Indirect Aerosol Effect Increases CMIP5 Models' Projected Arctic Warming

Phase 5 of the Coupled Model Intercomparison Project (CMIP5) climate models' projections of the 2014-2100 Arctic warming under radiative forcing from representative concentration pathway 4.5 (RCP4.5) vary from 0.9 degrees to 6.7 degrees C. Climate models with or without a full indirect aerosol effect are both equally successful in reproducing the observed (1900-2014) Arctic warming and its trends. However, the 2014-2100 Arctic warming and the warming trends projected by models that include a full indirect aerosol effect (denoted here as AA models) are significantly higher (mean projected Arctic warming is about 1.5 degrees C higher) than those projected by models without a full indirect aerosol effect (denoted here as NAA models). The suggestion is that, within models including full indirect aerosol effects, those projecting stronger future changes are not necessarily distinguishable historically because any stronger past warming may have been partially offset by stronger historical aerosol cooling. The CMIP5 models that include a full indirect aerosol effect follow an inverse radiative forcing to equilibrium climate sensitivity relationship, while models without it do not.