Researchers at the Jet Propulsion Laboratory (JPL) in Pasadena, Calif., have discovered that the Earth's shape is becoming squatter, with mass being redistributed to the equatorial regions.

In other words, the Earth is getting fatter.

Charles Ammon, associate professor of geosciences, said the return of glaciers to the polar regions following the last ice age, a motion known as post-glacial rebound, had, until recently, made the planet longer in the north-south direction.

The vast ice sheets that once covered the North "squished the Earth," Ammon said, making the planet wider.

Now, the gradual thinning caused by post-glacial rebound has begun to reverse, and the planet once again is putting on weight around its middle.

There are several explanations for the change, but geodesists, those who study the size and ashape of the Earth, are focusing on two main possibilities: glacial melting and a resultant shift in ocean currents, or a change in the density structure of Earth's liquid outer core.

Jean Dickey of JPL, whose paper was published in the Dec. 6 issue of Science Magazine, said glacial melting has increased the amount of water at the equator, causing the planet to become wider.

"I am dismayed that glaciers are melting so fast that we can measure it. I think that the gravity field is a good way to measure Earth's health," Dickey said. Scientists measure changes in gravity in order to find the shape of the planet.

Satellite data revealed that the change in dimension began in 1998. The team at JPL realized this coincided with the warmest year on record. The researchers also observed that 130 cubic miles of glacier melted that year, nearly double the count from the previous year.

In 1997-98, El Niño precipitated a worldwide shift in weather and oscillation between the east and west sides of the Pacific Ocean. This phenomenon occurs every two to seven years.

Dickey said the melting of glaciers in Alaska is an important part of Earth's variable shape. That state is affected nearly four times as much by global warming as is the rest of the world.

Melting glaciers could cause sea levels to rise, destroying islands and collapsing major ocean undercurrents. There have already been disappearances of small islands in the Chesapeake Bay.

The stretching of the Earth "hits at something people can visualize very easily," Dickey said.

Ammon explained that melted water stays at the equator due to the Earth's rotation. That rotation has been slowed by the increased mass, adding 45 millionths of a second to each day.

Another explanation for the expanding equator is the constant flow of molten iron in the Earth's core. This creates a density shift that is difficult to measure, and its effects are not seen for thousands of years.

Ammon said the discovery of the change would allow scientists an increased understanding of how ice caps, the atmosphere, oceans, and continents interact with climate. This could help explain past phenomena and help make better predictions in the future.

The launch last March of twin GRACE (Gravity Recovery and Climate Experiment) satellites should soon provide even greater accuracy to measurements of Earth's gravitational fields. If gravity is increased over a particular region of the planet, it means mass has increased there.