Temperature and Precipitation Extremes in the United States: Quantifying the Responses to Aerosols and Greenhouse Gases

Abstract:

Changes in extreme temperatures, heat waves, heavy rainfall events, and precipitation frequency can have adverse impacts on human health, air quality, agricultural productivity, and water resources. Using the aerosol only (AER) and greenhouse gas only (GHG) “single forcing” simulations (3 ensemble members each) from the GFDL CM3 chemistry-climate model, we investigate aerosol- versus greenhouse gas-induced changes in high temperature and precipitation extremes over the United States. We identify changes in these events from 1860 to 2005 and the associated large-scale dynamical conditions. Small changes in these extremes in the “all forcing” simulations reflect cancellations between the individual, opposite-signed effects of increasing anthropogenic aerosols and greenhouse gases. In AER, aerosols lead to lower extreme high temperatures and fewer warm spells over the western US (-2.1 K regional average; -20 days/year) and over the central and northeast US (-1.5 K; -12 days/year). In GHG, a similar but opposite-signed response pattern occurs (+2.7 K and +14 days/year over the western US; +2.5 K and +10 days/year in the central and northeast US). The similar spatial response patterns in AER versus GHG suggest a preferred regional mode of response that is largely independent of the regional distribution of the forcing agent. The influence of both greenhouse gases and aerosols on extreme high temperature is weakest in the southeast US, collocated with the observed “warming hole”. No statistically significant change occurs in AER, and a warming of only +1.8 K occurs in GHG. Warming in this region continues to be muted over the 21^st century under the RCP 8.5 scenario, with increases in extreme temperatures more than 1 K smaller than elsewhere. Aerosols induce decreases in the number of days per year with at least 10mm of precipitation (R10mm) over the eastern US in summer and winter and over the southern US in spring of roughly 1 day/year. In contrast, greenhouse gases induce increases in R10mm over the eastern US in winter (+0.8 days/year), the northern and central US during spring (+1 day/year), and the southeast US during summer (+0.5 days/year), but decreases over the northeast US in summer (-0.2 days/year). In RCP 8.5, the patterns of extreme temperature and precipitation associated with greenhouse gas forcing dominate.