Over the 2010 to 2011 winter, news stories suggested a potential connection between a warming climate, low Arctic sea ice extent, and unusually cold weather this winter in the U.S. and Europe. What do scientists know about how sea ice affects the weather?
Scientists have been exploring a possible link, but the question is far from settled. It makes sense that changing sea ice conditions could affect weather in the Arctic and other parts of the world. During the colder months, sea ice insulates the relatively warm ocean from the colder atmosphere. As sea ice declines, more heat can escape to the atmosphere in the fall and winter, affecting wind patterns, temperature, and precipitation.
But while sea ice affects the atmosphere, the atmosphere also affects sea ice. Warmer air temperatures help prevent ocean water from freezing over in the first place, and winds can push the ice together, keeping ice extent lower. “It’s a highly coupled system,” said NSIDC Director Mark Serreze, “Cause and effect are difficult to unravel.”
Separating cause and effect
When two or more things vary together in a measurable way—called correlation—scientists in all fields have to look more carefully to determine if it is mere coincidence, or if one event causes the other. For example, people who exercise more may be less likely to get sick. But does exercise prevent sickness, or do healthier people tend to exercise more simply because they feel better? To sort out correlation and causation, researchers collect data from observations, make hypotheses to explain their observations, and then run experiments to figure out if their hypotheses are correct.
Jennifer Francis, an atmospheric scientist at Rutgers University, has studied the connections between sea ice and weather. She said, “It’s very clear that the dramatic loss of sea ice during recent decades is affecting weather.” What’s not clear, she said, is how those changes affect weather outside the Arctic region.
In 2009, Francis found a correlation between low sea ice extent at the end of summer and altered storm tracks in the Northern Hemisphere in the fall. She said, “During years when there’s less ice, the temperature difference between high and mid-latitudes is weaker, which makes the jet stream weaker, which allows the storm track to meander farther south.”
Another study, by James Overland at the National Oceanic and Atmospheric Association (NOAA) Pacific Marine Environmental Laboratory, established a correlation between low summer sea ice and an Arctic weather system known as the Arctic Oscillation, which has behaved unusually during the last two winters.
Both of these recent studies showed correlations between sea ice conditions and weather in the Arctic and beyond. But a cause-effect relationship is not yet clear. Francis said, “We do not know if Arctic change is responsible for record cold outbreaks in Europe the past two years or the heavy snowstorms along the U.S. East Coast. All we know right now is that the behavior fits the current theory.”
It is not clear yet how much influence sea ice has on weather around the world—but researchers do know that the systems are closely connected. Continued study and observation will help them figure out where coincidence ends, and cause and effect begin.
Francis, J. A., W. Chan, D. J. Leathers, J. R. Miller, and D. E. Veron. 2009. Winter Northern Hemisphere weather patterns remember summer Arctic sea‐ice extent. Geophysical Research Letters 36, L07503, doi:10.1029/2009GL037274.
Overland, J.E., M.-Y. Wang. 2010. Large-scale atmospheric circulation changes are associated with the recent loss of Arctic sea ice. Tellus 62A, 1-9.