This website is the digital version of the 2014 National Climate Assessment, produced in collaboration with the U.S. Global Change Research Program.

For the official version, please refer to the PDF in the downloads section. The downloadable PDF is the official version of the 2014 National Climate Assessment.

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Welcome to the National Climate Assessment

The National Climate Assessment summarizes the impacts of climate change on the United States, now and in the future.

A team of more than 300 experts guided by a 60-member Federal Advisory Committee produced the report, which was extensively reviewed by the public and experts, including federal agencies and a panel of the National Academy of Sciences.

Explore the effects of climate change
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Floods

Flooding may intensify in many U.S. regions, even in areas where total precipitation is projected to decline. A flood is defined as any high flow, overflow, or inundation by water that causes or threatens damage.6 Floods are caused or amplified by both weather- and human-related factors. Major weather factors include heavy or prolonged precipitation, snowmelt, thunderstorms, storm surges from hurricanes, and ice or debris jams. Human factors include structural failures of dams and levees, altered drainage, and land-cover alterations (such as pavement).

Major Flood Types

All flood types are affected by climate-related factors, some more than others.

Flash floods occur in small and steep watersheds and waterways and can be caused by short-duration intense precipitation, dam or levee failure, or collapse of debris and ice jams. Most flood-related deaths in the U.S. are associated with flash floods.

Urban flooding can be caused by short-duration very heavy precipitation. Urbanization creates large areas of impervious surfaces (such as roads, pavement, parking lots, and buildings) that increased immediate runoff, and heavy downpours can exceed the capacity of storm drains and cause urban flooding.

Flash floods and urban flooding are directly linked to heavy precipitation and are expected to increase as a result of increases in heavy precipitation events.

River flooding occurs when surface water drained from a watershed into a stream or a river exceeds channel capacity, overflows the banks, and inundates adjacent low lying areas. Riverine flooding depends on precipitation as well as many other factors, such as existing soil moisture conditions and snowmelt.

Coastal flooding is predominantly caused by storm surges that accompany hurricanes and other storms that push large seawater domes toward the shore. Storm surge can cause deaths, widespread infrastructure damage, and severe beach erosion. Storm-related rainfall can also cause inland flooding and is responsible for more than half of the deaths associated with tropical storms.6 Climate change affects coastal flooding through sea level rise and storm surge, and increases in heavy rainfall during storms.

Increasingly, humanity is also adding to weather-related factors, as human-induced warming increases heavy downpours, causes more extensive storm surges due to sea level rise, and leads to more rapid spring snowmelt.

Trends in Flood Magnitude Trends in Flood Magnitude Details/Download

Worldwide, from 1980 to 2009, floods caused more than 500,000 deaths and affected more than 2.8 billion people.7 In the United States, floods caused 4,586 deaths from 1959 to 20058 while property and crop damage averaged nearly 8 billion dollars per year (in 2011 dollars) over 1981 through 2011.6 The risks from future floods are significant, given expanded development in coastal areas and floodplains, unabated urbanization, land-use changes, and human-induced climate change.7

References

  1. Ashley, S. T., and W. S. Ashley, 2008: Flood fatalities in the United States. Journal of Applied Meteorology and Climatology, 47, 805-818, doi:10.1175/2007JAMX1611.1. URL | Detail

  2. Doocy, S., A. Daniels, S. Murray, and T. D. Kirsch, 2013: The human impact of floods: A historical review of events 1980-2009 and systematic literature review. PLOS Currents Disasters, doi:10.1371/currents.dis.f4deb457904936b07c09daa98ee8171a. URL | Detail

  3. Hirsch, R. M., and K. R. Ryberg, 2012: Has the magnitude of floods across the USA changed with global CO2 levels? Hydrological Sciences Journal, 57, 1-9, doi:10.1080/02626667.2011.621895. URL | Detail

  4. ,, 2013: United States Flood Loss Report - Water Year 2011. 10 pp., National Oceanic and Atmospheric Administration, National Weather Service. URL | Detail

  5. Peterson, T. C. et al., 2013: Monitoring and understanding changes in heat waves, cold waves, floods and droughts in the United States: State of knowledge. Bulletin of the American Meteorological Society, 94, 821-834, doi:10.1175/BAMS-D-12-00066.1. URL | Detail

  6. Villarini, G., and J. A. Smith, 2010: Flood peak distributions for the eastern United States. Water Resources Research, 46, W06504, doi:10.1029/2009wr008395. URL | Detail

  7. Villarini, G., F. Serinaldi, J. A. Smith, and W. F. Krajewski, 2009: On the stationarity of annual flood peaks in the continental United States during the 20th century. Water Resources Research, 45, W08417, doi:10.1029/2008wr007645. URL | Detail

  8. Villarini, G., J. A. Smith, M. Lynn Baeck, and W. F. Krajewski, 2011: Examining flood frequency distributions in the Midwest U.S. JAWRA Journal of the American Water Resources Association, 47, 447-463, doi:10.1111/j.1752-1688.2011.00540.x. URL | Detail

The National Climate Assessment summarizes the impacts of climate change on the United States, now and in the future.

A team of more than 300 experts guided by a 60-member Federal Advisory Committee produced the report, which was extensively reviewed by the public and experts, including federal agencies and a panel of the National Academy of Sciences.

United States Global Change Research Program logo United States Global Change Research Program participating agency logos