How fast glaciers and ice caps are melting has long been a contentious issue. Previous estimates were based on extrapolations of on-site measurements and aircraft sampling of a relatively few of the earth’s 200,000 glaciers and ice caps. Now, new data from NASA’s GRACE satellites indicate that mountain glaciers and ice caps lost 148 billion tons of ice a year, 30% less than previous estimates. There are two satellites, 135 miles apart, which orbit the earth 16 times a day. As they encounter changes in earth’s mass, they detect changes in earth’s gravity, which causes first one and then the other to change speed and alter the distance between them. The satellites can determine that distance with great precision, which enables the team of NASA scientists to calculate the mass of each feature detected by the satellites. Over seven years, the team has tracked monthly changes in ice mass over all the continents. It has established wide regional differences, with Alaska, the Canadian Artic, Iceland, and Patagonia showing the biggest losses of mass. In a surprise, the loss rates from the Himalyas, Tien Shan and Pamir ranges in Asia were only 10% of previous estimates, because the losses previously attributed to glaciers actually came from agriculture and industrial activity. The new data make no predictions, but could help to validate climate models of glacier activity. The current GRACE satellites have exceeded their 5-year life span; however, they are not scheduled to be replaced until 2016.
GR
