It has been a subject of debate for years: How much has global warming contributed to a documented rise in temperatures along the West Coast?
A new study published Monday in a major research journal suggests the answer thus far, particularly in the Northwest, is: hardly any.
An average coastal temperature increase of 1 degree Celsius since 1900 along the West Coast appears more likely to be the result of changes in winds and air circulation over the eastern Pacific Ocean, two former University of Washington scientists found.
And the researchers said they could find no evidence that those weather patterns were being influenced by human greenhouse-gas emissions.
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“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,” said co-author Nate Mantua, now with NOAA Fisheries’ Southwest Fisheries Science Center. “That is taking people by surprise, and may generate some blowback.”
Climate scientists for years have acknowledged that Pacific Northwest weather can vary naturally year to year — or even decade to decade. But many have argued that human burning of fossil fuels is already a huge factor driving up regional temperatures.
But the new research by Mantua and lead author Jim Johnstone, formerly with the UW’s Joint Institute for the Study of the Atmosphere and Oceans, suggests that natural variation in weather accounts for the vast majority of regional temperature increases for the last 113 years.
The study found wind responsible for more than 80 percent of the warming from Northern California to the Northwest.
In Southern California, winds accounted for about 60 percent.
“It was a big eye-opener,” Johnstone said. “The winds have changed in a manner that explains virtually all of the coastal ocean warming. The winds appear to decide it all.”
Both authors were quick to point out that their study does not in any way refute that temperatures globally are on the rise or that humans are responsible for that trend. The Northwest just happens to be a region where wind and weather swing far more wildly than, say, the tropics, drowning out any climate signal.
“This doesn’t say that global warming is not happening,” Mantua said. “It doesn’t say human-caused climate change isn’t happening globally. It’s a regional story.”
Nor does it suggest that greenhouse gases won’t be a significant regional factor in the future.
But it does raise questions about how well global climate models can be used to predict the near-term for specific regions.
“There’s been so much pressure to get local and regional climate information, because that’s where people live and plan and experience the climate,” Mantua said. “But this raises some questions about whether the models are capturing the connection between human-caused changes and natural variability well enough to interpret these local and regional records.”
Said Johnstone: “There are projections, based on carbon-dioxide emissions, that predict substantial warming here for the next few decades. Having looked at the way the temperatures actually behave, I’d be hesitant to say if that’s the case. I don’t know whether it will warm or cool or stay flat.”
The study, published in the Proceedings of the National Academy of Sciences, raised eyebrows among several scientists.
“That sort of flies in the face of many, many years of research and modeling,” said John Abatzoglou, a climatologist at the University of Idaho, who co-authored a study earlier this year that deemed human issues a leading cause of temperature rise in the Northwest. “That leads me to question whether or not the results are robust.”
Others called the paper “significant” and said Johnstone and Mantua highlighted the thorny work of trying to tease apart the details that are driving global change.
“We can’t just assume that if it’s warming it must be climate change,” said UW atmospheric scientist John M. Wallace.
Mantua agreed: “The climate system isn’t that simple. History has been playing tricks on us.”
In fact, the road to their conclusions started in the fog.
A few years ago, Johnstone was trying to understand why fog in Northern California had declined so precipitously. He determined it was linked closely to sea and air temperatures and regional air-circulation patterns, which he tracked back to the early 1900s.
But then he had another question: How closely tied were temperatures with these regional winds?
”I didn’t have to dig hard for the answer at all,” Johnstone said. “It just seemed to present itself.”
He found that temperature dating back to 1900 was very consistently linked to coastal winds. But Johnstone and Mantua could find no link suggesting that climate change had altered those winds.
Guillaume Mauger, a UW climate scientist, said he had a few questions about the details of the study but didn’t dispute the findings.
“As these things go, it’s likely to get watered down the more people look into it, but it looks like they’ve done their homework well,” he said.
Abatzoglou, however, was skeptical about the quality of the early 20th-century data the study’s authors relied upon to reach their conclusions.
“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,” he said.
But he and Amy Snover, head of UW’s Climate Impacts Group, said the larger point is that the study doesn’t change the long-term trend.
“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.”
On that, the study’s authors agreed.
“Global warming is still proceeding,” Mantua said. “And it’s still a really huge deal that’s going to shape the future and be a bigger and bigger part of our story.”