Monday’s little rainshowers didn’t end the 2018 fire season, but they may have had a much bigger effect than last spring’s big snowpack.
A new study from the U.S. Forest Service’s Rocky Mountain Research Station in Missoula found that summer rains, or their lack, have 17 times more impact on forest fire acres burned than how much snow got stockpiled in the mountains the previous winter. The Missoula area just went 47 days without so much as a sprinkle. That’s the most since we started keeping that statistic in 1948.
“In the last 37 years, we’ve seen a substantial reduction in summertime precipitation,” said Zach Holden, one of the co-authors of the new study. “This definitely points to changing patterns of climate and weather as drivers of fire activity.”
Fire forecasters have long measured winter snowpack and summer temperature levels to predict the severity of the coming fire season. Warming summers reduced snowpacks, and thus soil moisture. Hotter air temperatures also dried out trees and brush faster, making them easier to catch fire.
Those predictions inform where to pre-position firefighters and equipment, allocate budget funds and warn communities of changing threat levels.
But looking back over several decades of past fire seasons, another trend poked up. Summer rains could have a big effect on how long the season lasted and how much got burned. In addition to dampening fuels, the cloudy weather increased humidity, cooled the ground and generally slowed fires. On the other hand, lack of rain made fuels dry out faster, increased ground temperatures and produced bigger fires.
Holden said adding the rain factor didn’t mean snow and temperature weren’t important. The new study just showed rain was a more influential force than the other measuring stick. And unfortunately, it’s a much harder one to predict.
But it appears to be tied to other trends in global conditions. In particular, the rapid shrinking of the Arctic icecap appears connected to more frequent droughts in the continental United States and temperature extremes. A new study from the Potsdam Institute for Climate Impact Research and Vrije Universiteit Amsterdam found that recent near-doubling of Arctic summer temperatures was slowing down winds that drive storms at lower latitudes, resulting in long-lasting heatwaves or downpours in North America, Europe and parts of Asia.
“Giant airstreams encircle our globe in the upper troposphere — we call them planetary waves,” Potsdam Institute Director Hans Joachim Schellnhuber told the Eureka Alert scientific news service. “Now evidence is mounting that humanity is messing with these enormous winds. Fueled by human-made greenhouse gas emissions, global warming is probably distorting the natural patterns.”
As an example, the Potsdam paper pointed to Alberta’s 2016 fire season, where unusually high temperatures and dry conditions resulted in the most destructive fire in Canadian history. The 1.5-million acre fire destroyed large parts of Fort McMurray and did more than $5 billion in damage.