Rising Sea Level

The rising sea level is a result of (1) the thermal expansion of the oceans, (2) the melting of glaciers and ice caps, (3) the melting of the Greenland and Antarctic ice sheets, and (4) changes in terrestrial storage. The effects of sea-level changes include (1) the increased intensity and frequency of storm surges and coastal flooding; (2) the increased salinity of rivers, bays, and coastal aquifers as a result of saline intrusion; (3) increased coastal erosion; (4) the loss of important mangroves and other wetlands (the exact response will depend on the balance between sedimentation and sea-level change); and (5) the impact on marine ecosystems, that is, coral reefs.

The global sea level rose by about 393.7 ft. (120 m) during the several millennia that followed the end of the last ice age (approximately 21,000 years ago) and stabilized between 3,000 and 2,000 years ago. Sea-level indicators suggest that the global sea level did not change significantly from that time until the late 19th century. The instrumental record of modern sea-level change shows evidence for the onset of rising sea levels during the 19th century. Estimates for the 20th century show that the global average sea level rose at a rate of about 1.7 mm/year–1.

Satellite data available since the early 1990s provides a more accurate sea-level measurement, with nearly global coverage. This decade-long satellite altimetry data set shows that since 1993, the sea level has been rising at a rate of around 3 mm/year–1, significantly higher than the average during the previous half century. Coastal tide gauge measurements confirm this observation and indicate that similar rates occurred in some earlier decades.

Currently, sea-level rise is determined by 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 due to ocean water volume and other oceanographic 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 Earth’s center of mass, and thus is not distorted by land motions, except for a small component due to large-scale deformation of ocean basins from GIA. The total 20th-century rise is estimated to be 0.17 m (0.12 to 0.22 m).

Rising sea levels are accelerating worldwide. Globally, 100 million people live within about one meter of present-day sea levels. The global average sea level rose at an average rate of 1.8 mm (1.3 to 2.3 mm) per year between 1961 and 2003 as a result of the global ocean temperature rise of 32.2 degrees F (0.10 degree C) from the surface to a depth of 2,297 ft. (700 m). This rate was faster between 1993 and 2003, about 3.1 mm (2.4 to 3.8 mm) per year. Whether the faster rate for 1993 to 2003 reflects decadal variability, or an increase in the long-term trend, is unclear. There is high confidence that the rate of observed sea-level rise increased from the 19th to the 20th century.