Each glacial period is followed by an interglacial period in which temperatures rise and the ice retreats. The Milankovitch cycles are not strong enough by themselves to cause the shift. What they do is get the ball rolling. A decrease in solar energy hitting the Arctic allows sea ice to form in winter and remain over summer, then to expand its reach the following year. The ice reflects more solar energy back to space. A colder ocean stores more CO2, which further dampens the greenhouse effect.
Conversely, when ocean temperatures raise, more CO2 escapes into the atmosphere where it traps more solar energy. With so many factors at play, each swing of the climate pendulum has produced its own unique conditions. Take, for example, the last interglacial, known as the Eemian, from 130 to 115,000 years ago. This happened at a time when CO2 was at preindustrial levels, and global temperatures had risen only modestly. But with higher solar energy striking the north, temperatures rose dramatically in the Arctic. The effect was amplified by the lower reflectivity of ice-free seas and spreading northern forests.
There is still uncertainty about how much these changes affected sea levels. Estimates range from 5 to 9 meters, levels that would be catastrophic today. That's one reason scientists today are intensively monitoring Earth's frozen zones, including the ice sheet that covers Greenland. Satellite radar shows the flow of ice from the interior of the island and into glaciers. In the eastern part of the island, glaciers push slowly through complex coastal terrain. In areas of higher snowfall in the northwest and west, the ice speeds up by a factor 10. The landscape channels the ice into many small glaciers that flow straight down to the sea.
Scientists are tracking the overall rate of ice loss with the Grace Satellite. They found that from 2003 to 2009, Greenland lost about a trillion tons, mostly along its coastlines. This number mirror's ice loss in the Arctic as a whole. By 2012, summer sea ice coverage had fallen to a little more than half of what it was in the year 1980. While the ice rebounded in 2013, the coverage was still well below the average of the last three decades. Analyzing global data from Grace, one study reports that Earth lost about 4,000 cubic kilometers of ice in the decade leading up to 2012.