Monday, 23 January 2017
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.
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