Global warming can’t be blamed for 2017’s wild swings in rainfall, but scientists say slightly wetter winters and drier summers might be more common in the future.
Remember when it seemed like the rain would never stop?
Now, folks in Western Washington are anxiously scanning for clouds and wondering if water will ever fall from the sky again.
After setting a record this year for the wettest rainy season, the Seattle area on Tuesday is expected to break the record for the longest streak with no measurable precipitation — 52 days in a row. There’s no rain in the forecast for the rest of the week.
“It has been a pretty amazing transition,” said Washington state climatologist Nick Bond. “Since the middle of June, the faucet has been turned off and the handle has been snapped off and thrown away.”
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But what’s to blame for the weather whiplash? Surely the juxtaposition of soggiest and driest can’t be just a coincidence?
Yes it can be — and it probably is, says Bond.
“It’s just a really unusual deal of the cards.”
Global warming doesn’t appear to be a factor, nor is there any sort of cosmic balancing out of wet and dry spells, he said.
Long-term climate models hint that slightly wetter winters and slightly drier summers could be more common across the Pacific Northwest by the latter half of the century. But the link between greenhouse gases and overall regional precipitation is weak, Bond said.
What’s more certain is that climate change combined with other factors is likely to spark more wildfires in the future — like those now casting a pall of smoke across the region.
“Maybe we are getting a bit of a taste of the future,” Bond said. “It’s like downtown Beijing out there.”
The Sea-Tac Airport dry streak record that is expected to fall this week was 51 days, set in 1951. In some ways, this year’s record is a bit of a geographical fluke, said National Weather Service meteorologist Gary Schneider.
“Most of Western Washington has had measurable rain,” he said. “It just missed Sea-Tac.”
Summers in the Puget Sound region are normally very dry, Schneider pointed out, so the total amount of moisture that didn’t materialize this year is small, so far.
“We’re probably running only about an inch below normal for the 52 days,” Schneider said.
For the year, the Seattle area still enjoys a water surplus thanks to the wet winter and spring. Total precipitation at Sea-Tac since Jan. 1 stands at 28.40 inches — more than 8.5 inches above normal.
While temperature is strongly influenced by rising greenhouse gases, there’s less impact on precipitation, said Philip Mote, director of the Oregon Climate Change Research Institute at Oregon State University.
In a 2013 analysis, Mote and his colleagues found that annual mean temperatures across the Pacific Northwest have risen about 1.3 degrees since 1895, due both to natural factors and greenhouse gases. They also found an increase in spring rainfall, but there was no obvious link to man-made global warming.
“We can identify the human contribution to regional warming,” Mote said. “But we tried the same thing with seasonal precipitation and didn’t find anything.”
Long-term models agree that temperatures will continue to rise in the coming decades, but are “all over the map” when it comes to Northwest precipitation, he said. The majority of models predict a slight uptick in the winter coupled with a modest downturn in summer. Some model projections show as much as a 30 percent drop in summer precipitation, while others see little or no effect.
Rising temperatures are expected to eat into the region’s overall water budget, however, by increasing evaporation, reducing the amount of precipitation that falls as snow, and causing the snow pack to melt earlier, Mote pointed out.
Heat and low snowpack can raise the risk of forest fires, which is also elevated by invasive species, development and a legacy of fire suppression that has left woodlands choked with flammable brush and small trees.
An analysis last year by researchers at the University of Idaho and Columbia University estimated that human-caused climate change alone was responsible for almost doubling the area burned since 1984.
Though dozens of fires are burning in British Columbia and Oregon, a paucity of thunderstorms in the Cascades has so far helped keep the number of fires in Washington low, Bond said. Now, conditions are tinder-dry in some areas, and the National Weather Service is warning of a possibility of thunderstorms in the mountains throughout the coming week.
“I’m going to be watching pretty closely what all this is going to mean for the rest of the fire season,” Bond said.
Global warming can also aggravate heat waves, like the one that seared the region with record high temperatures last week, Mote said. Quillayute, on the Washington coast, hit a record 98 on Aug. 2. Medford, Oregon, soared to 112. Seattle set back-to-back daily records, with highs of 92 on Aug. 3 and 4.
“Every heat wave is probably some amount hotter than it would have been without those increased greenhouse gases,” said Mote, who served on a National Academies of Sciences group that focused on extreme weather events.
The misery quotient was particularly intense last week because it didn’t cool off much at night. The lows at Sea-Tac Airport on Aug. 2 and 3 were 68 and 69 degrees — also records.
Bond and his colleagues analyzed a century’s worth of data and found that hot spells that linger after dark are becoming more common across the region. The change is consistent with what’s expected from global warming.
High temperatures will continue to hover in the low- to mid-80s through the rest of the week, Schneider said. Beyond that, models suggest a shift to a cooler, cloudier pattern that could bring rain early next week. But the outlook still remains uncertain — except in the long term.
“Even if it stays dry through the fall, it will be raining by November one way or another,” Schneider said.