Gravity Field

Earth's Middle Getting Fatter?
BBC, August, 2002

The changes are very, very slight Scientists have known for some time that the Earth is not a perfect sphere. It is shaped a little like a pumpkin - wider at the middle and narrower at the poles. It is a difference of more than 20 kilometres. But now new research published in the journal Science suggests our planet is getting even wider - if only by the odd millimetre. The scientists behind the report, Christopher Cox and Benjamin Chao, base their findings on space-based observations from past 25 years.

Since the early 1980s, satellite laser-ranging studies that have been used to work out the planet's gravity field have demonstrated how the Earth has lost a bit of its pumpkin look - it has actually become slightly more spherical. This has been put down to a rebound effect in the mantle - a thick layer of nearly molten rock between the Earth's crust and its core - following the loss of the heavy mass of ice at the poles after the last Ice Age. But Cox and Chao say their work suggests this trend reversed abruptly about four years ago. They are doubtful that phenomena such as further glacial melting or atmospheric changes can explain the rapid turnabout.

Instead, they suggest two possibilities. One is that changes in ocean circulation have shifted a larger mass of water towards equatorial regions; the other is that there has been a shift in mass at the boundary between the Earth's fluid outer core and the mantle. Further studies will be required to work out what is really happening. Cox says any increase in the Earth's girth is of the order of millimetres and may even be imperceptible given the rather technical way these things are monitored. "It depends on where the effect is, because it is measured in terms of a change in the shape of the gravity field," he says. "If it is in the ocean, it may be a few millimetres, but if it is in the core there could be no apparent change in the actual shape of the Earth."