Ice sheets melting faster than earlier estimates

By Brian Vastag
Washington Post Staff Writer
Thursday, March 10, 2011; A04

The vast ice sheets of Greenland and Antarctica are melting faster than previously estimated and that melting is accelerating, according to a new report that verifies 18 years of melting via two independent techniques.

Left unchecked, the extra water dumped into the oceans could push average global sea level six inches higher by 2050, the report finds. That would mark the ice sheets - defined as expanses of deep, long-term ice larger than 20,000 square miles - as the largest contributors to sea level rise, outstripping melting from Earth's other frozen reservoirs, namely mountain glaciers.

The new estimate of ice sheet melting and subsequent rise in sea level comes from an international team led by scientists at NASA's Jet Propulsion Laboratory. It outstrips more modest figures offered by the Intergovernmental Panel on Climate Change in 2007, the last time that international body published a comprehensive assessment of the ice sheets.

"It's going to be a concern for people in coastal areas," said Isabella Velicogna of JPL and the University of California, Irvine, a co-author of the report online at Geophysical Research Letters. "It looks like [the IPCC estimate] will easily be an underestimate of the sea level rise."

While six inches of additional sea height might sound small, the increase will distribute unevenly across the globe, Velicogna said, and have a disproportionate impact on low-lying countries such as Bangladesh.

"What impresses me is the fact that the independent data sets really match very well. This is a major point and major achievement," said Marco Tedesco, an ice sheet researcher at the City College of New York who was not involved in the study. "We knew that acceleration might occur over both ice sheets. The fact that they are coming out with numbers that are way bigger than the [melting of the] glaciers is very remarkable."

The study used two techniques to measure the melting of the ice sheets. The most thorough data set, from 1992 through the present, employed satellite radar readings of ice movement, soundings of ice thickness, and other ground-based observations to build a complete picture of the size of the ice sheets from month to month.

The second technique drew on twin satellites, called Grace, which measure minute differences in gravity over the entire planet. Because the density of the ice sheets differs from the density of surrounding areas, the ice sheets present a distinct gravity signature in Grace's readings.

The Grace satellites, an acronym for Gravity Recovery and Climate Experiment, were launched by NASA and the German Aerospace Center in 2002.

Eric Rignot, a co-author on the report and also of JPL and UC Irvine, said that year-to-year variability in snowfall, which can increase ice sheet mass, requires a long-term record to assess melting trends - something that researchers have been missing until now.

Combined, the two ice sheets dumped an average of 475 gigatonnes of ice (which then melted) into the ocean each year over the course of the study. (A gigatonne is 1 billion metric tons.) The pace of melting accelerated over time, increasing most rapidly after 2005.

A 2006 study found that the melting of mountain glaciers and the polar ice caps was also accelerating, but at a rate about three times slower than that of the ice sheets.

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