A growing percentage of U.S. precipitation now comes in the form of extreme events, with human-caused climate change directly responsible for much of this increase. Heavy rain contributes to flooding that is damaging buildings and roads, eroding soil, flushing pollutants into waterways in many parts of the United States, as well as straining the nation’s aging dam network.
Facts for Any Story
Rising air temperatures, caused primarily by the build-up of heat-trapping gases due to fossil fuel burning, are increasing evaporation from soil, plants, lakes, and oceans. Warm air holds more moisture than cooler air, creating a greater atmospheric reservoir of water vapor for release during rain and snow storms. For every 1ºC (1.8ºF) increase in atmospheric temperature, the air holds 6% to 7% more water vapor, causing downpour intensity to increase.1Wuebbles, D., et al. (eds.) (2017), Climate Science Special Report: Fourth National Climate Assessment, Volume I View Source
Heavy rains are getting heavier. Looking at the top 1% of rainfall events, as measured by the amount of rain that fell within a 24-hour period, the amount of precipitation falling in these heavy events has increased substantially across the United States since the 1950s — by 55% in the Northeast, 42% in the Midwest, 27% in the Southeast, 29% in the Northern Great Plains, 12% in the Southern Great Plains, 10% in the Southwest, and 9% in the Northwest.1Wuebbles, D., et al. (eds.) (2017), Climate Science Special Report: Fourth National Climate Assessment, Volume I View Source
Heavy rains are also getting more frequent in many parts of the United States, especially in the Northeast, and covering greater areas of land. The prevalence of record-breaking rainfall events and the amount of land they drenched was constant for most of the 20th century, but has doubled since the 1970s.2Archfield, S.A., et al. (2016), Fragmented patterns of flood change across the United States, Geophysical Research Letters, 43(19), 10232 View Source3U.S. Environmental Protection Agency. 2016. Climate change indicators in the United States, 2016. Fourth edition. EPA 430-R-16-004 View Source
Heavy rains are often lasting longer, too. Recent evidence suggests that climate-change-related atmospheric conditions may be causing storms to get “stuck” in place, causing them to linger longer over a given geographic region and lead to flooding.4Mann, M. E., et al. (2017), Influence of anthropogenic climate change on planetary wave resonance and extreme weather events, Scientific Reports, 7, 19831 View Source5Kossin, James P. (2018), A global slowdown of tropical-cyclone translation speed, Nature, 558, 104 View Source6Francis, J. A., et al. (2018), North American weather regimes are becoming more persistent: Is Arctic amplification a factor?, Geophysical Research Letters, 45, 11414-11422 View Source
Climate-change-related increases in heavy rain intensity and frequency, and the lingering persistence of these weather events over affected areas, have exacerbated flooding across the United States, especially in the Midwest, increasing the number of cities and expanses of land now at high risk. In coastal areas, rainfall-related flooding is exacerbating the documented doubling of high-tide-flooding caused by sea level rise over the past 30 years.1Wuebbles, D., et al. (eds.) (2017), Climate Science Special Report: Fourth National Climate Assessment, Volume I View Source7Sweet, W.V., et al. (2018), 2017 State of U.S. High Tide Flooding with a 2018 Outlook. Supplement to State of the Climate: National Overview for May 2018 View Source
Pitfalls to Avoid
Flooding is the result of many factors, including the amount of precipitation falling during a particular time interval and the kinds of surfaces on which it falls. Overstating the extent to which any single flood is attributable solely to climate change or heavy rain can unduly narrow the range of mitigation or resilience strategies considered.
Climate change is affecting flood trends differently in different regions, with a tendency for increases in wetter areas and decreases in drier ones. Avoid averaging such changes across regions, as increases and decreases can cancel each other out, falsely implying little or no change.
Be careful when using the term “100-year” storms or related short-cut constructions of probability. Define them carefully when you use them, or come up with your own explanatory language — for example: “By historical standards, storms of this intensity have only a 1% chance of happening in any given year, yet three have occurred in the past 12 months.”