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Wednesday, Feb. 22, 2012

Amazing GRACE can measure world's ice loss


SINGAPORE — One of the main climate change concerns for Japan and other Asian countries with valuable and densely-populated low-lying coastal land is how much of their land may be threatened by rising sea levels and storm surges as the century advances.

Humans burning fossil fuels and clearing forests are pumping huge amounts of carbon dioxide and other global warming gases into the atmosphere. The vast majority of climate scientists say this is warming the planet, an effect that is most pronounced in the polar regions where the bulk of the world's ice is located.

A warmer Earth leads to warmer oceans which expand. Melting land-ice that flows into the sea adds to the rise, potentially causing huge property and food production losses as well as forcing millions of coastal residents to move inland.

Since the start of the industrial revolution over 200 years ago, the measured rise in the average global land and sea surface temperature has been close to 1 C, with the most pronounced increases in recent decades. The panel of scientists advising the United Nations on climate change has warned that a warmer climate, even by just a couple of degrees Celsius, is likely to trigger more extreme and erratic weather, from droughts to floods and storms.

Most of the relatively small sea level rise recorded so far has come from thermal expansion of the ocean, and the melting of glaciers and ice in Asian mountain ranges and other high places on land, not from the great ice sheets covering Antarctica and Greenland. If they were to melt, the average sea level would rise by as much as 74 meters, with up to 67 meters coming from Antarctica alone.

Until recently, estimates of ice melting in Antarctica, Greenland and elsewhere have had to be made using ground measurements from relatively few glaciers to infer what all these slow moving rivers of ice are doing. Only a few hundred of approximately 200,000 glaciers worldwide have been monitored for longer than a decade.

A series of satellite measurements has changed this. Earlier this month, a team of U.S. and French researchers published the first comprehensive satellite survey of melting ice, and its contribution to global sea level rise. They found that total sea level rise from land-based ice, including Antarctica and Greenland, was about 1.5 mm per year, or about 12 mm in the eight years to 2010. If the ice continues to melt and flow into the ocean at this rate for the rest of the century, the average sea level will rise a further 135 mm by 2100.

But John Wahr, a physics professor at the University of Colorado Boulder and coleader of the study, said that the sea rise amount in his team's survey did not include expansion of water due to warming, which is the other key sea rise component.

The U.N. scientific panel reported in 2007 that the global sea level had risen since 1961 at an average rate of 1.8 mm per year, and since 1993 at 3.1 mm per year, with contributions from thermal expansion, melting glaciers and ice caps, and the polar ice sheets. After feeding anticipated increases in temperature and other projected changes into computer models, some scientists since then have warned that sea level rise could amount to a metre or more by 2100.

The volumes involved in the ice melting recorded by Wahr and his team are huge. The total global ice mass lost in the study period from 2003 — 2010 was about 4.2 trillion tons, enough to cover all of the United States with 0.5 metres of ice. The U.S. is the world's fourth biggest country by land mass, after Russia, Canada and China.

About 25 percent of the average annual ice loss, amounting to 148 billion tons, came from glaciers and ice caps outside Antarctica and Greenland. Ice loss from Antarctica, Greenland and their peripheral ice caps and glaciers averaged 385 billion tons per year.

The measurements were taken by sensors on twin Gravity Recovery and Climate Experiment (GRACE) satellites. Launched in 2002, the U.S.-German satellites orbit the Earth 16 times a day sensing subtle variations in its mass and gravitational pull.

"Earth is losing a huge amount of ice to the ocean annually, and these new results will help us answer important questions in terms of both sea rise and how the planet's cold regions are responding to global change, " Wahr said. "The strength of GRACE is it sees all the mass in the system, even though its resolution is not high enough to allow us to determine separate contributions from each individual glacier."

The good news for Asia from the survey is that some previous estimates of mountain glacier ice loss from ranges such as the Himalayas, Pamirs and Tien Shan appear to have been very exaggerated. These glaciers are vital because they feed into many of Asia's great rivers, providing water for humans, animals, crops and industry.

The GRACE measurements showed ice melting on Asian mountains at a rate of 4 billion tons annually, far below some previous ground-based estimates of up to 55 billion tons per year. A possible reason for this massive discrepancy is that previous estimates were based on measurements taken primarily from some of the lower, more accessible glaciers in Asia and extrapolated to infer the behavior of higher glaciers that are less affected so far by global warming.

Now that a system for a comprehensive satellite survey of ice loss is in place, the next challenge for climate scientists is to better predict how much sea levels will rise this century and beyond. What is still not clear is whether the recorded ice melt rates may increase or decrease, and how rapidly glaciers and polar ice sheets may change in coming decades. Asia has a big stake in the answers.

Michael Richardson is a visiting senior research fellow at the Institute of South East Asian Studies in Singapore.


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