Greenland and Antarctica are losing ice at a faster and faster rate, according to a new study that has tracked the rate of melting in two independent ways. At this rate, melting ice sheets could dominate sea level rise in the 21st century.

The most recent report by the Intergovernmental Panel on Climate Change suggested that sea levels could rise by 18 centimetres to 59 cm by 2100 – but that estimate didn’t take the Greenland and Antarctic ice sheets into account.

The new study, by Eric Rignot of the University of California, Irvine, and colleagues, could make things clearer. The team studied changes in the two ice sheets between 2002 and 2010 using two techniques. First they used data from the NASA GRACE satellite, which directly estimates the changes in the ice mass by measuring Earth’s gravity field over Greenland and Antarctica. The gravity field is influenced by changes in ice mass.

Next, they used a mass balance approach, which involves taking monthly measurements of glacier movement and ice thickness and plugging them into a regional climate model to estimate the net accumulation of snow and ice.


Fast ice loss

Both techniques broadly agreed on the quantity of ice being lost. They both confirmed, for example, that in 2006 the ice sheets of Greenland and Antarctica together lost a total of 475 gigatonnes. The two data sets also agree that the rate of mass loss from the ice sheets is increasing at about 36 gigatonnes per year – three times faster than the rate of mass loss from mountain glaciers and ice caps.

Because the two data sets agree both methods are validated. Measurements were taken using the mass balance approach for several years before the launch of GRACE in 2002, and indicate that melting has been on the increase for at least two decades.

“The mass loss is accelerating and it appears to have been accelerating for a while,” says Isabella Velicogna of University of California at Irvine, a member of the research team.

If the ice sheets continue to melt faster and faster, sea levels could rise 56 centimetres by 2100, “which is much bigger than what we were thinking just three years ago”, says Velicogna. As such, ice sheets would become the dominant contributor to sea level rise this century.

While the study cannot separate out the effects of climate change from natural variability, the trend is worrying. “It fits into a pattern that we’d expect of ice sheet response to climate change,” says glaciologist Ian Howat of the Ohio State University in Columbus. “Whatever processes are driving the mass loss are intensifying. The picture is getting worse, not better.”

Journal reference: Geophysical Research Letters, DOI:10.1029/2011GL046583