March 10th, 2011 by Joshua S Hill
In the longest study to date of changes in polar ice sheet mass, researchers funded by NASA have found that the Greenland and Antarctic ice sheets are losing mass at a drastic and accelerating rate, contrary to the findings in 2007 of the United Nations Intergovernmental Panel on Climate Change.
These findings suggest rather dramatically that the loss of ice into the oceans from ice sheets is soon going to overtake the loss of ice from Earth’s mountain glaciers as the largest contributor to global sea level rise a lot earlier than previous climate models had predicted.
“That ice sheets will dominate future sea level rise is not surprising — they hold a lot more ice mass than mountain glaciers,” said lead author Eric Rignot, jointly of NASA’s Jet Propulsion Laboratory, Pasadena, Calif., and the University of California, Irvine. “What is surprising is this increased contribution by the ice sheets is already happening. If present trends continue, sea level is likely to be significantly higher than levels projected by the United Nations Intergovernmental Panel on Climate Change in 2007. Our study helps reduce uncertainties in near-term projections of sea level rise.”
The study, which is published this month in the journal Geophysical Research Letters, found that the ice polar ice sheets are losing mass at a rapidly accelerating pace. In fact, according to the data, each year over the course of the 18 year study, the planet’s two largest ice sheets lost a combined average of 36.3 gigatonnes more than they did the year before.
The team under Rignot’s leadership combined monthly satellite measurements from 1992 to 2009 with advanced regional atmospheric climate model data to examine the changes in ice sheet mass and the accelerating loss of ice.
The researchers compared two independent measurement techniques, providing a firmer base from which to prove their findings.
The first technique characterized the difference between two different sets of data: “interferometric synthetic aperture radar data from European, Canadian and Japanese satellites and radio echo soundings, which were used to measure ice exiting the ice sheets; and regional atmospheric climate model data from Utrecht University, The Netherlands, used to quantify ice being added to the ice sheets.”
The second technique used eight years of data gathered by the NASA/German Aerospace Center’s Gravity Recovery and Climate Experiment (Grace) satellites which are tasked with tracking the smallest changes in Earth’s gravity field as a result of changes in Earth’s mass distribution, which includes ice movement.
The team found the two separate techniques to be in agreement, both for total amount and rate of mass loss over the eight years in which the two data sets overlapped, validating the data sets and establishing a consistent record of ice mass changes since 1992.
As a result, the team found that over the 18-year study the Greenland ice sheet lost mass faster than it did the year before, by an average of 21.9 gigatonnes each year, while in the southern hemisphere, the Antarctic ice sheet saw a speedup of ice mass loss of 14.5 gigatones each year.
“These are two totally independent techniques, so it is a major achievement that the results agree so well,” said co-author Isabella Velicogna, also jointly with JPL and UC Irvine. “It demonstrates the tremendous progress that’s being made in estimating how much ice the ice sheets are gaining and losing, and in analyzing Grace’s time-variable gravity data.”
Concluding predictions show that if current ice sheet melting rates continue as they are for the next four decades, the total amount of ice mass loss by 2050 will be 15 centimetres, or 5.9 inches. Add this to the 8 centimetres (3.1 inches) predicted to leech into the ocean from the glacial ice caps and 9 centimetres (3.5 inches) from ocean thermal expansion, the total sea level rise by 2050 could be as much as 32 centimetres (12.6 inches).
All of that being said, the authors caution that “considerable uncertainties remain in estimating future ice loss acceleration.”
Source: NASA Jet Propulsion Laboratory
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