If it seems like severe rain and snow storms are becoming more frequent in western Washington, that's because they are.
A study by Environment Washington released Tuesday, July 31, claims this trend toward more frequent and more intense severe storms is caused by global warming - a claim that is in line with mainstream climate research.
A local weather expert isn't so sure. Cliff Mass, professor of atmospheric sciences at the University of Washington, said linking global warming to a particular weather trend is risky business.
The 43-page study, "When It Rains, It Pours," looked at national rainfall data from 1948 to 2011 and concluded that the frequency of the most severe storms - those that occur once a year, on average - are now happening once every nine months. Also, the amount of precipitation coming out of a given year's biggest storm has increased by 10 percent over that period.
The national trend holds true in Washington. Those once-a-year storms are now happening once every 9.7 months in the state, and the big storms are producing 18 percent more precipitation compared to 1948.
The study goes on to recommend energy policy that would reduce the effects of global warming, including a 35 percent cut in greenhouse gas emissions by 2020.
"We need to heed scientists' warnings that this dangerous trend is linked to global warming, and do everything we can to cut carbon pollution today," said Samantha Cramer, field organizer for Environment Washington.
Mass said there is solid evidence that global warming is real. It's less certain whether it is causing the rainfall trends described in the environmental group's study.
"I think scientists in general believe that global warming is occurring," Mass said. "That's pretty clear, and the main effects are ahead of us."
But is it causing more frequent extreme weather, for example last January's major statewide snowstorm?
"This is something you've got to be really careful about," Mass said.
While Washington has experienced significant increases in extreme precipitation, these storms are occurring less often in Oregon and northern California. In New England, severe storms have increased in frequency by 85 percent - much more than global warming can account for.
Cramer said the report didn't attempt to explain differences among regions.
"We don't really know why New England had the largest trend," she wrote in an email to The Bellingham Herald. "There are a lot of possible relevant changes that could be at play. ... But overall the trends we are seeing are largely consistent with the expected consequences of global warming."
The new study's results may be consistent with global warming but are not necessarily caused by it, Mass said. What scientists glean from weather data sometimes depends on the time period chosen for study.
Mass recalled a dispute he had with a colleague at the University of Washington, who claimed a decade ago that smaller snow amounts in the Cascades since the 1950s were a sign of global warming. Mass countered that there was another, natural climate factor at work. In fact, the snowpack in the Cascades has not changed over the past 30 years.
To make the same point, there's no compelling reason to believe Environment Washington's study period, 1948 to 2011, was dominated by the global warming effect.
"There's all kinds of things happening, all kinds of natural variability," Mass said.
Whether or not global warming is causing more severe weather in western Washington, scientists generally believe the effects of climate change on this part of the state will be relatively small, Mass said. His research group took results from global climate computer models and applied them to the Pacific Northwest.
"Precipitation is not going to change that much here," Mass said. Eventually, global warming will catch up with the Cascade snowpack, as rain replaces snow at higher elevations. Also, there should be more low clouds in the spring and early summer, making for an even gloomier June, Mass said.
He described this prediction from his climate simulations in a 2011 blog post.
"Don't worry," he wrote, referring to the June gloom. "The rest of the summers will get much warmer if these simulations are correct."
ON THE WEB
The full Environment Washington study on rainfall and global warming is at environmentwashingtoncenter.org/reports/wae/when-it-rains-it-pours.
Reach RALPH SCHWARTZ at firstname.lastname@example.org or call 360-715-2266.