Present-day greenhouse gases could cause more frequent and longer Dust Bowl heatwaves

Substantial warming occurred across North America, Europe and the Arctic over the early twentieth century(1), including an increase in global drought(2), that was partially forced by rising greenhouse gases (GHGs)(3). The period included the 1930s Dust Bowl drought(4-7) across North America's Great Plains that caused widespread crop failures(4,8), large dust storms(9) and considerable out-migration(10). This coincided with the central United States experiencing its hottest summers of the twentieth century(11,12) in 1934 and 1936, with over 40 heatwave days and maximum temperatures surpassing 44 degrees C at some locations(13,14). Here we use a large-ensemble regional modelling framework to show that GHG increases caused slightly enhanced heatwave activity over the eastern United States during 1934 and 1936. Instead of asking how a present-day heatwave would behave in a world without climate warming, we ask how these 1930s heatwaves would behave with present-day GHGs. Heatwave activity in similarly rare events would be much larger under today's atmospheric GHG forcing and the return period of a 1-in-100-year heatwave summer (as observed in 1936) would be reduced to about 1-in-40 years. A key driver of the increasing heatwave activity and intensity is reduced evaporative cooling and increased sensible heating during dry springs and summers. The United States experienced two of its hottest recorded summers in 1934 and 1936, amplified by drier soils associated with the Dust Bowl drought. A large regional climate model ensemble estimates present-day GHGs would cause similarly extreme, 1-in-100-year heatwaves to occur about every 40 years.