This Year's Wildfires Are Bad. Climate Change Will Make Future Ones Worse

Once again, it is fire season in the western United States.

This month, the extreme fire that seems to test the bounds of our recent, place-based memory of how fires behave is the Sand Fire burning north of Los Angeles.

Last year, there was the Butte fire in northern California and the Okanogan complex in Washington. In 2014 and 2013, the King and Rim fires in Sierra Nevada forests. The Black Forest fire in 2013 in Colorado. In 2012, the High Park and Waldo Canyon fires among others in Colorado, the Long Draw and Miller Homestead fires in Oregon and the Whitewater-Baldy complex in New Mexico. In 2011, the Wallow and Horseshoe fires in Arizona and the Los Conchas in New Mexico. The list goes on.

The question naturally arises: are these fires affected by climate change? And the answer is simple: yes. Of course. A warming climate affects fire. Sure, there are other factors at play: invasive, fire-prone grasses are flourishing. Fire suppression led to more acres ready to ignite. But these factors compound or respond to the effect of climate on fire - they don't explain it away.

The statistics are dire. In western US federally managed forests (which is most of them), the number of large forest fires has increased over 500%, the burned area over 1,000%, since the 1970s and early 1980s. In some areas, the increase is much larger: area burned in northern US Rockies forests has increased 3,000%. In Pacific north-west forests, 5,000%. Grass and shrub-land fires across the region show lesser but still significant increases in burned area as well.

It's not as though climate change is creating conditions where all nature does is burn. We will continue to have droughts and wet periods, as we have had in the past.

But increasing temperatures will have measurable impacts. With warmer temperatures, evaporation increases. The atmosphere draws more moisture from soils, and from the live and dead vegetation that, drier than it used to be, helps fuel fires.

For decades now, through wet years and dry, more moisture has been evaporating from our forests, grasslands and shrub-lands than would have been the case without a warming climate. Less water is retained in wet years, and our droughts, like the current one, are made more extreme. The result is a drier, more flammable landscape, with larger and more extreme fires as the legacy fuels from a cooler, wetter era are consumed alongside the newer sources.

This process will continue with the accelerating changes we have already committed to by failing to address climate change as an emergency requiring immediate, sustained action. While fire - at its pre-climate change pace and tempo - is itself a natural process that our ecosystems adapted to, the changes in scale, timing, frequency and intensity we are seeing in the western US are not natural.

A changing climate is transforming our landscape, and fire is one of the tools it uses. Expect to see more of it, in more places, as temperatures rise.