Sea Level Rise
Changes in local sea level result from of a combination of global, regional, and local change. At the global scale, sea level is rising. Measured at tide gauges on every continent and by satellites on orbit, global average sea level has risen about 7 to 8 inches since 1900, with about 3 of those inches occurring since 1993.1
The observed global increase is due mainly to melting of glaciers and ice sheets on land and the thermal expansion of ocean water as it warms. In regions where land is rising faster than sea level (due to tectonic forces, for instance, as in southeast Alaska), local sea level change is decreasing. In other regions, where land is subsiding (often a result of pumping water or oil from underground), local sea level is increasing faster than the global average.
Global sea level rise scenarios
Human-caused climate change has made a substantial contribution to global mean sea level rise since 1900, contributing to a rate of rise that is greater than during any preceding century in at least 2,800 years.1 Scientists are highly confident global sea level will continue to rise between about 1 and 8 feet by 2100.2 Oceanographers and climatologists developed a set of six global sea level rise scenarios that reflect 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.
The table above, adapted from Table 5 in Global and Regional Sea Level Rise Scenarios for the United States, shows estimated global average sea level rise by the indicated years for each of the six scenarios. All values are in inches.
Regional and local sea level changes
At the regional scale, sea level is also influenced by ocean currents, wind patterns, and meltwater from glaciers and ice sheets on Greenland and Antarctica. On smaller scales, environmental factors such as shifts in ocean circulation patterns, changes in Earth's gravitational field and rotation, vertical land movements, sediment compaction, groundwater and fossil fuel withdrawals, and other nonclimatic factors can influence the direction, rate, and magnitude of local sea level changes.
To check estimated rates of regional sea level change in a region of interest to you, try the U.S Army Corps of Engineers' Sea Level Change Curve Calculator. Additionally, NOAA's Office of Coastal Management's Sea Level Rise Viewer offers a Local Scenarios tab to help users visualize changes projected for the coming decades in their communities.
Choosing a sea level rise scenario based on risk tolerance
For potential projects near a coast, one approach to determining which sea level rise scenario is most appropriate is to evaluate 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 highest 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 lowest 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. a. b. Sweet, W.V., R. Horton, R.E. Kopp, A.N. LeGrande, and A. Romanou, 2017: Sea level rise. In: Climate Science Special Report: Fourth National Climate Assessment, Volume I [Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 333-363, doi: 10.7930/J0VM49F2.
- 2. Sweet, W.V., R.E. Kopp, C.P. Weaver, J. Obeysekera, R.M. Horton, E.R. Thieler, and C. Zervas, 2017: Global and Regional Sea Level Rise Scenarios for the United States. NOAA Technical Report NOS CO-OPS 083. NOAA/NOS Center for Operational Oceanographic Products and Services. 75 pp.