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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.