Climate Projections Very Likely Underestimate Future Volcanic Forcing and Its Climatic Effects

Abstract

Standard climate projections represent future volcanic eruptions by a constant forcing inferred from 1850 to 2014 volcanic forcing. Using the latest ice‐core and satellite records to design stochastic eruption scenarios, we show that there is a 95% probability that explosive eruptions could emit more sulfur dioxide (SO2) into the stratosphere over 2015–2100 than current standard climate projections (i.e., ScenarioMIP). Our simulations using the UK Earth System Model with interactive stratospheric aerosols show that for a median future eruption scenario, the 2015–2100 average global‐mean stratospheric aerosol optical depth (SAOD) is double that used in ScenarioMIP, with small‐magnitude eruptions (<3 Tg of SO2) contributing 50% to SAOD perturbations. We show that volcanic effects on large‐scale climate indicators, including global surface temperature, sea level and sea ice extent, are underestimated in ScenarioMIP because current climate projections do not fully account for the recurrent frequency of volcanic eruptions of different magnitudes.

The Role of Small to Moderate Volcanic Eruptions in the Early 19th Century Climate

“…We also investigated the consistency between the reconstructions and our simulations, as small eruptions have recently been suggested to be included in future climate projections (Chim et al, 2023) but have not been included in pre-industrial simulations. For the large-scale anomalies, the small-to-moderate eruptions help explaining the long-lasting cooling after the two strong eruptions found in the reconstructions.…”

Volcanic Eruptions: A Source of Irreducible Uncertainty for Future Climates

“…Major volcanic eruptions are reported at the bottom. Note the lack of volcanic eruptions in the scenario period, the critical problem addressed by CHIM23.…”

“…The paper by Man Mei Chim and Colleagues (Chim et al., 2023, hereafter CHIM23) is a much‐needed step forward toward tackling this known unknown. Especially, their comprehensive approach allows for an unprecedented level of complexity in the description of volcanic forcing in a warmer climate, including potential effects of climate change on volcanic plume dynamics and aerosol life cycle, the so‐called climate‐volcano feedback (Aubry et al., 2019, 2021).…”

“…Missing small to moderate eruptions means that volcanic histories from ice‐core records are biased toward reduced volcanic activity and forcing, and so are scenarios based solely on such data. CHIM23 accounts for this potential bias by sampling small events from the satellite period only, when they are well observed. The study provides compelling evidence that including small volcanic eruptions leads to significantly more sulfur injections in the stratosphere till the end of the 21st century, hence more climatically relevant aerosol, compared to what currently assumed in CMIP6 scenarios.…”

“…Stochastic scenarios for large eruptions in CHIM23 build on a recent data set of volcanic aerosol depositions in ice cores covering the past 11,500 years (Sigl et al., 2022). Climatically relevant volcanic parameters from this record can vary across centuries by even more than an order of magnitude, revealing a very large potential uncertainty in volcanic forcing during the 21st century.…”