NCAR scientist suggests that if humans were still only hunters and gatherers, sea ice might have retreated anyway over the past 30 years

Human activities linked to warming, loss of sea ice: Weather | Alaska news at adn.com

About half the recent record loss of Arctic sea ice can be blamed on global warming caused by human activity, according to a new study by scientists from the nation's leading climate research center.
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The peer-reviewed study, funded by the National Science Foundation, is the first to attribute a specific proportion of the ice melt to greenhouse gases and particulates from pollution.

The study used supercomputers named Bluefire and Franklin and one of the world's most sophisticated climate models to reach its conclusions, said lead author Jennifer Kay, a staff scientist at the National Center for Atmospheric Research
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Kay said the climate model she chose, Community Climate System Model version 4, had been developed by teams of scientists over several decades. She ran some 4,000 years of data through the model, a period when volcanoes, solar variations and other factors were known or believed to have forced climate changes.
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The model accurately "predicted" what actually occurred historically, validating the method, she said.

But more to the point, by replaying the climate forces of the 20th Century over and over through the model, the scientists were able to show that variability can account for only half the loss of ice, she said.

That means that if humans were still only hunters and gatherers, sea ice might have retreated anyway over the past 30 years.
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As to the future, with the climate warming and the Arctic ice thinning still further, you can throw away the term "natural variability," she said. Where heavy sea ice once tended to dampen climate variability, that natural climate is gone. With the ice pack reduced, she said, year-to-year and decade-to-decade volatility increases.

"Our work really demonstrates that the variability in the climate model simulations is not entirely natural by the end of the 20th Century," she said. "That's why we call it in the paper, 'internal variability.' We're in a warmer state now, so we have different variability than we did before when it was just natural variability."