Professional Paper 1386–A
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Figure 57. Time series of the Earth’s oblateness (J2). J2 is decreasing over a long term, due to tidal friction, postglacial rebound, and other effects; as J2 decreases, the speed of the Earth’s rotation is increasing in order to preserve angular momentum. Dickey and others (2002) suggest that the marked increase in J2 around 1998 might have been caused by the recent acceleration of glacier wastage. The offset green line represents the change in J2 due to changes in surface height of the Greenland and Antarctic ice sheets; the blue line represents the changes implied by uniform fluctuations in sea level; and the purple line represents the changes implied by spatially variable changes in sea level. These changes, however, do not account for the 1998 increase in J2. Modified from Cox and Chao (2002, p. 832, fig. 2). Figure courtesy of Science magazine. Used with permission.