Climate Matters•May 8, 2024
Climate Change and Inland Flooding
KEY CONCEPTS
Flooding is costly, damaging, and deadly. Flood risk extends from coastlines to large areas far inland, disproportionately impacting racial minorities and low-income households in the U.S.
Climate change affects three key flood drivers: more intense rainfall, drier soils, and less snow. These trends can increase or decrease inland flooding depending on the location and season.
Precipitation changes linked to climate warming account for over one-third (37%) of inland flood damage in the U.S. since 1988.
Communities along the Gulf and Atlantic coasts, as well as inland in Appalachia and northern New England carry an outsized burden of current and future flooding.
Download KML map: Current and future average annual flood loss
Download the data for all states and counties
The high and inequitable costs of flooding
Floods are the second leading cause of weather-related deaths in the U.S., and their dangerous effects stretch far from the coast.
Floods happen not only along coastlines, where rising seas lead to more high tide and storm surge flooding, but also in urban and rural communities far inland. Here are a few reasons that floods matter:
1. Floods are damaging, costly, and not fully accounted for. The annual cost of flooding averages over $32 billion nationwide. But property values often don’t fully account for flood risk. Flood-prone U.S. homes are estimated to be overvalued by at least $121 billion. Such overvaluation puts many homeowners, and especially low-income households, at risk of losing value in their largest asset: their home.
2. Flooded homes put health at risk. Toxic or contaminated floodwaters are a serious health and safety risk, particularly in residential areas. After floodwaters recede, they often leave behind water-damaged homes. These damp conditions are ideal for the growth of mold and other microbes that can harm respiratory health and worsen allergies and asthma.
3. Flood risks are inequitable. Racial minorities and those living in mobile homes are disproportionately exposed to flooding, especially in rural areas and in the southern U.S. During flood recovery, renters, low-income households, and racial and ethnic minorities also face the highest barriers in accessing federal flood disaster assistance.
4. Flood damage is widespread. Inland flooding affects large areas of the country. About 94% (45 out of 48) of states in the continental U.S. experience more than $50 million in average annual flood damage.
What is inland flooding?
Inland floods occur when rivers overflow their banks (fluvial flooding) or when precipitation exceeds local capacity to absorb water (pluvial flooding) in soils or urban drainage systems.
Key drivers of inland floods include:
Precipitation accumulating over several days
Intense precipitation over a short period of time
Rapid melting of snow
Accumulation of debris in rivers
Dam or levee failure
Inland flooding and climate change
Inland flooding results from complex interactions between physical hazards (such as rainfall intensity) and other factors such as urbanization and land use as well as a growing number of people, homes, and businesses in flood-prone areas.
Climate change affects three key flood drivers, leading to both increases and decreases in inland flooding depending on the location and season:
1. Intense rainfall is increasing. Warmer air holds more moisture: 4% more water vapor for every 1°F of warming. This relationship supercharges the water cycle, bringing heavier rainfall extremes.
Increases flooding: Intense rainfall increases flood severity.
2. Droughts are lengthening, and soil is drier. With more heat comes more evaporation and transpiration of water from soils, leading to drought in some areas such as the southwestern U.S. This can have mixed effects on flood activity:
Decreases flooding: Drier soils have a greater capacity to soak up water from sustained rain.
Increases flooding: During intense downpours, however, dry and hardened soils can lead to more runoff and flash flooding.
3. Mountain snowpack is decreasing and melting earlier. Winter is the fastest-warming season for most of the U.S., resulting in less snow in many places.
Decreases flooding: Decreased snowpack leads to reduced river flow and less severe snowmelt-driven floods.
Increases flooding: Earlier snowmelt and more precipitation falling as rain than as snow can lead to more intense floods in cold regions.
Over one-third of inland flood damage is due to climate change
The relationship between climate change and flooding is complex. The most well-established connection between human-caused climate change and inland flooding is that more warming leads to more intense rainfall (even in relatively dry places such as the Southwest), which in turn increases flood severity.
This rainfall intensification effect accounts for more than one-third of all damages from inland flooding in the U.S. in recent 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 attributed to human-caused climate change.
Current and future flooding hotspots
A recent study (Wing et al., 2022) featured in the Fifth National Climate Assessment estimates current (2020) and projected future (2050) losses that could result from inland and coastal floods across the U.S.
According to this study, communities along the U.S. Gulf and Atlantic coasts, as well as inland in Appalachia and northern New England, carry an outsized burden of current and future flooding.
In the current (2020) climate, Allegheny County, Pennsylvania, where three rivers converge around the city of Pittsburgh, has estimated average annual flood losses of $1.1 billion — the highest among all counties in the continental U.S.
The next most flood-prone counties were: Miami-Dade County, Florida (where Miami is located); King County, Washington (Seattle); Charlotte County, Florida; and inland in Kanawha County, West Virginia.
With continued warming, Orange County, Texas and Richmond County, Virginia are expected to experience the greatest increase in average annual flood loss by 2050 (914% and 781% respectively).
Top five states for average annual flood losses in 2020 and expected increase by 2050:
State | Average annual flood loss (2020) | State | Expected increase (%) in average annual flood loss, 2020-2050 |
Florida | $4.3 billion | Louisiana | 147% |
Pennsylvania | $2.8 billion | Florida | 74% |
California | $1.7 billion | South Carolina | 58% |
West Virginia | $1.7 billion | Texas | 47% |
Texas | $1.5 billion | New Jersey | 46% |
Download the data for all states and counties.
Flood risk: beyond physical hazards
Human-caused climate change affects the timing, location, and intensity of physical flood hazards such as extreme rainfall. But hazards are only one part of overall flood risk.
Overall flood risk in any location is the result of three dynamic and interacting factors:
Hazards: changes in physical flood hazards, such as extreme rainfall, due to climate change
Exposure: increasing number of people, businesses, and infrastructure in flood-prone areas
Vulnerability: a community’s capacity to prepare for and recover from flooding — affected by local resources, access to recovery assistance, and the flood resilience of critical infrastructure
Some areas face higher risks from future flooding as a result of these three intersecting factors.
Urban flooding is a particular concern, especially as urban populations continue to grow. Urban flooding is likely to increase in a warmer world with more intense rainfall because paved streets and other impermeable surfaces in cities cannot absorb stormwater runoff, quickly overwhelming drainage systems.
Future flood risk also differs across demographic groups. In U.S. Census tracts with a Black population of at least 20%, future flood losses are projected to increase at about twice the rate in tracts where Black residents make up less than 1% of the population. This disproportionate flood risk is concentrated in the South and Southeast, extending from Texas across Georgia and up to Virginia.
LOCAL STORY ANGLES
What’s the flood risk in your local area?
Understand the flood risk in your state or county with First Street’s National flood assessment. Explore the climate risks for any U.S. address with Risk Factor or FEMA’s Flood Map Service Center.
How can communities adapt to flood risk?
Grist reports on how four communities in the U.S. have recovered and adapted to inland flooding. The Fifth National Climate Assessment presents resilience actions to help businesses and industries reduce the negative consequences of more heavy rainfall events. EPA’s Flood Resilience Checklist and related report Planning for Flood Recovery and Long-Term Resilience in Vermont provide strategies for communities to protect people and infrastructure from flood risks.
CONTACT EXPERTS
Carolyn Kousky, PhD (she/her)
Associate Vice President, Economics and Policy Analysis
Environmental Defense Fund
Relevant expertise: Climate risk management, climate adaptation, insurance
Media contact: ckousky@edf.org or Sommer Yesenofski, syesenofski@edf.org
Oliver Wing, PhD
Chief Research Officer
Fathom
Relevant expertise: Flood risk projections
Media contact: Jessica Roberts, j.roberts@fathom.global
Paul Bates, PhD
Professor of Hydrology
University of Bristol
Relevant expertise: Flood inundation modeling, flood risk and climate change
Media contact: Jessica Roberts, j.roberts@fathom.global
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
Estimates of current (2020) average annual flood loss and projected future (by 2050) change in flood loss for counties across the U.S. are from Wing et al., 2022. Data can also be accessed at: 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, via Figure 1.10c,d.