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Natural Wind Changes Warming West Coast

Sep 23, 2014 11:42 AM EDT

Naturally occurring changes in winds, not human-caused climate change, are responsible for most of the warming along the US West Coast over the last century, according to a new study.

Since 1990, coastal areas of the northwest United States have warmed by about 1 degree Fahrenheit. The majority assume that buildup of greenhouse gases in the atmosphere, thanks to us, is the main driver of increasing temperatures seen over the last few decades in the ocean and along the coastline from Alaska to California. But this new study, published in the journal PNAS, suggests that natural, wind-driven climate patterns in the Pacific Ocean, such as El Niño, play more of a part than is realized.

"There does seem to be a tendency for any [century-long] trend in temperature - a warming trend - to be interpreted in terms of human effects," study leader James Johnstone of the University of Washington told New Scientist. "We looked to see if we could verify that."

Together, Johnstone and his colleague Nathan Mantua compared the air pressure at sea level - an alternative for measuring wind strength - with air and sea surface temperatures since 1900. They found that weakening winds accounted for more than 80 percent of the warming trend along the Pacific Northwest coast between Washington and Northern California, while they were responsible for about 60 percent of the increased warming in Southern California.

"The most straightforward explanation is that changes in the wind have forced the changes in the temperature," Mantua added.

When coastal wind speeds weaken, they result in less evaporation from the sea surface and unusually low pressure that alters ocean currents and causes temperatures to rise over time. The researchers are not sure what exactly is causing these wind changes, but, according to Johnstone, there's no obvious explanation that links it to global warming, at least in this region.

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