Negligible climatic effects from the 2008 Okmok and Kasatochi volcanic eruptions

We used a general circulation model of Earth's climate to conduct simulations of climate response to the July 12, 2008, eruption of Okmok volcano and the August 8, 2008, eruption of Kasatochi volcano, which injected a total of 1.6 Tg of SO2 into the Arctic upper troposphere and lower stratosphere (0.1 Tg from Okmok and 1.5 Tg from Kasatochi). For the climate model simulations, we placed all the SO2 into the lower stratosphere. The temporal and spatial distribution of model predictions of sulfate aerosol optical depth agrees with measurements made by the Optical Spectrograph and InfraRed Imaging System (OSIRIS), a Canadian satellite instrument. After accounting for differences due to different wavelengths, different sampling volumes, and an overestimate of stratospheric SO2 loading in the model, the optical depths measured by OSIRIS are consistent with the modeled values. Although the shortwave radiative effects of the eruption are detectable in model output, perturbations in surface air temperature and precipitation were negligible, since the Okmok injection was quite small and the Kasatochi eruption was too late in the year for there to have been large radiative forcing in 2008 and was of insufficient magnitude for the sulfate aerosols to persist in the stratosphere into the following spring. We conducted further experiments with lower stratospheric injections of 3 and 5 Tg of SO2 into the Arctic on August 8, 2008. Although the sulfate aerosol optical depth and resulting shortwave radiative forcing increase linearly with atmospheric loading of SO2, the radiative forcing was still small due to the timing of the eruption, with little insolation by the time the sulfate aerosol cloud would form. High latitude eruptions of this size occurring in August or later in the calendar year would still be of insufficient magnitude for the sulfate aerosols to persist in the stratosphere into the following spring, and climate effects would be negligible.