When describing changes in sea level, it’s important to distinguish between global, regional, and local change. Globally, average sea level has risen eight inches since 1900. This observed global increase is due to melting of glaciers and ice sheets on land and the thermal expansion of ocean water as it warms.
Scientists are highly confident global sea level will continue to rise between 8 inches and 6.6 feet by 2100.1 Each of the four scenarios in the table below reflects different assumptions about the degree to which ocean warming and ice sheet loss will affect the rate and magnitude of global sea level rise. Choices the world makes about emissions will also influence sea level. Any amount of sea level rise will increase the frequency, magnitude, and duration of coastal flooding.
Global sea level rise scenarios
|Scenario||Global Average Sea Level Rise by 2100||Method for Determining Rate of Sea Level Rise|
|Highest||6.6 ft (2.0 m)||Increased rate caused by ocean warming combined with maximum potential glacier and ice sheet loss.|
|Intermediate-High||3.9 ft (1.2 m)||Increased rate caused by ocean warming and limited ice sheet loss.|
|Intermediate-Low||1.6 ft (0.5 m)||Increased rate caused by ocean warming only.|
|Lowest||0.7 ft (0.2 m)||Linear extrapolation of historical rate since 1900 (no increased rate).|
Regional and local sea level changes
On smaller scales, other environmental factors like tides, current patterns, and land movements influence the direction, rate, and magnitude of sea level changes. Sea level is rising much faster than the global average in some locations—like Norfolk, Virginia, and Grand Isle, Louisiana—because the land is sinking, while in other locations—like Neah Bay, Washington, and Kodiak Island, Alaska—local sea level is falling because the land is rising faster than the sea. At the regional scale, sea level is influenced by ocean currents, wind patterns, and melt water from glaciers and the ice sheets on Greenland and Antarctica.
Choosing an appropriate sea level rise scenario based on risk tolerance
For projects near a coast, one approach decision makers can use to determine which sea level rise scenario is most appropriate is evaluating the tolerance for risk associated with the project. In general, large, infrastructure-intensive projects that are expected to last for many decades have a low tolerance for risk. Conversely, smaller, less complex projects, or those that are unlikely to last more than a decade or so, have a higher tolerance for risk. Considering scenarios in the context of risk tolerance helps to improve transparency and credibility.
- For projects involving structures expected to have a long lifetime, where a loss would be catastrophic, or where there is limited flexibility for adaptation, there is little tolerance for risk, and the top two scenarios may be appropriate. Examples include power plants, ports, hospitals, and refuges for endangered species.
- Where local mean sea level is rising, the intermediate-low scenario should be considered as a minimum for project planning. Calculating the number of years a project is likely to be useful before being impacted by flooding or inundation can help decision makers evaluate tolerance for risk.
- Where a project is expected to have a short life span, require little infrastructure, or have the flexibility to make alternate choices, the tolerance for risk is relatively high, and the lower two scenarios may be appropriate. Examples of projects that may fall into this category include bike paths, golf courses, and parks.
- Where local relative sea level is falling, risk of coastal flooding is decreasing, so the use of the lowest scenario may be appropriate.
- 1. Parris, A., P. Bromirski, V. Burkett, D. Cayan, M. Culver, J. Hall, R. Horton, K. Knuuti, R. Moss, J. Obeysekera, A. Sallenger, and J. Weiss (2012): Global Sea Level Rise Scenarios for the United States National Climate Assessment. NOAA Tech Memo OAR CPO-1. 37 pp.