Karl Popper and Antarctic Ice
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IN 2006, A SURPRISING RESULT began to tilt the balance the other direction. Isabella Velicogna used measurements from the Gravity Recovery and Climate Experiment satellites to actually measure directly the gravity caused by the ice on Antarctica [Velicogna 2006]. Everything with mass creates a gravitational attraction, and that attraction is directly proportional to the amount of mass. Therefore, measuring the gravitational attraction is equivalent to directly “weighing” the ice.
Through an amazing feat of engineering, the GRACE satellites can actually directly measure the tiny variations in gravity that changes in Antarctic ice can cause. GRACE is a pair of satellites orbiting together, with one satellite a relatively small distance in front of the other. They have finely tuned equipment on board which can measure their distance from each other so carefully that they can notice a difference the size of a human hair. By measuring the way this distance changes over time, they can figure out the precise strength of gravity at that point on the Earth's surface.
Of course, there are many other potential causes of gravitational attraction. Velicogna had to account for the effect of the rest of the planet, as well as for changes in density in the ocean and even the density of the atmosphere. The biggest problem came from something called the “Post-Glacial Rebound,” which is the gradual expansion of landmasses that used to be crushed by glaciers in the last ice age. The effect of “PGR” on the gravitational attraction was very large, larger than the impact of the changes in ice mass in fact. Because of this, and the difficulty of modeling how exactly PGR works, Velicogna wasn't able to get a very precise number. However, no matter what model of PGR she used, she always got the same basic result: Antarctica was losing ice over time.
Since then, many more analyses of the GRACE data have been done, all agreeing that it seems Antarctica is losing ice mass. A few months ago, in November 2009, a team led by J. L. Chen calculated that it was losing between 123 and 267 gigatons of ice per year, and that the rate of loss was increasing over time [Chen 2009]. These studies (in tandem with many others) lend a great deal of weight to the claim that coastal ice flow is outweighing the central ice gains, and that Antarctica as a whole is losing mass. However, these results are not conclusive, and it would not be prudent to pass judgment yet either way.
