Dan Lunt is a paleoclimate scientist. That means he studies the earth's history to understand how the planet's climate works. A recent study led him to ask not why is the Greenland ice sheet melting, but rather: how did all that ice get there in the first place?
In its early days, 56 to 40 million years ago, Lunt says Greenland was a much different place from what it is today. He looked at similar latitudes from the present day and from modeling studies to conclude, "It is quite likely there were bare soil and unvegetated areas, but also a high probability that there were areas that were covered in grass and even forests."
Lunt says Greenland's ancient history was rubbed clean by glaciers – broad, slow-moving rivers of freshwater ice. Picking up rocks and other debris as they inched forward, Greenland's glaciers eventually reached the coasts, where they dropped huge icebergs, and their accumulated debris, into the sea.
"We can actually go to some of these places in the North Atlantic around the margins of Greenland and drill down and actually extract the sediment that's on the ocean floor," Lunt says. Around three million years ago the scientists find a sharp increase in the amount of small rocks and debris deposited by icebergs.
Four theories for ice analyzed
Lunt and colleagues at the University of Bristol in England used climate and ice sheet computer models to analyze four popular theories to explain the ice sheet formation. The hypotheses range from changes in ocean circulation, or the increasing height of the North American Rocky Mountains, to changes in the earth's orbit or the natural changes in atmospheric greenhouse gas concentration.
Lunt says carbon dioxide showed a more important effect than the others. "In some of the simulations that we looked at, it was actually the only one of those four theories that was able to cause a large increase in ice."
Lunt says the levels of CO2 in the atmosphere today are rapidly approaching 400 parts per million or the level of atmospheric CO2 when Greenland was ice-free. "But what you cannot say," he says, "is that as we approach 400 that the Greenland ice sheet will suddenly disappear because the ice sheets don't work in a linear fashion. It may be because the ice sheet is there now, at the moment it is more robust that we can go up that high and still maintain the ice sheet."
The new study is published in the British journal Nature. The work was carried out with funding from the British Antarctic Survey and Research Council.