Oceanographer James Overland, who reviewed Graversen's study for Nature, said the research dovetails with an upcoming article of his which concludes that the Arctic thawing is a combination of the two.
"If we didn't have the little extra kick from global warming then we wouldn't have gone past the threshold for the change in sea ice," said Overland, of the National Oceanic and Atmospheric Administration's lab in Seattle.
Other researchers said Graversen's study underestimates the effect of global warming because it relied on older data that stopped at 2001 and wasn't the most accurate.
Overland and scientist Mark Serreze disagree over which effect - man-made or natural - was the big shove that pushed the Arctic over the edge, but they agreed that overall it's a combined effort.
"Think of it as a boxer that's almost going down for the count ... and that one blow to the noggin comes and he's down for the count," said Serreze, a senior scientist at the government's snow and ice data center in Boulder, Colo.
