Volcanic flood basalt eruptions in Earth’s history have covered thousands of square kilometers with basalt deposits up to kilometers thick [Glaze et al., 2017]. The massive size and extended duration (up to centuries or millennia) result in enormous releases of climactically-relevant gas species such as SO₂ and CO₂ [e.g., Davis et al., 2017]. Historic flood basalt eruptions on Earth such as the Siberian and Deccan Traps are coincident with mass extinction events, although the casual linkages are still being studied [Courtillot et al., 1988; Wignall, 2001; Renne et al., 2015]. Additionally, flood basalt eruptions seem to be a common feature of terrestrial planets in our Solar System [Lancaster et al., 1995; O’Hara, 2000; Head et al., 2011; Jaeger et al., 2010] and are hence plausible on terrestrial exoplanets. Indeed, flood basalt eruptions may have made the ancient martian climate more habitable [e.g., Halevy and Head, 2014]. However, what is still unknown is precisely how flood basalt eruptions influence planetary climate via their eruption rates and cadence [Davis et al., 2017], height of the volcanic plumes [e.g., Glaze et al., 2017], and relative degassing abundance of climactically-relevant species like SO₂ [Self et al., 2006; Davis et al., 2017]. Once eruptions occur, the complex interplay of photochemistry (e.g., turning SO₂ into H₂SO₄ aerosols), greenhouse gas warming, changes to the atmospheric circulation, and aerosol-cloud interactions can only be properly simulated with a comprehensive global climate model (GCM). Previous work on the terrestrial climate response to large volcanic eruptions has settled on the initiation of “volcanic winter”, a cooling response to the reduced surface insolation caused by a widespread blanket of H₂SO₄ aerosols in the upper troposphere and stratosphere [e.g., Robock et al., 2009]. Smaller-scale eruptions produce more varied regional effects, but again, largely with cooler temperature anomalies at the surface [e.g., Oman et al., 2006]. However, these previous works have generally focused on explosive eruptions-single short-duration events that inject material into the stratosphere. This is in contrast to flood basalt eruptions, which have much longer durations and likely injected material both at the surface and at higher altitudes in the troposphere and lower stratosphere. Our ongoing work has simulated a short-duration Columbia River Flood Basalt (CRB)-like eruption, a medium-scale flood basalt eruption that occurred ~15-16 Mya in eastern Washington state and Oregon. While the CRB is not believed to have initiated an extinction event, it occurred in the midst of the Mid-Miocene climactic optimum [Kasbohm and Schoene, 2018] and there is some evidence of a coincident glaciation [e.g., Armstrong McKay et al., 2014]. The CRB eruption occurred in a variety of phases, the largest termed the Grande Ronde basalt formation. Following Davis et al. [2017], we created an eruption scenario for the Goddard Chemistry Climate Model (GEOSCCM) [Oman et al., 2013] that emits SO₂ in the near-surface atmosphere constantly and periodically (four times per year) an explosive eruption that emits much more SO₂ in the upper troposphere/lower stratosphere. The eruption lasts for 4 years and emits 30 Gt of SO₂ in total. This corresponds to approximately 1/10^th of what may have been emitted during the Grande Ronde eruption phase of the CRB [Davis et al., 2017]. Note that we have used a post-industrial atmosphere and ocean with modern continental configuration as our baseline, and simulations with a pre-industrial atmosphere are ongoing. Our simulations do not include SO₂ as a radiatively active species, however H₂SO₄ aerosols are radiatively active. Atmospheric CO₂ is set at 400 ppm. The massive flux of SO₂ into the atmosphere is quickly converted to H₂SO₄ aerosols. Global area-weighted mean visible band sulfate aerosol optical depth reaches 230 near the end of the eruption, comparable to cumulonimbus clouds. This reduces the surface shortwave radiative flux by 85% and top-of-atmosphere outgoing longwave flux by 70%. Contrary to our expectations, we find that the climate warms during and immediately following the eruption after a very brief initial cooling. Global mean surface temperature peaks 3-4 years after the eruption ends with a +7 K anomaly relative to a baseline simulation without the eruption. Post-eruption regional temperatures, particularly near-equatorial continental areas, see drastic rises of summertime temperatures with monthly mean temperatures equaling or exceeding 40°C, which are uninhabitable temperatures for mammals [Sherwood and Huber, 2010]. These temperature responses are radiative- and circulation-driven. The eruption warms and raises the tropical tropopause, allowing a massive flux of water vapor into the stratosphere. Stratospheric water vapor, usually ~3 parts per million reaches 1-2 parts per thousand. This increase results in increased thermal infrared flux from the stratosphere, which cools that portion of the atmosphere while also warming the surface and troposphere. Such a water flux into the stratosphere may have implications for historic water loss on planets such as Mars and Venus. Despite the massive perturbation to the climate during the four-year eruption, the climate approaches pre-eruption normal after seven years post-eruption. H₂SO₄ aerosols are nearly absent, surface radiative fluxes are near normal, and global temperatures are cooling toward normal levels. However, the stratospheric water vapor is more slowly returning to pre-eruption levels and remains more than one order of magnitude higher than pre-eruption levels after seven years post-eruption.

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