Climate Matters•May 1, 2024
Extreme Precipitation in a Warming Climate
KEY CONCEPTS
Climate change is supercharging the water cycle, bringing heavier precipitation extremes — and related flood risks — across the U.S.
As the climate has warmed from 1958 to 2021, the most extreme precipitation days have intensified in every major U.S. region, led by the Northeast (+60%) and Midwest (+45%).
This hazardous intensification is expected to continue with future warming.
With 2°C (3.6°F) of warming, 85% of 3,111 total U.S. counties are likely to experience a 10% or higher increase in precipitation falling on the heaviest 1% of days.
High future levels of extreme precipitation intensification are concentrated in: Alaska, Hawaii, Tennessee, Alabama, Georgia, Mississippi, Maine, North Carolina, and Kentucky.
People, ecosystems, and infrastructure in both wet and dry locations are facing the risks that come with short bursts of extreme rainfall.
Download KML map and XLSX data file: projected change in precipitation extremes under 2°C of warming for over 3,000 U.S. counties
Climate change is supercharging the water cycle
Climate change is bringing heavier rainfall extremes and increased, inequitable flood risk to many parts of the U.S.
For every 1°F of warming the air can hold an extra 4% of moisture, increasing the chances of heavier downpours that contribute to the risk of flash floods.
Heavy downpours bring more rain, faster — causing flash flooding and landslides that can displace families, drown crops, damage infrastructure, and expose people to hazardous debris, contaminants, and water-borne disease.
The rapid onset of flash floods limits time to get people out of harm’s way. In the U.S., racial minorities and those living in mobile homes are disproportionately exposed to flood risk, especially in the South and in rural areas.
In the U.S., extreme daily rainfall has become more frequent since the 1980s. Hourly rainfall intensity has also increased since 1970 — by 13% on average across 150 U.S. locations analyzed by Climate Central.
Risky rainfall extremes are the focus of analysis of past and projected future change in extreme rainfall across the U.S. featured in the Fifth National Climate Assessment.
Heaviest precipitation days have become heavier
As the climate has warmed over recent decades, the most extreme precipitation days have become more intense across the U.S.
As precipitation extremes intensify, the wettest days each year bring increasing flood hazards. And this intensification trend has been widespread.
In the Northeast and Midwest, the amount of precipitation falling on the heaviest 1% of days has increased 60% and 45%, respectively, from 1958 to 2021 according to the Fifth National Climate Assessment.
The heaviest rainfall events have become wetter across all other major regions of the continental U.S. from 1958 to 2021 as well, led by: the Southeast (+37%); the Northern Rockies and Plains (+24%); and the South (+21%).
Warmer future, heavier precipitation extremes
With continued warming, precipitation extremes are likely to increase globally — even in regions with decreasing average precipitation, according to the latest reports from the Intergovernmental Panel on Climate Change.
Most of the U.S. is projected to see increases in precipitation extremes with 2°C (3.6°F) of global warming, according to analysis in the Fifth National Climate Assessment. The planet has already warmed more than 1.1°C (2°F). Only by rapidly expanding and accelerating efforts to reduce heat-trapping emissions can we ensure that global warming levels remain below 2°C.
With 2°C (3.6°F) of warming, the majority (85% or 2,645) of the 3,111 total U.S. counties are likely to experience a 10% or higher increase in precipitation falling on the heaviest 1% of days.
On average, U.S. counties are likely to experience a 17% increase in precipitation falling on the heaviest 1% of days.
Counties likely to experience at least a 30% increase in extreme precipitation are concentrated in: Tennessee, Alabama, Georgia, Mississippi, Maine, North Carolina, and Kentucky.
Alaska and Hawaii are likely to experience some of the highest levels of extreme precipitation intensification (43% and 30%, respectively) at 2°C (3.6°F) of global warming.
A recent study suggests that, with high future levels of heat-trapping emissions (RCP8.5), U.S. flash floods could also intensify — especially in the Southwest — underscoring that the risks posed by short bursts of extreme rainfall affect both wet and dry regions.
These rising rainfall intensity risks are unlikely to be equally shared. A recent study suggests that the burdens of an estimated 26% increase in overall U.S. flood risk by 2050 could disproportionately impact Black communities along the Atlantic and Gulf coasts.
Three key points: extreme precipitation in a warming climate
1. For every 1°F of warming, the air can hold an extra 4% of moisture.
This relationship between air temperature and water vapor pressure is governed by the laws of thermodynamics and represented in the Clausius-Clapeyron equation.
The U.S. has already warmed 2.6°F since 1970 — meaning our atmosphere can already hold about 10% more moisture.
2. More moisture in warmer air increases heavier downpours.
Theory suggests that rainfall becomes more intense with warming. Both rain gauge data and modeling experiments support this theory.
Globally, the frequency and intensity of heavy rainfall events have increased since the 1950s, largely due to human-caused climate change, according to the latest reports from the Intergovernmental Panel on Climate Change.
In the U.S., extreme daily rainfall events have been on the rise since the 1980s.
3. Intensifying rainfall due to climate change has cost the U.S. billions in inland flood damages over the last three decades.
Inland flooding in the U.S. caused $230 billion in damages from 1988 to 2021. Over one-third (37%) of those damages are attributed to precipitation changes due to climate warming.
That’s $84 billion in past flood damage due to warming-induced rainfall intensification. And the most intense downpours have caused the largest damages.
Learn more: Extreme Weather Toolkit: Heavy Rain and Flooding
LOCAL STORY ANGLES
How vulnerable is your area to flooding?
Factors like local topography, age of infrastructure, watershed health, and precipitation trends influence flood vulnerability. See current flood risks in your neighborhood with FloodFactor’s ZIP code-level risk identifier. A 2021 report found that one-fourth of critical infrastructure is put at risk of failure by flooding. The American Society of Civil Engineers grades the integrity of infrastructures by state and category.
Who is most vulnerable to heavy rain-related hazards?
Racial minorities and mobile homes are disproportionately exposed to flooding, especially in rural areas and in the southern U.S. The American Flood Coalition has resources on adaptation, equitable disaster recovery, and funding opportunities for flood-resilient communities.
What are the health and safety risks from flooding?
Climate Central’s report, After the Storm: Health risks from damp, moldy homes, summarizes the potential health risks from poor indoor air quality following heavy precipitation and flooding. Mold is just one of many potential hazards in a storm-damaged home. Other safety hazards include debris, structural damage, or downed electrical wires. The CDC provides tips to protect health and safety following a storm or flooding.
What can you do to protect against flooding and extreme precipitation?
Firstly, know your own risk using the Federal Emergency Management Agency’s flood maps. Individual homeowners can insure their homes, maintain rainwater systems, reduce impervious surfaces on their property, elevate important utilities and equipment, and take on low-cost indoor projects recommended by FEMA. Communities can invest in flood management systems, green infrastructure and watershed restoration.
CONTACT EXPERTS
Greg Carbin
Branch Chief, Forecast Operations
National Weather Service
Contact: gregory.carbin@noaa.gov
Related expertise: short-term forecasting of extreme rainfall
FIND EXPERTS
Submit a request to SciLine from the American Association for the Advancement of Science or to the Climate Data Concierge from Columbia University. These free services rapidly connect journalists to relevant scientific experts.
Browse maps of climate experts and services at regional NOAA, USDA, and Department of the Interior offices.
Explore databases such as 500 Women Scientists, BIPOC Climate and Energy Justice PhDs, and Diverse Sources to find and amplify diverse expert voices.
Reach out to your State Climate Office or the nearest Land-Grant University to connect with scientists, educators, and extension staff in your local area.
METHODOLOGY
Data on past (1958–2021) and projected future (under 2°C of global warming) change in total precipitation (including rain and snow) falling on the heaviest 1% of days across the U.S. are from: USGCRP, 2023: Fifth National Climate Assessment. Crimmins, A.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, B.C. Stewart, and T.K. Maycock, Eds. U.S. Global Change Research Program, Washington, DC, USA. https://doi.org/10.7930/NCA5.2023. Data were accessed via Figure 2.8a and Figure 2.12a metadata and the Fifth National Climate Assessment Interactive Atlas.