Higher temperatures over the past two decades have caused the polar ice sheets to melt at an accelerating rate, contributing to an almost half-inch rise in global sea levels, according to the most comprehensive study done so far.
Scientists long have struggled to get a fix on whether the permanent ice sheets of Greenland and Antarctica are gaining or losing ice. Past satellite-based measurements either were limited in scope or suffered from methodological inconsistencies.
The new study, published Thursday in the journal Science, estimates that the melting of the ice sheets as a whole has raised global sea levels by 11.1 millimeters (0.43 inch) since 1992. That represents one-fifth of the total sea-level increase recorded in that period.
View Slideshow
Ian Joughin/University of WashingtonThe midnight sun casts a glow on an iceberg in Disko Bay, Greenland.
In the 1990s, melting of the polar ice sheets was responsible for about 10% of the global sea-level rise, but now it represents about 30%, the data suggest.
Higher temperatures can raise sea levels in several ways. Some estimates suggest that roughly half of the increase relates to the thermal expansion of the oceans: as the water warms, it becomes less dense and expands.
Another source is the runoff from melting glaciers. A third is the increased melting of the ice sheets of Antarctica and Greenland.
Greenland in particular has seen a greater melting of its permanent ice. One reason is that Northern Hemisphere ocean currents are warmer, which leads to more vigorous melting.
In addition, the air temperature in Greenland is much warmer than that in the Antarctic, so a rise in temperature in Greenland has a more profound effect.
Enlarge Image
Close"If you extrapolate these results, Greenland is going to be a serious contributor to global sea-level rise" in coming years, said Peter Wadhams, a professor of ocean physics at the University of Cambridge, England, who wasn't involved in the Science study. "Its contribution, relative to other sources, is becoming greater and greater," he said.
The issue of rising sea levels has gained more attention in recent months. The destructive flooding caused by the storm Sandy, which struck the U.S. East Coast in late October, revived anxieties about rising ocean levels in heavily populated coastal regions.
To assess the contribution from melting ice sheets, scientists try to measure "mass balance," which is the difference between the annual snow that falls on the permanent ice sheets each year, and the total mass of ice that melts or breaks off the sheets.
It is an extremely complex measurement, because there are so many factors at work: shifting ocean currents, the dynamics and movement of large ice shelves, and the varying temperature and saltiness of water at different places.
Consequently, researchers don't yet know exactly how much of the ice-sheet melt is caused by a warming atmosphere and how much by a warming ocean.
The last major assessment of mass balance was published in a 2007 report on climate change issued by the United Nations. But those findings were based on limited observations, and many scientists considered them to underestimate the melting.
The latest effort reconciles the differences among dozens of earlier measurements and includes new data to compile an estimate that is believed to be twice as accurate as previous ones, according to researchers involved.
"It allows us to make some firm conclusions," said Andrew Shepherd, a professor of earth observation at the University of Leeds in England and a lead author of the study. "It wasn't clear if Antarctica was gaining or losing ice. Now we can say with confidence it is losing ice."
The 2007 U.N. report, published by the Intergovernmental Panel on Climate Change, suggested that similar amounts of ice were being lost at both the polar regions. By contrast, the new study concludes that two-thirds of the ice loss was in Greenland and the remainder in Antarctica.
The latest findings show that the rate of ice loss in Greenland has increased almost fivefold since the mid-1990s, while Antarctica overall has been losing relatively small amounts of ice at a more or less constant rate.
"Antarctica is so cold that even if warming occurs it won't melt" at the rate seen in Greenland, said Ian Joughin, a glaciologist at the University of Washington in Seattle and a co-author of the new paper.
One tricky question is whether the overall accelerated melting of the ice sheets can be linked to man-made climate change.
The shrinkage of the permanent ice sheets can't entirely be explained by any of the decadeslong or centurylong natural shifts in climate cycles, according to Prof. Shepherd.
Scientists note that current climate-change models predict that some parts of the Antarctic ice sheet will grow while other parts will shrink, and that parts of the Greenland ice also will melt. Observations have borne out these projections so far.
"The signals suggest there is no immediate threat" from rising sea levels, Prof. Shepherd said. "But we can at least warn people that there are instabilities that need to be investigated."
The study involved 26 laboratories and was supported by the European Space Agency and the U.S. National Aeronautics and Space Administration. The data used for the study were based on measurements from 10 separate satellite missions.
Write to Gautam Naik at gautam.naik@wsj.com
No comments:
Post a Comment