Ongoing research by climatologists and marine scientists confirms that global sea level—the average height of the ocean's surface, apart from tidal changes—is rising. In the late 1800s, after two or three thousand years of stability, sea level began to rise at about 1.7 millimeters (0.07 inch) per year. From 1993 to the early 2000s, the rate of sea level rise increased to about 3 millimeters (0.12 inch) per year. Sea level is predicted to rise for at least the next few centuries because of global warming processes already set in motion, but how much farther it rises will depend on factors that are difficult to predict.
Sea level rise is a general term based upon a global average. Shifts in Earth's gravity, spin, and other factors determine whether a particular coastline will experience more or less sea level rise than the global average.
Sea level rise threatens fresh water supplies, coastal lands and settlements, and vital economic activity. For low-lying islands, significant sea level rise could be ecologically and economically devastating.
Warming of the ocean causes the body of water to expand. This effect, confined mostly to the ocean's top 701 meters (2,300 feet), is called thermal or thermosteric expansion. In addition, melting of glaciers and other bodies of ice lying on land causes the amount of water in the ocean to increase. The breakup and melting of floating sea ice does not raise sea level. Discounting minor contributions related to the difference in the density of fresh water and salt water, floating ice has approximately the same effect on water level whether it melts or not. Melting glaciers, however, add new water to the ocean, raising sea level. Retention of liquid water on the continents—as, for example, the damming of rivers or other activities that withhold water from the oceans—also makes a small negative contribution to sea level rise.
The energy that causes both thermal expansion and ice melting comes from solar warming of the atmosphere. According to a strong consensus of scientific data, solar warming is increased by human-caused emissions of carbon dioxide and other greenhouse gases.
Sea levels have also risen and fallen due to natural causes many times over Earth's history. During the last half-million years, Earth has gone through about half a dozen ice ages. These have alternated with warmer periods called interglacial periods. During the last four glacial cycles, each of which has lasted about 100,000 years, sea level has fallen about 122 meters (400 feet) when the ice was at its height and then has risen again by the same amount as the ice melted. These patterns of ancient sea level change are not merely a matter of abstract interest. Scientists study them for clues to the changes that humans may face in coming decades and centuries.
In 2007, the Intergovernmental Panel on Climate Change (IPCC) sponsored by the United Nations predicted that sea levels would rise between 18 and 59 centimeters (7 and 23 inches) by 2100, a reduction of predicted sea level rise from IPCC's 2001 estimate of 8 to 88 centimeters (3.1 to 34.7 inches). However, no other figure in the IPCC report has been so disputed by scientists as the prediction for sea level rise. Critics contend that the range calculated by the IPCC for its 2007 report did not include the possibility of increased contributions to sea level rise from accelerated melting of the Greenland and West Antarctica ice sheets, which has been observed since the report was published. A number of scientists have argued in leading scientific journals that future sea level rise may be greater than the IPCC's 2007 forecast but is unlikely to be less.
Several studies released in 2009 directly and indirectly challenged that the 2007 IPCC report underestimated the extent and rapidity of sea level rise. A report released at a Climate Change Congress in Copenhagen in March 2009 reported that sea levels could rise as much as one meter (39.4 inches) by 2100. Scientists attributed the higher estimate to faster warming oceans and greater thermal expansion. Other studies predicted that ocean temperatures would rise much faster than anticipated and that glaciers and ice sheets in Greenland and Antarctica are melting much more rapidly than expected.
In September 2009, a study published by the British Antarctic Survey in the journalNature indicated that ice sheets are melting much more rapidly than anticipated. One factor contributing to the accelerated melting was termed dynamic thinning. In this process, warmer ocean temperatures melt the underside of ice shelves, just as warmer atmospheric temperatures increase surface melting.
In late November 2009, a study released by the Scientific Committee on Antarctic Research (SCAR) warned that elevated sea temperatures are accelerating melting along the west coast of Antarctica. The study also predicted that global sea levels are likely to rise by about 1.4 meters (4.6 feet) before 2100 as a result of accelerating Antarctic ice melts. A rise of the magnitude predicted by the SCAR report will endanger many well-populated coastal cities and areas around the world.
As of February 2010, a wide range in estimates of sea level rise continued to challenge researchers. A 2009 study published in Nature Geoscience comparing sea level changes with historical temperature changes estimated that by 2100 sea level could rise as little as 7 centimeters (approximately 2.8 inches) with a temperature increase of 1.1 degrees Celsius (2 degrees Fahrenheit) to as much as 84 centimeters (33.1 inches) as a result of a 6.4 degrees Celsius (11.5 degrees Fahrenheit) rise in average global temperatures.
While predictions are expected to shift with new data, the vast majority of models predict significant sea level rise that will result in perilous consequences. Most experts and government leaders argue that an increase of just half a meter (1.6 ft) in global sea level would be a geopolitical and economic catastrophe for many densely settled coastal areas. In July 2009, United Nations officials released studies predicting that climate change could lead to human displacement and mass migration. As many as 700 million people might be forced to relocate by 2050 if the most severe climate change models prove accurate.
During 2009 and the first half of 2010, glaciers in Greenland, including the prominent Jakobshavn Isbrae glacier, recorded record retreats. Such glacial retreats directly contribute to sea level rise. Since 2000, melting of Greenland's glaciers has contributed enough water to raise global sea levels by 5 mm (.2 inches).
Christina Robev October 27/10
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