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Ocean Surface Topography from Space
NASA Satellites Measure and Monitor Sea Level
July 07, 2005

For the first time, NASA has the tools and expertise tounderstand the rate at which sea level is changing, some ofthe mechanisms that drive those changes and the effects thatsea level change may have worldwide.

"It's estimated that more than 100 million lives are potentiallyimpacted by a one-meter (3.3-foot) increase in sea level," saidDr. Waleed Abdalati, head of the Cryospheric Sciences Branch at NASA's Goddard Space Flight Center, Greenbelt, Md. "When youconsider this information, the importance of learning how andwhy these changes are occurring becomes clear," he added.

Although scientists have directly measured sea level since theearly part of the 20th century, it was not known how many of theobserved changes in sea level were real and how many were relatedto upward or downward movement of the land. Now satellites havechanged that by providing a reference by which changes in oceanheight can be determined regardless of what the nearby land isdoing. With new satellite measurements, scientists are able tobetter predict the rate at which sea level is rising and thecause of that rise.

"In the last 50 years sea level has risen at an estimated rateof .18 centimeters (.07 inches) per year, but in the last 12 yearsthat rate appears to be .3 centimeters (.12 inches) per year. Roughlyhalf of that is attributed to the expansion of ocean water as it hasincreased in temperature, with the rest coming from other sources,"said Dr. Steve Nerem, associate professor, Colorado Center forAstrodynamics Research, University of Colorado, Boulder.

Another source of sea level rise is the increase in ice melting.Evidence shows that sea levels rise and fall as ice on land growsand shrinks. With the new measurements now available, it's possibleto determine the rate at which ice is growing and shrinking.

"We've found the largest likely factor for sea level rise ischanges in the amount of ice that covers the Earth. Three-fourthsof the planet's freshwater is stored in glaciers and ice sheets orthe equivalent of about 67 meters (220 feet) of sea level," said Dr.Eric Rignot, principal scientist for the Radar Science and EngineeringSection at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "Icecover is shrinking much faster than we thought, with over half ofrecent sea level rise due to the melting of ice from Greenland,West Antarctica's Amundsen Sea and mountain glaciers," he said.

Additionally, NASA scientists and partner researchers now are ableto measure and monitor the world's waters globally in a sustainedand comprehensive way using a combination of satellite observationsand sensors in the ocean. By integrating the newly available satelliteand surface data, scientists are better able to determine the causes andsignificance of current sea level changes.

"Now the challenge is to develop an even deeper understanding of what isresponsible for sea level rise and to monitor for possible future changes.That's where NASA's satellites come in, with global coverage and abilityto examine the many factors involved," said Dr. Laury Miller, chief ofthe National Oceanic and Atmospheric Administration Laboratory forSatellite Altimetry, Washington, D.C.

NASA works with agency partners such as the National Oceanic and AtmosphericAdministration and the National Science Foundation to explore and understandsea level change. Critical resources that NASA brings to bear on thisissue include such satellites as:

  • Topex/Poseidon and Jason, the U.S. portions of which are managed by JPL,which use radar to map the precise features of the oceans' surface,measuring ocean height and monitoring ocean circulation;

  • Ice, Cloud and Land Elevation Satellite (IceSat), which studiesthe mass of polar ice sheets and their contributions to global sea level change;

  • Gravity Recovery And Climate Experiment (Grace), also managed by JPL,which maps Earth's gravitational field, allowing us to better understandmovement of water throughout the Earth.
For more information about sea level change:

For information about Topex/Poseidon and Jason:

For information about Grace:

For information about NASA and agency programs:
Source: NASA

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