Climate: Arctic and Antarctic ice melt are accelerating

Greenland and Antarctica are now losing more than three times as much ice as they were in the 1990s

Antarctic sea ice may be growing, but ice sheets on the frozen continent’s edge are losing mass and contributing to sea-level rise. Bob Berwyn photo.

By Summit Voice

SUMMIT COUNTY — Climate scientists say they’re closer to pinpointing exactly how much of Greenland’s and Antarctica’s ice is melting, and after producing the most accurate assessment of ice losses to-date, a team of satellite experts say they’ve ended 20 years of uncertainty about how much that melting ice contributes to global sea level rise.

According to the landmark study, published on Nov. 30 in the journal Science, the that melting has contributed 11.1 millimeters to global sea levels since 1992. This amounts to 20 percent of all sea level rise during the survey period. About two thirds of the ice loss was from Greenland, and the remainder was from Antarctica.

Together, Greenland and Antarctica are now losing more than three times as much ice (equivalent to 0.95 mm of sea level rise per year) as they were in the 1990s (equivalent to 0.27 mm of sea level rise per year). The rate of melting increased dramatically in the late 1990s.

“In the 1990s, not very much was happening. Sometime around 1999, the ice sheets started losing more mass, and probably have been losing mass more rapidly over time since then,” said Benjamin Smith, a research scientist at the UW’s Applied Physics Laboratory who co-authored a companion research paper describing some the physical reasons for accelerated ice loss.

“We are just beginning an observational record for ice,” said co-author Ian Joughin, a glaciologist in the University of Washington’s Applied Physics Laboratory. “This creates a new long-term data set that will increase in importance as new measurements are made.”

Although the ice sheet losses fall within the range reported by the IPCC in 2007, the spread of the IPCC estimate was so broad that it was not clear whether Antarctica was growing or shrinking. The new estimates are more than twice as accurate, confirming that both Antarctica and Greenland are losing ice.

“The success of this venture is due to the cooperation of the international scientific community, and due to the provision of precise satellite sensors by our space agencies,” said Professor Andrew Shepherd, with the University of Leeds. “Without these efforts, we would not be in a position to tell people with confidence how the how the Earth’s ice sheets have changed,” he said.

The study also found differences in the pace of change at each pole.

“The rate of ice loss from Greenland has increased almost five-fold since the mid-1990s,” said Dr Erik Ivins, with NASA’s Jet Propulsion Laboratory. “In contrast, while the regional changes in Antarctic ice over time are sometimes quite striking, the overall balance has remained fairly constant, at least within the certainty of the satellite measurements we have to hand.”

The Ice Sheet Mass Balance Inter-comparison Exercise (IMBIE) is a collaboration between 47 researchers from 26 laboratories, and was supported by the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA).

The researchers were able to reconcile the differences between dozens of earlier ice sheet studies through careful use of matching time periods and survey areas, and by combining measurements collected by different types of satellites.

“This project is a spectacular achievement,” said Professor Richard Alley, a climate scientist with Penn State University who was not involved in the study. “The data will support essential testing of predictive models, and will lead to a better understanding of how sea-level change may depend on the human decisions that influence global temperatures.”

Understanding why the ice sheets have been shedding mass faster in the last decade is an area of intense research. The accelerated ice loss was not predicted by the models, leading the latest International Panel on Climate Change to place no upper limit on its estimate for future ice-sheet loss.

Understanding ice sheets is central to modeling global climate and predicting sea-level rise. Even tiny changes to sea level, when added over an entire ocean, can have substantial effects on storm surges and flooding in coastal and island communities.

The West Antarctic Ice Sheet could trigger abrupt changes globally if it were to become unstable, and although Greenland is thought to be more stable, the recent calving of glaciers has led to some alarm.

Joughin believes the recent activity is a reason to pay attention, but not to panic.

“We don’t fully understand why it’s accelerating,” Joughin said. “But the longer-term observations we have, the more solid predictions we will be able to make.”

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