Solar and volcanic forcings in the GISS GCM
Gavin Schmidt

The largest natural forcings of climate over interannual to decadal timescales are variations in solar irradiance and volcanic aerosols. In the pre-industrial period, these forcings were likely dominant. The availability of multi-proxy seasonal reconstructions of climate of the last few hundred years now allow model simulations using these forcings to be validated against climate statistics unaffected by anthropogenic forcings. I will discuss some recent results using the GISS suite of climate models with sets of forcings that encompass the best estimates of the pre-industrial forcing history. Notably, we find that the spectral variation of solar forcing and ozone chemical feedbacks are of crucial importance in realistically modelling the climate response. The model response to solar forcing resembles a forced shift in the Arctic Oscillation/NAO over multiple decades, but are insignificant on shorter timescales. The interannual response to volcanic forcing is also largely related to the AO/NAO "winter warming" pattern, however, the regional impact over multiple decades of volcanos is very small. The results suggest that in the global mean, both forcings were of roughly equal importance for the Little Ice Age (LIA), but that solar forcing dominated the regional patterns, and may have been responsible for the particular severity of LIA climate in Europe.

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