SAN FRANCISCO — The first global analysis of melting glaciers and ice caps showed that while approximately 1,000 cubic meters of water poured into the world’s oceans between 2003 and 2010, a smaller fraction of that water came from mountain glaciers than scientists had expected.
“Our estimates for ice loss in Greenland and Antarctica, which are the dominant sources of sea-level rise, were accurate,” said John Wahr, physics professor at the University of Colorado, Boulder, and an author of the study. However, the world’s mountain glaciers are warming more slowly than scientists had predicted using evidence from ground-based observation, he added.
The study, published Feb. 8 in the journal Nature, does not contradict reports that polar ice mass loss is contributing significantly to sea-level rise, but it does highlight the need for ongoing efforts to study the many variables influencing the Earth’s climate, said Mike Watkins, project scientist for the joint U.S.-German Gravity Recovery and Climate Experiment (GRACE) mission and the GRACE Follow-On Mission at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.
Wahr and his University of Colorado colleagues used data gathered by the twin GRACE satellites to identify changes in the mass of ice. GRACE mission officials produce monthly reports on the Earth’s gravity field based on slight variations in the distance between the GRACE satellites.
Since the GRACE mission was launched in 2002, researchers have continued to refine the satellite’s data products and scientists have become increasingly adept at using GRACE data to study a variety of phenomena, including aquifers, ice sheets and ocean currents, Watkins said. Although the GRACE mission does not produce data with high enough resolution to pinpoint the amount of ice lost by a specific glacier in a mountain range, it does provide data on variations in regional and global ice mass, Wahr said.
Wahr, who has been involved with the GRACE mission since its inception, said the most surprising result of the University of Colorado study was that ice melt from mountain glaciers was at least 30 percent lower than scientists anticipated. He attributed much of that disparity to evidence that Asia’s high mountain glaciers, including those in the Himalayan, Tien Shan and Pamir mountain ranges, are warming much more slowly than researchers thought.
In previous reports, scientists estimated that Asia’s tens of thousands of mountain glaciers were losing as much as 50 billion tons of ice each year. The University of Colorado study suggested that ice loss was closer to 4 billion tons annually, Wahr said.
While additional research will be needed to determine why the results are so far below previous estimates, Wahr said those estimates were often based on ground-based observation of a small fraction of Asia’s glaciers. Most Asian glaciers are at high elevations, but many of the glaciers scientists monitored were found at lower elevations. While warming temperatures have caused the lower-elevation glaciers to melt, many of the higher-elevation glaciers may be in areas where temperatures have stayed consistently below freezing. Glaciers at higher elevations also may be receiving more snowfall, Wahr said.
“To determine the amount of ice loss, you need to know about temperature and precipitation,” Wahr said. “It’s difficult to figure out what’s going on in all these tens of thousands of glaciers by monitoring a handful of them. You also have to take into account all the different elevations and climate regimes.”
The University of Colorado researchers plan to continue using data obtained by GRACE to study global and regional ice loss. “The ice loss from all the regions continues to evolve,” Wahr said. “It undoubtedly won’t be the same next year as it is this year. So we will keep adding to the data record and continuing to monitor what happens.”
That research also will be used to improve the accuracy of global climate change models. “You can’t use an eight-year study to tell you what will happen to the ice-covered part of the world over the next several generations,” Wahr said. “But it does tell people about the state of the cryosphere today. If you know what it looks like today, you have a better chance of understanding the dynamics and predicting into the future.”
The GRACE mission, launched in 2002, was designed to last five years. As GRACE approaches its 10th anniversary, the satellites are “showing signs of old age,” Watkins said. The most serious concern for mission officials is failing battery cells. GRACE satellite batteries no longer can retain a full charge. As a result, the satellites do not collect data when they travel in the Earth’s shadow twice each year for periods of approximately 30 days. NASA is working with the German space agency, DLR, to plan the GRACE Follow-On mission, scheduled for launch in 2016 or 2017, Watkins added.