The NASA and German Aerospace Centre Gravity Recovery and Climate Experiment (GRACE) launched in 2002 aims to measure variations over time in the Earth’s gravity field by gauging tiny changes in distance between two satellites using radar. Briefly, mass in the Earth tugs first on the leading satellite and then on the one trailing it, so if mass distribution stays constant so does the separation between the craft. If mass below a point on the Earth’s surface does change, GRACE detects this from a change in separation between the two craft. Between April 2002 and February 2009, monthly measurements over Greenland and Antarctica reveal losses in the amount of ice, and the rate at which the ice caps are shrinking is accelerating (Velicogna, I. 2009. Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE. Geophysical Research Letters, v. 36, L19503 doi:10.1029/2009GL040222). Isabella Velicogna of NASA/JPL shows that the Greenland ice cap (total mass~3 x 1015 t) lost 1.37 x 1011 t a-1 in 2002–3, rising to 2.86 x 1011 t a-1 in 2007–9 , the loss is accelerating at 3.0 ± 1.1 x 1010 t a-2). The ten times more massive Antarctic ice cap lost 1.04 x 1011 t a-1 in 2002–6 rising to 2.46 x 1011 t a-1 in 2006–9, giving an acceleration of 2.6 ± 1.4 x 1010 t a-2. Proportionate to size the Greenland ice cap is dwindling faster than Antarctica, but at these rates it still has 10 thousand years before it disappears.
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