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CU researchers document loss of Earth’s ice cover

The loss of glacial and polar ice on Earth has been so extensive in recent years that the melt water would flood the entire United States.

That’s the conclusion of a research team led by University of Colorado at Boulder scientists, who pioneered the use of satellites as a mechanism for measuring reductions in the planet’s ice cover.

“The Earth is losing an incredible amount of ice to the oceans annually, and these new results will help us answer important questions in terms of both sea rise and how the planet’s cold regions are responding to global change,” Dr. John Wahr, a CU-Boulder physics professor and co-leader of the study, said.

The Gravity Recovery and Climate Experiment (GRACE), a system of two satellites that fly about 135 miles apart, measures fluctuations in the planet’s gravitational field caused by variations in its mass. Researchers used data gathered between 2003 and 2010 to calculate that melting glaciers and ice caps are raising sea levels around the world by about 0.4 millimeters per year.

The annual loss of ice during those seven years was about 148 billion tons.

Before GRACE was launched researchers generally relied on periodic sampling of a few of Earth’s glaciers to obtain ice loss data. The general approach involved inferring world-wide ice melt from changes in that small number of glaciers.

“The strength of GRACE is that it sees everything in the system,” Wahr said. “Even though we don’t have the resolution to look at individual glaciers, GRACE has proven to be an exceptional tool.”

Wahr and a co-author, Thomas Jacob of France’s Bureau de Recherches Géologiques et Minières, sectioned Earth into “mascons” to facilitate calculation of ice mass loss in each region.

They did not include in the total figure for worldwide ice loss melt occurring in individual glaciers and ice caps located on the periphery of Greenland and Antarctica. Loss of ice in those areas accounts for another 80 million tons of lost ice each year.

An unanticipated result of the study was a conclusion that the high mountains of Asia are not losing as much ice as scientists previously thought.

The research team found that ice in the Himalaya, Pamir, and Tien Shan mountains is melting at an annual rate of about 4 billion tons.

“One possible explanation is that previous estimates were based on measurements taken primarily from some of the lower, more accessible glaciers in Asia and extrapolated to infer the behavior of higher glaciers,” Wahr said. “But unlike the lower glaciers, most of the high glaciers are located in very cold environments and require greater amounts of atmospheric warming before local temperatures rise enough to cause significant melting. This makes it difficult to use low-elevation, ground-based measurements to estimate results from the entire system.”

Sea level rise from the melting of all ice caps and glaciers on Earth is about 1.5 millimeters per year, the study’s authors report. That total does not include increases in sea level due to expansion of the water as the ocean warms.

The study does not predict the extent of ice melt in the future and, consequently, does not provide much guidance as scientists continue to work on a model capable of predicting future sea level increases attributable to global warming.

“If we could understand the physics more completely and perfect numerical models to simulate all of the processes controlling sea level — especially glacier and ice sheet changes — we would have a much better means to make predictions,” Dr. Tad Pfeffer, a co-author of the study and a member of the faculty in CU-Boulder’s civil, environmental, and architectural engineering department who specializes in the study of glaciers, said. “But we are not quite there yet.”

The study was published online Feb. 9 in the journal Nature.

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