April 17, 2012

Are icebreakers changing the climate?

On July 20, 2011, the U.S. Coast Guard Cutter Healy traveled through a break in the sea ice and melt ponds in the Arctic Ocean, during the NASA Impacts of Climate on Ecosystems and Chemistry of the Arctic Pacific Environment (ICESCAPE) mission, a field survey aimed at understanding the ecology of the Arctic Ocean. Credit: NASA/Kathryn Hansen
In summer months, icebreaking ships head north into the Arctic Ocean, tearing through the sea ice and leaving trails of open water in their wakes. Readers occasionally write in to ask us whether the trails left by these ships contribute to the melting of sea ice.

Breaking trail

Arctic sea ice reflects most of the sun’s rays, helping to keep the Arctic and the whole Northern Hemisphere cool. Open water has a lower albedo —or reflectivity—than sea ice, and so it absorbs more heat from the sun. Researchers have found that as Arctic sea ice melts in summer, leaving more areas of open water, the open water absorbs more of the sun’s energy, warming the water and melting more ice.  This is one of the positive feedback loops that scientists say could lead to increased warming and sea ice loss in the Arctic.

NSIDC scientist Walt Meier said, “Even in the summer, when the ice is melting, sea ice reflects at least 50 percent of the sun’s energy. The ocean only reflects about 10 percent of the sun’s energy, and 90 percent is absorbed, warming the ocean and the atmosphere.” It makes sense that a strip of open water left by an icebreaker would absorb more heat from the sun, and melt away the sea ice along that trail. Meier said, “It’s certainly true locally, that the open water in the wake of an icebreaker absorbs more of the sun’s energy than the ice around it.” But what is the effect on the ice cover as a whole?

The 420-foot Coast Guard Cutter Healy breaks ice in the Bering Sea on. 8, 2012. The Healy is currently the Coast Guard's only operational polar icebreaker. Researchers say that there are rarely more than three icebreakers in the Arctic at any given time. Credit: U.S. Coast Guard photo by Petty Officer 3rd Class Jonathan Lally

How much ice does an icebreaker break?

Meier decided to crunch some numbers and find out. While his numbers are an estimate, he said, they provide a helpful comparison of just how much icebreakers might contribute to summer ice loss. Meier said, “In late June, when the sun’s energy is strongest, the total sea ice extent is around 10 million square kilometers or 3.9 million square miles. An icebreaker cruising through the ice for 1,000 kilometers (620 miles) and leaving an ice-free wake of 10 meters (33 feet) would open an area of water 10 square kilometers (3.9 square miles) over the entire cruise. In contrast, the Arctic sea ice cover decreases by an average of over 9 million square kilometers or 3.5 million square miles each year during its melt season—an area larger than the contiguous United States.  In total, researchers estimate that the number of icebreakers traversing the Arctic at any given time is usually less than three. So, Meier said, “The actual contribution is miniscule—only one part in a million of the total ice cover.”