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 (∼0.56°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°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. Key Points We use large ensembles of simulations to determine whether anthropogenic aerosols were responsible for the U.S. warming hole The large ensembles suggest that the warming hole was a rare event that contained a large contribution from unforced internal variability Anthropogenic aerosols increased the likelihood of the warming hole, but their effects were canceled to a large extent by greenhouse gases