Washington ? An estimated 24 cubic miles of ice are disappearing annually from Alaskan glaciers, turning some imposing ice mountains into minor hills and adding to the steady rise in global sea level, a study shows.

Researchers at the University of Alaska surveyed 67 major glaciers using an airborne laser system and found that the rate of melting in the last five years was rapidly growing.

“From the mid-1950s to the mid-1990s, the glaciers lost about 52 cubic kilometers (13 cubic miles) a year,” said Anthony A. Arendt, first author of the study appearing in the journal Science. “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 last 100 years, and some experts say the rate is increasing. Arendt said that would be consistent with what he and his co-authors have found in their study of the Alaskan glaciers.

“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.”

Many believe that the burning of fossil fuels is causing an increase in atmospheric carbon dioxide, triggering what is called the greenhouse effect.