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A Negative feedback mechanism for the long-term stabilization of Earths surface-temperature
Abstract
It is suggested that the partial pressure of carbon dioxide in the atmosphere is buffered, over geological time scales, by a negative feedback mechanism, in which the rate of weathering of silicate minerals (followed by deposition of carbonate minerals) depends on surface temperature, which in turn depends on the carbon dioxide partial pressure through the greenhouse effect. Although the quantitative details of this mechanism are speculative, it appears able to partially stabilize the earth's surface temperature against the steady increase of solar luminosity, believed to have occurred since the origin of the solar system.
... Therefore, we emphasize the importance of silicate weathering as a carbon sink process in the global carbon cycle, which significantly consumed atmospheric CO 2 and consequently contributed to global cooling and ice sheet development in Antarctica and the Northern Hemisphere during the late Cenozoic ( Fig. 7a and 7b) (Lear et al., 2000;Westerhold et al., 2020;Rae et al., 2021). Moreover, the consistent dominance of the silicate weathering rate by the physical erosion rate in both tectonically inactive regions (e.g., South China) and orogenic belts (e.g., Himalayas), as revealed by sediment records in the northern SCS and Bay of Bengal, strongly suggests that the "thermostat" hypothesis (Walker et al., 1981) and "uplift-weathering" hypothesis (Raymo and Ruddiman, 1992) can be essentially reconciled in the context of large-scale and variable terrains . In tectonically active regions, such as the Himalayas and Tibetan Plateau, strong tectonic activity induces higher physical erosion, which enhances the silicate weathering rate and leads to a pronounced increase in atmospheric CO 2 consumption and global cooling (Raymo and Ruddiman, 1992;Misra and Froelich, 2012). ...
... For example, Ding & Wordsworth (2020) have shown that nightside cold trapping might be avoided on planets with high CO 2 values, due to increased nightside temperatures. Constraining the exoplanet CO 2 levels remains a challenge, due to their dependence on outgassing and weathering rates (Walker et al. 1981). Therefore, future work may wish to consider both terminator habitability and high-CO 2 "eye" habitability scenarios, while exploring additional factors, such as the impact of groundwater flow and long-term climate sensitivity in the presence of M-dwarf flare activity, to quantify prospects for sustained habitability. ...
Rocky planets orbiting M-dwarf stars are among the most promising and abundant astronomical targets for detecting habitable climates. Planets in the M-dwarf habitable zone are likely synchronously rotating, such that we expect significant day–night temperature differences and potentially limited fractional habitability. Previous studies have focused on scenarios where fractional habitability is confined to the substellar or “eye” region, but in this paper we explore the possibility of planets with terminator habitability, defined by the existence of a habitable band at the transition between a scorching dayside and a glacial nightside. Using a global climate model, we show that for water-limited planets it is possible to have scorching temperatures in the “eye” and freezing temperatures on the nightside, while maintaining a temperate climate in the terminator region, due to reduced atmospheric energy transport. On water-rich planets, however, increasing the stellar flux leads to increased atmospheric energy transport and a reduction in day–night temperature differences, such that the terminator does not remain habitable once the dayside temperatures approach runaway or moist greenhouse limits. We also show that while water-abundant simulations may result in larger fractional habitability, they are vulnerable to water loss through cold trapping on the nightside surface or atmospheric water vapor escape, suggesting that even if planets were formed with abundant water, their climates could become water-limited and subject to terminator habitability.
The dozens of rocky exoplanets discovered in the Circumstellar Habitable Zone (CHZ) currently represent the most suitable places to host life as we know it outside the Solar System. However, the presumed presence of liquid water on the CHZ planets does not guarantee suitable environments for the emergence of life. According to experimental studies, the building blocks of life are most likely produced photochemically in presence of a minimum ultraviolet (UV) flux. On the other hand, high UV flux can be life-threatening, leading to atmospheric erosion and damaging biomolecules essential to life. These arguments raise questions about the actual habitability of CHZ planets around stars other than Solar-type ones, with different UV to bolometric luminosity ratios. By combining the "principle of mediocricy" and recent experimental studies, we define UV boundary conditions (UV-habitable Zone, UHZ) within which life can possibly emerge and evolve. We investigate whether exoplanets discovered in CHZs do indeed experience such conditions. By analysing Swift-UV/Optical Telescope data, we measure the near ultraviolet (NUV) luminosities of 17 stars harbouring 23 planets in their CHZ. We derive an empirical relation between NUV luminosity and stellar effective temperature. We find that eighteen of the CHZ exoplanets actually orbit outside the UHZ, i.e., the NUV luminosity of their M-dwarf hosts is decisively too low to trigger abiogenesis - through cyanosulfidic chemistry - on them. Only stars with effective temperature >3900 K illuminate their CHZ planets with enough NUV radiation to trigger abiogenesis. Alternatively, colder stars would require a high-energy flaring activity.
The advent of oxygenic photosynthesis represents the most prominent biological innovation in the evolutionary history of the Earth. The exact timing of the evolution of oxygenic photoautotrophic bacteria remains elusive, yet these bacteria profoundly altered the redox state of the ocean-atmosphere-biosphere system, ultimately causing the first major rise in atmospheric oxygen (O2 )-the so-called Great Oxidation Event (GOE)-during the Paleoproterozoic (~2.5-2.2 Ga). However, it remains unclear how the coupled atmosphere-marine biosphere system behaved after the emergence of oxygenic photoautotrophs (OP), affected global biogeochemical cycles, and led to the GOE. Here, we employ a coupled atmospheric photochemistry and marine microbial ecosystem model to comprehensively explore the intimate links between the atmosphere and marine biosphere driven by the expansion of OP, and the biogeochemical conditions of the GOE. When the primary productivity of OP sufficiently increases in the ocean, OP suppresses the activity of the anaerobic microbial ecosystem by reducing the availability of electron donors (H2 and CO) in the biosphere and causes climate cooling by reducing the level of atmospheric methane (CH4 ). This can be attributed to the supply of OH radicals from biogenic O2 , which is a primary sink of biogenic CH4 and electron donors in the atmosphere. Our typical result also demonstrates that the GOE is triggered when the net primary production of OP exceeds >~5% of the present oceanic value. A globally frozen snowball Earth event could be triggered if the atmospheric CO2 level was sufficiently small (<~40 present atmospheric level; PAL) because the concentration of CH4 in the atmosphere would decrease faster than the climate mitigation by the carbonate-silicate geochemical cycle. These results support a prolonged anoxic atmosphere after the emergence of OP during the Archean and the occurrence of the GOE and snowball Earth event during the Paleoproterozoic.
Planetary geodynamics may have an important influence over planetary habitability and the boundaries of the circumstellar habitable zone (CHZ) in space and time. To investigate this we use a minimal parameterized model of the co-evolution of the geosphere and atmosphere of Earth-like planets around F, G, K and M main sequence stars. We found the CHZ for the present Solar System located between 0.92 and 1.09 au for a 1.0 M $_{\oplus }$ Earth-like planet, extendible to 1.36 au for a 4.0 M $_{\oplus }$ planet. In the literature, the CHZ varies considerably in width and border location, but the outer edges tend to be more spread out than the inner edges, showing a higher difficulty in determining the outer edge. Planetary mass has a considerable effect on planetary geodynamics, with low-mass planets cooling down faster and being less capable of maintaining a rich carbon dioxide atmosphere for several billions of years. Age plays a particularly important role in the width of the CHZ as the CHZ contracts in both directions: from the inner edge (as stellar luminosity increases with time), and from the outer edge (as planetary heat flux and seafloor spreading rate decrease with time). This strongly affects long-lived habitability as the 5 Gyr continuous CHZ may be very narrow or even non-existent for low-mass planets (<0.5 M $_{\oplus }$ ) and fast-evolving high-mass stars (>1.1 M $_{\odot }$ ). Because of this, the mean age of habitable terrestrial planets in our Galaxy today may be younger than Earth's age. Our results suggest that the best targets for future surveys of biosphere signatures may be planets between 0.5 and 4.0 M $_{\oplus }$ , in systems younger than the Solar System. These planets may present the widest and long-lived CHZ.
A class of mean annual, zonally averaged energy-balance climate models
of the Budyko-Sellers type are studied by a spectral (expansion in
Legendre polynomials) method. Models with constant thermal diffusion
coefficient can be solved exactly, The solution is approached by a
rapidly converging sequence with each succeeding approximant taking into
account information from ever smaller space and time scales. The first
two modes represent a good approximation to the exact solution as well
as to the present climate. The two-mode approximation to a number of
more general models are shown to be either formally or approximately
equivalent to the same truncation in the constant diffusion case. In
particular, the transport parameterization used by Budyko is precisely
equivalent to the two-mode truncation of thermal diffusion. Details of
the dynamics do not influence the first two modes which fortunately seem
adequate for the study of global climate change. Estimated ice age
temperatures and ice line latitude agree well with the model if the
solar constant is reduced by 1.3%.
This study investigates the response of a global model of the climate to the quadrupling of the CO2 concentration in the atmosphere. The model consists of (1) a general circulation model of the atmosphere, (2) a heat and water balance model of the continents, and (3) a simple mixed layer model of the oceans. It has a global computational domain and realistic geography. For the computation of radiative transfer, the seasonal variation of insolation is imposed at the top of the model atmosphere, and the fixed distribution of cloud cover is prescribed as a function of latitude and of height. It is found that with some exceptions, the model succeeds in reproducing the large-scale characteristics of seasonal and geographical variation of the observed atmospheric temperature. The climatic effect of a CO2 increase is determined by comparing statistical equilibrium states of the model atmosphere with a normal concentration and with a 4 times the normal concentration of CO2 in the air. It is found that the warming of the model atmosphere resulting from CO2 increase has significant seasonal and latitudinal variation. Because of the absence of an albedo feedback mechanism, the warming over the Antarctic continent is somewhat less than the warming in high latitudes of the northern hemisphere. Over the Arctic Ocean and its surroundings, the warming is much larger in winter than summer, thereby reducing the amplitude of seasonal temperature variation. It is concluded that this seasonal asymmetry in the warming results from the reduction in the coverage and thickness of the sea ice. The warming of the model atmosphere results in an enrichment of the moisture content in the air and an increase in the poleward moisture transport. The additional moisture is picked up from the tropical ocean and is brought to high latitudes where both precipitation and runoff increase throughout the year. Further, the time of rapid snowmelt and maximum runoff becomes earlier.
A relatively simple numerical model of the energy balance of the earth-atmosphere is set up and applied. The dependent variable is the average annual sea level temperature in 10° latitude belts. This is expressed basically as a function of the solar constant, the planetary albedo, the transparency of the atmosphere to infrared radiation, and the turbulent exchange coefficients for the atmosphere and the oceans. The major conclusions of the analysis are that removing the arctic ice cap would increase annual average polar temperatures by no more than 7C, that a decrease of the solar constant by 2–5% might be sufficient to initiate another ice age, and that man's increasing industrial activities may eventually lead to a global climate much warmer than today.
A numerical model of the energy balance of the earth-atmosphere is set up and applied. The dependent variable is the average annual sea level temperature in 10//0 latitude belts. This is expressed basically as a function of the solar constant, the planetary albedo, the transparency of the atmosphere to infrared radiation, and the turbulent exchange coefficients for the atmosphere and the oceans. The major conclusions of the analysis are presented.
This study investigates the influences of the seasonal variation of solar radiation based upon the results of numerical experiments with a mathematical model of climate. The model consists of (1) a general circulation model of the atmosphere, (2) a heat- and water-balance model of continents, and (3) a simple mixed layer model of the ocean. It has a limited computational domain and idealized geography. Two versions of the model are constructed. In the first version of the model (the seasonal model), a seasonal variation of insolation is imposed at the top of the model atmosphere. On the other hand, an annual mean isolation is prescribed for the second version of the model (the annual model). The response of the seasonal model to the q quadrupling of CO2-concentration in air is compared to the corresponding response of the annual model. It is found that the response of the annual mean surface air temperature of the seasonal model is significantly less than the corresponding response of the annual model. The smaller sensitivity of the seasonal model is attributed to the absence of strongly reflective snow cover (or sea ice) during the summer when the isolation has a near-maximum intensity. A comparison between the hydrologic responses of the seasonal and the annual models indicates that the latitudinal distributions of these responses have qualitatively similar zonal mean features. However, the zonal mean response of the seasonal model is found to have considerable seasonal variations. For example, in summer the zonal mean soil wetness is reduced extensively over two seperate zones of middle and high latitudes in response to the CO2 increase, respectively. Owing to the seasonal variation mentioned above, the latitudinal variation of the annual mean hydrologic response of the seasonal model is less than that of the corresponding response of the annual model.
The thermodynamic ocean of the Sillen school offers little incentive to those who search the sedimentary record for evidence of changes in ocean chemistry during Cenozoic time. Their models predict a uniform chemical composition. However as the sediments presently accumulating in the ocean show little evidence of equilibration with the overlying water, the possibility that kinetic factors play an important role must be seriously explored. Such a model is presented in this paper. Material balance restrictions are substituted for some of the usual chemical equilibria. The role of organisms is shown to be dominant for at least some of the important components of sea salt (i.e., C, N, P, Si, …). If, as proposed here, the chemistry of sea water is dependent on rates of supply of individual components, the rate of vertical mixing in the sea, and the type of material formed by organisms, then substantial changes in the chemical composition have almost certainly taken place. Several means by which such changes might be reconstructed from chemical and isotopic measurements on marine sediments are discussed.
The surface temperature of a planet with an atmosphere depends, amongst other factors, on the atmospheric chemical composition and surface pressure. However, the detailed calculation of surface temperature variations as a function of atmospheric composition is extremely complex. A simplified model is presented which can be used to follow surface temperature changes over periods up to the lifetime of the solar system. This model is applied to a number of chemical constituents of interest in studying the evolution of planetary atmospheres (with special reference to the earth).
A study has been made of the release of Si and Al to solution from the alteration of a potassic feldspar in solutions buffered at pH values between 4 and 10. Release of both Si and Al is consistent with diffusion from an altered layer, presumably formed by rapid initial hydration and exchange of H + for K + . In a limited volume of solution diffusion ceases when the Al concentration in the external solution reaches a fixed value at each pH; this value is reasonably consistent with the solubility of Al(OH) 3 . The Si concentration tends to reach a maximum at each pH. The interpretation is made that the maximum in Si concentration corresponds to a balance between Si diffusion into the solution, and Si removed from the solution by reaction with Al(OH) 3 to form a hydrated silicate. The calculated equilibrium value for the reaction Al(OH) 3 + SiO 2aq = Al-silicate is 5 ppm SiO 2 .