Gravitational Attraction of Ice Sheets on the Sea

Click the image to watch a computer animation showing how a melting ice sheet causes sea level to rise globally while lowering it locally. Graphic by Simon Wermuller von Elgg

It might be hard to imagine the huge scale of Earth’s polar ice sheets if you live far from the planet’s poles. They’re more than two miles thick in places and cover a combined surface area 50 percent bigger than that of the US. They weigh millions of billions of tons. During the recent ice ages they were three times larger than today.

The huge mass of water frozen in ice sheets exerts a powerful gravitational force that pulls on nearby seawater. This gravitational attraction must be taken into account when considering changes in sea level as ice sheets advance or retreat. Jerry Mitrovica likes to illustrate the importance of the gravitational attraction of ice sheets with a thought experiment: what would happen if Greenland suddenly melted? Averaged around the world, sea level would go up by about 23 feet, as water poured into the ocean around Greenland and spread throughout the oceans. The local story, however, would be quite different. Ocean water near the melting ice sheet would experience two opposing effects. On the one hand, the addition of melt water would raise the ocean higher. On the other hand, the loss of the ice sheet’s mass, and thus its powerful gravitational attraction, would cause the ocean’s surface to relax away from the former position of the ice sheet, lowering local sea level. Mitrovica discovered that within about 1,000 miles of Greenland, the balance of forces would favor lower sea level, leading to the counter-intuitive conclusion that sea level falls even though water is being added to the ocean. At the distance of Scotland, the opposing effects would counter each other and no net change in sea level would be observed. Even very far away from Greenland we would observe this gravitational effect -- since sea level would go down near the melting ice sheet, to make up the difference it would have to go up even higher, than the 23-foot average, on distant shores.

Because the polar ice sheets were smaller during the Pliocene (though nobody knows exactly how much smaller), the Pliomax team will have to consider the lower gravitational attraction they exerted on sea water when assessing elevation measurements from fossil beaches.

Learn about other adjustments the Pliomax team makes to its field measurements: