Over at the work blog today, I shared some comments from Julio Betancourt, a USGS ecologist who knows his way around arid climate, about our recent fires and their connection to climate change. It’s something he wrote a month ago, before Las Conchas, but seems relevant, and I share here with his permission:
I think what some reporters and a good part of the public don’t understand is that ultimate causes for individual fires are extremely difficult to disentangle. A single fire or even a single fire season in a given region (Arizona) or even across the whole West is difficult to attribute to any single cause. Over the past few days, I’ve had reporters ask me to list and rank the causes of the Wallow Fire, including climate change, and I’ve declined comment. I’m comfortable talking about possible causes of long-term trends in regional fires, and even complex interactions between these causes. However, it would be really tricky to justify and rank in order of importance the eventual causes for individual burns like Arizona’s Wallow Fire. As of today (June 15), the Wallow fire has burned nearly half a million acre and is still barely contained.
In the Southwest, long-term trends in wildfires, specifically in ponderosa pine forests, could have multiple causes. Since Europeans got here, livestock grazing reduced the grass cover necessary for light and episodic surface fires that cleared the understory and spared the older trees. Aggressive suppression of wildfires also added to this unnatural accumulation of fuel. Open forests became cluttered with different-aged trees capable of laddering surface fires into the forest canopy or crown.
There are probably other human factors, but here we can only speculate. Maybe human ignitions have increased in ways that heighten the probabilities for large wildfires. Maybe in the past, conducive weather and fuel conditions would lead to big wildfires only once in a while, but now it happens all the time. Or maybe it has to do with the changing flammability of vegetation, something as subtle (and unstudied) as long-term trends in the terpene concentrations of leaf litter or live vegetation.
Climate (and weather) have a lot to do with fire occurrence, and is likely involved in variations and trends. In the Southwest, for example, a good fire year alternates with a bad year depending on whether El Nino or La Nina made for a wet or dry winter. This affects fuel moisture conditions in the subsequent fire season, which here generally runs from April to July. But weather, including the temperature, relative humidity and wind speed at the time of the fire, also matters a lot. Yes, the last few months have been pretty dry, but the Wallow Fire has been fanned by unusually high winds since it started May 29.
Since the late 70?s and 80?s, there has been a sharp trend in springtime (and summertime) temperatures across the West. Averaged across the West, spring now comes nearly 2 weeks earlier than before, and the warming has yielded reduced snowpack, earlier snowmelt, and earlier plant growth to boot. As plants begin growing earlier, there is less and less water left in the soil as summer approaches and the live fuels go dry. Year to year and over the long term, the number of large fires across the West seem to be in lockstep with springtime temperatures.
But all of this gets complicated when we try attribute the cause of a single fire. This La Nina winter has been unusually dry in Arizona. But like most La Ninas, spring was unusually cool and late across the West, including Arizona. We can talk about central tendencies in wildfire activity year to year and over the past century, but when it comes to individual fires, the jury is still out.