The United States Summertime "Warming Hole": Quantifying the Forced Aerosol Response Given Large Internal Variability

It has been asserted that the observed United States summertime "warming hole" was caused by increasing anthropogenic aerosol emissions. We test this claim using a large initial condition ensemble of climate model simulations run under all forcing and aerosol-only forcing conditions. In our model, the response to increasing anthropogenic aerosols between 1951-1975 is a surface cooling over the southeastern region of 0.21±0.08°C/decade (mean and 5-95% confidence interval), which cannot alone explain the observed cooling of 0.60°C/decade. Moreover, two thirds of the aerosol-driven cooling is canceled by other forcings. In a long unforced control simulation, the likelihood of obtaining an event as extreme as the warming hole is small (p<0.01), and is increased only slightly under anthropogenic aerosol and all forcing conditions (by 6% and 1%, respectively). Thus, we conclude that the warming hole was primarily a rare episode of large internal variability, rather than due to changes in external forcing.