Volcanic Impacts on Climate

Over the course of the last millennium, the largest natural influence on the climate system is the emission of aerosols (essentially, dust and soot) from strong volcanic eruptions. Based on reconstructions from aerosol layers in ice cores, we believe that past eruptions were much stronger than anything seen during the 20th century: they can cool the world by up to several degrees! By simulating these strong eruptions in climate models, we can gain an understanding of how aspects of eruptions (their location, size, and other characteristics) affect their influence on climate, and what that might mean for the risks associated with any future eruptions.

 
 

Volcanic Effects on Hydroclimate

'Hydroclimate' refers to changes in moisture conditions around the world; we hav a good sense of how it has varied in the past in many places, since paleoclimatologists have constructed gridded "drought atlases" for much of the world's land surface based on tree rings (see Links for more details). Based on these atlases, people have concluded that volcanic eruptions sometimes lead to preferential El Nino initiation the following year; but the exact mechanisms are still under some debate, and the effect is somewhat small compared to internal variability.  

I have been using the CESM Last Millennium Ensemble to understand how reliable these 'El Nino-like' responses to eruptions are: because this set of simulations contains 30 different, physically plausible realizations of the past millennium, we can separate the climate responses to the same eruption based on the state of the ocean the following year. And it turns out that the hydroclimate patterns which form in response to eruptions can look 'El Nino-like' even when there is no El Nino... changes to the atmospheric circulation due to aerosol influences lead to patterns that look similar to what happens during El Nino events. 
(Stevenson et al. 2016)

Figure 7 from Stevenson et al. (2016), Journal of Climate PDSI composites for Tropical and Northern eruptions during JJA +1. (a) Tropical eruptions followed by ENSO-neutral conditions during DJF +0 to +1. (b) As in (a), but for eruptions followed by El Niño conditions. (c) Northern eruptions followed by ENSO-neutral conditions during DJF +0 to +1. (d) As in (c), but for eruptions followed by El Niño conditions. Boxes indicate the southwest United States and monsoon Asia regions discussed in the text, and the numbers alongside are the pattern correlations between the PDSI composite and the El Niño teleconnection pattern over the appropriate region. Boldface red text indicates a pattern correlation significant at 95% relative to internal variability, as computed via a Wilcoxon rank-sum test.

Figure 7 from Stevenson et al. (2016), Journal of Climate
PDSI composites for Tropical and Northern eruptions during JJA +1. (a) Tropical eruptions followed by ENSO-neutral conditions during DJF +0 to +1. (b) As in (a), but for eruptions followed by El Niño conditions. (c) Northern eruptions followed by ENSO-neutral conditions during DJF +0 to +1. (d) As in (c), but for eruptions followed by El Niño conditions. Boxes indicate the southwest United States and monsoon Asia regions discussed in the text, and the numbers alongside are the pattern correlations between the PDSI composite and the El Niño teleconnection pattern over the appropriate region. Boldface red text indicates a pattern correlation significant at 95% relative to internal variability, as computed via a Wilcoxon rank-sum test.

 
Figure 3 from Stevenson et al. (2017), PNAS Comparison of CESM simulations against proxy reconstructions. (C) Palmer Drought Severity Index anomaly for the boreal summer of the eruption year, derived from the Monsoon Asia Drought Atlas (Cook et al. 2010) (Left) and the North American Drought Atlas (Cook et al. 2004) (Right). (D) Zero- to 30-cm soil moisture anomaly averaged over the period 0–5 mo after the eruption, for CESM simulations with eruptions in April (Left) and July (Right). All CESM anomalies are computed relative to the 30 y before the eruption; in D, stippling indicates regions where the anomalies are insignificant relative to internal variability at 90% based on a Wilcoxon rank-sum test.

Figure 3 from Stevenson et al. (2017), PNAS
Comparison of CESM simulations against proxy reconstructions. (C) Palmer Drought Severity Index anomaly for the boreal summer of the eruption year, derived from the Monsoon Asia Drought Atlas (Cook et al. 2010) (Left) and the North American Drought Atlas (Cook et al. 2004) (Right). (D) Zero- to 30-cm soil moisture anomaly averaged over the period 0–5 mo after the eruption, for CESM simulations with eruptions in April (Left) and July (Right). All CESM anomalies are computed relative to the 30 y before the eruption; in D, stippling indicates regions where the anomalies are insignificant relative to internal variability at 90% based on a Wilcoxon rank-sum test.

How Eruption Season Affects Atmosphere/Ocean Circulation

Every volcanic eruption is different: the location of the volcano, the strength of the eruption, and the prevailing winds blowing when the eruption goes off can all make a big difference to how that eruption affects weather and climate patterns. But we do not know all of these things perfectly, since they were not directly observed at the time of the eruption. So in order to simulate eruptions in climate models, we must make assumptions: one important assumption is the time of year when the eruption occurred.

For many historical eruptions, we know only the year of the eruption, and not its starting month - so in sets of model simulations, the start date is set in April, when the 1815 eruption of Mt. Tambora occurred. Our recent article in the Proceedings of the National Academy of Sciences (Stevenson et al. 2017) shows that the impacts of tropical volcanoes can be quite different from one another depending on that starting month. We ran idealized simulations of the 1815 Tambora and 1258 Samalas eruptions starting in four different months: the eruption-year "La Nina-like" cooling depends strongly on eruption month, and so do the patterns of hydroclimate changes occurring within that year. This means that previous work suggesting that climate models don't properly capture eruption-year influences may not be telling the full story: if we knew more about the details of past eruptions, we could get a much better sense of how well we are doing in simulating their effects.