Scientists using data from the U.S.-French oceanography satellite TOPEX/Poseidon say they appear to have detected a rise in the average global sea level over the past two years. This and several other findings were announced today at the American Geophysical Union's fall meeting in San Francisco.
Other topics at the meeting related to TOPEX/POSEIDON include an experiment to study phenomena that can disrupt offshore drilling and studying deep-ocean tides.
If scientists detect a global, long-term rise in sea level, they will be able to test theories about the effects of climate change.
"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 Dr. 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 data (from December 1992 to September 1994) show a rise in average sea level of about 3 millimeters (0.12 inches) per year, which is in reasonable agreement with the tide gauge results," Nerem explained.
Data collected from tide gauges over the last century suggest that average sea level has been rising at a rate of about 1 to 2 millimeters (0.04 to 0.08 inches) per year, roughly equivalent to the rate expected from global warming, 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. TOPEX/Poseidon measures the sea-surface height along a ground track that repeats once every 10 days. The satellite measures absolute sea level relative to the center-of-mass of the Earth, and the 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," Nerem said.
In the Gulf of Mexico, TOPEX/Poseidon data are helping scientists and a U.S. oil company study potentially dangerous ocean phenomena that can disrupt off-shore oil drilling.
"Oil industry operations in deep water can be adversely affected by high ocean currents," said Dr. George Born, of the University of Colorado's Colorado Center for Astrodynamics Research (CCAR). "In the gulf, eddies spun off from the Loop Current have caused serious operational problems, including delays of several weeks, lost and damaged equipment and millions of dollars in unproductive expense."
The Loop Current is a strong ocean current that circulates around the Gulf of Mexico. Roughly once a year, a large eddy is spun off from the Loop Current and moves west through the gulf.
"An eddy can be thought of as a hurricane in the water," Born said. "The Gulf of Mexico has some of the largest eddies in the world, ranging up to 300 to 400 kilometers (180 to 250 miles) in diameter."
In late summer 1994, a large eddy, called Eddy Yucatan, broke off from the Loop Current. Oceanographers from Texaco Inc. used TOPEX/Poseidon data to augment their drifting buoy data and track this eddy. As of late October, the center of Eddy Yucatan was located approximately 150 kilometers (93 miles) southeast of the Texaco drilling site and it appeared to be moving away from the site.
"However, the predictive models available for the gulf are still under development and it is impossible to predict the future path of the eddy with great certainty," Born said. "Work is currently under way to use TOPEX/Poseidon data to produce a more reliable forecast. This work ultimately will benefit not only off-shore oil drilling, but many oceanographic activities in the Gulf of Mexico."
In another application, the precision of the TOPEX/Poseidon ocean measurements has enabled scientists to calculate global tides across all the open oceans, an important step toward monitoring global ocean circulation from space and understanding the complexities of global climate change.
"Using the first year of data we can now predict deep ocean tides everywhere over the areas covered by the satellite with an accuracy of 2 to 3 centimeters (0.8 to 1.1 inches)," said Dr. Christian Le Provost, of the National Scientific Research Center in France.
"We need to precisely understand deep ocean tides so that we can remove their influence from the ocean circulation data," said Dr. Lee-Lueng Fu, the TOPEX/Poseidon project scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "It is ocean circulation that is the key to understanding global warming and global climate change."
Precise knowledge of global ocean tides helps scientists better understand how pollutants are dispersed by tidal currents. The information also has applications for shipping, oil drilling and exploration, and fishery development.
TOPEX/Poseidon is part of NASA's Mission to Planet Earth, a coordinated, long-term research program to study the Earth as a single global environment. TOPEX/Poseidon's sea-surface height data are essential to understanding the role oceans play in regulating global climate, one of the least understood areas of climate research. The Jet Propulsion Laboratory manages the TOPEX/Poseidon mission for NASA.
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