A new study zeroes in on just how much of the Arctic sea ice's precipitous decline in recent decades is attributable to manmade global warming and how much is due to natural processes, finding that between 50 and 70 percent is caused by mankind.
By studying air current patterns, researchers determined that between 30 and 50 percent of the decline in summer sea ice in the Arctic since 1979 may be due to natural processes, for the first time breaking down just how much melting could be attributed to each cause. The study was published Monday in the journal Nature Climate Change.
"If we look to Earth as a whole, the global temperature rise is definitely due to anthropogenic forcing in the last 100 years. There's no question of that," said author Qinghua Ding, a professor at the University of California Santa Barbara. "But if we zoom in, some internal role comes in to have some impact."
Arctic sea ice has been rapidly declining since satellites first started tracking it in 1979, and according to NASA, roughly 13.3 percent of the ice disappears every decade. Models have projected that manmade global warming would heat the Arctic faster than it would heat more temperate regions, and observation has borne that out. The Arctic is warming at least twice as fast as the rest of the globe, and the first two months of this year both had the lowest levels of sea ice on historical record.
Though models have accurately predicted for decades that the ice would decline as the amount of carbon in the atmosphere increases, in recent years, the actual rate of melting has outpaced the models.
That's where Ding's study comes in. What he and his coauthors found is that air currents that are a part of Earth's natural variability have played a significant role in melting the ice, which helps explain why the models have underestimated the melting.
In an earlier study, Ding looked at how changes in air circulation in the tropical Pacific have accelerated warming over Greenland and northeastern Canada. For this study, Ding and his colleagues created a complex model that took into account all of the variables that can influence the melting of sea ice. One by one, they turned off variables that are driven by manmade global warming, until they were left with just the amount of ice melt that has been caused by air circulation.
"Nobody knew how important the air circulation was," Ding said. "Probably 10 or 20 percent, they thought, but here we show that it's much more."
"Their study certainly illustrates the complicated nature of the climate system," said NASA senior climate change scientist Claire Parkinson, who was among the first to model the relationship between sea ice and warming. She was not a part of Ding's study.
Ding said his findings in no way minimize the role of anthropogenic warming in melting sea ice.
"I don't want people to take the wrong message in our study, that we're not to blame for Arctic warming," said Ding. "The message is that it's more complex than we expected. In the long term, maybe 100 years from now, the Arctic will become ice free in summer because eventually this internal variability will be overwhelmed by anthropogenic forcing."
But in the short term, he said, both factors will be important.
Ding explained the phenomena like this: Imagine the Arctic is a room and the floor is the sea ice. Manmade global warming is the equivalent of turning up the thermostat. If you then put a thick blanket over the ice/floor, that represents that secondary role played by Earth's natural cycle.
As time goes on and mankind emits more CO2, causing more warming, eventually the man-made aspect—the thermostat—will be what causes the ice to disappear.
Mark Serreze, the director of the National Snow and Ice Data Center, called the paper "a good effort to try and sort out the relative roles of changes in atmospheric circulation and anthropogenic forcing."
"But we still have a lot to learn," Serreze said. "And in the end, given current greenhouse gas emission rates, natural variability will not halt the slide towards a seasonally ice-free Arctic Ocean."
A recent study found that if the world warms 2 degrees Celsius over preindustrial times—the lofty goal laid out in the Paris climate agreement— there is still a 39 percent chance that the summer sea ice will disappear.