Sea-level rise has the potential for significant impacts on societies and ecosystems around the world. Climate change causes sea level to rise by affecting both the density and the amount of water in the oceans. First and most significantly, water expands as it warms, and less-dense water takes up more space. This “thermal expansion” is projected to be the largest component of sea-level rise over the next 100 years and will persist for many centuries. Secondly, warming increases melting of glaciers and ice caps (land-based ice), adding to the amount of water flowing into the oceans.
Global average sea level rose almost three millimeters per year during the 1990s, up from about two millimeters per year in the several decades prior to that. This rate is, in turn, 10 to 20 times faster than the estimated rate of rise over the past few thousand years. The primary factors contributing to this rise are thermal expansion due to ocean warming and melting of land-based ice that increases the total amount of water in the ocean.
Global average sea level is projected to rise 10 to 90 centimeters during this century, with the rate of rise accelerating as the century progresses. Over the longer term, much larger increases in sea level are projected. Sea-level rise is expected to vary around the globe, with the largest increases projected to occur in the Arctic, in part due to the projected increase in freshwater input to the Arctic Ocean and the resulting decrease in salinity and thus density.
Sea-level rise is projected to have serious implications for coastal communities and industries, islands, river deltas, harbors, and the large fraction of humanity living in coastal areas worldwide. Sea-level rise will increase the salinity of bays and estuaries. It will increase coastal erosion, especially where coastal lands are soft rather than rocky.
Extensive coastal lowlands and delta areas contain important ecosystems that will be affected by rising sea levels. Wetlands will be forced farther inland, and the incidence of coastal floods will increase.
The impacts of sea-level rise are likely to be most severe along gently sloping coastal lands, inland areas bordering estuaries, and coastlines that are subsiding due to tectonic forces, sedimentation, or extraction of oil or groundwater. Low-lying islands in the Pacific Ocean (Marshall, Kiribati, Tuvalu, Tonga, Line, Micronesia, Cook), Atlantic Ocean (Antigua, Nevis), and Indian Ocean (Maldives), are very likely to be severely affected.
In Bangladesh, about 17 million people live less than one meter above sea level and are already vulnerable to flooding. In Southeast Asia, a number of very large cities including Bangkok, Bombay, Calcutta, Dhaka, and Manila (each with populations greater than five million), are located on coastal lowlands or on river deltas. In the United States, Florida and Louisiana are particularly susceptible to impacts of future sea-level rise.