If you have been following the topic of Arctic sea ice, you have probably seen a few headlines warning that the Arctic is about to lose its summer ice cover. Although scientists agree that Arctic sea ice is declining, they have published several different estimates of when the last of the sea ice will melt in summer, leaving more ocean surface open to absorb heat and add to global warming. What is behind all these different predictions?
What does ice-free mean?
When researchers say the Arctic could soon be nearly ice free, they do not mean that every last ice floe will melt, turning the Arctic Ocean into a new Caribbean Sea. Ice will still form each winter, as the temperature falls far below freezing. Even in the summer, new studies indicate that some of that ice will remain, pushed by the prevailing winds to pile up along the northern Canadian coast.
Stephanie Pfirman, professor of oceanography at Barnard College, says “Many climate scientists use the term ‘ice-free’ in a functional way, as a shorthand to mean very little reflective surface at the end of the summer when sea ice is at its minimum.”
In its 2007 report, the Intergovernmental Panel on Climate Change (IPCC) said that in the “business as usual” scenario, the Arctic could become ice free in summer around 2100. But later that year, Arctic sea ice hit an unprecedented record low, and for the last three years sea ice has remained far below IPCC projections. Why was reality so different than the IPCC expected?
Predicting future sea ice extent is a tricky task. Just as past data and computer models help forecasters predict weather in the short term, researchers take the best data they have on Arctic sea ice, and combine it with climate models for the region to try to forecast ice extent years into the future. But none of the models perfectly replicate the Arctic climate, and weather causes big variations from year to year that are hard to predict. For example, the models that IPCC scientists studied lacked sufficient data on changes in sea ice thickness. On top of those challenges, changes in Arctic climate may alter the way weather and sea ice interact with each other, leading to unexpected effects.
Studies by James Overland at NOAA and Muyin Wang at the University of Washington predict that the Arctic will be nearly ice free in three or four decades. Overland and Wang combined observational data with climate models to reach their conclusions. However, other research suggests that the Arctic could lose its ice even sooner. For example, a study by Wieslaw Maslowski, of the Naval Postgraduate School in Monterey, California, suggests that the Arctic could lose its summer ice cover by the end of this decade. Maslowski’s work is different from Overland/Wang’s in that it analyzed changes in recent observations and estimates of sea ice volume to project—rather than predict—future sea ice loss. That is, Maslowski’s study estimated how fast sea ice volume might decline if the current loss rate continues on a linear path.
What is the right number?
Nobody knows exactly when the Arctic will lose its summer ice cover, because changes in the ice are introducing even more changes to the sea ice and Arctic climate. However, most researchers agree that it is a question of when, rather than if we will see ice-free summers. Maslowski said, “If ice is melting, the ocean can absorb more and more solar energy.” And that extra heat will have to go somewhere – limiting winter ice growth and accelerating summer melt, affecting weather patterns, melting glaciers in Greenland, or disrupting ocean circulation patterns. Overland said, “When sea ice goes from a present loss of 30 percent to an 80 percent loss in the future, the added heat storage could have larger effects on climate.”
Pfirman, S. L., B. Tremblay, R. Newton, and C. Fowler. 2010. The Last Arctic Sea Ice Refuge. American Geophysical Union, Fall Meeting 2010, abstract #C43E-0592
Stroeve, J., M. M. Holland, W. Meier, T. Scambos, and M. Serreze. 2007. Arctic sea ice decline: Faster than forecast, Geophys. Res. Lett., 34, L09501, doi:10.1029/2007GL029703.
Wang, M., and J. E. Overland. 2009. A sea ice free summer Arctic within 30 years? Geophysical Research Letters, 36, L07502, doi:10.1029/2009GL037820.