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Regional climate scientists, studying temperatures and other weather factors since 1901, concluded that humans have caused the Pacific Northwest to warm by an of 1.3 degrees Fahrenheit over the past century-plus. The study is one of the first to isolate the role of greenhouse gases associated with regional warming.

The 1.3-degree increase may not sound like much, but it has lengthened the “freeze-free” season by two to three weeks and is the equivalent of moving the snowline 600 feet higher, says Philip Mote, director of the Oregon Climate Change Research Institute at Oregon State University, and a co-author of the study.

Authors found the warming trend has accelerated over the past three to four decades.

“At the rate the temperature is increasing, the next 1.3-degree bump will happen much more quickly,” Mote says.

Scientists, led by John Abatzoglou of the University of Idaho, examined four different factors to determine the influence of human activities: greenhouse gases and aerosols, solar cycles, volcanic eruptions, El Niño events and the the Pacific Decadal Oscillation. They found volcanic activity led to cooler temperatures in 1961, 1982 and 1991. El Niño events led to warming in several years.

“Climate is a bit like a symphony where different factors like El Niño, solar variability, volcanic eruptions and manmade greenhouse emissions all represent different instruments,” Abatzoglou says. “At regional scales like in the Northwest, years or decades can be dominated by natural climate variability, thereby muffling or compounding the tones of human-induced warming.”

However, he adds, “Once you silence the influence of natural factors, the signal of warming due to human causes is clear – and it is only getting louder.”

Authors found that the Northwest experienced relatively cool periods from 1910 to 1925 and 1945 to 1950, and a warm period around 1940 and from the mid-1980s to the present. The warmest 10-year period in the Northwest was from 1998 to 2007.

Authors found the coldest night of the year has warmed significantly in recent decades.

“Natural variation can explain much of the change from year to year, but it cannot account for this long-term warming trend,” says David Rupp, a research associate with the Oregon Climate Change Research Institute and co-author on the report.

“Climate is complex and you can get significant variations from year to year,” Mote says. “You have to step back and look at the big picture of what is happening over time. Clearly the Northwest, like much of the world, is experiencing a warming pattern that isn’t likely to change and, in fact, is accelerating.”

The study was published in a recent issue of the Journal of Climate, a publication of the American Meteorological Society. It was funded by the U.S. Department of Agriculture and the National Oceanic and Atmospheric Administration.

Steve Law can be reached at 503-546-5139 or This email address is being protected from spambots. You need JavaScript enabled to view it..

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