August 8, 2011

Climate change or variability: What rules Arctic sea ice?

Arctic sea ice is near its annual low extent for the year. Will it reach a new record low? While many people are watching this year’s ice extent closely, the effect of climate change on ice extent in a single year is different than its effect in the long term. Arctic sea ice has declined more than 30 percent in summer since satellite measurements started in 1979. But from year to year, ice extent jumps up and down quite a lot. Scientists now believe that the already-weakened ice cover was pushed to a record low ice extent in September 2007 by a perfect storm of conditions: persistent weather patterns that favored ice loss combined with thinner and therefore more vulnerable sea ice. Just how much of the recent Arctic ice loss was caused by persistent weather conditions, and how much is because of climate change resulting from increased greenhouse gases in the atmosphere?

This photo, taken during the NASA ICESCAPE mission in summer 2011, shows melt ponds on the surface of Arctic sea ice. Weather patterns in the Arctic this summer have favored ice loss, leading to near-record low ice extent over most of the summer. New research is explaining how much ice loss is caused by variable conditions, and how much can be pinned on human-caused climate change. Credit: NASA/Kathryn Hansen

A recent study from Jennifer Kay and colleagues at the National Center for Atmospheric Research in Boulder, Colorado, helps answer that question. The team used a computer climate model called the Community Climate System Model (CCSM4) to study how much natural variability and human-caused climate change contribute to changes in sea ice. Kay said, “We found that observed Arctic sea ice loss during the satellite era cannot be explained by natural causes alone. That’s consistent with previous studies. But we were surprised to see so much variability in the sea ice trends in our simulations.” The study showed that approximately half of the decline in Arctic sea ice from 1979 to 2005 could be blamed on increased greenhouse gases: increasing air temperatures thinned the ice pack over many seasons, leading to other changes like more open water and a warmer ocean surface that promote even more melt. But the other half of that decline resulted from variability that is a part of the climate system, such as changes in wind patterns. That means that even in a warming world persistent periods of weather that encourage ice growth—cold temperatures or ice-spreading winds—could  cause the ice to expand for a few years before declining again.

Researchers often use models to study climate change to see how complex factors work together. The CCSM4 model that Kay and her colleagues used includes atmospheric, land, and ocean processes that all interact, allowing the researchers to run experiments that can simulate how sea ice behaves. Kay said, “Models let us replay the 20th century multiple times to quantify the relative contributions of climate variability and greenhouse gas increases to observed and modeled hemispheric sea ice trends.” The new study suggests that inherent variability in sea ice extent will make it impossible for scientists to predict exactly when the Arctic will lose its ice. “What happens on short timescales depends a lot on inherent variability that may be impossible to predict,” said Kay. But in the long term, the influence of climate change means that sea ice will continue to decline. Kay said, “There is no escaping that we will see an Arctic with no summer sea ice this century if we continue to rapidly increase greenhouse gases in the atmosphere.”

Kay, J. E., M. M. Holland, and A. Jahn. 2011. Inter-annual to multi-decadal Arctic sea ice extent trends in a warming world. Geophysical Research Letters 38, L15708, doi:10.1029/2011GL048008.