OUR planet has started getting fatter around its middle.
The Earth is wider at the equator than from pole to pole, mainly because the centrifugal forces generated by its rotation make it bulge outwards. For most of the past 20 years, observations showed that the Earth was getting more rounded at the poles, but in 1998 that trend unexpectedly reversed. Christopher Cox, a geophysicist contracted by NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland, and his colleague Benjamin Chao discovered the effect in a study of observations from nine satellites (Science, vol 297, p 831).
The new trend implies there has been a transfer of mass from high to low latitudes, but Cox isn鈥檛 sure why. Melting polar ice and a resulting sea level rise across the world don鈥檛 explain it. You鈥檇 need a block of ice 10 kilometres wide and 5 kilometres high to melt every year to explain the change. A shift in matter at the boundary between the Earth鈥檚 core and mantle might be one factor. But more likely, an unexpected change in ocean circulation has moved more water to low latitudes.
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