The Gravity Recovery and Climate Experiment (GRACE) satellite mission, launched by NASA and the German Space Agency in March 2002, is providing global maps of the Earth's gravity field to astonishing accuracy every month. Because the Earth's gravity field is caused by its mass distribution, time-variations in gravity as determined from GRACE data can be used to estimate monthto- month changes in the Earth's mass distribution. GRACE can recover mass variability at scales of about 250-300 km and larger.
We have been using these data to look at a number of geophysical signals, particularly those that involve the storage of water (including snow and ice) on continents and in the polar ice sheets.
For example, because of its large effective footprint and its sensitivity to mass, GRACE offers the best available method for measuring the total mass balance of the polar ice sheets. The top figure at right shows monthly GRACE results (black line; the orange line is a smoothed version) for the mass variability summed over the entire Greenland Ice Sheet, between April 2002 and December 2009. The trend of the best fitting straight line is about 260 km3/yr of ice volume lost per year, which generates enough meltwater each year to cover all of Colorado to a depth of 90 cm. There is a notable downward curvature to the results, indicating that the mass loss rate has been increasing during this time period. The lower figure shows how this mass loss rate is distributed across Greenland, as determined from the GRACE solutions. By far the largest rates occur in the southeast (note the break in the color scale), where dramatic acceleration of outlet glaciers and accompanying ice thinning has been observed over the last few years. But mass loss has also been occurring up along the western ice sheet margin, particularly within the last 2-3 years. There appears to have been a modest mass gain in the northern interior, presumably associated with increased accumulation rates there.
For other land areas, GRACE recovers changes in the total stored water: the sum of water on the surface, in the soil, and beneath the soil layer. Before GRACE, there was no practical way of measuring total water storage at regional- to global-scales.
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