- The risk of flooding has increased in most coastal regions of the United States and its island territories since 1900, and that risk is projected to grow even more this century.
- Coastal lifelines, such as water and energy infrastructure, and nationally important assets, such as ports, tourism, and fishing sites, are increasingly vulnerable to sea level rise, storm surge, erosion, flooding, and related hazards. Socioeconomic disparities create uneven vulnerabilities.
- Coastal ecosystems are particularly vulnerable to climate change because many have already been dramatically altered by human stresses; climate change will result in further reduction or loss of the services that these ecosystems provide, including potentially irreversible impacts.
- There is no one-size-fits-all solution to reduce risk and improve resilience. Every community should develop its own plan of action, but can learn from other communities about effective approaches.
Every year, at multiple locations along the coast of the United States, events such as storm surges, high tides, strong waves, heavy precipitation, increased river flow, and tsunamis cause damaging coastal floods. As global sea level rises, higher water levels will exacerbate the impacts of these incidents, resulting in deeper floods that last longer and extend further inland. Additionally, as climate changes, some coastal hazards are projected to increase. For instance, coasts may see more severe or more frequent storms and heavier rainfall events.
Average global sea level rose eight inches during the last century, and scientists are highly confident that it will continue rising in the future. By 2100, global sea level is projected to be between 8 inches and 6.6 feet higher than it was in 1992. At regional and smaller scales, relative sea level is also affected by vertical land movement and ocean currents, but any amount of global sea level rise will increase the frequency and magnitude of coastal flooding impacts, posing an increasing threat to people, infrastructure, and coastal economies.
The greatest damage from coastal flooding generally occurs when high waves and storm surge occur during high tide. In many locations, small increases in sea level over the past few decades have already increased the height of storm surge and wind-waves. Thus, considering the impact of different weather events combined with sea level rise is crucial for effective planning.
The preceding text is abridged from the report Global Sea Level Rise Scenarios for the United States National Climate Assessment.
People and infrastructure in harm's way
Nearly five million people in the United States live within four feet of the local high-tide level. In the next several decades, many of these regions will experience coastal flooding. Past coastal flooding events illustrate that public safety and human well-being are jeopardized by the disruption water and energy systems, and evacuation routes. As sea level continues to rise, repeated disruptions by coastal flooding will burden people and aggravate existing impacts on infrastructure and natural systems. Planning for these changes, while balancing different and often competing demands, are vexing challenges for decision makers.
Coastal infrastructure is exposed to climate change impacts from both the landward and ocean sides. Infrastructure is built to site-specific design standards that take account of historical variability in climate, coastal, and hydrologic conditions. Impacts exceeding these standards can shorten the expected lifetime, increase maintenance costs, and decrease services. In general, higher sea levels, especially when combined with inland changes from flooding and erosion, will result in accelerated infrastructure impairment, with associated indirect effects on regional economies and a need for infrastructure upgrades, redesign, or relocation.
Though sea level rise scenarios specify an amount of sea level rise projected for 2100, seas won’t stop rising then. The ocean takes a very long time to respond to warmer conditions at Earth’s surface, so ocean waters will continue to warm and sea level will continue to rise for many centuries at rates equal to or higher than that of the current century.
The preceding text is abridged from Climate Change Impacts in the United States: The Third National Climate Assessment (Chapter 25, Coastal Zone Development and Ecosystems).
Managing coasts into the future
With higher seas, the past is no longer the key to the present or future. Historical erosion data and historical flood data simply aren't sufficient for managing coasts with higher water levels. Future flood risk must take sea level rise into account.
The Coastal Zone Management Act of 1972, as amended, states that “because global warming may result in a substantial sea level rise with serious adverse effects..., coastal states must anticipate and plan for such an occurrence.” In particular, the act calls for states to protect natural resources and manage coastal development to minimize the loss of life, property, and other coastal zone assets caused by development in hazardous areas. Additionally, the Biggert-Waters Flood Reform Act of 2012 allows the Federal Emergency Management Agency (FEMA) to update its federal insurance rate maps (FIRMs) to include “relevant information and data” on flood hazards caused by land-use changes and “future changes in sea levels, precipitation, and intensity of hurricanes.”
To learn more about the impacts of climate change and variability in coastal areas, visit the subtopic pages: