By Tim Radford

By the century's end, some of Greenland's ice will have vanished forever.

New research shows that the coastal glaciers and ice caps are melting faster than ever before and may have already reached the point of no return two decades ago. That is because they have passed the stage at which they can refreeze their own meltwater.

These peripheral glaciers and icecaps cover an estimated 100,000 square kilometers of the island. And when they have gone, the world's oceans will have risen by four centimeters.

Body of Greenland ice

But scientists reporting in Nature Communications journal said most of the Greenland ice—the biggest body of ice in the northern hemisphere—is still safe. Were all of its ice to melt, sea levels would rise by at least seven meters.

"Higher altitudes are colder, so the highest ice caps are still relatively healthy at the moment," said study leader Brice Noël, a PhD student of polar glaciology and Arctic climate modeling at the University of Utrecht in the Netherlands.

"However, we see melting occur higher and higher. That's a big problem, because that 'melting line' is moving towards the altitude where most of the ice mass is.

"The main ice sheet in the interior of Greenland is much more elevated and isn't doing too bad yet. But we can already see an increase in the altitude of the 'melting line' there as well."

The coastal research concentrated on the mechanics of ice loss. Normally, glaciers and ice caps grow because summer meltwater drains through into the deeper frozen snow and freezes again. The icecap retains its mass and even increases.

But 20 years ago, the firn, or older snow, became saturated, freezing right through, and more summer meltwater now runs to the sea. The rate of increase varies from 17 to 74 percent and the icecaps each year are losing three times the mass loss measured in 1997.

Concern about Greenland ice and glaciers being in retreat is not new. In fact, glaciers in both hemispheres are observed to be in retreat, and the Geological Society of America has just published telltale imagery and an analysis based on observations of more than 5,200 glaciers in 19 regions around the world, showing that the loss of ice mass this century is without precedent.

So Greenland's glaciers are just part of a bigger picture. But since Greenland is home to the second largest volume of ice on the planet, what happens there concerns the entire world.

Testimony to climate change

Researchers observed years ago that the rivers of Greenland ice are in spate and rates of melting are thought likely to accelerate. The latest report is another piece of testimony to climate change in the far north.

"These peripheral glaciers and ice caps can be thought of as colonies of ice that are in rapid decline, many of which will likely disappear in the near future," said Ian Howat, a glaciologist at Ohio State University in the U.S. and a co-author of the report.

"In that sense, you could say that they're 'doomed.' However, the ice sheet itself is still not 'doomed' in the same way. The vast interior ice sheet is more climatologically isolated than the surrounding glaciers and ice caps.

"Also, since this 'tipping point' was reached in the late 1990s before warming really took off, it indicates that these peripheral glaciers are very sensitive and, potentially, ephemeral relative to the timescales of response of the ice sheet."