Glaciers and Ice Caps Quickly Melting Into the Seas
BOULDER, Colorado, July 20, 2007 (ENS) - Sea level rise this century may be greater than previously thought, posing risks to hundreds of millions of people who live close to the world's oceans, concludes a new study of ice loss from glaciers and ice caps. The researchers say that in the near future, the giant Greenland and Antarctic ice sheets will contribute less to sea level rise than glaciers and ice caps.
Scientists with the University of Colorado-Boulder's Institute of Arctic and Alpine Research, INSTAAR, and the Russian Academy of Sciences conclude that glaciers and ice caps now contribute about 60 percent of the ice melting into the oceans and the rate has been accelerating over the past decade.
"One reason for this study is the widely held view that the Greenland and Antarctic ice sheets will be the principal causes of sea-level rise," says lead author Emeritus Professor Mark Meier, former INSTAAR director and CU-Boulder professor in geological sciences.
"But we show that it is the glaciers and ice caps, not the two large ice sheets, that will be the big players in sea rise for at least the next few generations, he says.
Alaska's Columbia glacier is melting into Prince William Sound. (Photo by Ken Tape)
The Columbia Glacier has thinned up to 1,300 feet in places. It has shrunk by about nine miles since 1980 and is expected to shrink by another nine miles in the next two decades.
The team estimates the accelerating melt of glaciers and ice caps could add from four inches to 9.5 inches of additional sea level rise globally by 2100.
This does not include the expansion of warming ocean water, which could potentially double those numbers.
A one foot rise in sea level rise typically causes a shoreline retreat of 100 feet or more, and about 100 million people now live within about three feet of the world's shorelines.
Anderson says that although the volume of ice locked up in Greenland is equal to roughly 23 feet in sea level rise, only a small fraction of that amount is likely to be "pulled out" during the next century, most of it through outlet glaciers.
The glaciers and ice caps are presently contributing about 100 cubic miles of ice annually to sea level rise - a volume nearly equal to the water in Lake Erie. This volume is rising by about three cubic miles per year, the study shows.
By contrast, the CU-Boulder team estimated Greenland is now contributing about 28 percent of the total global sea rise from ice loss and Antarctica is contributing about 12 percent.
Greenland is not expected to contribute as much to sea level rise as glaciers and ice caps until the end of the century.
The accelerating contribution of glaciers and ice caps is due in part to rapid changes in the flow of tidewater glaciers that discharge icebergs directly into the ocean, says Anderson.
Many tidewater glaciers are undergoing rapid thinning, stretching and retreat, which causes them to speed up and deliver increased amounts of ice into the world's oceans, he says.
Water controls how rapidly glaciers slide along their beds, he explains. When a glacier with its "toe in the water" thins, a larger fraction of its weight is supported by water and it slides faster and calves more ice into the ocean at the glacier terminus.
"While this is a dynamic, complex process and does not seem to be a direct result of climate warming, it is likely that climate acts as a trigger to set off this dramatic response," he says.
Oceanside cities such as San Diego, California are most at risk from rising sea levels. (Photo courtesy U.S. Coast Guard)
Meier estimated there are several hundred thousand small glaciers and small, pancake-shaped ice caps in polar and temperate regions. They range from modest, high mountain glaciers to huge glaciers like the Bering Glacier in Alaska, which measures about 5,000 square miles in area and is nearly half a mile thick in places.
The researchers used a mathematical "scaling" process to estimate more remote glacier volumes, thicknesses and trends by factoring in data like altitude, climate and geography. They used data gathered from around the world, including cold regions in Russia, Europe, China, Central Asia, Canada and South America.
While warming temperatures will likely cause many small high mountain glaciers in North America and Europe to disappear by the end of the century, large ice fields and ice caps will continue to produce large amounts of melt water, Meier says.
The scientists also believe many "cold" polar glaciers and ice caps will soon warm up enough to begin melting and contributing to sea rise.
The retreat of the Greenland and Antarctic ice sheets also is giving birth to new, smaller glaciers that are prime candidates for study by scientists.
"It is incorrect to assume that the small glaciers will simply go away next century," says Anderson. "They will continue to play a key role in the sea level story."
Many smaller "benchmark" glaciers around the world that have been under study for decades are expected to disappear by the end of the century, said Anderson.
He says scientists need to start gathering benchmark information on some of the larger glaciers that are unlikely to disappear, for a long-term record of their behavior.
"Since the world is becoming increasingly aware that sea-level rise is a very real problem," he said, "we need to acknowledge the role of all of the ice masses and understand the physical mechanisms by which they deliver water to the sea."
Co-authors include CU-Boulder INSTAAR researchers Mark Dyurgerov, Ursula Rick, Shad O'Neel, Tad Pfeffer, Robert Anderson and Suzanne Anderson, as well as Russian Academy of Sciences scientist Andrey Glazovsky.
Funded by NASA and the National Science Foundation, the research appears in the July 19 issue of "Science Express," the online edition of the journal "Science."
"At the very least," wrote the researchers in "Science Express," "our projections indicate that future sea level rise may be larger than anticipated, and that the component due to glaciers and ice caps will continue to be substantial."
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