Earth isn't losing its "spare tire" as fast as it should be, according to new research—and it's definitely not because the planet's not getting enough water.

In fact, melting ice from Antarctica and Greenland (map) is giving the oceans huge infusions of water, which then gets pulled toward the Equator—counteracting a millennia-old slimming trend around the planet's middle, experts say.

Researchers have long known that Earth isn't a perfect sphere. Rotational forces cause the planet to bulge at its waistline. A person standing at the North Pole, for example, is about 13 miles (21 kilometers) closer to the center of Earth than someone at the Equator.

That difference has been shrinking, at least over the long term. Ever since scientists have been measuring the equatorial bulge, it's been retracting at a rate of seven millimeters (about a quarter of an inch) a decade—part of a long rebounding from the Ice Age, which lasted from about 2.6 million to 11,700 years ago.

During the Ice Age, "all that ice is sitting there, and it's sitting there for tens of thousands of years," study co-author John Wahr said. At the Poles, "it pushes down on the Earth, and the Earth sinks down underneath it."

The extra Ice Age weight even squished Earth's malleable mantle outward, further enlarging the planet's bulge.

Once the Ice Age ice melted, though, the Poles began to slowly spring back, and they've been doing so ever since.

But now "there's something else going on that offsets [the bulge's shrinkage]," said Wahr, a geophysicist at the University of Colorado.

Straight to the Waistline

Beginning in the 1990s, the post-Ice Age rebound began to slow and now appears to have stopped, according to the new study, which relied on data from the twin Gravity Recovery and Climate Experiment (GRACE) satellites. GRACE takes ultra-precise readings of Earth's gravitational field, allowing for researchers to gauge, for example, changes in ice mass and the amount of water in the ocean.

The culprit? "It looks like that is Greenland and Antarctica losing mass," Wahr said.

According to Wahr and his collaborator Steve Nerem, the two regions are losing a combined 382 billion tons of ice a year.

That ice falls into the ocean, melts, and is pulled toward the Equator by the same forces that create Earth's spare tire. And while the reduced weight on Greenland and Antarctica will eventually allow Earth to spring back a bit more, it'll take thousands of years for that adjustment to take place, the researchers say.

Overall, the current ice loss is causing Earth's bulge to grow at a rate of seven millimeters a decade, Wahr and Nerem found—exactly enough to counteract the long-term rebound, at least temporarily.