The team of researchers sought to find a climatic reason for the dramatic changes in Earth's gravity field observed since 1997.

These changes have resulted from large redistributions of mass around the globe.

And these are characterised by an increased bulge in the Earth's equator and mass movement away from the poles.

This is an occurrence known as oblateness, which can be thought of as the difference between an American football and a soccer ball.

The football has a larger radius at the equator. The results of the experiments were published in the journal Science.

"During the last ice age, glaciers covered much of the land, reaching down far into the United States, deforming the solid Earth," Dr. Jean Dickey of the Royal Observatory of Belgium.

As the ice age ended, the number of ice sheets gradually were reduced, leaving behind large remnant glaciers in the earth.

The Earth is inelastic and readily responds to changes by very slowly recovering on time scales of tens of thousands of years.

This results in a redistribution of mass toward the higher latitudes. Also this makes the earth effectively elastic.

This phenomenon, called post-glacial rebound, still continues today, due to the slow response of the Earth's mantle."

In 1998, however, data from the satellites began to show an increase in the oblateness of the Earth's gravity field.

To find the catalyst for these changes, Dickey and her colleagues looked to the oceans, where strong El Niño and Southern Oscillation events and a shift in a climate cycle called the Pacific Decadal Oscillation had begun to occur in 1997.

Sea-surface height measurements from the NASA's Topex/Poseidon oceanography satellite combined with ocean circulation models showed a movement of water from the Southern Pacific Ocean to the tropical regions in the Pacific and Indian Oceans.

The researchers also studied sub-polar and mountain glaciers, using information from the National Snow and Ice Data Centre, to determine their effect on gravity changes over a period of 37 years.

The record shows substantial glacier melting from the 1960s to the 1980s, accelerating in the late 1990s. From 1980 to 1989, the average melting rate was approximately 100 cubic kilometres (26.4 trillion gallons) per year.

However, by the years 1997 and 1998 the yearly rate had increased dramatically by nearly 320 cubic kilometres (85 trillion gallons) and about 540 cubic kilometres (143 trillion gallons), respectively.

This melting, in combination with changes in the ocean, was found to account for the observed Earth gravity field changes.

``The rapid surge in glacial melting and changes in oceanic mass distribution in 1997-98 coincided with an intense El Niño, and with the highest global mean surface air temperatures on record.

The world is certainly changing in a major way,'' Dickey said. ``The links between these relatively rapid mass shifts and concurrent climate anomalies, however, remain to be established.''