Still hope for Arctic sea ice

Steffen Tietsche, lead author of the study that appeared this week in the scientific journal Geophysical Research Letters, said that he was quite surprised when he found this result: "It seems so obvious that a tipping point for Arctic summer sea ice exists: with a smaller sea-ice cover, more sunlight is absorbed by the dark open water of the polar ocean. This water therefore warms efficiently during summer, which leads to additional melting of sea ice and even more open water. This feedback loop can in principle cause the loss of Arctic sea ice to become at some point self-amplified and hence independent of the prevailing climate conditions."

To examine the validity of this concept, the researchers used a climate model in which they removed the Arctic sea-ice cover completely at the beginning of summer. In doing so, they maximized the absorption of solar radiation. "We expected the ocean to remain ice free after the initial ice removal, because so much more heat would be absorbed by the open water during summer," Tietsche said. However, in the model simulations the ice always recovered within about three years to the conditions it had before the artificial removal. This indicates that sea ice extent closely resembles the prevailing climate conditions at any given time, which makes the existence of a tipping point unlikely.

The researchers find that two processes contribute most to the recovery of the ice cover: First, during winter the ocean loses most of the additional heat it gained during summer. This heat loss is very efficient after the removal of the insulating sea-ice cover, because then the ocean is in winter directly in contact with the cold atmosphere. Second, the thin ice that eventually forms during winter grows very fast, because thin ice does not insulate as efficiently as thick ice. The heat that is released from the ocean through the thin ice leads to an increased heat loss from the atmosphere into space and to a decreased heat transport from the South into the Arctic. The combination of these stabilizing feedbacks more than compensates for the additional absorption of sunlight during summer. The findings of this new study confirm work that was carried out by American scientists with a much simpler model. "This general agreement of models with substantially different complexity usually indicates that the result is robust," says Jochem Marotzke, director at the Max Planck Institute and co-author of the new study.

The researchers underline that their results do not question the dramatic loss of Arctic sea ice or its relation to anthropogenic climate change. "If we don't slow down global warming extensively, we will lose the summer sea-ice cover in the Arctic within a few decades," says Tietsche. "Our research shows that the speed of sea-ice loss is closely coupled to the speed of global warming. We think that it's important to know that we can still do something about slowing down or possibly even stopping the loss of the sea-ice cover."