Rising seas may herald global warming.

Rising seas may herald global warming 

Have increasing amounts of greenhouse gases in the atmosphere warmed the planet enough to cause sea levels to rise? Though the answer remains far from clear, a new study hints it may be yes.

According to theory, a greenhouse warming could raise sea levels by melting continental glaciers and ice caps and by thermally expanding the oceans. Tide gauge records do indeed indicate that the sea levels around the world have risen over the last century by 10 to 20 centimeters. But scientists cannot be sure the oceans are truly swelling because a tide gauge doesn't measure absolute sea level; it only shows the relative levels between the coastlines and the oceans. If a coastline moves up or down while the ocean stays the same, a gauge will still indicate a change in relative sea level.

Many forces can affect the height of a coastline. In fact, Scandinavia and Alaska are rising because of a process called glacial rebound. Thousands of years after ice age glaciers retreated from these regions, the now unweighted crust is rising to its former level. This effect strongly influences sea levels around the globe, even in regions that remained ice free, says W. Richard Peltier of the Unviersity of Toronto in Ontario.

To get a clearer picture of the real sea-level trend, Peltier used a computer model to calculate glacial rebound around the globe and then subtracted this contamination from the tide gauge readings. The analysis indicates that sea levels are rising globally at a fairly uniform rate of 2.4 [plus-or-minus] 0.9 millimeters a year, report Peltier and A. Mark Tushingham in the May 19 SCIENCE. "This signal could constitute an indication of global climate warming," Peltier says.

Tim P. Barnett at the Scripps Institution of Oceanography in La Jolla, Calif., says the Toronto researchers are the first to tackle the problem of quantifying the glacial rebound effect. But even with this removed from the records, it is not clear how much the seas are rising because other effects such as tectonic uplift and subsidence are altering relative sea level, he says.