This raises a question: Are wildfires in the Western United States getting bigger and more severe? There's a fair bit of evidence that yes, they have been. And, ecologists and fire experts say, that's not a fluke. Thanks to both climate change and shifting forestry practices, humans may bear some responsibility here.
First, the numbers: A 2009 report (pdf) from the U.S. Global Change Research Program describes how "both the frequency of large wildfires and the length of the fire season have increased substantially in recent decades." Here's a chart showing the sharp uptick since the 1980s:
Note that there's plenty of variability. This year, for instance, even though the fire season in the Rocky Mountain got off to an early and ferocious start, the number of fires and the amount of acreage burned across the country as a whole is still below the 10-year average. That said, there does appear to be an overall upward trend. Wildfires are getting bigger and more frequent. In areas such as the Sierra Nevadas, they're becoming far more destructive to the forest habitat as well.
Why is that? Craig Allen, a research ecologist at the U.S. Geological Survey in Los Alamos, New Mexico, explains that the trends appear to have been driven, at least in part, by a confluence of three factors:
1) Global warming. Huge wildfires are, of course, more likely during droughts, when the forests are dried out and filled with kindling. And many parts of the West are facing "severe" or "extreme" droughts right now. But, Allen notes, data from tree-ring studies suggest that there have often been large droughts in the West. "What's different today," he says, "is that it's also getting warmer, which can amplify the fire severity in the West."
One 2006 Arizona State University study found that rising temperatures appear to be a major driver of increased Western wildfires in recent years. One reason is that, as winters get warmer and warmer, the snowpack in the mountains has gotten smaller and is melting earlier in the year.
Historically, Allen says, the snowy mountains have acted as giant "fire towers" that release water slowly throughout the spring and summer. But when there's less snow to go around, soils and forests get parched more quickly, which exacerbates droughts and can make large wildfires more likely. This year, snowpack in the Colorado Rockies peaked on March 6, a full month earlier than the historical average date. (Note, however, that there are still plenty of other, non-climate-change reasons why the current Colorado fires are so vicious, such as a high-pressure system that has prevented clouds from forming.)
2) Sprawl has pushed more people into forest areas, increasing the odds of fires. Many forest fires are caused by lightning. But others are caused by human activities. And as more and more people push deep into forested regions, that increases the risk of accidents. "More smokers, more ignition from motorized vehicles... even more arson," says Allen. He offers up one salient example: The record-setting Las Conchas fire in New Mexico last year, which consumed 40,000 acres, began when an aspen tree toppled onto a power line that was serving just six homes in a remote area.
3) Changing forestry practices have made wildfires more destructive. A look at tree-ring data shows that the Southwest has seen massive fires going back for centuries. But, in the past, many of these fires were low-intensity "surface" fires that mostly cleared out underbrush and prevented forests from building up too thickly.
That all changed around the 1900s, when the area's population grew and forest managers began suppressing these natural fires. Southwestern forests became much more dense with trees and brush. And that means when fires do break out, there's more fuel to burn and the fires are often far more destructive. The scale of these "megafires," which can spread all the way up to the tree canopy, appears to be unprecedented in the historical record.
"The basic story has been a convergence of climate and changing fuels in the forests due to land use," says Allen. "And this wasn't even a partisan issue — it's been the standard practice of forest management for the past century."
So can anything stop wildfires from getting worse? For starters, humans could try to put less carbon dioxide into the atmosphere and not heat up the planet so rapidly. A recent study by a team of researchers at the University of California at Berkeley looked at 16 different climate models and concluded that wildfire activity was likely to become much more common in regions such as the Western United States if the world keeps warming.
Better forest management could also help. Scott Stephens, a professor of fire science at UC Berkeley, recently published a study (pdf) in Biosphere showing that intentionally setting "prescribed fires" in forests was a viable method of clearing out the surface brush and preventing even more catastrophic, canopy-killing fires from breaking out. (That's right, you can fight fire with fire.) His paper found that it was possible for forest managers to set prescribed fires without causing undue damage to ecosystems.
The current U.S. Forest Service chief, Tom Tidwell, has proposed new forestry practices along those lines. That would include a greater use of prescribed fires, as well as "mechanical thinning" to clear out the tangled overgrowth in many forests. That won't come cheap: The price tag to treat 4 million acres this year alone comes to about $1 billion. But dealing with the aftermath of severe fires — burned homes, evacuations, death — certainly isn't cheap, either.