West Coast warming blamed on natural causes, not human activity

A century-long warming of the West Coast of North America has occured mostly due to natural changes in winds and not human-induced activity such as greenhouse gas emissions, a new study suggests.

The average temperature along the West Coast increased by about 1 degree Fahrenheit between 1900 and 2000. The study, published in the Proceedings of the National Academy of Sciences, found that changing winds that affect ocean circulation were responsible for more than 80 percent of the warming trend along the Pacific Northwest coast between Washington and Northern California. In Southern California, wind patterns accounted for about 60 percent of the increased warming.

"Changing winds appear to explain a very large fraction of the warming from year to year, decade to decade and the long-term," study author James Johnstone, an independent climatologist who did most of the work for the study when he was at the University of Washington's Joint Institute for the Study of the Atmosphere and Ocean, told the Los Angeles Times. The paper explains that a weakening of coastal winds slowed down evaporation and altered ocean currents, which boosted local temperatures.

The researchers determined that most of the temperature increase in the region happened before 1940, and that human activity such as greenhouse gas emissions was not a major factor.

"It's a simple story, but the results are very surprising: We do not see a human hand in the warming of the West Coast," study co-author Nate Mantua, now with NOAA Fisheries' Southwest Fisheries Science Center, told the Seattle Times. "That is taking people by surprise, and may generate some blowback."

But the results do not suggest there's no relationship between human activity and global climate change, the researchers stressed.

"This [study] doesn't say that global warming is not happening," Mantua told the paper. "It doesn't say human-caused climate change isn't happening globally. It's a regional story."

However, the new findings do raise questions about how well climate change models can predict information about changes in local temperatures.

Some experts who were not involved in the study expressed skepticism of the quality of the early 20th-century data the scientists used in the study.

"The principles they are putting forth in the paper I agree with, but as you go back further and further in time you start to increase the amount of error inherent in the data," John Abatzoglou, a climatologist at the University of Idaho, told the Seattle Times. Abatzoglu co-authored a study earlier this year that determined that human issues were a leading cause of temperature rise in the Northwest.

In addition, Amy Snover, the head of Climate Impacts Group at University of Washington, told the paper that the study doesn't contradict the long-term trend of global climate change.

"I think what it does show is that there are aspects of regional climate that these models could do better at," Snover said. "But we know we're in for a bumpy ride. We know that the influence of humans on climate is only growing over time. We expect over coming decades for that influence to get bigger and bigger."

The new study was published Monday in the journal PNAS.