The earth is more like a pumpkin than a perfect sphere: it's a bit wider around the middle. Scientists have been tracking the earth's girth for more than two decades. For most of this time, the planet was becoming more spherical because the melting of the polar ice caps at the end of the last glaciation allowed the earth's mantle to relax slightly in a process known as postglacial rebound. Now researchers writing in the current issue of the journal Science say this trend has reversed.

Using data from 10 satellites, Christopher M. Cox of Raytheon Information Technology and Scientific Services (ITSS) and Benjamin F. Cho of the NASA Goddard Space Flight Center determined that the gravitational forces near the equator have grown since 1998. This implies that the circumference of the planet has increased as a result of a shift of mass from high altitudes to lower ones. "The three areas that can trigger large changes in the earth's gravitational field are oceans, polar and glacial ice and atmosphere," Cox explains. The researchers say they have ruled out atmospheric causes, but beyond that, the reason for the shift is unclear. Because a concomitant rise in sea level hasn't been detected, it is unlikely that the transfer of earth-bound ice to the oceans is driving the change. A redistribution of mass within the oceans, however, could be the culprit. Further measurements of sea level height, water temperature and salinity should help scientists unravel the mystery of the bulge.