The rotation of the Earth and convection in its mantle make our planet about 0.3% wider around its equator than it is around its meridian. Changes in this mass distribution -- which can be determined from the strength of the local gravitational field -- have been tracked for decades using satellite laser ranging. In this technique, laser pulses are reflected back to Earth by a satellite. The time this takes is used to calculate the location of the satellite, which is related to the local gravitational field of the Earth.

Geophysicists know from such measurements that the Earth had slowly been getting more spherical, an effect thought to arise from "post-glacial rebound". This occurs at the end of an ice age, when ice melts into water and returns to the sea and the atmosphere. This reduces the pressure on the land beneath the glaciers, and this land is slowly pushed back to its former position by the Earth’s mantle.

But when Cox and Chao studied laser-ranging data collected between 1979 and 2001, they found that this trend reversed sharply in 1998. This means that mass must be moving away from the poles and towards the equator – and any such process must involve either the atmosphere, the ocean or the Earth’s mantle.

El Niño is a good candidate for this process, say the researchers. El Niño and La Niña are alternating hot and cold periods in the atmosphere and ocean of the Pacific, each lasting about six months. Although the effects of these phenomena on flow patterns in the ocean are not well understood, the strongest El Niño this century coincided with the abrupt change in oblateness that took place in 1998.

Another possible cause is a sudden change in the Earth's magnetic field, which would affect the flow of the Earth’s liquid outer core. Such an event was observed in 1999, and Cox and Chao speculate that changes in this layer preceding the flip could have triggered the rise in oblateness.

But the researchers ruled out a link with global warming, which they initially thought could have raised sea levels significantly by melting polar ice. Records dating back to 1992 showed that this rise was too small to account for the observed shift in mass.