US scientists have found that glaciers in Alaska are retreating much faster than originally thought.

The researchers say the resulting melt waters are sufficiently large to drive up global sea levels by 0.14 millimetres per year.

Over the last five to 10 years there has been an acceleration

Dr Keith Echelmeyer

The study by Dr Keith Echelmeyer, of the University of Alaska Fairbanks Geophysical Institute, and colleagues used laser altimetry to measure the volume changes of 67 Alaskan glaciers from the mid-1950s to the-mid 1990s.

Their work, published in the journal Science, adds to the growing evidence that the level of recent glacier wastage - from polar regions to the tropics - has been underestimated.

Short of data

"There is some historical evidence that at the turn of the last century glaciers were thinning but not so that people noticed it much," Dr Keith Echelmeyer told the BBC.

Malaspina Glacier is losing over 2.7 cubic km of water per year

"What we see over the last 50 years is that they have thinned quite substantially and then over the last five to 10 years there has been an acceleration."

Scientists who suspect human activities such as the burning of fossil fuels are causing an unnatural global temperature rise believe glacier wastage may be a good indicator of what is happening.

But Dr Echelmeyer is hesitant to say the recent changes his team have seen are the result of a warmer climate because he feels there is currently insufficient data to come to firm conclusions.

Greatest loss

"Climate is changing and this is affecting the glaciers - and they are being a good indicator of that," he said.

"Now, whether it's warming up of the climate or less snowfall, it is hard to say. That will take further investigation and an analysis of glacier flow, for example."

Glaciers in Alaska and neighbouring Canada cover 90 thousand square kilometres, or approximately 13% of the mountain glacier area on Earth.

Dr Echelmeyer's team surveyed the volume and area changes of part of this region from an aircraft equipped with a laser altimetry system. The researchers measured the volume loss by checking glacier elevation and volume data on US Geological Survey maps from the 1950s.

"Most glaciers have thinned several hundred feet at low elevations in the last 40 years and about 60 feet at higher elevations," Dr Echelmeyer said.

Higher levels

The team has calculated that Alaskan glaciers are responsible for at least 9% of the global sea-level rise during the past century, and Alaska's glaciers raise the level of Earth's oceans by more than one-tenth of a millimetre each year.

The study fits with a review of data by Professor Meier and Mark Dyurgerov, of the University of Colorado at Boulder, US.

They said glacier wastage had been seriously underestimated by the UN's Intergovernmental Panel on Climate Change, which is trying to assess humanity's influence on the global climate.

One of the reasons for this, they said, was that the IPCC had not had access to the latest Alaskan data.

"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," Professor Meier said after the Fairbanks study was published.

The contribution from Alaska's glaciers to the worldwide sea level rise "is even more that what we had expected," he added.

Currently, measured sea levels are going up by about 0.8 millimetres per year with no apparent acceleration in that increase.