World's glaciers 'out of balance'

McCall Glacier, N Alaska The retreat of McCall Glacier in North Alaska. The left panel is 1958; the right panel is 2003

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Earth's glaciers are seriously out of balance with the global climate and are already on their way to losing almost 40% of their volume.

That is the assessment of scientists after studying a representative group of 144 small and large glaciers around the world.

Their figure assumes no further warming of the climate.

However, if temperatures continue to rise as models predict, the wastage will be even higher, the team says.

"When we look at the data, we can see that the glaciers are out of balance, meaning the climate has actually changed faster than the changes we've seen in ice area and volume," explained Sebastian Mernild from Los Alamos National Laboratory, New Mexico, US.

"Our data suggests the glaciers will commit about 30% of their area and about 38% of their volume to global sea level rise."

Dr Mernild's group calculates this figure to be on the order of 22cm.

"This will happen in the next decades to centuries," he told BBC News.

Dr Mernild, from the Climate, Ocean, and Sea Ice Modeling (COSIM) Group at Los Alamos, was describing his team's work here at the European Geosciences Union (EGU) meeting in Vienna, Austria.

A glacier is in balance when the snow at higher, colder elevations equals the volume of snow and ice lost through melting at lower, warmer elevations.

If precipitation is greater, the glacier will increase its mass; if melting dominates, the glacier will thin and retreat until it reaches a state of equilibrium again.

Dr Mernild's team says its assessment of the glacier sample indicates climate conditions have changed so fast that many ice bodies have not yet had time to fully adjust to their new equilibrium position.

Glacier de la Meije (Ecrins massif) Alpine glaciers are said to be more out of balance than the global average

This means a certain amount of mass loss is already locked into the system even if there is no further warming.

"Glaciers will move up in the terrain, they will become smaller and thinner and they will adjust to the climate conditions.

"On the other hand, we expect the climate will warm continuously in the future, meaning that the glaciers will become even more out of balance, and that means the glaciers will commit even more volume to sea level rise."

If the models are correct and further warming is seen during the next several decades and longer, the study projects that the Earth's glaciers could ultimately lose more than half their mass.

The picture is described as regional, with some areas said to be more out of balance than others.

Dr Mernild cites the Alps as one glacier group that is farther from balance than the global average.

He says alpine glaciers are likely to lose most of their mass by 2100.

"But if you take into account the volume of ice in the glaciers here in the Alps, it won't have the same impact on the global sea level rise compared to if we see the same out of balance conditions in other places on the globe where we know there is more ice located.

"So, the contribution to sea level rise will not be that big from the Alps region."

The COSIM assumption is that there is a sea level equivalent of 60cm locked away in all the world's glaciers (the number excludes the ice sheets of Antarctica and Greenland).

That figure has been debated here at EGU. Dr Matthias Huss, from the University of Fribourg, Switzerland, presented new data suggesting there was only 48cm of sea level tied up in all the world's glaciers (and the larger glaciers referred to as ice caps).

"Our number is 192,000 cu km. This is about 25% less than some previous estimates. It is a total potential sea-level rise of 0.48m," Dr Huss told BBC News.

"We're using a physically based approach. Until now, people have relied on simple statistical methods."

The Fribourg researcher said his team's work captured much better the range of thicknesses in the world's glaciers, of which there are estimated to be about 160,000.

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter

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