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1. Severe Global Cooling After Volcanic Super-Eruptions? The Answer Hinges on Unknown Aerosol Size

   https://doi.org/10.1175/JCLI-D-23-0116.1  

   McGraw, Zachary ; DallaSanta, Kevin ; Polvani, Lorenzo M. ; Tsigaridis, Kostas ; Orbe, Clara ; Bauer, Susanne E. ( February 2024 , Journal of Climate)

   Volcanic super-eruptions have been theorized to cause severe global cooling, with the 74 kya Toba eruption purported to have driven humanity to near-extinction. However, this eruption left little physical evidence of its severity and models diverge greatly on the magnitude of post-eruption cooling. A key factor controlling the super-eruption climate response is the size of volcanic sulfate aerosol, a quantity that left no physical record and is poorly constrained by models. Here we show that this knowledge gap severely limits confidence in model-based estimates of super-volcanic cooling, and accounts for much of the disagreement among prior studies. By simulating super-eruptions over a range of aerosol sizes, we obtain global mean responses varying from extreme cooling all the way to the previously unexplored scenario of widespread warming. We also use an interactive aerosol model to evaluate the scaling between injected sulfur mass and aerosol size. Combining our model results with the available paleoclimate constraints applicable to large eruptions, we estimate that global volcanic cooling is unlikely to exceed 1.5°C no matter how massive the stratospheric injection. Super-eruptions, we conclude, may be incapable of altering global temperatures substantially more than the largest Common Era eruptions. This lack of exceptional cooling could explain why no single super-eruption event has resulted in firm evidence of widespread catastrophe for humans or ecosystems.

   Whether volcanic super-eruptions pose a threat to humanity remains a subject of debate, with climate models disagreeing on the magnitude of global post-eruption cooling. We demonstrate that this disagreement primarily stems from a lack of constraint on the size of volcanic sulfate aerosol particles. By evaluating the range of aerosol size scenarios, we demonstrate that eruptions may be incapable of causing more than 1.5°C cooling no matter how much sulfur they inject into the stratosphere. This could explain why archaeological records provide no evidence of increased human mortality following the Toba super-eruption. Further, we raise the unexplored possibility that the largest super-eruptions could cause global-scale warming.

    

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2. Stratospheric Climate Anomalies and Ozone Loss Caused by the Hunga Tonga‐Hunga Ha'apai Volcanic Eruption

   https://doi.org/10.1029/2023JD039480  

   Wang, Xinyue ; Randel, William ; Zhu, Yunqian ; Tilmes, Simone ; Starr, Jon ; Yu, Wandi ; Garcia, Rolando ; Toon, Owen B. ; Park, Mijeong ; Kinnison, Douglas ; et al ( November 2023 , Journal of Geophysical Research: Atmospheres)

   The Hunga Tonga‐Hunga Ha'apai (HTHH) volcanic eruption in January 2022 injected unprecedented amounts of water vapor (H₂O) and a moderate amount of the aerosol precursor sulfur dioxide (SO₂) into the Southern Hemisphere (SH) tropical stratosphere. The H₂O and aerosol perturbations have persisted during 2022 and early 2023 and dispersed throughout the atmosphere. Observations show large‐scale SH stratospheric cooling, equatorward shift of the Antarctic polar vortex and slowing of the Brewer‐Dobson circulation. Satellite observations show substantial ozone reductions over SH winter midlatitudes that coincide with the largest circulation anomalies. Chemistry‐climate model simulations forced by realistic HTHH inputs of H₂O and SO₂qualitatively reproduce the observed evolution of the H₂O and aerosol plumes over the first year, and the model exhibits stratospheric cooling, circulation changes and ozone effects similar to observed behavior. The agreement demonstrates that the observed stratospheric changes are caused by the HTHH volcanic influences.

    

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3. CO2 and tectonic controls on Antarctic climate and ice-sheet evolution in the mid-Miocene

   https://doi.org/10.7275/9wxa-yz16  

   Halberstadt, A. R. ( April 2021 , Earth and planetary science letters)

   null (Ed.)

   Antarctic ice sheet and climate evolution during the mid-Miocene has direct relevance for understanding ice sheet (in)stability and the long-term response to elevated atmospheric CO2in the future. Geologic records reconstruct major fluctuations in the volume and extent of marine and terrestrial ice during the mid-Miocene, revealing a dynamic Antarctic ice-sheet response to past climatic variations. We use an ensemble of climate – ice sheet – vegetation model simulations spanning a range of CO2concentrations, Transantarctic Mountain uplift scenarios, and glacial/interglacial climatic conditions to identify climate and ice-sheet conditions consistent with Antarctic mid-Miocene terrestrial and marine geological records. We explore climatic variability at both continental and regional scales, focusing specifically on Victoria Land and Wilkes Land Basin regions using a high-resolution nested climate model over these domains. We find that peak warmth during the Miocene Climate Optimum is characterized by a thick terrestrial ice sheet receded from the coastline under high CO2concentrations. During the Middle Miocene Climate Transition, CO2episodically dropped below a threshold value for marine-based ice expansion. Comparison of model results with geologic data support ongoing Transantarctic Mountain uplift throughout the mid-Miocene. Modeled ice sheet dynamics over the Wilkes Land Basin were highly sensitive to CO2concentrations. This work provides a continental-wide context for localized geologic paleoclimate and vegetation records, integrating multiple datasets to reconstruct snapshots of ice sheet and climatic conditions during a pivotal period in Earth’s history. 

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4. Modeling the 1783–1784 Laki Eruption in Iceland: 2. Climate Impacts

   https://doi.org/10.1029/2018JD029554  

   Zambri, Brian ; Robock, Alan ; Mills, Michael J. ; Schmidt, Anja ( July 2019 , Journal of Geophysical Research: Atmospheres)

   The Laki eruption in Iceland, which began in June 1783, was followed by many of the typical climate responses to volcanic eruptions: suppressed precipitation and drought, crop failure, and surface cooling. In contrast to the observed cooling in 1784–1786, the summer of 1783 was anomalously warm in Western Europe, with July temperatures reaching more than 3 K above the mean. However, the winter of 1783–1784 in Europe was as cold as 3 K below the mean. While climate models generally reproduce the surface cooling and decreased rainfall associated with volcanic eruptions, model studies have failed to reproduce the extreme warming in western Europe that followed the Laki eruption. As a result of the inability to reproduce the anomalous warming, the question remains as to whether this phenomenon was a response to the eruption or merely an example of internal climate variability. Using the Community Earth System Model from the National Center for Atmospheric Research, we investigate the “Laki haze” and its effect on Northern Hemisphere climate in the 12 months following the eruption onset. We find that the warm summer of 1783 was a result of atmospheric blocking over Northern Europe, which in our model cannot be attributed to the eruption. In addition, the extremely cold winter of 1783–1784 was aided by an increased likelihood of an El Niño after the eruption. Understanding the causes of these anomalies is important not only for historical purposes but also for understanding and predicting possible climate responses to future high‐latitude volcanic eruptions.

    

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5. Intra‐Annual Climate Anomalies in Northwestern North America Following the 1783–1784 CE Laki Eruption

   https://doi.org/10.1029/2020JD033544  

   Edwards, Julie ; Anchukaitis, Kevin J. ; Zambri, Brian ; Andreu‐Hayles, Laia ; Oelkers, Rose ; D'Arrigo, Rosanne ; von Arx, Georg ( February 2021 , Journal of Geophysical Research: Atmospheres)

   The 1783–1784 CE Laki eruption in Iceland was one of the largest, in terms of the mass of SO₂emitted, high‐latitude eruptions in the last millennium, but the seasonal and regional climate response was heterogeneous in space and time. Although the eruption did not begin until early June, tree‐ring maximum latewood density (MXD) reconstructions from Alaska suggest that the entire 1783 summer was extraordinarily cold. We use high‐resolution quantitative wood anatomy, climate model simulations, and proxy systems modeling to resolve the intra‐annual climate effects of the Laki eruption on temperatures over northwestern North America. We measured wood anatomical characteristics of white spruce (Picea glauca) trees from two northern Alaska sites. Earlywood cell characteristics of the 1783 ring are normal, while latewood cell wall thickness is significantly and anomalously reduced compared to non‐eruption years. Combined with complementary evidence from climate model experiments and proxy systems modeling, these features indicate an abrupt and premature cessation of cell wall thickening due to a rapid temperature decrease toward the end of the growing season. Reconstructions using conventional annual resolution MXD likely over‐estimate total growing season cooling in this year, while ring width fails to capture this abrupt late‐summer volcanic signal. Our study has implications not only for the interpretation of the climatic impacts of the Laki eruption in North America, but more broadly demonstrates the importance of timing and internal variability when comparing proxy temperature reconstructions and climate model simulations. It further demonstrates the value of developing cellular‐scale tree‐ring proxy measurements for paleoclimatology.

    

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