The rate of polar ice-sheet melting is key to calculating how fast sea levels are rising. Now, scientists have a new analysis of polar ice loss in Greenland and Antarctica—and the numbers aren't good for the planet.

Image courtesy of Ian Joughin Surface water rushes along a crack in the Greenland ice sheet. Melting from the polar ice sheets has accelerated sea level rise

Superstorm Sandy, which made landfall on the East Coast a month ago yesterday, wasn’t a particularly powerful storm. But what it did have was water—lots of it. Sandy pushed record storm surges in places like lower Manhattan, and it was the flooding triggered by those surges—much more than the winds accompanying the storm—that caused the tens of billions of dollars in damages attributed to the Superstorm.

But our coastlines were already primed for those kinds of catastrophic floods, thanks in part to the gradual rise in sea level over the past century caused chiefly by man-made global warming. Sea levels have risen by about half a foot over the past century—and are likely rising even faster along the U.S. Northeastern coast—which amplified the effects of Sandy’s storm surges. That’s why scientists are so worried about the impacts that climate change to come may have on sea levels. The higher the seas rise, the more devastating coastal storms will become.

Unfortunately, trying to predict how rapidly the seas will rise as the climate warms is extremely difficult. We know that as ocean temperatures increase—which goes hand in hand with global warming—water expands and sea levels rise. But the big X factor is the polar ice sheets chiefly found in Greenland and Antarctica. As that massive land ice melts, the water flows directly into the seas, causing the water to rise. By contrast, sea ice melting—which has been occurring at a record pace in the Arctic this summer—does not raise the sea levels, just as the melting of an ice cube in a glass of scotch doesn’t raise the overall level of liquid.

The problem is that scientists have struggled to nail down just how quickly the polar ice sheets are melting. There have been more than 30 different estimates of sea level contributing due to polar ice sheet melting made since 1989. But in a new paper published in the November 29 Science, a team of researchers have gone through all of those estimates and come to a broadly agreed conclusion that melting from the ice sheets have contributed an average of 0.023 in (0.59 mm) to sea-level rise since 1992, with an uncertainty of 0.008 in. (0.2 mm) per year. That might not sound like much—ice-sheet melting has only added about half an inch (12.7 mm) to sea levels in that time span—but the new analysis means that polar ice sheets are melting three times faster today than they did in the 1990s, with much of the ice loss happening in Greenland. “This will give the wider climate science community greater confidence in ice losses and lead to improved mode predictions of future sea-level rise,” said Andrew Shepherd, a professor of earth observation at the University of Leeds and a co-author of the Science paper.

(PHOTOS: Portraits of Melting Glaciers)

Most of that melting appears to be happening to the Greenland ice sheet, which holds nearly 700,000 cu. miles of ice, although the even more massive Antarctic ice sheet is melting as well—though not as uniformly as in the Arctic. (In Antarctica—which has an ice sheet the size of the U.S. and Mexico combined—parts of the continent are melting, while other parts seem to be growing.) In the 1990s, the two ice sheets combined on average to lose 110 billion tons of ice a year. That sounds like a lot—actually, that is a lot—but keep in mind that it takes about 10 trillion tons of ice melting to raise sea levels by an inch. But that rate increased to 379 billion tons a year between 2005 and 2010—and the Science paper doesn’t include information from 2012, when the Greenland ice sheet experienced a record thaw.

(MORE: Unfrozen Tundra)

The study may not seem earth-shattering, but it does what earth science needs to do: give us a clearer picture of what’s happening to our planet. The mechanics of polar ice-sheet melting were so confusing that the 2007 Intergovernmental Panel on Climate Change assessment simply didn’t include ice loss estimates in its attempt to model future sea level rise. Said co-author Benjamin Smith, a research scientist at the University of Washington:

It provides a simpler picture. In the 1990s, not very much was happening. Sometime around 1999, the ice sheets started losing more mass, and probably have been losing mass more rapidly over time since then.

Of course, what the data doesn’t tell us is what we should actually be doing about ice-sheet melting, sea-level rise and climate change. (Although if the sleep-walking delegates at the U.N. climate summit in Doha this week and next week are any measure, the answer is “not much.”) But we shouldn’t kid ourselves that there won’t be major costs if we fail to slow the pace of warming—and that we’ll eventually have to pay the bill. Sandy helped teach us that.

MORE: Arctic Sea Ice Larger Than U.S. Melted This Year