Monday, 24 December 2012

Global warming


West Antarctica Warming Twice As Fast As Previously Believed: Study
West Antarctica is warming almost twice as fast as previously believed, adding to worries of a thaw that would add to sea level rise from San Francisco to Shanghai, a study showed on Sunday.


23 December, 2012

Annual average temperatures at the Byrd research station in West Antarctica had risen 2.4 degrees Celsius (4.3F) since the 1950s, one of the fastest gains on the planet and three times the global average in a changing climate, it said.

The unexpectedly big increase adds to fears the ice sheet is vulnerable to thawing. West Antarctica holds enough ice to raise world sea levels by at least 3.3 metres (11 feet) if it ever all melted, a process that would take centuries.

"The western part of the ice sheet is experiencing nearly twice as much warming as previously thought," Ohio State University said in a statement of the study led by its geography professor David Bromwich.

The warming "raises further concerns about the future contribution of Antarctica to sea level rise," it said. Higher summer temperatures raised risks of a surface melt of ice and snow even though most of Antarctica is in a year-round deep freeze.

Low-lying nations from Bangladesh to Tuvalu are especially vulnerable to sea level rise, as are coastal cities from London to Buenos Aires. Sea levels have risen by about 20 cms (8 inches) in the past century.


The United Nations panel of climate experts projects that sea levels will rise by between 18 and 59 cms (7-24 inches) this century, and by more if a thaw of Greenland and Antarctica accelerates, due to global warming caused by human activities.

GLACIERS

The rise in temperatures in the remote region was comparable to that on the Antarctic Peninsula to the north, which snakes up towards South America, according to the U.S.-based experts writing in the journal Nature Geoscience.

Parts of the northern hemisphere have also warmed at similarly fast rates.

Several ice shelves - thick ice floating on the ocean and linked to land - have collapsed around the Antarctic Peninsula in recent years. Once ice shelves break up, glaciers pent up behind them can slide faster into the sea, raising water levels.

"The stakes would be much higher if a similar event occurred to an ice shelf restraining one of the enormous West Antarctic ice sheet glaciers," said Andrew Monaghan, a co-author at the U.S. National Center for Atmospheric Research.

The Pine Island glacier off West Antarctica, for instance, brings as much water to the ocean as the Rhine river in Europe.

The scientists said there had been one instance of a widespread surface melt of West Antarctica, in 2005. "A continued rise in summer temperatures could lead to more frequent and extensive episodes of surface melting," they wrote.

West Antarctica now contributes about 0.3 mm a year to sea level rise, less than Greenland's 0.7 mm, Ohio State University said. The bigger East Antarctic ice sheet is less vulnerable to a thaw.

Helped by computer simulations, the scientists reconstructed a record of temperatures stretching back to 1958 at Byrd, where about a third of the measurements were missing, sometimes because of power failures in the long Antarctic winters.



Ice Sheet Loss at Both Poles Increasing, Study Finds


29 November, 2012



PASADENA, Calif. - An international team of experts supported by NASA and the European Space Agency (ESA) has combined data from multiple satellites and aircraft to produce the most comprehensive and accurate assessment to date of ice sheet losses in Greenland and Antarctica and their contributions to sea level rise.

In a landmark study published Thursday in the journal Science, 47 researchers from 26 laboratories report the combined rate of melting for the ice sheets covering Greenland and Antarctica has increased during the last 20 years. Together, these ice sheets are losing more than three times as much ice each year (equivalent to sea level rise of 0.04 inches or 0.95 millimeters) as they were in the 1990s (equivalent to 0.01 inches or 0.27 millimeters). About two-thirds of the loss is coming from Greenland, with the rest from Antarctica.

This rate of ice sheet losses falls within the range reported in 2007 by the Intergovernmental Panel on Climate Change (IPCC). The spread of estimates in the 2007 IPCC report was so broad, however, it was not clear whether Antarctica was growing or shrinking. The new estimates, which are more than twice as accurate because of the inclusion of more satellite data, confirm both Antarctica and Greenland are losing ice. Combined, melting of these ice sheets contributed 0.44 inches (11.1 millimeters) to global sea levels since 1992. This accounts for one-fifth of all sea level rise over the 20-year survey period. The remainder is caused by the thermal expansion of the warming ocean, melting of mountain glaciers and small Arctic ice caps, and groundwater mining.

The study was produced by an international collaboration -- the Ice Sheet Mass Balance Inter-comparison Exercise (IMBIE) -- that combined observations from 10 satellite missions to develop the first consistent measurement of polar ice sheet changes. The researchers reconciled differences among dozens of earlier ice sheet studies by carefully matching observation periods and survey areas. They also combined measurements collected by different types of satellite sensors, such as ESA's radar missions; NASA's Ice, Cloud and land Elevation Satellite (ICESat); and the NASA/German Aerospace Center's Gravity Recovery and Climate Experiment (GRACE).

"What is unique about this effort is that it brought together the key scientists and all of the different methods to estimate ice loss," said Tom Wagner, NASA's cryosphere program manager in Washington. "It's a major challenge they undertook, involving cutting-edge, difficult research to produce the most rigorous and detailed estimates of ice loss from Greenland and Antarctica to date. The results of this study will be invaluable in informing the IPCC as it completes the writing of its Fifth Assessment Report over the next year."

Professor Andrew Shepherd of the University of Leeds in the United Kingdom coordinated the study, along with research scientist Erik Ivins of NASA's Jet Propulsion Laboratory in Pasadena, Calif. Shepherd said the venture's success is because of the cooperation of the international scientific community and the precision of various satellite sensors from multiple space agencies.

"Without these efforts, we would not be in a position to tell people with confidence how Earth's ice sheets have changed, and to end the uncertainty that has existed for many years," Shepherd said.

The study found variations in the pace of ice sheet change in Antarctica and Greenland.

"Both ice sheets appear to be losing more ice now than 20 years ago, but the pace of ice loss from Greenland is extraordinary, with nearly a five-fold increase since the mid-1990s," Ivins said. "In contrast, the overall loss of ice in Antarctica has remained fairly constant, with the data suggesting a 50-percent increase in Antarctic ice loss during the last decade."

For more on ICESat, visit: 
http://icesat.gsfc.nasa.gov . For more on GRACE, visit: http://www.csr.utexas.edu/grace .

The California Institute of Technology in Pasadena manages JPL for NASA.

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