Scientists have cautioned that a warming planet could melt Greenland's vast ice sheet, a potentially catastrophic event that would raise sea levels and inundate coastal communities around the globe.

Yet while they puzzle over when and whether this might happen, they're also mystified over how the giant island formed so much ice in the first place. Greenland's ice sheet is the second largest in the world, behind only Antarctica.

Strangely, other parts of the globe at similar latitudes, including northern Canada and Siberia, don't have year-round patches of ice anywhere near as extensive or thick.

A new study finds that a mysterious drop in greenhouse gases around 3 million years ago allowed Greenland's ice to proliferate. The research could help with forecasts about the fate of the ice and the potential for rising seas.

Why it matters

If all of Greenland's ice were to melt, perhaps as quickly as in a few centuries, seas would rise 21 feet (6.5 meters) all around the planet, according to the U.S. Geological Survey. And already, an alarming melt is under way there, other studies find. In 2007, the ice melted at a rate of 150 percent of the average going back to 1988.

Recent studies have found that as the ice melts more rapidly, water pours through fissures and gets under glaciers, acting like a lubricant to allow the ice to race ever-faster toward the sea. In addition, when snow melts at high altitudes and then refreezes, it can absorb up to four times more sunlight, creating even more melting the next year.

Some scientists fear a snowball effect could exacerbate the ice sheet's disintegration in as little time as a matter of decades.

But given a lack of understanding about the mechanisms, and questions about how warm the planet will get and how much ice will melt, scientists' estimates for the extent of sea-level rise by the end of this century range from just inches to perhaps 6 feet or so.

The new study

Around 3 million years ago, there was an increase in the amount of rock and debris deposited on the ocean floor around Greenland. The stuff must have been deposited by ice that had originated on land and then become icebergs, indicating that large amounts of ice on Greenland only began to form around that time, scientists say.

"Prior to that, Greenland was largely ice-free and probably covered in grass and forest," said Dan Lunt of the University of Bristol in England. "Furthermore, atmospheric carbon dioxide levels were relatively high. So the question we wanted to answer was why did Greenland become covered in an ice-sheet?"

Theories abound, from changes in ocean circulation to changes in the Earth's orbit or tectonic uplift of the planet's surface. Another idea is that atmospheric greenhouse gas concentrations changed. Lunt and his colleagues used computer models of climate and ice sheets to test the theories.

Each theory got some support. But the only one producing effects large enough to explain the current reality was that carbon dioxide, a greenhouse gas that acts like a blanket to keep heat in and warm our world, fell to levels closer to those of pre-industrial times.

The research, funded by the British Antarctic Survey, is detailed in the Aug. 28 issue of the journal Nature.

What it means

Here's the really interesting part: When Greenland was ice-free, carbon dioxide was at 400 parts per million by volume in the atmosphere, Lunt explained via email. The level was 280 ppm before the Industrial Revolution. Now it has climbed back to 385 ppm.

Next year, Lunt and colleagues plan to publish a study that applies their finding to the question of what this means for the future of the ice and the potential for higher sea levels. For now, he told LiveScience, "The work does certainly indicate that the ice sheet is sensitive to changes in atmospheric carbon dioxide concentrations."

The study does not answer why Greenland got covered in ice but other northern locales did not. Lunt has an idea, however:

"The answer is most likely related to the fact that underneath the ice on Greenland are some high-altitude mountains on the east coast, which are high enough to be cold enough that ice can form, which then flows slowly down the slopes and eventually covers the entire island," Lunt explained. "In certain time periods [for example about 20,000 years ago], when the Earth's orbit is aligned in a certain way, ice does start to form in Canada and Siberia — for example in the last Ice Age."

Yet another big question has now been raised: Why did elevated atmospheric carbon dioxide concentrations fall so dramatically 3 million years ago?

Lunt's colleague, Alan Haywood of the University of Leeds, tackles that one: "That is the million dollar question which researchers will no doubt be trying to answer during the next few years."