It’s one of the most pressing questions facing climate scientists today: how vulnerable are the vast ice caps on Greenland and Antarctica to rising temperatures? An unfathomable amount of ice is stored on those two land masses, and as that ice melts and flows into the oceans, global sea levels rise—if all the ice on Greenland melted tomorrow, it would raise global sea levels by more than 20 ft. (The sea ice over the North Pole is melting too, and quickly, but since it’s already in the water, the change doesn’t raise sea levels any more than an ice-cube melting in a gin and tonic raises the level of the drink.) Getting a proper fix on how quickly polar land ice is melting would help scientists better model how quickly sea levels could rise in a warmer world–and that information would be vital for policymakers trying to adapt to climate change. But the dynamics of the Greenland and Antarctic ice sheets are extraordinarily complex—so much so that the Intergovernmental Panel on Climate Change decided not to try to calculate the potential increase in melt rate for those ice caps when it was estimating sea level rise for the next century. Nor does it help that carrying out science in these parts of the world is difficult and expensive—and cold, as I learned when I visited the heart of Greenland’s ice cap in 2008.
Still, over the past several years scientists have been able to use data from the twin Gravity Recovery and Climate Experiment (GRACE) satellites to begin to get better estimates of just how fast we’re losing polar ice. (Changes in the mass of polar ice can actually affect the Earth’s gravitational field, and the GRACE satellites can detect those changes and use it to help calculate ice loss.) Recent studies using GRACE data estimated that Greenland was losing around 230 billion metric tons of ice a year, while West Antarctica was losing around 132 billion metric tons a year. Together that would account for 0.2 in. of sea level rise a year—which might seem like a small figure, but it’s far higher than the 0.07 in. that seas rose annually in the 1960s.)
Those are scary numbers, but a new study published in the September issue of Nature Geoscience suggests that the true melt rate might be much slower than that. (Access a PDF of the study here.) A joint team of American and Dutch scientists took another look at the GRACE data and found that Greenland and West Antarctica may be melting just half as fast the earlier studies estimated. As researcher Bert Vermeersen, a professor at Delft University of Technology in the Netherlands, told the AFP, the earlier estimates failed to account for glacial isostatic adjustment—the rebounding of the Earth’s crust after the end of the last Ice Age:
A good analogy is that it’s like a mattress after someone has been sleeping on it all night,” [Vermeersen] said.The weight of the sleeper creates a hollow as the material compress downwards and outwards. When the person gets up, the mattress starts to recover. This movement, seen in close-up, is both upwards and downwards and also sideways, too, as the decompressed material expands outwards and pulls on adjacent stuffing.
Often ignored or considered a minor factor in previous research, post-glacial rebound turns out to be important, says the paper.
Vermeersen and his colleagues—led by Xiaoping Wu of NASA’s Jet Propulsion Laboratory—used ground-based GPS stations and sea floor pressure, along with GRACE data, to get a better sense of what’s happening to the land in Greenland and Antarctica. They found that southern Greenland is actually subsiding, or falling, as it’s pulled down by the rebound of land in North America, while West Antarctica is rising, but not as fast as earlier studies estimated. As a result, they estimate Greenland is losing around 104 billion metric tons of ice a year, while West Antarctica is losing around 64 billion tons a year.
Climate change skeptics were quick to seize on the results—Rush Limbaugh, for what I’m guessing is the first and last time, was able to use the term “glacial isostatic adjustment.” But the Nature Geoscience study won’t be the final word on the subject. It’s own estimates of ice loss come with significant uncertainty, and as David Bromwich of Ohio State points out in a commentary on the study, the estimates rely on data from a very small number of GPS sites, all of which are located on the edges of the ice sheets. The Nature Geoscience study also doesn’t change the essential fact that we are losing ice on a daily basis from Greenland and West Antarctica—104 billion metric tons is still a lot of water to be adding to the global seas each year. Most of all the study underscores the need to keep researching the impact of warming on our polar regions—which is why it’s good news that the GRACE mission was just extended through 2015.