Annual average precipitation has increased by 4% since 1901 across the entire United States, with strong regional differences: increases over the Northeast, Midwest, and Great Plains and decreases over parts of the Southwest and Southeast (Figure 2.5),94 consistent with the human-induced expansion of the tropics.95 In the future, the greatest precipitation changes are projected to occur in winter and spring, with similar geographic patterns to observed changes: increases across the Northern Great Plains, the Midwest, and the Northeast and decreases in the Southwest (Figure 2.5, bottom). For 2070–2099 relative to 1986–2015, precipitation increases of up to 20% are projected in winter and spring for the north central United States and more than 30% in Alaska, while precipitation is projected to decrease by 20% or more in the Southwest in spring. In summer, a slight decrease is projected across the Great Plains, with little to no net change in fall.
The frequency and intensity of heavy precipitation events across the United States have increased more than average precipitation (Figure 2.6, top) and are expected to continue to increase over the coming century, with stronger trends under a higher as compared to a lower scenario (Figure 2.6).94 Observed trends and model projections of increases in heavy precipitation are supported by well-established physical relationships between temperature and humidity (see Easterling et al. 2017,94 Section 7.1.3 for more information). These trends are consistent with what would be expected in a warmer world, as increased evaporation rates lead to higher levels of water vapor in the atmosphere, which in turn lead to more frequent and intense precipitation extremes.
For heavy precipitation events above the 99th percentile of daily values, observed changes for the Northeast and Midwest average 38% and 39%, respectively, when measured from 1901, and 55% and 42%, respectively, when measured with the more robust network available from 1958. The largest observed increases have occurred and are projected to continue to occur in the Northeast and Midwest, where additional increases exceeding 40% are projected for these regions by 2070–2099 relative to 1986–2015. These increases are linked to observed and projected increases in the frequency of organized clusters of thunderstorms and the amount of precipitation associated with them.96,97,98
Trends in related types of extreme events, such as floods, are more difficult to discern (e.g., Hirsch and Ryberg 2012, Hodgkins et al. 201799,100). Although extreme precipitation is one of the controlling factors in flood statistics, a variety of other compounding factors, including local land use, land-cover changes, and water management also play important roles. Human-induced warming has not been formally identified as a factor in increased riverine flooding and the timing of any emergence of a future detectable human-caused change is unclear.101
Declines have been observed in North America spring snow cover extent and maximum snow depth, as well as snow water equivalent (a measurement of the amount of water stored in snowpack) in the western United States and extreme snowfall years in the southern and western United States.102,103,104 All are consistent with observed warming, and of these trends, human-induced warming has been formally identified as a factor in earlier spring melt and reduced snow water equivalent.101 Projections show large declines in snowpack in the western United States and shifts to more precipitation falling as rain rather than snow in many parts of the central and eastern United States. Under higher future scenarios, assuming no change to current water resources management, snow-dominated watersheds in the western United States are more likely to experience lengthy and chronic hydrological drought conditions by the end of this century.105,106,107
Across much of the United States, surface soil moisture is projected to decrease as the climate warms, driven largely by increased evaporation rates due to warmer temperatures. This means that, all else being equal, future droughts in most regions will likely be stronger and potentially last longer. These trends are likely to be strongest in the Southwest and Southern Great Plains, where precipitation is projected to decrease in most seasons (Figure 2.5, right) and droughts may become more frequent.101,108,109,110,111,112 Although recent droughts and associated heat waves have reached record intensity in some regions of the United States, the Dust Bowl of the 1930s remains the benchmark drought and extreme heat event in the historical record, and though by some measures drought has decreased over much of the continental United States in association with long-term increases in precipitation (e.g., see McCabe et al. 2017113), there is as yet no detectable change in long-term U.S. drought statistics. Further discussion of historical drought is provided in Wehner et al. (2017).101
Few analyses consider the relationship across time and space between extreme events; yet it is important to note that the physical and socioeconomic impacts of compound extreme events can be greater than the sum of the parts.25,114 Compound extremes can include simultaneous heat and drought such as during the 2011–2017 California drought, when 2014, 2015, and 2016 were also the warmest years on record for the state; conditions conducive to the very large wildfires, that have already increased in frequency across the western United States and Alaska since the 1980s;115 or flooding associated with heavy rain over snow or waterlogged ground, which are also projected to increase in the northern contiguous United States.116