This summer, the Western United States saw a truly devastating wildfire season. Across the country, more than 48,000 wildfires raged, damaging more than six million hectares of land. It would be nice to think that humans weren’t the primary cause of these events and that natural changes in weather patterns contributed to how dry and fire-prone parts of the world have become.
But the reality isn’t so nice. Climate change is likely the cause of the wildfires, according to new research that aimed to quantify just how much blame we can lay at the feet of natural causes when it comes to the increasing rates of wildfires in the US’s West. “We want to know how much this increase in fire weather is just changing weather patterns and how much cannot be explained by changing weather patterns,” Rong Fu, one of the paper’s authors and a professor at UCLA’s Department of Atmospheric and Oceanic Sciences, told Ars.
The research began around a year ago. Fu and some of her colleagues live in California and were all impacted by the wildfires, so they wanted to investigate what is causing them.
How bad is it?
Fu and her team deployed a technique called an “ensemble constructed flow analogue.” In short, they looked back in time between 1979 and 2010 in the region and found cases in which the naturally occurring weather looked much as it does now. From there, they looked at a figure called the vapor pressure deficit (VPD)—basically, how dry and “thirsty” the area is—both in the historic and modern cases.
VPD is the leading cause of forest fires on the US West Coast and many other places around the world. So if the modern VPD is higher than it was in historic cases with similar weather patterns, it would suggest that there is a non-natural component to how fire-prone things are currently. “Basically, the difference between [the present VPD] and the [VPD] we can get from the same weather patterns in the past is due to climate change,” she said.
Fu noted that these cases won’t have the same weather patterns, however. She also added that the work took into consideration various other factors that might have played a role, such as changes in vegetation.
In all, the research suggests that only around 32 percent of VPD trends can be attributed to natural causes. The remaining 68 percent or so cannot—and it’s likely that climate change is the culprit.
Ruh-roh
To make matters worse, 68 percent is a conservative estimate. The paper notes that the number could be as high as 88 percent. Fu noted that the past weather data her team used was likely also impacted by the fact that humans were emitting carbon and otherwise mucking around with the climate even back then. “The reference period is already being affected by greenhouse gases,” she said.
As a result, their estimates of climate change impacts is not as large as it would be in reality; it’s on the more conservative side of things. “We want to be as conservative as possible. That way, when we say, ‘Climate change contributes two-thirds of [the increase to] fire weather,’ we know that’s likely to be true and only an underestimate of climate change,” she said.
Trying to suss out exactly how much human activity impacts climate change is a tricky business because the climate changes to some degree on its own. But this research is another step toward understanding just how much humans are responsible. Further, according to Fu, the methods used in this paper could be deployed elsewhere around the world. “I think this approach can be generalized to other areas,” she said.
PNAS, 2021. DOI: 10.1073/pnas.2111875118 (About DOIs)