Sea ice in the Arctic has been melting at a record-breaking pace. Scientists blame a warming climate for most of that, but researchers have now teased out a natural cycle for how Arctic sea ice melts year-to-year.

Based on that cycle, they conclude that 30 percent to 50 percent of the melting is due to natural causes, while human-caused warming is responsible for the rest.

Climate scientists have always acknowledged a natural cause for shifts in the rate of Arctic melting. But nailing down just how much nature contributes compared with greenhouse gases rising into the atmosphere has been difficult.

[The results] do not call into question whether human-induced warming has led to Arctic sea-ice decline — a wide range of evidence shows that it has.

Neil Swart of the Canadian Centre for Climate Modeling and Analysis

Researchers at the University of California, Santa Barbara, and several other institutions focused on shifts in atmospheric circulation above the Arctic over the past four decades. In particular, they studied circulation over Greenland and the Arctic Ocean during summers. They found a lot of long-term variation in temperature and humidity, which influences how much solar radiation reaches the surface.

The group concludes that "decadal trends in the hemispheric circulation are an important driver of Arctic climate change, and therefore a significant source of uncertainty in projections of sea ice," according to lead author Qinghua Ding of UCSB.

Writing in the journal Nature Climate Change, the authors note that human-caused warming due to the rise in greenhouse gases still causes at least half of the increase in melting. In fact, surface temperatures in the Arctic have been rising at twice the rate as the rest of world at least.

Neil Swart, a climate scientist at the Canadian Centre for Climate Modeling and Analysis who commented on the study, says the results "do not call into question whether human-induced warming has led to Arctic sea-ice decline — a wide range of evidence shows that it has."

"Rather, the implication is that Arctic sea-ice is less sensitive to human-induced forcing than if one assumes that all loss observed to date is anthropogenically driven," he adds.

But the authors also note that the increase in greenhouse gases in the atmosphere could influence this natural variation in circulation — thus making it something less than "natural."

They say their research should help scientists make better predictions of whether and how soon the Arctic Ocean might lose all its floating sea ice.

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