"A rise in global mean sea level is an important indicator of global change, because it can be caused by thermal expansion of the oceans and melting of glaciers and the polar ice caps," said R. Steven Nerem of NASA's Goddard Space Flight Center, Greenbelt, Md. "Therefore, if a long-term rise in global mean sea level were detected, this would provide further evidence to support the global warming predicted by some climate models due to an increase in the 'greenhouse' gases." The findings were announced recently at the American Geophysical Union meeting in San Francisco.
Insights from TOPEX/Poseidon add to data collected from tide gauges over the last century that suggest that the average sea level has been rising at a rate of about .04 to .08 inches (one to two millimeters) per year, roughly equivalent to the rate expected from global warming, Nerem said. "The data from Dec. 1992 to Sept. 1994 show a rise in average sea level of about .12 inches (three millimeters) per year, which is in reasonable agreement with the tide gauge results," Nerem said. However, tide gauge measurements can be affected by the movement of land masses and are too sparsely distributed to provide global coverage. TOPEX-Poseidon was launched on Aug. 10, 1992 to study how long-term ocean circulation affects climate change. It measures sea-surface height along a groundtrack that repeats once every 10 days. The satellite also measures absolute sea level relative to the center of mass of the Earth. The measurement data is unaffected by land movements. Nerem cautioned that the results are preliminary and could change as a longer time series is collected and as the measurement errors are better understood. "It should also be noted that since the sea level rise is only measured over two years, it could represent a short-term variation unrelated to the long-term signal expected from global warming. Nevertheless, TOPEX/Poseidon appears to be providing corroborating evidence that global sea level is indeed rising," he said.