Sea Level, Rising

SEA LEVEL RISE is caused by thermal expansion of the oceans, melting of glaciers and ice caps, melting of the Greenland and Antarctic ice sheets, and changes in terrestrial storage. Changes in sea level will be felt through increases in the intensity and frequency of storm surges and coastal flooding; increased salinity of rivers, bays, and coastal aquifers resulting from saline intrusion; increased coastal erosion; loss of important mangroves and other wetlands (the exact response will depend on the balance between sedimentation and sea level change) and its effect on marine ecosystems (i.e., coral reefs).

Global sea level rose by about 394 ft. (120 m.) during the several millennia that followed the end of the last Ice Age (approximately 21,000 years ago) before stabilizing between 3,000 and 2,000 years ago. Sea level indicators suggest that global sea level did not change significantly from then until the late 19th century, when the instrumental record of modern sea level change shows evidence for onset of sea level rise. Estimates for the 20th century show that global average sea level rose at a rate of about 1.7 mm. per year.

Satellite observations available since the early 1990s provide more accurate sea level data with nearly global coverage. This decade-long satellite altimetry data set shows that since 1993, sea level has been rising at a rate of around 3 mm. per year—significantly higher than the average during the previous half century. Coastal tide gauge measurements confirm this observation and indicate that similar rates have occurred in some earlier decades.

Sea level rise is currently determined by the employment of two techniques: the use of tide gauges and satellite altimetry. Tide gauges provide sea level variations with respect to the land on which they lie. To extract the signal of sea level change resulting from ocean [Page 890]water volume and other océanographie change, land motions need to be removed from the tide gauge measurement. Sea-level change based on satellite altimetry is measured with respect to the Earths center of mass and thus is not distorted by land motions, except for a small component resulting from large-scale deformation of ocean basins. The total 20th-century rise is estimated to be around 0.5 ft. (0.17 m.).

The Lower Patuxent River in Maryland, showing the flooding of low-lying areas by extreme high tides. If climate change causes sea level to continue to rise, this type of flooding will become increasingly common.

Sea-level rise is accelerating worldwide. Globally, 100 million people live within about 3 mi. (1 m.) of sea level. Eight to 10 million people live within 3 mi. (1 m.) of high tide in each of the unprotected river deltas of Bangladesh, Egypt, and Vietnam. Intergovernmental Panel on Climate Change (IPCC) reports estimate that the global average sea-level rose at an average rate of 1.8 (1.3–2.3) mm. per year between 1961 and 2003, and within that period, the rate of rise was faster between 1993 and 2003—about 3.1 (2.4–3.8) mm. per year. Overall, the IPCC concludes that there is high confidence that the rate of observed sea-level rise has risen from the 19th to the 20th century. The total 20th-century rise is estimated to be 0.17 (0.12–0.22) m. In 2001, IPCC projections were for a sea-level rise of between 9 and 88 cm. between 1990 and 2100 and a global average surface temperature rise of between 2.5–10.4 degrees F (1.4–5.8 degrees C). In 2007, IPCC projections based on different scenarios predict seal level rise from 0.18 to up to 0.59 mm. by 2099.

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