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Scientists wonder why Earth bulges at the middle 11-14-02 Earth is fighting the battle of the bulge. Like a squashed Nerf ball, our blue and green planet has a wider middle, mostly due to glaciers that weighed down the North and South Poles during the last ice age. Today, thousands of years later, in a warmer world, Earth has slowly been springing back into shape. But the road to true sphere-hood faces a mysterious hurdle. Land that was underneath ice sheets rises. "The Earth behaved much like putting your finger into a sponge ball and watching it slowly bounce back," says Christopher Cox, a scientist with Raytheon Information Technology and Scientific Services. Despite this trend, which would make it more sphere-like, Earth is actually getting wider around the middle. "Something else has been happening since 1998 opposite of post-glacial rebound; it has to be big, really big, to notice it," Cox said. In the August 2, 2002 issue of Science, published by the American Association for the Advancement of Science, Cox and Benjamin Chao, of NASA's Goddard Space Flight Center, describe evidence for this recent change in the direction mass moves on the Earth's surface. While the exact causes are not completely understood, Cox has a hunch that underground dynamics or the composition of the ocean may be causing changes in the globe's girth. Using 25 years of data from 10 different satellites, Cox and Chao analyzed changes in earth's gravity, which help to measure how flat the Earth is at the poles, known as its oblateness. Although Earth will always be slightly wider at the equator because of the way it spins on its axis, the post-glacial rebound has, overall, made the Earth more spherical. "The earth has been doing that for several thousand years because it's a very long term process. Even today, it's plodding right on that way," Cox said.
The new opposing trend occurs at the same time as the rebound and overshadows its effects. The researchers have already ruled out some reasons for the increasing oblateness, leading them to believe the reason has something to do with activities in the earth's core or in the oceans. "Breakup of ice shelves already in the water can't cause a gravity change like this. They're already floating in the ocean and it's just part of the ocean at that point. There is melting occurring, but not enough to explain what we're seeing," Cox said. The scientists have also ruled out the atmosphere as a direct cause. Changes beneath the Earth's surface may relate to a combination of things, such as a magnetic field jerk in 1998, ocean mass transport caused by El Niño between 1997 and 1998, or changes in the ocean's composition, such as its salinity, which changes water's density. The magnetic field jerk and El Niño occurred roughly at the same time, but the effect of one alone is not enough to bring about a change large enough to counter the effect of the post-glacial rebound, say the researchers. Thus, ocean composition may have more secrets that need to be unlocked to fully understand the complexities behind this, Cox says. For example, future studies should also calibrate for the effect of temperature on water, which makes a water column in the ocean longer or shorter, he says. This could change the way scientists assess mass. "It may be tempting to search for an oceanic (and hence climatic) origin for the observed change… but as yet there is no evidence," writes Anny Cazenave at the French Centre National d'Etudes Spatiales and R. Steven Nerem at University of Colorado in a related article in Science. The increasing oblateness is not noticeable to the naked eye and while it is a dramatic shift in the earth's shape over geologic time, the changes will not alter our sense of gravity any time soon. Says Cox, "You will never notice changes like this in your day-to-day life directly, but it's the kind of indirect result that comes from our improved knowledge of our climate.""Measurements of sudden changes in oblateness such as these, remind us that Earth is dynamically coupled from the crust to the core and our solid ground is constantly moving beneath our feet," says Linda Rowan, a senior editor at Science.