Sea ice around Antarctica is very variable from year to year—we've seen the highest winter extents of the last few decades a few years ago, but record low extents have occurred in recent months. Although the trend of shrinking sea ice in the Arctic has been clearer, this doesn’t mean natural variability isn’t significant there, too. In any given year, a few storm systems can bring warm weather or blow sea ice around, deciding whether a new record low is reached. But can some meaningful portion of the Arctic trend over the last 40 years be blamed on longer-term variability?

It’s difficult to tease apart the human-caused and natural effects precisely here, so we haven’t had a great answer to that question. A new study led by the University of California, Santa Barbara’s Qinghua Ding worked out a different way to examine the question, looking for interesting patterns of atmospheric circulation.

The researchers looked for correlations between the summer sea ice low point in September and atmospheric conditions over the preceding summer months. They found a connection between the area of fastest sea ice loss—the Beaufort, Chukhchi, and East Siberian Sea side of the Arctic Ocean—and high-level atmospheric circulation over Greenland and northeastern Canada. High pressure systems in the upper troposphere there cause air to descend as it circles around the Arctic, warming as it comes down to a lower altitude.

That particular pattern was more likely in years with lower sea ice extent and became gradually more likely over the past few decades. The warmer air, which picks up more water vapor, increases the amount of infrared (heat) radiation being emitted toward the surface, causing more sea ice to melt.

You could imagine, though, that shrinking sea ice is responsible for these atmospheric patterns, since exposed ocean absorbs more sunlight. To test this, the researchers ran an array of climate model simulations that tweaked one variable or another. Most forced the upper atmosphere to roughly mirror the observed circulation patterns, with sea ice and an ocean that either mirrored observations or was free to respond to the simulated weather conditions. In other simulations, sea ice was controlled while the atmosphere responded more freely.

The results showed that while this pattern of atmospheric circulation does reliably cause more sea ice melt, small differences in sea ice loss didn’t have much effect on local circulation in the summer.

In one simulation in which this atmospheric circulation pattern was effectively deleted, sea ice loss was only about 40 percent as bad as simulations that included it. That means that the increasingly common pattern the researchers identified above Greenland could be responsible for as much as 60 percent of Arctic sea ice loss since satellite observation began in 1979. But there’s one final hurdle—what if that circulation pattern isn’t entirely natural, but is itself a consequence of global warming?

To attempt to account for this, the researchers examined the trends in winds in all the model simulations that were compiled for the last Intergovernmental Panel on Climate Change report. Then they ran another simulation with the average of those wind trends subtracted from the virtual atmosphere—essentially simulating a world that warmed, but in which humans had no influence on atmospheric circulation. This change resulted in about a 30 percent reduction in sea ice loss.

Combining that result with the 60 percent of sea ice loss caused by the circulation pattern above Greenland, the researchers come to their final conclusion: about 30 to 50 percent of the observed loss of Arctic sea ice was the result of natural variability. Put another way, natural variability caused additional sea ice loss, by chance, but at least half of the loss was due to human activities.

This has some interesting implications, starting with the fact that sea ice loss has generally outpaced climate model projections so far. Much discussion of that fact has centered on the need for improvements to sea ice models. This study suggests that the models might be doing better than they appear.

The results also provide a reminder that predictions of when the Arctic Ocean will first become ice-free in the summer can’t get much more precise than a range of a few decades, because natural variability will have a strong say in the matter. Even so, human-caused global warming has at least as strong a say in how quickly sea ice shrinks in the Arctic.

Nature Climate Change, 2016. DOI: 10.1038/NCLIMATE3241 (About DOIs).