Monitoring and Understanding Changes in Heat Waves, Cold Waves, Floods and Droughts in the United States: State of Knowledge

Weather and climate extremes have been varying and changing on many different time scales. In recent decades,heat waves have generally become more frequent across the U.S. while cold waves have been decreasing. While this is in keeping with expectations in a warming climate it turns out that decadal variations in the number of U.S. heat and coldwaves do not correlate well with the observed U.S. warming during the last century. River flooding trends on the century-scale do not show uniform changes across the country. While floods in the Southwest have been decreasing, floods in the Northeast and North Central have been increasing.Confounding the analysis of trends in river flooding is multiyear and even multidecadal variability likely caused by both large-scale atmospheric circulation changes and basin-scale “memory” in the form of soil moisture, together with human modifications of the catchments. Droughts also have long-term trends as well as multi-year and decadal variability. Instrumental data indicate that the Dust Bowl of the 1930s and the drought in the 1950s were the most significant 20th century droughts in the U.S. while tree ring data indicate that the megadroughts over the 12th century exceeded anything in the 20th century in both spatial extent and duration. The state of knowledge of the factors that cause these four phenomena to change is fairly good with heat waves being the best understood.