Study Finds Ice Sheets Becoming Dominant Contributor to Sea Level Rise

By Michael D. Lemonick

A tidewater glacier in Greenland, pictured in 2008. Credit: Michael Lemonick.

About 110,000 years ago, global sea level began to drop as the planet cooled, and evaporating seawater was transformed into massive ice sheets that covered large parts of the Northern Hemisphere. About 10,000 years ago, the Earth warmed up again. The ice retreated dramatically, and sea level rose. Since then, the planet’s ice, and the level of the ocean have been more or less stable.

Not any more, though. Thanks largely to human-generated greenhouse gases, the ice that remains in mountain glaciers and ice caps — and more significantly, in the massive ice sheets that smother Antarctica and Greenland under frigid blankets up to two miles thick in places — is moving to the sea once again. Just how high and how fast global sea level will rise as a result is still uncertain, though. One big reason: scientists haven’t been able to get a firm handle on how ice melting has already changed as a consequence of the warming that’s already taken place.

A new paper in the journal Geophysical Research Letters is sure to help, though. Using two different measurement techniques, a team of geophysicists from the U.S. and Netherlands has shown that the ice in Antarctica and Greenland is not only vanishing into the sea: the rate of disappearance has been accelerating over an 18-year period, with about 36.3 billion metric tons more ice lost each year compared to the year before.

By 2006, a year in which a total of about 475 billion metric tons of ice were lost, the acceleration in ice mass loss from the ice sheets had already surpassed acceleration in ice mass loss from mountain glaciers and ice caps — and that lead is likely to grow over the coming century, the study indicates, to the point where ice sheets will be “the dominant contributor to sea level rise in the 21st century.”

What makes this study so important, says co-author Isabella Velicogna, of the University of California, Irvine and NASA’s Jet Propulsion Laboratory in Pasadena, is that the ice loss was measured in two entirely independent ways. The first involved the Gravity Recovery and Climate Experiment, or GRACE. It’s a pair of satellites that measure the local gravity at every spot on Earth. In both Greenland and Antarctica, the ice generates some of that gravity — and as the ice melts in response to warming air and ocean temperatures, the gravity diminishes. “GRACE basically weighs the ice every 30 days,” says Velicogna, “and sees how much it’s changing.”

How Do We Know: Greenland’s Melting Ice Sheet from Climate Central on Vimeo.

The second technique looks at the ice sheets in the same way you’d look at your bank account. The deposits, in the form of snowfall, are calculated using a combination of observations and models that estimate annual precipitation. The withdrawals — physical shrinkage of the ice as it melts and as tidewater glaciers dump icebergs into the ocean — are measured with satellite-mounted radars. If you withdraw money faster than you deposit it, your bank balance shrinks. Similarly, if the shrinkage of the ice outpaces the growth from precipitation, the so-called “mass-balance” shrinks.

“There’s very solid agreement between the two [measurements],” says lead author Eric Rignot, also of UC-Irvine and NASA. And while the GRACE satellites have only been orbiting since 2000, that agreement gives the scientists confidence that the mass-balance estimates, which go back some 18 years, are reliable throughout that whole period.

What that means for sea-level rise over the coming century, however, is still unclear. However, in light of this study and other recent findings, the projections in the 2007 U.N. Intergovernmental Panel on Climate Change (IPCC) report of up to a half-meter of sea-level rise by 2100 may be too low. “If you take our 18 years of good records,” says Rignot, “and extend them forward, you’re going to get to a meter easily. Beyond that, it’s difficult to say.”

“It all depends,” says Rignot, “on whether the ice loss continues at this rate, or slows, or accelerates.”

Nevertheless, says Velicogna, “I personally don’t think it’s going to slow down. I believe we should be a little more concerned than we are now. It’s going to take many years to prepare for this degree of sea-level rise. It’s happening — so what are we going to do about it?”

Use our “Envisioning Ice Loss” tool to find out how recent Greenland ice loss would look if it occurred in your home state.