The United States "warming hole": Quantifying the forced aerosol response given large internal variability

Twenty-five years of large summer cooling over the southeastern United States ending in the mid-1970s coincided with rapidly increasing anthropogenic aerosol emissions. Here we assess the claim that the cooling in that period was predominantly due to such aerosols. We utilize two 50-member sets of coupled climate model simulations, one with only anthropogenic aerosol forcings and another with all known natural and anthropogenic forcings, together with a long control integration. We show that, in the absence of aerosol forcing, none of the model simulations capture the observed surface cooling rate (similar to 0.56 degrees C decade(-1)), whereas with increasing aerosol emissions 2 (of 50) of the simulations do. More importantly, however, we find that the cooling from aerosols (0.20 degrees C decade(-1)) is insufficient to explain the observation. Our results therefore suggest that, while aerosols may have played a role, the observed cooling was a rare event that contained a large contribution from unforced internal variability.