The rate of melting by the ice sheets covering Greenland and Antarctica may throw existing projections for sea level rise out the window. Unfortunately for Maldivians and other idyllic, but altitude-challenged islands, the ice sheets are melting faster than anyone expected.

Sea levels have been rising nearly unabated since the late 19th century, but rates have been increasing in recent years. The bulk of the change has been attributed to water expanding due to rising ocean temperatures, while melting ice in Greenland and Antarctica was considered to contribute a relatively small amount. But a new study published in Geophysical Research Letters reports the ice sheets may be contributing to sea level rise at a rate three times that which was previously suspected. The result? Sea levels could reach predicted heights 50 years earlier than experts thought, and total sea level rise may exceed previous estimates.

Ice is primarily lost from the Greenland and Antarctic sheets when it calves into the ocean. When ice sheets move faster toward the ocean, they shuck more ice into the water. And if snowfall over the ice sheets fails to replenish the amount lost, the sheet shrinks in size over time, transferring the water it once held into the ocean.

The new study used two independent approaches to arrive at the result. The first used interferometric synthetic aperture radar, commonly known as IfSAR or InSAR, from three satellites (the European Space Agency's ERS-1 and 2, Canada’s Radarsat-1, and Japan’s ALOS). These satellites can track the speed at which ice sheets move. In conjunction with radio echo sounding to determine depth of the sheet and regional weather models to estimate snowfall, the researchers determined how the amount of ice on Greenland and Antarctica was changing.

The second approach used NASA and the German Space Agency’s GRACE satellite system, which detects small variations in the Earth’s gravitational pull. Two identical GRACE satellites travel in the same orbit and use microwaves to calculate how far apart they are. Fluctuations in the Earth’s gravity, such as those caused by the loss of ice in Greenland and Antarctica, alter distance between the satellites subtly. They can detect shifts between themselves as small as 10 micrometers.

Using these approaches essentially allowed the researchers to verify their results. The researchers compared the same areas over the same periods of time for both ice sheets. The verification test produced results that were within 20 gigatons per year in Greenland and 150 gigatons per year in Antarctica, well under previously observed error estimates for each individual approach.

That both Greenland and Antarctica are melting faster than previously suspected—and therefore contributing more to sea level rise than expected—will certainly change estimates of sea level rise over the next century. Both ice sheets were expected to melt due to climate change, but experts anticipated the melting would occur later in the century. Given the pace of their current melting, sea levels may rise more quickly than experts had predicted and will likely overtop current end of the century estimates.

Quickly rising sea levels will give low lying areas less time to prepare. Results from the new study indicate 32cm of sea level rise by 2050, 15cm of which is attributable to the Greenland and Antarctic melting. Though one foot may not sound like much, a FEMA study (PDF) estimated storm surges under such a scenario would cause about 40 to 60 percent more damage than at present. That seems like small potatoes compared to the kinds of problems Bangladesh, the Maldives and many Pacific islands will likely face in just 40 years.

Geophysical Research Letters 2011. DOI: (About DOIs).

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