A new study suggests that natural variability has been an extra push for Arctic sea ice to decline faster than predicted by most models of climate change in recent decades.
The study, by researchers with the University of Washington (UW), the University of California, Santa Barbara, and U.S. government, blames natural variability in the atmosphere over the Arctic Ocean for a substantial chunk of summer sea ice loss in recent decades.
It builds on previous work by Qinghua Ding, a climate scientist at the UC Santa Barbara who holds an affiliate position at the UW, where he worked with other researchers and found changes in the tropical Pacific Ocean have in recent decades created a "hot spot" over Greenland and the Canadian Arctic that has boosted warming in that region. The hot spot is a large region of higher pressure where air is squeezed together so it becomes warmer and can hold more moisture, both of which bring more heat to the sea ice below.
The new project focused specifically on what this atmospheric circulation means for Arctic sea ice in September, when the ocean reaches its maximum area of open water.
Ding designed a new sea ice model experiment that combines forcing due to climate change with observed weather in recent decades. The model shows that a shift in wind patterns is responsible for about 60 percent of sea ice loss in the Arctic Ocean since 1979. Some of this shift is related to climate change, but the study finds that 30-50 percent of the observed sea ice loss since 1979 is due to natural variations in this large-scale atmospheric pattern.
"Anthropogenic forcing is still dominant -- it's still the key player," Ding, first author of a paper published in Nature Climate Change, was quoted as saying in a news release from UW. "But we found that natural variability has helped to accelerate this melting, especially over the past 20 years."
"What we've found is that a good fraction of the decrease in September sea ice in the past several decades is most likely natural variability. That's not really a surprise," said co-author David Battisti, a UW professor of atmospheric sciences. "The method is really innovative, and it nails down how much of the observed sea ice trend we've seen in recent decades in the Arctic is due to natural variability and how much is due to greenhouse gases."
The long-term natural variability is thought to be driven by the tropical Pacific Ocean. Conditions in the tropical Pacific set off ripple effects, and atmospheric waves snake around the globe to create areas of higher and lower air pressure. However, "in the long term, say 50 to 100 years, the natural internal variability will be overwhelmed by increasing greenhouse gases," Ding said. "But to predict what will happen in the next few decades, we need to understand both parts."
The tropical Pacific Ocean could stay in its current phase or it could enter an opposite phase, causing a low-pressure center to develop over Arctic seas that would temporarily slow the long-term loss of sea ice due to increased greenhouse gases. "The idea that natural or internal variability has contributed substantially to the Arctic sea ice loss is not entirely new," said second author Axel Schweiger, a UW polar scientist who tracks Arctic sea ice. "This study provides the mechanism, and uses a new approach to illuminate the processes that are responsible for these changes."
