Published online 9 January 2011 | Nature | doi:10.1038/news.2011.3

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IPCC estimates of sea level rise corroborated, but large ice sheets might endure.

New Zealand's mountain ranges could lose up to 85% of their glaciers by 2100. Rob Brown/Minden Pictures/FLPA

In the most comprehensive study of mountain glaciers and small ice caps to date, a team of US and Canadian scientists has projected that most of the world's smaller glaciers will be gone by 2100. The finding confirms that the Intergovernmental Panel on Climate Change (IPCC) — the scientific group assessing climate risk — was correct in estimating that by that date, complete or partial melting of smaller glaciers will contribute about the same amount to sea-level rise as meltwater from the giant ice sheets of Antarctica and Greenland. The study also confirms that the IPCC was wrong in stating that Himalayan glaciers would disappear by 2035.

The study, in Nature Geoscience1, found that half of the world's smallest glaciers, with a surface area less than 5 square kilometres, will disappear entirely, with possible implications for communities dependent on them for water supplies. Overall, the melting of small glaciers and ice caps alone will contribute about 12 centimetres of sea level rise by 2100, based on an average of ten global climate models.

"While the IPCC review process clearly failed in the Himalayan instance," says IPCC member and forest ecologist Steve Running of the University of Montana in Missoula, "the estimate of overall mountain-glacier contribution to sea-level rise was about right."

Earlier studies used by the IPCC had found approximately the same result, but were forced to extrapolate from a relatively small number of glaciers. The new study was based on a total of 120,229 mountain glaciers and 2,638 ice caps.

Although even these represent only 40% of the world's total, it is by far the most comprehensive study on the subject to date, says lead author Valentina Radić, a glaciologist at the University of British Columbia, Vancouver, Canada.

Their research shows that "when we are speaking about sea-level rise, small glaciers are really very important contributors", she says. That's key, she says, because these glaciers are only 1% the size of the ice sheets in Antarctica and Greenland, making them easy to overlook.

Many small melts

Radić and coauthor Regine Hock of the University of Alaska in Fairbanks conducted their study by modelling the effect of climate change on every mapped mountain glacier or ice cap, using a middle-of-the-road IPCC scenario for future emissions of greenhouse gases. They then extrapolated the results to account for the fact that while Earth's total glaciated areas are well mapped, many sections have yet to be divided into individual glaciers.

The projected contribution of each glacier's partial or complete melting to sea level rise ranges from 8.7 cm to 16 cm, depending on the model. The IPCC's estimates for sea level rise by 2100 ranged from 7 to 17 centimetres in its 2007 fourth assessment report.

Glaciologist Ted Scambos of the US National Snow and Ice Data Center in Boulder, Colorado says it is reassuring that the IPCC and the new study have independently reached the same conclusion. "Both could be wrong, but it gives more confidence that both are approximately right," he says.

"The authors clearly invested a lot of effort to get it right," says another IPCC member Richard Alley, a geoscientist at Pennsylvania State University in University Park. Nevertheless, he notes, even finer tuning is possible. "We still don't know the depth distribution of many mountain glaciers," he says — thicker glaciers will take longer to melt away.

And, he says, as glaciers melt, rocky debris can accumulate on their surfaces. "If the debris is thick enough, it slows further melting. The numbers might require a little revision as these processes are understood better."

Himalayan failing

Rather than finding that Himalayan glaciers are on the verge of disappearing, as the IPCC had suggested, Radić and Hock found that, on average, the models predict only a 10–15% decline by 2100. In fact, Radić says, some models predict that these glaciers might even grow slightly.

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That, she says, is because of the prospect of increased snowfall, something that varies from model to model. "These glaciers are very sensitive to changes in snow precipitation."

Among the world's shrinking glaciers, those in the Himalayas seem to be among the robust exceptions. By 2100, Radić says, the European Alps could lose 50–90% of their glacial ice. The Caucasus Mountains could see a 45–90% decline and in New Zealand's ranges the figure could be 60–85%.

The brunt of the demise will occur in regions with small glaciers, Radić says. By 2100, she says, "Most of them will be gone by 2100."

Scambos, however, says the new study suggests that melting glaciers only make a modest contribution to sea-level rise. "It's not something that should alarm people, all by itself," he says. Of much greater concern, he says, is how warming will affect the Antarctic and Greenlandic ice sheets. "They are the things that could really push sea level rise to levels that could be difficult to adapt to."