As smoky, apocalyptic-looking skies spread across western North America, researchers are trying to improve their predictions of wildfire risk. A changing climate means not only higher temperatures, but also new patterns of rain and snow, which interact in complex ways to increase the risk of fire.
A paper in PNAS this week argues that the role of temperature has been exaggerated, while the importance of rain has been overlooked. Zachary Holden and a team of collaborators report that, among a range of different factors, extended dry spells were the best predictor of wildfire risks. However, other researchers argue that the article has overlooked previous research and that the results are not definitive. Everyone agrees that the risk of wildfires will increase as a result of climate change, but the grainy details are harder to pin down.
Hot, dry and fiery
This investigation is in a strange place between the blindingly obvious and the intensely obscure. The contributions of rising temperatures to the risk of wildfires in summer are intuitive: less snow, earlier snowmelt, faster evaporation of water from the environment, warmer and drier vegetation. The role of drought in making things warmer and drier is also clear.
If every place kept getting hotter and drier at exactly the same rate, it would be easy. The complication is that the projections of changes in rain, snow and temperature are different for different regions and even for different elevations in those regions, so predicting risk means calculating the relative contributions of all those factors. “Meaningful use of climate projections can only happen if we can accurately link the different climate elements to fire,” Holden and colleagues write.
To estimate those contributions to overall risk, the researchers looked at historical fire data from 1979 to 2016 in the western US, and compared it with data on snow, temperature and rainfall. They found evidence that less rain fell over time and also that the number of days with a fair amount of rain (just over 2.5mm) decreased. “We’re pretty quietly breaking records here for really long dry spells,” Holden says.
When they compared the different factors, they found that the biggest contributors came from the dry spells: the more days without significant rainfall, the greater the chance that a larger area in the region will be covered by wildfires. The researchers, many of whom are based at the US Forest Service, plan to use the data to improve predictions. “The primary focus of this work was to improve the way we characterize wildfire hazards so that fire managers can make better decisions,” explains Holden.
The temperature hasn’t gone down yet
However, other researchers are not convinced by the results. LeRoy Westerling, whose work on wildfire risk has emphasized temperature as a primary factor, points to his own research that identified the decline in rainfall, but he didn’t find that it played such a large role in wildfires.
The two studies made different decisions when choosing and analyzing their data. For example, Holden and his colleagues studied three different ways to measure snowfall: the peak amount of snow in a given year, the number of snow-free days between March and September, and the amount of snow remaining on April 1. snowfall to use in their analysis, as it had the strongest correlation with wildfire. Westerling and his colleagues, on the other hand, used the latest permanent snowpack date. These different stats are all related, but not identical.
Westerling also points to the inclusion of 2016, a record-breaking drought year, in the dataset. While the drought itself is clearly part of the history of the changing climate, the drought was “followed by one of the wettest years,” he says. “We don’t really know if we’re going to have more droughts or more wet years; the trend we have seen is towards more variability.”
It’s helpful to think of the different findings as boundaries that set the boundaries of what’s likely to be true, says Park Williams, whose work has explored the role of drought in wildfires. His interpretation, he explains, is that his work “should be interpreted as an upper bound on the importance of atmospheric heat/drought,” while the new research should be interpreted as a lower bound.
“The reality is somewhere in between,” he explains. “This paper will motivate future work to more thoroughly unravel the closely intertwined effects.”
PNAS2018. DOI: doi.org/10.1073/pnas.1802316115 (About DOIs).