USA — The north-western US has experienced 100 years of warming, leading to wildfires and infestations of pests like pine beetles. But the extra heat seems to be linked to natural changes in the winds of the Pacific rather than human-caused climate change.
Since 1900, the north-west US, including northern California and Oregon, has warmed by about 0.8 °C. James Johnstone of the University of Washington in Seattle wanted to see if global warning was responsible. “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,” he says. “We looked to see if we could verify that.”
With colleague Nathan Mantua, Johnstone compared the air pressure at sea level a proxy for wind strength with the temperatures of the sea surface and the air since 1900. They found that every change in sea surface temperature was preceded by a change in the winds.
“The most straightforward explanation is that changes in the wind have forced the changes in the temperature,” he says. This is the same mechanism that is known to drive sea surface temperature changes across the Pacific, which flip every 20 or 30 years in what is known as the Pacific Decadal Oscillation, which was first described by Mantua.
Wind of change
It’s not clear what’s causing the changes in the winds, but Johnstone says there’s no obvious explanation that links it to global warming. Climate change is sometimes blamed for changes in this region, including earlier snowmelt, and increased fire risk and pest attacks. Johnstone says that, to the extent that local temperature rises are responsible for these, global warming is not to blame. The role of climate change in rainfall and drought in the north-west, however, could be a different story, because these are both driven by temperature gradients over much larger areas.
It is not surprising that some regional warming trends are natural, says Axel Timmermann of the University of Hawaii in Honolulu.
Wenju Cai of the CSIRO, Australia’s national research agency, in Melbourne, agrees. He says that huge eddies in the Pacific pull heat west across it, so the coastal north-west of the US isn’t getting much warmth from climate change.