A study, likely to add to the already heated debate about humans' role in global warming, suggests a century of increasing temperatures around the U.S. Northwest is the result of a natural trend, not human activity.

The rising temperatures are not solely human-caused but can be explained by natural alterations in ocean wind patterns, say researchers who report analysis of ocean surface temperature between 1900 and 2012 and wind measurements for the same period yield a close correlation.

Scientists have been aware for some time that sea temperatures drop when ocean waves are stirred up by strong winds, but increase when winds drop and seas become calm.

Now researchers, writing in the journal Proceedings of the National Academy of Sciences, suggest the U.S. Northwest has warmed because ocean winds in the region have become weaker and have shifted direction.

"The concept of winds controlling or affecting ocean temperature in that very way is not controversial, but the strength of that relationship was quite amazing," says study leader and climatologist James Johnstone.

"What we found was the somewhat surprising degree to which the winds can explain all the wiggles in the temperature curve," he says.

The findings suggest there are factors stronger than human-emitted greenhouse gases driving the temperature increases, he concludes.

Not all climate scientists were willing to accept the conclusion at face value, disputing the researchers' assertion that the observed alterations in wind patterns and speeds were completely natural and were not themselves a result of climate change.

"This may say more about the state of climate modeling than it says about causes of warming in the Pacific Northwest," says Ken Caldeira, atmospheric scientist at the Carnegie Institution for Science. "The authors ... have not established the causes of these atmospheric pressure variations. Thus, claims that the observed temperature increases are due primarily to 'natural' processes are suspect and premature, at best."

Johnstone responded by pointing out that an extreme warming period recorded between 1920 and 1940 predates any significant increase in global greenhouse gases, while in the period 1998-2013 -- when average global temperatures hovered at all-time high levels -- there was a marked stretch of ocean cooling.

The researchers acknowledge that the findings may not apply to climate change everywhere, and the area chosen for the study may not be typical.

"Look at a map of the world's hot spots for wind variations, and the North Pacific shows up as a pretty special place, one of those hot spots," says study co-author, Nathan J. Mantua of the federal government's Southwest Fisheries Science Center in California.

Still, the researches assert, a large percentage of the temperature increases in coastal and ocean areas of the region could be linked to a weakening in the North Pacific high-pressure zone dictating weather in the northeast Pacific Ocean.