Alaska's major glaciers are eroding so rapidly that researchers estimate they are losing 24 cubic miles of ice each year, adding to the steady rise in global sea level.
In a study appearing Friday in the journal Science, University of Alaska researchers said a survey of 67 major glaciers using an airborne laser system found that the rate of melting has hastened over the past five years.
"From the mid-1950s to the mid-1990s, the glaciers lost about 13 cubic miles a year," said Anthony A. Arendt, first author of the study. "In the last five years, that rate has almost doubled."
Over almost a half century, he said, the glaciers have lost some 500 cubic miles of ice.
The new measurements show that the glaciers of Alaska are contributing about half of the water worldwide flowing into the oceans from shrinking mountain glaciers, said Arendt.
Studies have suggested that the global sea level has risen about 7.8 inches over the past 100 years, and some experts say the rate is increasing. Arendt said that would be consistent with what he and his co-authors found in their study.
"The next question is what has been causing this glacier thinning. Is it because there is less snowfall in the winter or are the summers warmer?" said Arendt. "Glacier changes are linked to the climate, so this indicates that something has changed about the Alaskan climate."
Alaska's glaciers grow if they receive more snow in the winter than what melts in the summer. Because the glaciers are shrinking, then one end of this ice equation has changed. Arendt said that more study is needed to find out the causes.
Mark F. Meier, a glacier expert at the University of Colorado, Boulder, said the Alaskan study is an important advance in the efforts of science to understand the global climate.
"For the first time we have some hard data from these glaciers which we have suspected, but didn't know for sure, are major contributors to the sea level change caused by glacier melt," Meier said.
The contribution from Alaska's glaciers to the worldwide sea level rise "is even more that what we had expected," he said.
Although Alaska contains 13 percent of the world's glacier-bound ice, the melt from its glaciers is greater than all the other glacier fields put together, excluding the ice fields in Greenland and Antarctica.
"Greenland is actually contributing less runoff than are these Alaskan glaciers," Meier said. "Greenland is much bigger, but it is much colder."
Experts have attributed sea level rise to two primary effects: runoff from the melting of ancient ice fields, such as the Alaskan glaciers, and an ocean expansion due to warming. Some have attributed the warming of the ocean to a general global trend caused by human action.
Many believe that the burning of fossil fuels is causing an increase in atmospheric carbon dioxide, triggering what is called the greenhouse effect. A higher concentration of CO2 in the atmosphere would trap more of the sun's heat, possibly causing the Earth to warm.