Torrential rain, flooding, and climate change

Top Line

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, and straining the nation’s aging dam network.

Facts for Any Story

• Extreme precipitation events are increasing in frequency, duration, and extent as the globe warms.¹Rohde, Melissa M., (2023), Floods and droughts are intensifying globally, Nature Water, 1, 226-227 View Source
• Rising air temperatures, caused primarily by the build-up of heat-trapping gases due to fossil fuel burning and land use change, 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.²Wuebbles, 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 60% in the Northeast, 45% in the Midwest, 37% in the Southeast, 24% in the Northern Great Plains, 21% in the Southern Great Plains, 17% in the Southwest, and 1% in the Northwest.³Marvel, K., et al. (2023), Chapter 2: Climate Trends, Fifth National Climate Assessment, 19 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.⁴Archfield, S.A., et al. (2016), Fragmented patterns of flood change across the United States, Geophysical Research Letters, 43(19), 10232 View Source⁵U.S. Environmental Protection Agency (2016), Climate change indicators in the United States, Fourth edition. EPA 430-R-16-004 View Source
• Heavy rains now often last longer, too. There is growing evidence that climate change-related atmospheric conditions, including a wavier jet stream, are causing storms to get “stuck” in place. When they linger longer over a given region, it drives an increase in extreme rainfall that can lead to flooding.⁶Mann, M. E., et al. (2017), Influence of anthropogenic climate change on planetary wave resonance and extreme weather events, Scientific Reports, 7, 45242 View Source⁷Francis, J. A., et al. (2018), North American weather regimes are becoming more persistent: Is Arctic amplification a factor?, Geophysical Research Letters, 45 (20), 11414-11422 View Source⁸Moon, Woosock, et al. (2022), Wavier jet streams driven by zonally asymmetric surface thermal forcing, PNAS 119(38) 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 in the frequency of high-tide-flooding caused by sea level rise over the past 30 years.²Wuebbles, D., et al. (eds.) (2017), Climate Science Special Report: Fourth National Climate Assessment, Volume I View Source⁹Sweet, 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, NOAA View Source
• From 1988 to 2017, the increase in precipitation due in part to human-caused climate change was responsible for about one-third of the cumulative cost of U.S. flood damage, with an impact totaling an estimated $73 billion.¹⁰Davenport, Frances V., et al. (2021), Contribution of historical precipitation change to US flood damages, PNAS, 118(4) View Source
• Atmospheric rivers (ribbons of atmosphere with high water vapor concentrations) are the primary drivers of flood damage in the U.S. West, with damage increasing about 10-fold with each one-step increase in the AR scale (a measure of atmospheric river intensity and duration) from 1978 to 2017. ¹¹Corringham, Thomas W., et al. (2019) Atmospheric rivers drive flood damages in the western United States, Science Advances, Volume 5, Issue 12 View Source The Oroville Dam crisis of 2017 in northern California, which forced the evacuation of more than 180,000 people, is one example of damage caused by atmospheric river precipitation intensified by human-caused warming, according to simulations.¹²Michaelis, Allison C., et al. (2022) Atmospheric River Precipitation Enhanced by Climate Change: A Case Study of the Storm That Contributed to California’s Oroville Dam Crisis, Earth’s Future 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 (e.g., impervious roads and parking lots) 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, such as urban rain gardens.

• 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.”