From 2 Satellites, the Big Picture on Ice Melt

JOANNA M. FOSTER

Melting glaciers and ice caps are perhaps the most striking illustrations of the effects of global climate change. Surprisingly, however, there is relatively little data on just how fast the ice is disappearing.

Now, a new paper from researchers at the University of Colorado, Boulder, offers the most up-to-date and comprehensive numbers on glacier and ice cap melt worldwide. The research, published in the journal Nature, calculates that from 2003 to 2010, the world’s glaciers and ice caps lost about 150 billion tons of ice each year. This ice loss was responsible for an average rise of four-tenths of a millimeter in sea level every year over the eight-study period.

The new numbers come from measurements made by the two Gravity Recovery and Climate Experiment satellites, or Grace, a joint project of NASA and German scientists. The tandem satellites, which are usually 137 miles apart, are sensitive to regional changes in the Earth’s mass and gravitational pull caused by the distribution of water and ice on the planet.

When the lead satellite flies over an area of increased mass, it will sense the increase in gravity and pull slightly away from the trailing satellite. Researchers can detect changes of just one micron between the two satellites, giving them new insights into the dynamics of the Earth’s aquifers and glaciers.

While the loss from glaciers and ice caps certainly isn’t trifling, over the same time period Antarctica and Greenland and their peripheral glaciers and ice caps lost about 385 billion tons of ice annually.

John Wahr, a professor of physics who helped lead the study, said the results provided an important baseline, although he cautioned that there were so many variables at play that the next eight years could look very different than the past eight years.

“Eight years of data is great, but it’s still hard to see any kind of trend or to say whether ice loss is accelerating,” Dr. Wahr said. “All I can say is that we have a better sense of the current state of the cryosphere,” or frozen regions of the planet.

Eric Rignot, a professor of earth systems science at the University of California, Irvine, who was not involved in the study, said the research was noteworthy because it was the first time that researchers were able to use the same technique to gather measurements of all of the world’s glaciers and ice caps.

“All the studies done in the past were based on just a few data points from the few accessible glaciers in the world where you can do ground-based analysis,” he said. “We were using data from 120 glaciers to extrapolate for about 160,000 different sites.”

Scientists are hopeful that the Grace satellites, which were launched in 2002 on what was expected to be a mission of just three years, will keep orbiting for at least a few more years and that funding can be assured for a follow-up mission to be launched at the end of 2016.

“I hope these kinds of studies provide the incentive to ensure that there is no gap in the Grace coverage of the Earth,” Dr. Rignot said.

While the contribution of melting glaciers and ice caps to sea-level rise may be dwarfed by the potentially catastrophic melt of Greenland and Antarctica, their loss will affect the water supplies of billions of people worldwide.