Ice loss from the massive ice sheets covering Greenland and Antarctica is accelerating, according to a new study.

If the trend continues, ice sheets could become the dominant contributor to sea level rise sooner than scientists had predicted, concludes the research, which will be published this month in the journal Geophysical Research Letters.

"The traditional view of the loss of land ice on Earth has been that mountain glaciers and ice caps are the dominant contributors, and ice sheets are following behind," said study co-author Eric Rignot, a glaciologist at NASA's Jet Propulsion Laboratory and the University of California, Irvine. "In this study, we are showing that ice sheets, mountain glaciers and ice caps are neck-and-neck."

But that could soon change, Rignot said, because the rate at which ice sheets are losing mass is increasing three times faster than the rate of ice loss from mountain glaciers and ice caps.

"I don't think we expected ice sheets to run neck-and-neck with mountain glaciers, which basically sit in a warmer climate, this soon," he said. "At the same time, the mass loss on the ice sheet is not very large compared to how much mass they store."

Rignot was part of a research team that also included scientists from Utrecht University in the Netherlands and the National Center for Atmospheric Research in Boulder, Colo.

The researchers based their analysis on a comparison of two different methods to measure ice loss.

Sea level rise estimates likely to increase
One source was NASA's twin GRACE satellites, which orbit the Earth about 200 kilometers apart from each other. Small changes in the planet's gravity field can push the satellites together, ever so slightly, or pull them apart -- variations that scientists use to interpret the terrain below.

The second method combined different satellite data that measure the speed at which the ice sheets flow to the ocean, airborne measurements of the ice sheets' thickness and a regional climate model. Combining the speed and thickness measurements allowed the scientists to determine how much ice was flowing into the ocean, while the climate model allowed them to estimate how much snow was falling on the ice sheet. Subtracting one from the other produced a "mass-balance" picture of net ice loss or growth for each ice sheet.

The two data sets overlapped for an eight-year period, from 2002 to 2010, and showed similar results. Based on that close agreement between the two measurement methods, the scientists had confidence that the full 18-year record produced by the mass-balance method was generally accurate.

Rignot said the results are "probably going to provide more incentive for the next [Intergovernmental Panel on Climate Change report] to revise sea level prediction a little bit upwards."

He cautioned that it's hard to extrapolate his new results to the end of the century, because "18 years of data is not too much."

In its last major report, released in 2007, the IPCC predicted seas would rise between 7 and 23 inches by 2100 -- but couched that estimate with a giant caveat. The IPCC cautioned that an additional rise could come from rapid and unpredictable melting in Greenland and Antarctica, which it didn't attempt to estimate.

Since that report was released, scientists have worked hard to improve their understanding of ice sheet behavior and improve estimates of future sea level rise. Many researchers now believe the sea could rise an average of 3 to 6 feet by the end of the century, with the more likely amount at the low end of that range.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500