Alaska's glaciers are melting so badly that they are raising the world's overall sea level more than any other single source, scientists said on Friday.
The melting glaciers are responsible for at least 9 percent of the rise in the world's sea level over the past century -- adding more than one-tenth of a millimeter (.04 inch) each year to overall sea level, the researchers said.
That is far more than anyone thought, and more than what was produced by what was until now believed to be the biggest single known source -- the melting Greenland ice sheet.
"Most glaciers have thinned several hundred feet at low elevations in the last 40 years and about 60 feet at higher elevations," Keith Echelmeyer, a glaciologist at the University of Alaska Fairbanks Geophysical Institute who led the study, said in a statement.
"Data from our study indicates that Alaska glaciers are contributing the most to the sea level rise that has been measured," said Anthony Arendt, a graduate student listed as first author on the study.
The team of researchers used a laser altimetry system rigged up to a small airplane, which Echelmeyer flew over 67 of Alaska's mountain glaciers. They compared their readings to measurements taken by the U.S. Geological Survey in the 1950s.
Writing in the journal Science, they said the glaciers had lost, on average, more than half a meter a year in height, or more than a foot and a half.
ENOUGH WATER TO RAISE SEA LEVEL
This added up to a lot of water -- enough to raise the overall level of the world's oceans measurably.
"(One-tenth of a millimeter) seems like small amount but that can cause a fair amount of transgression of water onto an area where people live near coastal regions," Arendt said -- especially if the shoreline is flat.
Many people in countries such as Bangladesh and some island nations live along flat coastlines and estuaries that can be severely affected by small changes in sea level.
In 2001 Echelmeyer's team flew over the same glaciers they had measured in the early 1990s, and to their amazement found that the glaciers were thinning at double the rates of 40 years before.
Echelmeyer, Arendt and colleagues said they could not go so far as to blame the melting on global warming.
"We can't really make that link," Arendt said. "It's not really our job. But (global warming) is consistent with other things going on, with increasing rates of warming in other parts of the world," he added.
"Certainly the fact that the thinning rate of the glaciers has doubled in the past 10 years indicates that something is going on in Alaska -- warmer summers or less precipitation in winter."
There is, however, plenty more water locked up in these glaciers, the researchers noted. "Glaciers in Alaska and neighboring Canada cover 90,000 square kilometers, or about 13 percent of the mountain glacier area on Earth, and include some of the largest ice masses outside of Greenland and Antarctica," they wrote.
Some Antarctic glaciers are also melting, as are some ice sheets, but parts of the continent are adding ice, so the picture there is complicated.