| Summary: | Although the 2010 volcanic eruptions of Eyjafjallajokull did not exert a large climate forcing, several features of their emissions favored weaker aerosol cooling or stronger warming than commonly attributed to volcanic events. These features include a high ratio of fine ash to sulfur dioxide, occurrence near reflective surfaces exposed to strong insolation, and the production of very little stratospheric sulfate. We derive plausible ranges of optical properties and top-of-atmosphere direct radiative forcing for aerosol emissions from these events, and find that shortwave cooling from sulfate was largely offset by warming from ash deposition to cryospheric surfaces and longwave warming from atmospheric ash and sulfate. Shortwave forcing from atmospheric ash was slightly negative in the global-mean under central estimates of optical properties, though this forcing term was uniquely sensitive to the simulated distribution of clouds. The forcing components sum to near climate-neutral global-mean 2010 instantaneous (-1.9 milliwatts per square meter (mW/m^2)) and effective (-0.5 mW/m^2) radiative forcing, where the latter is elevated by high efficacy of snow-deposited ash. Ranges in net instantaneous (-7.3 to +2.8 mW/m^2) and effective (-7.2 to +4.9 mW/m^2) forcing derived from sensitivity studies are dominated by uncertainty in ash shortwave absorptivity. Forcing from airborne ash decayed quickly, while sulfate forcing persisted for several weeks and ash deposits continued to darken snow and sea-ice surfaces for months following the eruption. Despite small global forcing, monthly-averaged net forcing exceeded 1 watt per square meter (W/m^2) in some regions. These findings indicate that ash can be an important component of climate forcing from high-latitude volcanic eruptions, and in some circumstances may exceed sulfate forcing. Data provided here include the derived ash optical properties that were used in the climate and radiative forcing calculations of this study.
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