The science connecting wildfires to climate change

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The science connecting wildfires to climate change

Climate change has led to bigger and more intense fires across the American West over the past few decades. Increasing heat, changing rain and snow patterns, shifts in plant communities, and other climate-related changes have greatly increased the likelihood that fires will start more often and burn more intensely than they have in the past.

Fire has three components: weather and climate conditions, plenty of burnable fuel, and a spark. Climate change has affected the first two components (and in some cases, the third) in clear, measurable ways.

The clearest connection is with warming air temperatures. Warming has accelerated since the 1980s to just under 0.3 degrees per decade. Just a little warming can create a big change. Hot air, if it's not at 100 percent humidity, is like a thirsty sponge: It soaks up water from whatever it touches—plants (living or dead) and soil, lakes, and rivers. The hotter and drier the air, the more it sucks up. The amount of water air can hold increases exponentially as the temperature rises. A brief heat wave will dry out the smallish stuff or the already dead stuff—and maybe even some of the bigger tinder. When there's no moisture left to evaporate, the soil or vegetation, dead and alive, absorbs that heat instead—feeding back into the drying-out process that increases fire risk.

Climate change also affects the seasonal rain and snow patterns across the Western U.S. Springtime begins earlier. Snowpack, which usually provides about 30 percent of the state's summer water needs, is melting earlier in year, giving the plants and soils longer to dry out.

The hot drying-out season is stretching out; the western fire season has extended by at least 84 days since the 1970s. California's fire protection service has said publicly that it no longer considers there to be a wildfire "season," because the season is now the entire year. The character of the fires has also changed, growing larger and more intense, and that in turn can accelerate future fire risk. Even plants that need fire to propagate, like many high-elevation conifers, are now often finding themselves in fires more intense and powerful than they're adapted for. That's a problem because when vast stretches of forest burn, we can no longer count on them to self-regenerate. The seed sources and gentle shade that may have been normal in the past are gone. The conditions may give rise to highly flammable species, like non-native grasses and shrubs, to move in.

So climate change has increased fire risk in both direct and indirect ways. When an ignition happens, even if it's natural, the chances of it growing into a big fire are much higher than they would have been without climate change.