Sea-level changes (Encyclopedia of Environmental Issues, Revised Edition)
The world’s oceans and seas constantly rise and fall over short periods because of tides, currents, winds, air pressure, seasonal heating, and weather phenomena, such as El Niño and La Niña events. Moreover, because cold water is denser than warm water, the sea levels in polar oceans are generally lower than sea levels in tropical oceans. Nevertheless, scientists use data gathered by tidal gauges and satellites to determine an overall mean sea level (MSL), purely a statistical value. In some areas available tidal gauge data extend back more than one hundred years. These data and more recent data sets show that the MSL has been rising since the mid-nineteenth century after about six thousand years of relative stability. The twentieth century saw an average rise of 1-2 millimeters (0.04-0.08 inch) per year, with an increase to 3 millimeters (0.12 inch) per year at the end of the century. If that rate accelerates because of human-caused global warming, as many scientists have concluded will happen, sea levels could become high enough to imperil coastal cities, agriculture, ecosystems, and fisheries during the twenty-first century.
(The entire section is 183 words.)
Long-Term Change (Encyclopedia of Environmental Issues, Revised Edition)
Scientists distinguish two types of long-term change in sea level. The first, secular (or isostatic) change, comes from local movement of the land and seabed: land rebounding after the ice sheets of the last ice age melted (glacial isostatic adjustment), land subsiding as groundwater is removed, the deposit of sediments, and the effects of volcanoes, earthquakes, and tectonic plate movement. The second type, eustatic change, involves the volume of water in the oceans. The volume varies based on water temperature and on how much of the world’s water supply is land-bound in lakes, aquifers, and ice (glaciers, ice caps, and ice sheets).
About half of the rise in sea level during the twentieth century came from warming water (thermal expansion). Water added to the oceans from melting glaciers and ice caps contributed most of the rest, a small amount coming from the ice sheets covering Greenland and Antarctica. These changes arise from temperature change caused by several factors influencing the amount of heat energy in the atmosphere, land surface, and water, principally orbital cycles varying the earth’s distance from and attitude toward the sun and the concentration of greenhouse gases. For example, when global cooling locked greater volumes of water in land ice during the depths of the last ice age, twenty thousand years ago, the MSL was about 120 meters (394 feet) lower than that of the early twenty-first century. During...
(The entire section is 250 words.)
Future Change (Encyclopedia of Environmental Issues, Revised Edition)
If all the icebound water on the earth melted and ran into the oceans, the MSL would increase an estimated 64-70 meters (210-230 feet) above the contribution of thermal expansion. This is not likely to happen. Thermal expansion and melting will continue, however, accelerated by an anticipated increase in global temperatures of 3 degrees Celsius (5.4 degrees Fahrenheit) because of greenhouse gas emissions.
Projections about sea-level changes in the near future are controversial, but a majority of climate scientists agree that the MSL will rise faster in the twenty-first century than it did during the twentieth century. In 2007, the Intergovernmental Panel on Climate Change estimated that the rise will be somewhere between 18 and 59 centimeters (7 and 23 inches) by 2100. However, that projection did not include possible contributions from the Antarctic and Greenland ice sheets, because scientists did not have sufficient understanding of the ice sheets’ dynamics. Subsequent studies found increasing instability in parts of these ice sheets, and in 2009 the Scientific Committee on Antarctic Research forecast accelerated melting in Greenland and the Antarctic; the committee estimated that the MSL could rise as much as 1.4 meters (4.6 feet) by the end of the twenty-first century. Such a rise could bring serious disruptions to both agriculture and cities in coastal areas, principally because low-lying lan will be much more vulnerable...
(The entire section is 291 words.)
Further Reading (Encyclopedia of Environmental Issues, Revised Edition)
Archer, David. The Long Thaw: How Humans Are Changing the Next 100,000 Years of Earth’s Climate. Princeton, N.J.: Princeton University Press, 2009.
Langwith, Jacqueline, ed. Water. Farmington Hills, Mich.: Greenhaven Press, 2010.
McGranahan, Gordon, Deborah Balk, and Bridget Anderson. “The Rising Tide: Assessing the Risks of Climate Change and Human Settlements in Low Elevation Coastal Zones.” Environment and Urbanization 19, no. 1 (2007): 17-37.
Pilkey, Orrin H., and Rob Young. The Rising Sea. Washington, D.C.: Island Press, 2009.
Pugh, David. Changing Sea Levels: Effects of Tides, Weather, and Climate. New York: Cambridge University Press, 2004.
(The entire section is 90 words.)