of uncertain affinities. Four new species are erected: a lycophyte (Neoraistrickia loconiensis sp. nov.), a salvinialean (Thecaspora polygonalis sp. nov.), and two fern species of unknown affinities (Clavatosporis varians sp. nov. and Microreticulatisporites patagonicus sp. nov.). The recorded palynoflora reinforces previous environmental interpretation of the La Colonia deposits as coastal plains bathed by shallow seas and barrier island/lagoon complexes and the presence of freshwater bodies where aquatic plant communities developed. The vegetational history of the bryophytes, lycophytes, and ferns in the studied sections of the La Colonia Formation indicates the lack of a significant floristic change across the K–Pg interval at the local scale.
The direct effects of nuclear war would be horrific, with blasts, fires, and radiation killing and injuring many people. But in 1983, United States and Soviet Union scientists showed that a nuclear war could also produce a nuclear winter, with catastrophic consequences for global food supplies for people far removed from the conflict. Smoke from fires ignited by nuclear weapons exploded on cities and industrial targets would block out sunlight, causing dark, cold, and dry surface conditions, producing a nuclear winter, with surface temperatures below freezing even in summer for years. Nuclear winter theory helped to end the nuclear arms race in the 1980s and helped to produce the Treaty on the Prohibition of Nuclear Weapons in 2017, for which the International Campaign to Abolish Nuclear Weapons received the 2017 Nobel Peace Prize. Because awareness of nuclear winter is now widespread, nuclear nations have so far not used nuclear weapons. But the mere existence of nuclear weapons means that they can be used, by unstable leaders, accidently from technical malfunctions, such as in computers and sensors, due to human error, or by terrorists. Because they cannot be used without the danger of escalation (resulting in a global humanitarian catastrophe), because of recent threats to use them by Russia, and because nuclear deterrence doctrines of all nuclear-armed states are based on the capability and readiness to use nuclear weapons, it is even more urgent for scientists to study these issues, to broadly communicate their results, and to work for the elimination of nuclear weapons.
The direct effects of nuclear war would be horrific, with blast, fires, and radiation killing and injuring many people. But in 1983, United States and Soviet Union scientists showed that a nuclear war could also produce a nuclear winter, with catastrophic consequences for global food supplies for people far removed from the conflict. Smoke from fires ignited by nuclear weapons exploded on cities and industrial targets would block out sunlight, causing dark, cold, and dry surface conditions, producing a nuclear winter, with surface temperatures below freezing even in summer for years. Nuclear winter theory helped to end the nuclear arms race in the 1980s and to produce the Treaty on the Prohibition of Nuclear Weapons in 2017, which led to the 2017 Nobel Peace Prize. Because awareness of nuclear winter is now widespread, nuclear nations have so far not used nuclear weapons. But the mere existence of nuclear weapons means that they can be used, by unstable leaders or because of an accident, computer malfunction, sensor malfunction, human error, or terrorism. Because they cannot be used without the danger of escalation and a global humanitarian catastrophe, and because of recent threats to use them by Russia, it is even more urgent for scientists to broadly communicate their results and work for the elimination of nuclear weapons.
Researchers at Tanis, North Dakota, U.S.A., cited faulting associated with soft sediment deformation of the K/Pg boundary clay at Madrid East in the Raton Basin, U.S.A., as additional evidence of “more-instantaneous effects” that preceded the longer-term, climatic effects of the Chicxulub impact. However, Madrid East is among numerous localities where the initial phase of the K/Pg boundary fern spike initiated within rather than above the K/Pg boundary clay crosscut by this fault. Accordingly, this phase of the K/Pg boundary fern spike must have preceded the cessation of seismic aftershocks of the Chicxulub impact—a conclusion also reached by researchers at the deep marine depositional setting of Gorgonilla Island in Colombia, South America. The observation that the Chicxulub impact winter was shorter in duration than post-impact seismicity need not signify a major dilemma for contemporary climate models of the Chicxulub impact, a dilemma originally exacerbated by the common perception that this phase of the fern spike was dominated by tropical tree ferns. Because tree ferns have exceptionally long generation times (ca. 100 y according to contemporary estimates), they could not have had ample time to reach sporing stage if they were killed back to rhizomes or other belowground reserves by a complete shutdown of photosynthesis and subsequent impact winter. Alternatively, the dennstaedtiaceous affinity of spores at Madrid East suggests that these ferns had a generation time of only 2–4 y, which better fits contemporary estimates on how long it took for the effects of Chicxulub to wind down.
The Chicxulub crater is the site of an asteroid impact linked with the Cretaceous‐Paleogene (K‐Pg) mass extinction at ∼66 Ma. This asteroid struck in shallow water and caused a large tsunami. Here we present the first global simulation of the Chicxulub impact tsunami from initial contact of the projectile to global propagation. We use a hydrocode to model the displacement of water, sediment, and crust over the first 10 min, and a shallow‐water ocean model from that point onwards. The impact tsunami was up to 30,000 times more energetic than the 26 December 2004 Indian Ocean tsunami, one of the largest tsunamis in the modern record. Flow velocities exceeded 20 cm/s along shorelines worldwide, as well as in open‐ocean regions in the North Atlantic, equatorial South Atlantic, southern Pacific and the Central American Seaway, and therefore likely scoured the seafloor and disturbed sediments over 10,000 km from the impact origin. The distribution of erosion and hiatuses in the uppermost Cretaceous marine sediments are consistent with model results.
The global heterogeneity in export productivity after the Cretaceous‐Paleogene (K‐Pg) mass extinction is well documented, with some sites showing no change on geologic timescales, some demonstrating sustained decline, and a few showing a somewhat surprising increase. However, observational data come from sites so widespread that a key outstanding question is the geographic scale of changes in export productivity, and whether similar environments (e.g., open ocean gyres) responded similarly or whether heterogeneity is unrelated to environment. To address this, we developed three new Ba/Ti export productivity records from sites in the Gulf of Mexico and Caribbean which, combined with published data from a fourth site in the Chicxulub Crater itself, allow us to reconstruct regional changes in post K‐Pg export productivity for the first time. We find that, on a regional scale, export productivity change was homogenous, with all four sites showing a ∼300 Kyr period of elevated export production just after the boundary, followed by a longer period of decline. Interestingly, this interval of elevated export production appears to coincide with the post K‐Pg global micrite layer, which is thought to at least partially have been produced by blooms of carbonate‐producing cyanobacteria and other picophytoplankton. Global comparison of sites shows that elevated export productivity appears to have been most common in oligotrophic gyres, which suggests that changing plankton ecology evidenced by the micrite layer altered the biological pump, leading to a temporary increase in export production in these settings.
The Chicxulub impact triggered a global impact winter at the Cretaceous-Paleogene (K-Pg) boundary 66 million years ago. Yet, the exact killing mechanisms of the K-Pg mass extinction including the wipe-out of non-avian dinosaurs, remain poorly constrained. Here, we present paleoclimate simulations based on new sedimentological constraints from an expanded K-Pg boundary deposit in North Dakota, to evaluate the relative and combined effects of impact-generated sulfur and silicate dust as well as soot from global wildfires on the post-impact photosynthetic activity. In prior works, the relative contribution of dust was considered peripheral compared to the other types of fine-grained ejecta. However, our results show that a massive plume of micrometer-sized silicate dust was a key factor driving the K-Pg impact winter due to a long atmospheric lifetime at least 20 years. The dust-induced photosynthetic shut-down, together with additional effects of soot and sulfur, led to the catastrophic collapse of primary productivity on land and in the ocean, steering the mass extinction in the direct aftermath of the Chicxulub impact.
The extinction of the dinosaurs and around three-quarters of all living species was almost certainly caused by a large asteroid impact 66 million years ago. Seismic data acquired across the impact site in Mexico have provided spectacular images of the approximately 200-kilometre-wide Chicxulub impact structure. In this Review, we show how studying the impact site at Chicxulub has advanced our understanding of formation of large craters and the environmental and palaeontological consequences of this impact. The Chicxulub crater’s asymmetric shape and size suggest an oblique impact and an impact energy of about 1023 joules, information that is important for quantifying the climatic effects of the impact. Several thousand gigatonnes of asteroidal and target material were ejected at velocities exceeding 5 kilometres per second, forming a fast-moving cloud that transported dust, soot and sulfate aerosols around the Earth within hours. These impact ejecta and soot from global wildfires blocked sunlight and caused global cooling, thus explaining the severity and abruptness of the mass extinction. However, it remains uncertain whether this impact winter lasted for many months or for more than a decade. Further combined palaeontological and proxy studies of expanded Cretaceous–Palaeogene transitions should further constrain the climatic response and the precise cause and selectivity of the extinction. The Chicxulub impact 66 million years ago caused catastrophic environmental changes, leading to the extinction of three-quarters of plant and animal species, including the dinosaurs. This Review explores how the Chicxulub impact structure provides insight into cratering processes and events leading to the Cretaceous–Palaeogene extinction. The Chicxulub impact ended the Mesozoic era and was almost certainly the principal cause of the Cretaceous–Palaeogene (K–Pg) mass extinction.Seismic images of the approximately 200-km-wide Chicxulub impact structure reveal that it has the same morphology as the largest impact basins on other solid planetary bodies, such as the Lise Meitner and Klenova craters on Venus.Rocks from the impact site and asteroid were ejected within an impact plume and ejecta curtain. Ejection velocity is a function of shock pressure, with the most-shocked rocks leaving the impact site at >11 km s–1 (escape velocity).The high-velocity ejecta interacted with the Earth’s atmosphere to form a fast-moving cloud that carried dust, soot, sulfate aerosols and other ejecta around the Earth within 4–5 hours of impact.Ejecta within the cloud, along with soot from wildfires, caused the Earth to become dark and cold for about a decade, and induced longer-term (decadal to millennial) temperature changes and chemical changes in the ocean.This extended impact winter explains the abruptness and severity of the mass extinction, as well as its selective impact on different organisms. The Chicxulub impact ended the Mesozoic era and was almost certainly the principal cause of the Cretaceous–Palaeogene (K–Pg) mass extinction. Seismic images of the approximately 200-km-wide Chicxulub impact structure reveal that it has the same morphology as the largest impact basins on other solid planetary bodies, such as the Lise Meitner and Klenova craters on Venus. Rocks from the impact site and asteroid were ejected within an impact plume and ejecta curtain. Ejection velocity is a function of shock pressure, with the most-shocked rocks leaving the impact site at >11 km s–1 (escape velocity). The high-velocity ejecta interacted with the Earth’s atmosphere to form a fast-moving cloud that carried dust, soot, sulfate aerosols and other ejecta around the Earth within 4–5 hours of impact. Ejecta within the cloud, along with soot from wildfires, caused the Earth to become dark and cold for about a decade, and induced longer-term (decadal to millennial) temperature changes and chemical changes in the ocean. This extended impact winter explains the abruptness and severity of the mass extinction, as well as its selective impact on different organisms.
An approximately 14-km diameter asteroid is implicated in the Cretaceous/Paleogene (K/Pg) mass extinction 1 . The bolide impact caused global temperature fluctuations 1 , large aerosol 2 , soot and dust plumes 3 , and wildfires from ejecta re-entering the atmosphere 4,5 . Drilling cores from the International Continental Drilling Program (ICDP) and the Integrated Ocean Drilling Program 6 (IODP) revealed the exact physical and geophysical nature of the crater and its peak ring and facilitated the modeling of the impact event 7 . There have been regional tsunami simulations of the impact region of the Chicxulub impact within the Gulf of Mexico by Ward 8 and Matsui et al. 9 Here we present the first global simulation of the Chicxulub impact tsunami from initial contact of the projectile to global propagation using a hydrocode to model the displacement of water, sediment, and crust over the first ten minutes, and a shallow-water ocean model from that point onwards. The tsunami due to the impact and subsequent submarine landslides on the marine shelf 10 was approximately 2700 times more energetic than the December 26, 2004 Indian Ocean tsunami, one of the largest tsunamis in the modern record. Flow velocities exceeded 20 cm/s along shorelines worldwide and disturbed sediments over 6000 km from the impact origin.
The global heterogeneity in export productivity after the Cretaceous-Paleogene (K-Pg) mass extinction is well documented, with some sites showing no change on geologic timescales, some demonstrating sustained decline, and a few showing a somewhat surprising increase. However, these records come from sites so widespread that a key outstanding question is the geographic scale of changes in export productivity, and whether similar environments (open ocean gyres, western boundary currents) responded similarly or whether heterogeneity is unrelated to environment. To address this, we developed three new Ba/Ti export productivity records from sites in the Gulf of Mexico and Caribbean which, combined with published data from a fourth site in the Chicxulub Crater itself, allows us to reconstruct regional changes in post K-Pg export productivity for the first time. We find that, on a regional scale, export productivity change is homogenous, with all four sites showing a ~300 kyr period of elevated export production just after the boundary, followed by a longer period of decline. Interestingly, this interval of elevated export production appears to coincide with the post K-Pg global micrite layer, which is thought to at least partially have been produced by blooms of carbonate-producing cyanobacteria and other picophytoplankton. We note from a global comparison of sites that elevated export productivity appears to be most common in tropical waters, which suggests that changing plankton ecology evidenced by the micrite layer altered the biological pump in a way that encouraged a temporary increase in export production in the tropics.
About 66 million years ago, an asteroid about 10 km in diameter struck the Yucatan Peninsula creating the Chicxulub crater. The crater has been dated and found to be coincident with the Cretaceous–Paleogene (K-Pg) mass extinction event, one of six great mass extinctions in the last 600 million years. This event precipitated one of the largest episodes of rapid climate change in Earth's history, yet no modern three-dimensional climate calculations have simulated the event. Similarly, while there is an ongoing effort to detect asteroids that might hit Earth and to develop methods to stop them, there have been no modern calculations of the sizes of asteroids whose impacts on land would cause devastating effects on Earth. Here, we provide the information needed to initialize such calculations for the K-Pg impactor and for a 1 km diameter impactor.
There is considerable controversy about the details of the events that followed the Chicxulub impact. We proceed through the data record in the order of confidence that a climatically important material was present in the atmosphere. The climatic importance is roughly proportional to the optical depth of the material. Spherules with diameters of several hundred microns are found globally in an abundance that would have produced an atmospheric layer with an optical depth around 20, yet their large sizes would only allow them to stay airborne for a few days. They were likely important for triggering global wildfires. Soot, probably from global or near-global wildfires, is found globally in an abundance that would have produced an optical depth near 100, which would effectively prevent sunlight from reaching the surface. Nanometer-sized iron particles are also present globally. Theory suggests these particles might be remnants of the vaporized asteroid and target that initially remained as vapor rather than condensing on the hundred-micron spherules when they entered the atmosphere. If present in the greatest abundance allowed by theory, their optical depth would have exceeded 1000. Clastics may be present globally, but only the quartz fraction can be quantified since shock features can identify it. However, it is very difficult to determine the total abundance of clastics. We reconcile previous widely disparate estimates and suggest the clastics may have had an optical depth near 100. Sulfur is predicted to originate about equally from the impactor and from the Yucatan surface materials. By mass, sulfur is less than 10 % of the observed mass of the spheres and estimated mass of nanoparticles. Since the sulfur probably reacted on the surfaces of the soot, nanoparticles, clastics, and spheres, it is likely a minor component of the climate forcing; however, detailed studies of the conversion of sulfur gases to particles are needed to determine if sulfuric acid aerosols dominated in late stages of the evolution of the atmospheric debris. Numerous gases, including CO2, SO2 (or SO3), H2O, CO2, Cl, Br, and I, were likely injected into the upper atmosphere by the impact or the immediate effects of the impact such as fires across the planet. Their abundance might have increased relative to current ambient values by a significant fraction for CO2, and by factors of 100 to 1000 for the other gases.
For the 1 km impactor, nanoparticles might have had an optical depth of 1.5 if the impact occurred on land. If the impactor struck a densely forested region, soot from the forest fires might have had an optical depth of 0.1. Only S and I would be expected to be perturbed significantly relative to ambient gas-phase values. One kilometer asteroids impacting the ocean may inject seawater into the stratosphere as well as halogens that are dissolved in the seawater.
For each of the materials mentioned, we provide initial abundances and injection altitudes. For particles, we suggest initial size distributions and optical constants. We also suggest new observations that could be made to narrow the uncertainties about the particles and gases generated by large impacts.
Here we use a coupled atmospheric-ocean-aerosol model to investigate the plume development and climate effects of the smoke generated by fires following a regional nuclear war between emerging third-world nuclear powers. We simulate a standard scenario where 5 Tg of black carbon (BC) is emitted over 1 day in the upper troposphere-lower stratosphere. However, it is likely that the emissions from the fires ignited by bomb detonations include a substantial amount of particulate organic matter (POM) and that they last more than 1 day. We therefore test the sensitivity of the aerosol plume and climate system to the BC/ POM ratio (1:3, 1:9) and to the emission length (1 day, 1 week, 1 month). We find that in general, an emission length of 1 month substantially reduces the cooling compared to the 1-day case, whereas taking into account POM emissions notably increases the cooling and the reduction of precipitation associated with the nuclear war during the first year following the detonation. Accounting for POM emissions increases the particle size in the short-emission-length scenarios (1 day/1 week), reducing the residence time of the injected particle. While the initial cooling is more intense when including POM emission, the long-lasting effects, while still large, may be less extreme compared to the BC-only case. Our study highlights that the emission altitude reached by the plume is sensitive to both the particle type emitted by the fires and the emission duration. Consequently, the climate effects of a nuclear war are strongly dependent on these parameters.
The mass extinction of life 66 million years ago at the Cretaceous/Paleogene boundary, marked by the extinctions of dinosaurs and shallow marine organisms, is important because it led to the macroevolution of mammals and appearance of humans. The current hypothesis for the extinction is that an asteroid impact in present-day Mexico formed condensed aerosols in the stratosphere, which caused the cessation of photosynthesis and global near-freezing conditions. Here, we show that the stratospheric aerosols did not induce darkness that resulted in milder cooling than previously thought. We propose a new hypothesis that latitude-dependent climate changes caused by massive stratospheric soot explain the known mortality and survival on land and in oceans at the Cretaceous/Paleogene boundary. The stratospheric soot was ejected from the oil-rich area by the asteroid impact and was spread globally. The soot aerosols caused sufficiently colder climates at mid–high latitudes and drought with milder cooling at low latitudes on land, in addition to causing limited cessation of photosynthesis in global oceans within a few months to two years after the impact, followed by surface-water cooling in global oceans in a few years. The rapid climate change induced terrestrial extinctions followed by marine extinctions over several years.
About 66 million years ago an asteroid about 10 km in diameter struck the Yucatan Peninsula creating the Chicxulub crater. The crater has been dated and found to be coincident with the Cretaceous-Paleogene (K-Pg) mass extinction event, one of 6 great mass extinctions in the last 600 million years. This event precipitated one of the largest episodes of rapid climate change in Earth history, yet no modern three-dimensional climate calculations have simulated the event. Similarly, while there is an on-going effort to detect asteroids that might hit Earth and to develop methods to stop them, there have been no modern calculations of the sizes of asteroids whose impacts on land would cause devastating effects on Earth. Here we provide the information needed to initialize such calculations for the K-Pg impactor and for a 1 km diameter impactor. There is considerable controversy about the details of the events that followed the Chicxulub impact. We proceed through the data record in the order of confidence that a climatically important material was present in the atmosphere. The climatic importance is roughly proportional to the optical depth of the material. Several hundred-micron diameter spherules are found globally in an abundance that would have produced an atmospheric layer with an optical depth around 20, yet their large sizes would only allow them to stay airborne for a few days. They were likely important for triggering global wildfires. Soot, probably from global or near-global wildfires, is found globally in an abundance that would have produced an optical depth near 100, which would effectively prevent sunlight from reaching the surface. Nanometer sized iron particles are also present globally. Theory suggests these particles might be remnants of the vaporized asteroid and target that initially remained as vapor rather than condensing on the hundred-micron spherules when they entered the atmosphere. If present in the abundance suggested by theory, their optical depth would have exceeded 1000. Clastics may be present globally, but only the quartz fraction can be quantified since shock features can identify it. However, it is very difficult to determine the total abundance of clastics. We reconcile previous widely disparate estimates and suggest the clastics may have had an optical depth near 100. Sulfur is predicted to originate about equally from the impactor and from the Yucatan surface materials. By mass, sulfur is less than 10 percent of the mass of the spheres and nano-particles. Since the sulfur probably reacted on the surfaces of the soot, nano-particles, clastics and spheres, it is likely a minor component of the climate forcing; however, detailed studies of the conversion of sulfur gases to particles are needed to determine if sulfuric acid aerosols dominated in late stages of the evolution of the atmospheric debris. Numerous gases, including CO2, SO2 (or SO3), H2O, CO2, Cl, Br, and I, were likely injected into the upper atmosphere by the impact or the immediate effects of the impact such as fires across the planet. Their abundance might have increased relative to current ambient values by a significant fraction of current values for CO2, and by factors of 100 to 1000 for the other gases. For the 1 km impactor, nano-particles might have had an optical depth of 1.5 if the impact occurred on land. If the impactor struck a densely forested region, soot from the forest fires might have had an optical depth of 0.1. Only S and I would be expected to be perturbed significantly relative to ambient gas phase values. 1 km asteroids impacting the ocean may inject seawater into the stratosphere as well as halogens that are dissolved in the seawater. For each of the materials mentioned we provide initial abundances and injection altitudes. For particles we suggest initial size distributions and optical constants. We also suggest new observations that could be made to narrow the uncertainties about the particles and gases generated by large impacts.
2015-01 A major challenge in paleoclimatology is disagreement between data and models for periods of warm climate. Data indicate equable conditions and reduced latitudinal temperature gradients, while models produce colder conditions and steeper latitudinal gradients when using realistic levels of CO2. Here we demonstrate congruence between temperature indicators and climate model output for the cool greenhouse interval of the latest Cretaceous (Maastrichtian) using a comprehensive database of terrestrial and marine indicators and fully coupled simulations with the Community Climate System Model, version 3 (CCSM3). In these simulations we explore the potential roles of greenhouse gases, vegetation, and properties of pre-anthropogenic liquid clouds in creating warm equable conditions. Our model simulations successfully reproduce warm polar temperatures and the latitudinal temperature gradient without overheating the tropics, using geologically realistic levels of atmospheric CO2. The best fit for Mean Annual Temperature is a simulation that prescribes 560 ppm CO2 (2x preindustrial), 2000 ppb CH4, realistic vegetation, and liquid cloud properties that may reflect pre-anthropogenic levels of cloud condensation nuclei. The most problematic region is the Siberian interior, which may relate in part to reconstructed elevation and the presence of a large lake not included in model simulations. Data and models together indicate an average equator to pole temperature difference of 25–30°C, and a mid-latitudinal gradient of mean annual temperature of ~0.4°C. This is consistent with suggestions that intervals of greenhouse climate were characterized by significant equator to pole temperature differences and moderate latitudinal temperature gradients.
The NCAR Community Earth System Model (CESM) now includes an atmospheric component that extends in altitude to the lower thermosphere. This atmospheric model, known as the Whole Atmosphere Community Climate Model (WACCM), includes fully interactive chemistry, allowing, for example, a self-consistent representation of the development and recovery of the stratospheric ozone hole and its effect on the troposphere. This paper focuses on analysis of an ensemble of transient simulations using CESM1(WACCM), covering the period from the preindustrial era to present day, conducted as part of phase 5 of the Coupled Model Intercomparison Project. Variability in the stratosphere, such as that associated with stratospheric sudden warmings and the development of the ozone hole, is in good agreement with observations. The signals of these phenomena propagate into the troposphere, influencing near-surface winds, precipitation rates, and the extent of sea ice. In comparison of tropospheric climate change predictions with those from a version of CESM that does not fully resolve the stratosphere, the global-mean temperature trends are indistinguishable. However, systematic differences do exist in other climate variables, particularly in the extratropics. The magnitude of the difference can be as large as the climate change response itself. This indicates that the representation of stratosphere-troposphere coupling could be a major source of uncertainty in climate change projections in CESM.
The Community Earth System Model (CESM) is a flexible and extensible community tool used to investigate a diverse set of Earth system interactions across multiple time and space scales. This global coupled model significantly extends its predecessor, the Community Climate System Model, by incorporating new Earth system simulation capabilities. These comprise the ability to simulate biogeochemical cycles, including those of carbon and nitrogen, a variety of atmospheric chemistry options, the Greenland Ice Sheet, and an atmosphere that extends to the lower thermosphere. These and other new model capabilities are enabling investigations into a wide range of pressing scientific questions, providing new foresight into possible future climates and increasing our collective knowledge about the behavior and interactions of the Earth system. Simulations with numerous configurations of the CESM have been provided to phase 5 of the Coupled Model Intercomparison Project (CMIP5) and are being analyzed by the broad community of scientists. Additionally, the model source code and associated documentation are freely available to the scientific community to use for Earth system studies, making it a true community tool. This article describes this Earth system model and its various possible configurations, and highlights a number of its scientific capabilities.
We present the first study of the global impacts of a regional nuclear war with an Earth system model including atmospheric chemistry, ocean dynamics, and interactive sea-ice and land models. A limited, regional nuclear war between India and Pakistan in which each side detonates 50 15-kt weapons could produce about 5 Tg of black carbon. This would self-loft to the stratosphere, where it would spread globally, producing a sudden drop in surface temperatures and intense heating of the stratosphere. Using the Community Earth System Model with the Whole Atmosphere Community Climate Model (CESM1(WACCM)), we calculate an e-folding time of 8.7 years for stratospheric black carbon, compared to 4–6.5 years for previous studies. Our calculations show that global ozone losses of 20-50% over populated areas, levels unprecedented in human history, would accompany the coldest average surface temperatures in the last 1000 years. We calculate summer enhancements in UV indices of 30-80% over Mid-Latitudes, suggesting widespread damage to human health, agriculture, and terrestrial and aquatic ecosystems. Killing frosts would reduce growing seasons by 10–40 days per year for 5 years. Surface temperatures would be reduced for more than 25 years, due to thermal inertia and albedo effects in the ocean and expanded sea ice. The combined cooling and enhanced UV would put significant pressures on global food supplies and could trigger a global nuclear famine. Knowledge of the impacts of 100 small nuclear weapons should motivate the elimination of the more than 17,000 nuclear weapons that exist today.
This study examines modeled properties of black carbon (BC), tar ball
(TB), and soil dust (SD) absorption within clouds and aerosols to
understand better Cloud Absorption Effects I and II, which are defined
as the effects on cloud heating of absorbing inclusions in hydrometeor
particles and of absorbing aerosol particles interstitially between
hydrometeor particles at their actual relative humidity (RH),
respectively. The globally and annually averaged modeled 550 nm aerosol
mass absorption coefficient (AMAC) of externally mixed BC was 6.72
(6.3-7.3) m2/g, within the laboratory range (6.3-8.7
m2/g). The global AMAC of internally mixed (IM) BC was 16.2
(13.9-18.2) m2/g, less than the measured maximum at 100% RH
(23 m2/g). The resulting AMAC amplification factor due to
internal mixing was 2.41 (2-2.9), with highest values in high RH
regions. The global 650 nm hydrometeor mass absorption coefficient
(HMAC) due to BC inclusions was 17.7 (10.6-19) m2/g,
˜9.3% higher than that of the IM-AMAC. The 650 nm HMACs of TBs and
SD were half and 1/190th, respectively, that of BC. Modeled aerosol
absorption optical depths were consistent with data. In column tests, BC
inclusions in low and mid clouds (CAE I) gave column-integrated BC
heating rates ˜200% and 235%, respectively, those of interstitial
BC at the actual cloud RH (CAE II), which itself gave heating rates
˜120% and ˜130%, respectively, those of interstitial BC at
the clear-sky RH. Globally, cloud optical depth increased then decreased
with increasing aerosol optical depth, consistent with boomerang curves
from satellite studies. Thus, CAEs, which are largely ignored, heat
clouds significantly.
The effect of introducing a new longwave radiation parameterization, RRTM, on the energy budget and thermodynamic properties of the National Center for Atmospheric Research (NCAR) community climate model (CCM3) is described. RRTM is a rapid and accurate, correlated k, radiative transfer model that has been developed for the Atmospheric Radiation Measurement (ARM) program to address the ARM objective of improving radiation models in GCMs. Among the important features of RRTM are its connection to radiation measurements through comparison to the extensively validated line-by-line radiative transfer model (LBLRTM) and its use of an improved and validated water vapor continuum model. Comparisons between RRTM and the CCM3 longwave (LW) parameterization have been performed for single atmospheric profiles using the CCM3 column radiation model and for two 5-year simulations using the full CCM3 climate model. RRTM produces a significant enhancement of LW absorption largely due to its more physical and spectrally extensive water vapor continuum model relative to the current CCM3 water continuum treatment. This reduces the clear sky, outgoing longwave radiation over the tropics by 6-9 W m-2. Downward LW surface fluxes are increased by 8-15 W m-2 at high latitudes and other dry regions. These changes considerably improve known flux biases in CCM3 and other GCMs. At low and midlatitudes, RRTM enhances LW radiative cooling in the upper troposphere by 0.2-0.4 K d-1 and reduces cooling in the lower troposphere by 0.2-0.5 K d-1. The enhancement of downward surface flux contributes to increasing lower tropospheric and surface temperatures by 1-4 K, especially at high latitudes, which partly compensates documented, CCM3 cold temperature biases in these regions. Experiments were performed with the weather prediction model of the European Center for Medium Range Weather Forecasts (ECMWF), which show that RRTM also impacts temperature on timescales relevant to forecasting applications. RRTM is competitive with the CCM3 LW model in computational expense at 30 layers and with the ECMWF LW model at 60 layers, and it would be relatively faster at higher vertical resolution.
Micrometeorites that ablate in the lower thermosphere and upper mesosphere are thought to recondense into nanometer-sized smoke particles and then coagulate into larger dust particles. Previous studies with one-dimensional models have determined that the meteoric dust size distribution is sensitive to the background vertical velocity and have speculated on the importance of the mesospheric meridional circulation to the dust spatial distribution. We conduct the first three-dimensional simulations of meteoric dust using a general circulation model with sectional microphysics to study the distribution and characteristics of meteoric dust in the mesosphere and upper stratosphere. We find that the mesospheric meridional circulation causes a strong seasonal pattern in meteoric dust concentration in which the summer pole is depleted and the winter pole is enhanced. This summer pole depletion of dust particles results in fewer dust condensation nuclei (CN) than has traditionally been assumed in numerical simulations of polar mesospheric clouds (PMCs). However, the total number of dust particles present is still sufficient to account for PMCs if smaller particles can nucleate to form ice than is conventionally assumed. During winter, dust is quickly transported down to the stratosphere in the polar vortex where it may participate in the nucleation of sulfate aerosols, the formation of the polar CN layer, and the formation of polar stratospheric clouds (PSCs). These predictions of the seasonal variation and resulting large gradients in dust concentration should assist the planning of future campaigns to measure meteoric dust.
Recognition of elevated concentrations of aciniform soot in Cretaceous-Paleogene (K-P) boundary sediments worldwide led to the hypothesis that global-scale forest wildfires could have been generated by the K-P boundary bolide impact and might have contributed directly to the extinction event. The wildfires are estimated to have injected 10(13) t of CO(2) into the atmosphere, resulting in an interval of greenhouse warming. Yet minimal amounts of charred plant remains and abundant noncharred material occur in various K-P boundary locations across North America. This refutes the inference that wildfires occurred on a global scale, and requires an alternative explanation for the aciniform soot. Here we describe significant concentrations of carbon cenospheres in K-P boundary sediments from New Zealand, Denmark, and Canada. Carbon cenospheres are thought to derive solely from incomplete combustion of pulverized coal or fuel-oil droplets, which suggests that the impact may have combusted organic-rich target crust. The Chicxulub impact crater is located adjacent to the Cantarell oil reservoir, one of the most productive oil fields on Earth. This indicates that abundance of organic carbon in the Chicxulub target crust was likely to have been above global mean values. But even if we discount Chicxulub's organic-rich locality, the global mean crustal abundance for fossil organic matter is more than adequate to account for observed concentrations of both carbon cenospheres and aciniform soot, therefore making the global wildfire hypothesis unnecessary.
Viable diatom and dinoflagellate resting stages were recovered from sediments in Koljö Fjord on the west coast of Sweden. To determine the maximum survival time of buried resting stages, samples from sediment depths down to 50 cm were incubated at temperatures of 3, 10 and 18 °C. Sediment cores were dated by Pb and the age of samples containing viable resting stages was determined using the constant rate of supply model. Dilution cultures of surface sediments allowed semiquantitative estimates of the potential seed bank. Dinoflagellate cysts from species such as Diplopsalis sp., Gymnodinium nolleri, Oblea rotunda and Protoceratium reticulatum were viable down to 15 cm depth, or 37 years old. Spores and resting cells of the diatoms Chaetoceros spp., Detonula confervacea and Skeletonema costatum were viable to over 40 cm depth, and may have been buried for many decades. The seed bank of living resting stages in surficial sediments was found to be rich (c. 57000 diatom resting stages g wet weight and c. 200 dinoflagellate cysts g wet weight), and the percentage of viable resting stages was higher for spore- and cyst-forming species. The oxygen-deficient sediments in Koljö Fjord appear to be a natural conservator of cell viability, a condition not easily simulated in laboratory studies. These results are ecologically important since spores and cysts are a repository of genetic material able to repopulate waters if resuspended and exposed to suitable light, temperature and nutrients.
“ It takes a village to finish (marine) science these days ”
Paraphrased from Curtis Huttenhower (the Human Microbiome project)
The rapidity and complexity of climate change and its potential effects on ocean biota are challenging how ocean scientists conduct research. One way in which we can begin to better tackle these challenges is to conduct community-wide scientific studies. This study provides physiological datasets fundamental to understanding functional responses of phytoplankton growth rates to temperature. While physiological experiments are not new, our experiments were conducted in many laboratories using agreed upon protocols and 25 strains of eukaryotic and prokaryotic phytoplankton isolated across a wide range of marine environments from polar to tropical, and from nearshore waters to the open ocean. This community-wide approach provides both comprehensive and internally consistent datasets produced over considerably shorter time scales than conventional individual and often uncoordinated lab efforts. Such datasets can be used to parameterise global ocean model projections of environmental change and to provide initial insights into the magnitude of regional biogeographic change in ocean biota in the coming decades. Here, we compare our datasets with a compilation of literature data on phytoplankton growth responses to temperature. A comparison with prior published data suggests that the optimal temperatures of individual species and, to a lesser degree, thermal niches were similar across studies. However, a comparison of the maximum growth rate across studies revealed significant departures between this and previously collected datasets, which may be due to differences in the cultured isolates, temporal changes in the clonal isolates in cultures, and/or differences in culture conditions. Such methodological differences mean that using particular trait measurements from the prior literature might introduce unknown errors and bias into modelling projections. Using our community-wide approach we can reduce such protocol-driven variability in culture studies, and can begin to address more complex issues such as the effect of multiple environmental drivers on ocean biota.
The high resolution sampling across the Cretaceous/Tertiary (K/T) boundary of the most expanded and continuous sections located in Spain and Tunisia allows us to test and elucidate the extinction model of Cretaceous planktic foraminifera in subtropical and temperate latitudes. The planktic foraminiferal extinction occurred over a short period, with 5% of the species disappearing in the late Maastrichtian, 70% of the species because extinct at the K/T boundary and about 25% of the species are ranging into the early Danian. The species that became extinct at the K/T boundary were large, complex tropical and subtropical forms that dwelled in deep and intermediate water depths. Their disappearance constitutes the largest and most sudden extinction even in the history of planktic foraminifera. Nevertheless, the small cosmopolitan surface dwellers with simple morphologies appear to have survived and the last of them gradually disappeared in the early Danian. The planktic foraminiferal extinction model can be interpreted as a catastrophic mass extinction that was centred at the K/T boundary, and was superimposed on a less evident and controversial gradual mass extinction which apparently began in the late Maastrichtian and continued into the early Danian. The catastrophic pattern of extinction of 70% of the species at the K/T boundary is very compatible with the effect of a large asteroid impact.
Cretaceous-Tertiary boundary clays rich in iridium from five sites in Europe and New Zealand were investigated. The clays are found to be 100-10,000-fold-enriched in elemental carbon (mainly soot), which is isotopically uniform and apparently comes from a single global fire. The soot layer coincides with the iridium layer, suggesting that the fire was triggered by meteorite impact and began before the ejecta had settled.
The Chicxulub crater ejecta stratigraphy is reviewed, in the context of the stratigraphy of underlying and overlying rock sequences. The ejecta sequence is regionally grouped in (a) thick polymict and monomict breccia sequences inside the crater and within 300 km from the rim of the crater known from drill holes in and close to the breater, and exposures near the border of Yucatan and Belize; (b) Gulf of Mexico region, <2500 m from the crater, with up to 9 m thick, complex, tsunami-wave influenced, tektite-bearing sequences in shallow marine (<500 m deep) environments and tektite bearing, decimeter thick gravity-flow deposits in deep water sites; (c) an intermediate region between 2500 and 4000 km from the crater where centimeter thick, tektite-bearing layers occur, and (d) a global distal region with a millimeter thin ejecta layer. The distal ejecta layer is characterized by sub-millimeter sized microkystites, often rich in Ni-rich spinels and (altered) clinopyroxene. Wherever present, the ejecta layers mark exactly the sudden mass-mortality horizon of the K/T boundary. What exactly caused the mass mortality is still uncertain, but it appears the main event leading to the K/T mass extinctions.
We present an analysis of the evolution of the smoke plume caused by the Black Saturday bushfires, which started on 7 February 2009 in the Australian state of Victoria. Within 3 days this smoke plume was located at altitudes between 15 and 20 km thousands of kilometers away from its source region. Standard explanations for high tropospheric and lower stratospheric absorbing aerosols are either volcanic eruptions or pyroconvection. We performed a detailed analysis of various satellite observations, forward trajectory calculations, and meteorological conditions during the fire episode, yet we could not find evidence of either of these standard mechanisms explaining the observed plume evolution. Pyroconvection observed within the initial smoke plumes remained predominantly below 10 km altitude. Furthermore, there are not active volcanoes in the region. We postulate that the subsequent rise beyond approximately 10 km altitude during the first 3 days after the fires started was caused by absorption of short-wave solar radiation in the plume. Observations indicate that the plume was highly absorptive and optically very thick. One-dimensional plume height radiative transfer calculations with realistic assumptions about the optical properties of the smoke show that the plume could rise to 16–18 km after 5 days and up to 20 km after 10 days. The plume rise exhibits a characteristic step-like time evolution that tracks the variation in diurnal insolation and resembles an escalator. We argue that this is the first time that this mechanism, known as “self-lifting,” has been observed on a large scale. The key features of this mechanism and its implications are briefly discussed.
1] Twenty years ago, the results of climate model simulations of the response to smoke and dust from a massive nuclear exchange between the superpowers could be summarized as ''nuclear winter,'' with rapid temperature, precipitation, and insolation drops at the surface that would threaten global agriculture for at least a year. The global nuclear arsenal has fallen by a factor of three since then, but there has been an expansion of the number of nuclear weapons states, with additional states trying to develop nuclear arsenals. We use a modern climate model to reexamine the climate response to a range of nuclear wars, producing 50 and 150 Tg of smoke, using moderate and large portions of the current global arsenal, and find that there would be significant climatic responses to all the scenarios. This is the first time that an atmosphere-ocean general circulation model has been used for such a simulation and the first time that 10-year simulations have been conducted. The response to the 150 Tg scenario can still be characterized as ''nuclear winter,'' but both produce global catastrophic consequences. The changes are more long-lasting than previously thought, however, because the new model, National Aeronautics and Space Administration Goddard Institute for Space Studies ModelE, is able to represent the atmosphere up to 80 km, and simulates plume rise to the middle and upper stratosphere, producing a long aerosol lifetime. The indirect effects of nuclear weapons would have devastating consequences for the planet, and continued nuclear arsenal reductions will be needed before the threat of nuclear winter is removed from the Earth. Citation: Robock, A., L. Oman, and G. L. Stenchikov (2007), Nuclear winter revisited with a modern climate model and current nuclear arsenals: Still catastrophic consequences, J. Geophys. Res., 112, D13107, doi:10.1029/2006JD008235.
While the abrupt climate events of the last deglaciation are well defined in ice core records from the polar regions of both hemispheres, their manifestation elsewhere is less well constrained. Here we compile 104 high-resolution paleoclimate records to characterize the timing and spatial pattern of climate change during the last deglaciation. This compilation indicates relatively concurrent timing of the Last Glacial Maximum (LGM; peak glacial conditions) and the Altithermal (peak interglacial conditions) in the Northern (22.1 ± 4.3 ka and 8.0 ± 3.2 ka) and Southern (22.3 ± 3.6 ka and 7.4 ± 3.7 ka) Hemispheres, suggesting the hemispheres were synchronized by greenhouse gases, local insolation, and/or Northern Hemisphere induced ocean circulation changes. The magnitude of the glacial–interglacial temperature change increases with latitude, reflecting the polar amplification of climate change, with a likely minimum global mean cooling of ∼−4.9 °C during the LGM relative to the Altithermal.
To date, global models of direct radiative forcing have treated elemental carbon (EC) as completely externally mixed or well-mixed internally. No global study has treated EC as a core in an internal mixture. It is hypothesized that the well-mixed treatment is unphysical and reality lies between the externally-mixed and core treatments. It is also suggested, but not proven, that most EC particles are coated to some degree; hence, the core treatment may be more representative than the external-mixture treatment. Global simulations with the core treatment resulted in EC forcing 50% higher and 40% lower than forcings obtained with the externally-mixed and well-internally-mixed treatments, respectively. In the core case, EC's positive forcing more than offset negative forcing due to all other anthropogenic aerosol components combined. Further studies are needed to understand the mixing state of EC and determine the accuracy of the core treatment. Copyright 2000 by the American Geophysical Union. Paper number 1999GL010968.
Sixty-six million years ago, the end-Cretaceous mass extinction ended the reign of the dinosaurs. Flood basalt eruptions and an asteroid impact are widely discussed causes, yet their contributions remain debated. Modeling the environmental changes after the Chicxulub impact can shed light on this question. Existing studies, however, focused on the effect of dust or used one-dimensional, noncoupled atmosphere models. Here we explore the longer-lasting cooling due to sulfate aerosols using a coupled climate model. Depending on aerosol stratospheric residence time, global annual mean surface air temperature decreased by at least 26°C, with 3 to 16 years subfreezing temperatures and a recovery time larger than 30 years. The surface cooling triggered vigorous ocean mixing which could have resulted in a plankton bloom due to upwelling of nutrients. These dramatic environmental changes suggest a pivotal role of the impact in the end-Cretaceous extinction.
Proxy temperature reconstructions indicate a dramatic cooling from the Cenomanian to Maastrichtian. However, the spatial extent of and mechanisms responsible for this cooling remain uncertain, given simultaneous climatic influences of tectonic and greenhouse gas changes through the Late Cretaceous. Here we compare several climate simulations of the Cretaceous using two different Earth system models with a compilation of sea surface temperature proxies from the Cenomanian and Maastrichtian to better understand Late Cretaceous climate change. In general, surface temperature responses are consistent between models, lending confidence to our findings. Our comparison of proxies and models confirms that Late Cretaceous cooling was a widespread phenomenon and likely due to a reduction in greenhouse gas concentrations in excess of a halving of CO2, not changes in paleogeography.
Models of atmospheric transmission allow the estimation of spectral and biologically-weighted ultraviolet (UV) radiation reaching the Earth’s surface. The theory of radiative transfer is well established, but information about the atmosphere (e.g., ozone profiles, cloud morphology), which is required as input to the models, is often incomplete. Still, model sensitivity studies provide many useful insights that, when combined with measurements, give us a more complete understanding of the complex UV environment.