Kirstin Krüger’s research while affiliated with University of Oslo and other places

Effusive, long-lasting volcanic eruptions impact climate through the emission of gases and the subsequent production of aerosols. Previous studies, both modelling and observational, have made efforts to quantify these impacts and untangle them from natural variability. However, due to the scarcity of large and well-observed effusive volcanic eruptions, our understanding remains patchy. Here, we use an Earth system model to systematically investigate the climate response to high-latitude, effusive volcanic eruptions, similar to the 2014–2015 Holuhraun eruption in Iceland, as a function of eruption season and size. The results show that the climate response is regional and strongly modulated by different seasons, exhibiting midlatitude cooling during summer and Arctic warming during winter. Furthermore, as eruptions increase in size in terms of sulfur dioxide emissions, the climate response becomes increasingly insensitive to variations in emission strength, levelling off for eruptions between 20 and 30 times the size of the 2014–2015 Holuhraun eruption. Volcanic eruptions are generally considered to lead to surface cooling, but our results indicate that this is an oversimplification, especially in the Arctic, where warming is found to be the dominant response during autumn and winter.

Plain Language Summary Volcanic eruptions can cause large climate fluctuations and pose environmental risks. The 10th‐century Eldgjá eruption in Iceland is the largest of its type in the past two thousand years, but our understanding of its consequences for the climate, environment, and human societies at the time is limited. Using the newest information about the eruption, from field observations of the volcano in Iceland and analyses of ice‐cores from Greenland, we infer a plausible scenario for the eruption with emissions several times higher than what is typically used in climate model simulations. We simulate the effects of the volcanic gas emissions described in this scenario in a complex climate model, indicating a strong temperature drop of around 2° $2{}^{\circ}$C in the extratropics of the Northern Hemisphere. In addition to this cooling, our results suggest that the eruption thinned the protective ozone layer and increased air pollution, the simultaneous occurrence of which would have posed serious challenges to societies at the time. By examining the Eldgjá eruption's consequences for the climate and environment, our study not only improves our understanding of historical volcanic activity, but also informs current efforts to predict and mitigate the impacts of future eruptions of this type.

Effusive, long-lasting volcanic eruptions impact climate through emission of gases and subsequent production of aerosols. Previous studies, both modelling and observational, have made efforts in quantifying these impacts and untangle them from natural variability. However, due to the scarcity of large and well observed effusive volcanic eruptions, our understanding remains patchy. Here we use an Earth system model to systematically investigate the climate response to high-latitude, effusive volcanic eruptions, similar to the 2014–15 Holuhraun eruption in Iceland, as a function of eruption season and eruptive size. The results show that the climate response is regional and strongly modulated by different seasons, with mid-latitude cooling during summer and Arctic warming during winter. Furthermore, as eruptions become larger in terms of sulfur dioxide emissions, the climate response becomes increasingly insensitive to variations in the emission strength, levelling out for eruptions between 20 and 30 times the size of the 2014–15 Holuhraun eruption. Volcanic eruptions are generally considered to lead to surface cooling, but our results indicate that this is an oversimplification, especially in the Arctic where we find warming to be the dominating response during fall and winter.

The Eldgjá eruption is the largest basalt lava flood of the Common Era. It has been linked to a major ice‐core sulfur (S) spike in 939–940 CE and Northern Hemisphere summer cooling in 940 CE. Despite its magnitude and potential climate impacts, uncertainties remain concerning the eruption timeline, atmospheric dispersal of emitted volatiles, and coincident volcanism in Iceland and elsewhere. Here, we present a comprehensive analysis of Greenland ice‐cores from 936 to 943 CE, revealing a complex volatile record and cryptotephra with numerous geochemical populations. Transitional alkali basalt tephra matching Eldgjá are found in 939–940 CE, while tholeiitic basalt shards present in 936/937 CE and 940/941 CE are compatible with contemporaneous Icelandic eruptions from Grímsvötn and Bárðarbunga‐Veiðivötn systems (including V‐Sv tephra). We also find four silicic tephra populations, one of which we link to the Jala Pumice of Ceboruco (Mexico) at 941 ± 1 CE. Triple S isotopes, Δ³³S, spanning 936–940 CE are indicative of upper tropospheric/lower stratospheric transport of aerosol sourced from the Icelandic fissure eruptions. However, anomalous Δ³³S (down to −0.4‰) in 940–941 CE evidence stratospheric aerosol transport consistent with summer surface cooling revealed by tree‐ring reconstructions. Tephra associated with the anomalous Δ³³S have a variety of compositions, complicating the attribution of climate cooling to Eldgjá alone. Nevertheless, our study confirms a major S emission from Eldgjá in 939–940 CE and implicates Eldgjá and a cluster of eruptions as triggers of summer cooling, severe winters, and privations in ∼940 CE.

Oceanic bromoform (CHBr3) is an important precursor of atmospheric bromine. Although highly relevant for the future halogen burden and ozone layer in the stratosphere, global CHBr3 production in the ocean and its emissions are still poorly constrained in observations and are mostly neglected in climate models. Here, we newly implement marine CHBr3 in the second version of the state-of-the-art Norwegian Earth System Model (NorESM2) with fully coupled interactions of ocean, sea ice, and atmosphere. Our results are validated using oceanic and atmospheric observations from the HalOcAt (Halocarbons in the Ocean and Atmosphere) database. The simulated mean oceanic concentrations (6.61 ± 3.43 pmol L-1) are in good agreement with observations from open-ocean regions (5.02 ± 4.50 pmol L-1), while the mean atmospheric mixing ratios (0.76 ± 0.39 ppt) are lower than observed but within the range of uncertainty (1.45 ± 1.11 ppt). The NorESM2 ocean emissions of CHBr3 (214 Gg yr-1) are within the range of or higher than previously published estimates from bottom-up approaches but lower than estimates from top-down approaches. Annual mean fluxes are mostly positive (sea-to-air fluxes); driven by oceanic concentrations, sea surface temperature, and wind speed; and dependent on season and location. During winter, model results imply that some oceanic regions in high latitudes act as sinks of atmospheric CHBr3 due to their elevated atmospheric mixing ratios. We further demonstrate that key drivers for oceanic and atmospheric CHBr3 variability are spatially heterogeneous. In the tropical West Pacific, which is a hot spot for oceanic bromine delivery to the stratosphere, wind speed is the main driver for CHBr3 fluxes on an annual basis. In the North Atlantic, as well as in the Southern Ocean region, atmospheric and oceanic CHBr3 variabilities interact during most of the seasons except for the winter months, when sea surface temperature is the main driver. Our study provides an improved process-based understanding of the biogeochemical cycling of CHBr3 and more reliable natural emission estimates, especially on seasonal and spatial scales, compared to previously published model estimates.

Volcanic eruptions impact the climate and environment. The volcanic forcing is determined by eruption source parameters, including the mass and composition of volcanic volatiles, eruption season, eruption latitude, and injection altitude. Moreover, initial atmospheric conditions of the climate system play an important role in shaping the volcanic forcing and response. However, our understanding of the combination of these factors, the distinctions between tropical and extratropical volcanic eruptions, and the co-injection of sulfur and halogens remains limited. Here, we perform ensemble simulations of volcanic eruptions at 15 and 64° N in January, injecting 17 Mt of SO2 together with HCl and HBr at 24 km altitude. Our findings reveal that initial atmospheric conditions control the transport of volcanic volatiles from the first month and modulate the subsequent latitudinal distribution of sulfate aerosols and halogens. This results in different volcanic forcing, surface temperature and ozone responses over the globe and Northern Hemisphere extratropics (NHET) among the model ensemble members with different initial atmospheric conditions. NH extratropical eruptions exhibit a larger NHET mean volcanic forcing, surface cooling and ozone depletion compared with tropical eruptions. However, tropical eruptions lead to more prolonged impacts compared with NH extratropical eruptions, both globally and in the NHET. The sensitivity of volcanic forcing to varying eruption source parameters and model dependency is discussed, emphasizing the need for future multi-model studies to consider the influence of initial conditions and eruption source parameters on volcanic forcing and subsequent impacts.

Understanding climate variability across interannual to centennial timescales is critical, as it encompasses the natural range of climate fluctuations that early human agricultural societies had to adapt to. Deviations from the long-term mean climate are often associated with both societal collapse and periods of prosperity and expansion. Here, we show that contrary to what global paleoproxy reconstructions suggest, the mid to late-Holocene was not a period of climate stability. We use mid- to late-Holocene Earth System Model simulations, forced by state-of-the-art reconstructions of external climate forcing to show that eleven long-lasting cold periods occurred in the Northern Hemisphere during the past 8000 years. These periods correlate with enhanced volcanic activity, where the clustering of volcanic eruptions induced a prolonged cooling effect through gradual ocean-sea ice feedback. These findings challenge the prevailing notion of the Holocene as a period characterized by climate stability, as portrayed in multi-proxy climate reconstructions. Instead, our simulations provide an improved representation of amplitude and timing of temperature variations on sub-centennial timescales.

Oceanic bromoform (CHBr3) is an important precursor of atmospheric bromine. Although highly relevant for the future halogen burden and ozone layer in the stratosphere, the global CHBr3 production in the ocean and its emissions are still poorly constrained in observations and are mostly neglected in climate models. Here, we newly implement marine CHBr3 in the state-of-the-art Norwegian Earth System Model (NorESM2) with fully coupled ocean-sea-ice-atmosphere biogeochemistry interactions. Our results are validated with oceanic and atmospheric observations from the HalOcAt (Halocarbons in the Ocean and Atmosphere) data base. The simulated mean oceanic concentrations (6.61±3.43 pmol L-1) are in good agreement with observations in open ocean regions (5.02±4.50 pmol L-1), while the mean atmospheric mixing ratios (0.76±0.39 ppt) are lower than observed but within the range of uncertainty (1.45±1.11 ppt). The NorESM2 ocean emissions of CHBr3 (214 Gg yr-1) are in the range of or higher than previously published estimates from bottom-up approaches but lower than estimates from top-down approaches. Annual mean emissions are mostly positive (sea-to-air), driven by oceanic concentrations, sea surface temperature and wind speed, dependent on season and location. During low-productivity winter seasons, model results imply some oceanic regions in high latitudes as sinks of atmospheric CHBr3, because of its elevated atmospheric mixing ratios. We further demonstrate that key drivers for the oceanic and atmospheric CHBr3 variability are spatially heterogeneous. In the tropical West Pacific, which is a hot spot for oceanic bromine delivery to the stratosphere, wind speed is the main driver for CHBr3 emissions on annual basis. In the North Atlantic as well as in the Southern Ocean region the atmospheric and oceanic CHBr3 variabilities are interacting during most of the seasons except for the winter months where sea surface temperature is the main driver. Our study provides improved process understanding of the biogeochemical cycling of CHBr3 and more reliable natural emission estimates especially on seasonal and spatial scales compared to previously published model estimates.

High-latitude explosive volcanic eruptions can cause substantial hemispheric cooling. Here, we use a whole-atmosphere chemistry-climate model to simulate Northern Hemisphere (NH) high-latitude volcanic eruptions of magnitude similar to the 1991 Mt. Pinatubo eruption. Our simulations reveal that the initial stability of the polar vortex strongly influences sulphur dioxide lifetime and aerosol growth by controlling the dispersion of injected gases after such eruptions in winter. Consequently, atmospheric variability introduces a spread in the cumulative aerosol radiative forcing of more than 20%. We test the aerosol evolution’s sensitivity to co-injection of sulphur and halogens, injection season, and altitude, and show how aerosol processes impact radiative forcing. Several of these sensitivities are of similar magnitude to the variability stemming from initial conditions, highlighting the significant influence of atmospheric variability. We compare the modelled volcanic sulphate deposition over the Greenland ice sheet with the relationship assumed in reconstructions of past NH eruptions. Our analysis yields an estimate of the Greenland transfer function for NH extratropical eruptions that, when applied to ice core data, produces volcanic stratospheric sulphur injections from NH extratropical eruptions 23% smaller than in currently used volcanic forcing reconstructions. Furthermore, the transfer function’s uncertainty, which propagates into the estimate of sulphur release, needs to be at least doubled to account for atmospheric variability and unknown eruption parameters. Our results offer insights into the processes shaping the climatic impacts of NH high-latitude eruptions and highlight the need for more accurate representation of these events in volcanic forcing reconstructions.