Volcanic Radiative Forcing From 1979 to 2015

Abstract

Using volcanic sulfur dioxide emissions in an aerosol‐climate model, we derive a time series of global‐mean volcanic effective radiative forcing (ERF) from 1979 to 2015. For 2005–2015, we calculate a global multiannual mean volcanic ERF of −0.08 W/m2 relative to the volcanically quiescent 1999–2002 period, due to a high frequency of small‐to‐moderate‐magnitude explosive eruptions after 2004. For eruptions of large magnitude such as 1991 Mt. Pinatubo, our model‐simulated volcanic ERF, which accounts for rapid adjustments including aerosol perturbations of clouds, is less negative than that reported in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) that only accounted for stratospheric temperature adjustments. We find that, when rapid adjustments are considered, the relation between volcanic forcing and volcanic stratospheric optical depth (SAOD) is 13–21% weaker than reported in IPCC AR5 for large‐magnitude eruptions. Further, our analysis of the recurrence frequency of eruptions reveals that sulfur‐rich small‐to‐moderate‐magnitude eruptions with column heights ≥10 km occur frequently, with periods of volcanic quiescence being statistically rare. Small‐to‐moderate‐magnitude eruptions should therefore be included in climate model simulations, given the >50% chance of one or two eruptions to occur in any given year. Not all of these eruptions affect the stratospheric aerosol budget, but those that do increase the nonvolcanic background SAOD by ~0.004 on average, contributing ~50% to the total SAOD in the absence of large‐magnitude eruptions. This equates to a volcanic ERF of about −0.10 W/m2, which is about two thirds of the ERF from ozone changes induced by ozone‐depleting substances.

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

“…Figure shows the zonally averaged volcanic effective radiative forcing (ERF) from the Laki eruption. We calculate the volcanic ERF as in Schmidt et al (), and include contributions from aerosol‐radiation interactions (ERF_ari), aerosol‐cloud interactions (ERF_aci), and atmospheric adjustments and surface albedo forcing (dLW_ERFa). The SW radiation anomalies in Figure a indicate a maximum reduction of insolation—approximately 1 month after the eruption onset and lasting 2 months—of more than 31 W/m 2 poleward of 50°N.…”

The Community Earth System Model Version 2 (CESM2)

“…As described in Mills et al (), this new feature provides CESM2 with stratospheric aerosol forcing that is much more consistent with observations than previous climatologies developed for climate models. This self‐consistent treatment of volcanic aerosols and chemistry has been shown to produce realistic radiative responses to eruptions over the 1979–2015 period (Mills et al, ; Schmidt et al, ). For CMIP6, volcanic SO 2 emissions for 1850–2015 from VolcanEESM database (Neely & Schmidt, ) are used.…”

A New Volcanic Stratospheric Sulfate Aerosol Forcing Emulator (EVA_H): Comparison With Interactive Stratospheric Aerosol Models

“…Last, we fit SAOD increases as a function of corresponding stratospheric SO injections using a power law (Figure .a). We find an exponent of . We use the 1979–2015 experiments run with the Community Earth System Model version 1 with a prognostic aerosol scheme (Whole Atmosphere Community Climate Model, WACCM) using the Neely and Schmidt () volcanic sulfur emission inventory (Mills et al, ; Schmidt et al, ), with adjusted mass of 10 Tg of SO (instead of 18 Tg in Neely & Schmidt, ) and height of 18–20 km (instead of 23–27 km in Neely & Schmidt, ) for the 1991 eruption of Mount Pinatubo. We fit the monthly mean values of global mean SAOD anomaly (i.e., the difference between runs with and without volcanic emissions) at 550 nm to the stratospheric SO burden anomaly using a power law fit and find an exponent of (Figure b). We use 30 experiments from the MAECHAM5‐HAM interactive stratospheric aerosol model, where 8.5 Tg S were injected at six different sets of altitudes and latitudes (Toohey et al, ).…”

“…(a) Global mean SAOD increase (GloSSAC) as a function of corresponding stratospheric SO loadings (Carn et al, ). (b) Global mean SAOD anomaly as a function of the global stratospheric SO burden anomaly in WACCM 1979–2015 run (Schmidt et al, ). (c) Same as (b) but for MAECHAM's runs (Toohey et al, ) and using global mean SAOD anomaly.…”

“…Volcanic eruptions can inject sulfur dioxide (SO ) into the stratosphere and form long‐lived (1–3 years) sulfate aerosol that modify Earth's radiative balance, causing a net cooling at the surface and affecting major modes of climate variability (e.g., Kremser et al, ; Robock, ; Timmreck, ). Recently, it has emerged that even relatively small eruptions (injecting less than around 1 teragram (Tg) of SO ) of the early 21st century exert small but significant radiative forcing (e.g., Schmidt et al, ) and have a statistically discernible cooling effect on sea surface and tropospheric temperatures (Santer et al, ).…”