The science connecting wildfires to climate change

Climate change has inexorably stacked the deck in favor of bigger and more intense fires across the American West over the past few decades, science has incontrovertibly shown. Increasing heat, changing rain and snow patterns, shifts in plant communities, and other climate-related changes have vastly increased the likelihood that fires will start more often and burn more intensely and widely than they have in the past.

The scale and intensity of the wildfires burning across the western U.S. right now is “staggering,” says Philip Higuera, a wildfire scientist and paleoecologist at the University of Montana. More than five million acres have already burned this year—and much more may be yet to come.

Noah Diffenbaugh, a climate scientist at Stanford University, makes a baseball analogy to describe increase in risk. “If there’s a three-run home run in baseball, it’s the home run that definitely caused the runners to round the bases and score. The home run is the proximal cause of the event. But people being on base matters," he says, and global warming is putting people on base.

Other factors also hike fire risk, like forest management decisions that have allowed for the buildup of vast amounts of vegetation that can quickly turn into fuel, as well as more problematic issues like the slow creep of houses and other infrastructure into risky areas. But for fires near that so-called wildland-urban interface, as well as more remote, forest-centered burns, climate change has significantly heightened the baseline risks.

Heat like a thirsty sponge

In some ways, fire is simple. It takes three components: the right weather and climate conditions, plenty of burnable fuel, and a spark.

“People are changing all three of those,” says Jennifer Balch, a fire ecologist at the University of Colorado, Boulder. “Climate change is not the only thing going on, but it is a big and important part of the story.” (Human-caused ignitions are clearly a major part of the risk: A study published in September, on which Balch was a co-author, found that humans were responsible for 97 percent of the ignitions that caused fires that then threatened homes in the wildland-urban interface, between 1992 and 2015).

Climate change has affected the first two components (and in some cases, the third) in clear, measurable ways that have become increasingly obvious over the past few decades.

The clearest connection is with warming air temperatures. The planet has heated up nearly continuously since the start of the Industrial Revolution in the late 1800s, when humans started burning massive quantities of fossil fuels, releasing carbon dioxide that traps excess heat in the atmosphere. Since then, global average temperatures have ticked up roughly 1.8 degrees Fahrenheit (1 degree Celsius); California’s change is closer to 3 degrees Fahrenheit. Warming has accelerated since the 1980s to just under 0.2 degrees Celsius (0.3 degrees F) per decade, and it's likely to accelerate further in the future.

That might not seem like very much warming, but just a little can go a long way. 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, and the amount of water it can hold increases exponentially as the temperature rises; small increases in the air's heat can mean big increases in the intensity with which it pulls out water. Scientists can measure this "vapor pressure deficit"—the difference between how much water the air holds and how much it could hold. If that deficit is cranked up for a long time, soils and vegetation will parch.

A brief heat spell will dry out the smallish stuff or the already dead stuff—and maybe even some of the bigger tinder. Intense, record-breaking heat waves like the ones that encompassed the West during August and early September likely caused major crisping of burnable material, as the regional vapor pressure deficit and associated drought climbed to record levels.

“In a lot of places, you have a lot of ‘flashy’ fuel on the ground,” says Balch. “This stuff that’s as thin as paper—(like) grasses. Short-term drought events or heat waves are really impactful for drying those out.” That small stuff ignites so easily that it can often help speed along a fire’s spread.

When excess heat stays in place for months or longer, the wildfire risk rises even further.

An early, warm spring can jump-start a summer drought by extending the season of heat and growth, increasing the amount of water vapor that is shed by plant leaves or that evaporates directly from soil. Lower soil moisture, in turn, can feed back into the local warming cycle and intensify it, since evaporating moisture usually takes up a lot of the energy the sun beams down. 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.

This year, the snow melted early; across the West, snow cover in February and March was well below its long-term average.

Then, the heat kicked in and stayed. Many western states had their hottest summers on record; the average temperature across the U.S. was 2.6 degrees Fahrenheit above the 20th-century average.

But even before that, a longer, deeper aridity had California and much of the West in its grips from 2011 until a brief period of reprieve last year. Not coincidentally, five of the state’s hottest years on record occurred in the past decade.

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A particularly severe phase of that persistent drought, fueled by climate change and of an intensity not seen for the preceding 1,200 years, set in between 2012 to 2016. It stressed out the region’s trees more and more as the water deficit dragged on. In the grand conifer forests of the Sierra Nevada, as in many other forests across the state, the damage accumulated.

By 2014, millions of trees had died, pushed beyond repair by the record-breaking temperatures and dryness, which reached so far into the soil that even the deep-rooted trees could find no moisture. By 2015, mass die-off was obviously underway; by 2016, the mortality count soared to about 100 million. At high elevations, nearly 80 percent of the trees died. And across the state, some 150 million trees have died since the drought’s onset. Many of those trees are still there, drying out, a major fuel source ready to burn hot and bright when a fire arrives.

Since the 1970s, a recent study found, human-caused climate change caused more than half of the drying-out of burnable materials and consequent fire risk.

“These most recent heat waves are coming on top of an already hotter period, and it’s all coming together and sucking moisture out of dead and live fuels, into the atmosphere,” says Matthew Hurteau, a climate scientist at the University of New Mexico.

Changing rains, changing snows

Climate change is messing with the seasonal rain and snow patterns across the Western U.S., too—one of the other factors that controls fire risk.

Springtime is often coming 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. A 2016 study found that over 70 percent of the area burned in forest fires between 1970 and 2012 occurred in years where the winter snows disappeared early.

The hot drying-out season is stretching on the tail end, too, according to research published in August. Higher autumn temperatures and less precipitation—in particular, a growing delay in the onset of winter rains, which usually puts an end to the fire season in California—have led to a 20 percent increase in the number of autumn days ripe for burning.

In all, the western fire season has extended by at least 84 days since the 1970s. Cal Fire, 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 very 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, says Scott Stephens, a forest ecologist and fire expert at the University of California, Berkeley.

“One of the very alarming trends we’re starting to see is that these fires are killing very large patches of conifers: 200, 300, 500, 1,000-acre patches, and some even larger,” he says. In contrast, research from his group and others found that in the Sierra Nevada forests, before European colonizers arrived and started changing the landscape, the patches burned were small: less than an acre in many cases, or sometimes a bit bigger. And, Stephens says, the increase in fire size has accelerated in the climate-changed present, particularly since the 1990s.

That’s a problem because when vast swaths 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, and the conditions become ripe for highly flammable species, like non-native grasses and shrubs, to move in. Similar plant transitions are also occurring across other fire-prone habitat, like Southern California’s chaparral and Colorado’s forests.

The bottom line

So climate change has increased fire risk in both direct and indirect ways. When an ignition happens, even if it’s natural— like the unusual and dramatic lightning swarm that hit the Bay Area in August—the chances of it spawning a big fire are much higher than they would be, absent climate change. Overall, over the past few decades in California, the annual average area burned increased fivefold.

Today’s fires are both shocking and wholly expected, say many researchers. “That’s the tricky thing about fires—it isn’t any one thing that’s causing them, it’s multiple puzzle pieces fitting together,” says Balch. Climate change. Forest management. Human behavior. Learning to adapt to the new reality and mitigate risks requires swift, decisive action from many different angles, she says.

“What this year is showing me is the nature of fires here is changing, and changing really fast,” says Higuera. “We need to be doing like five things at once: patting our heads, rubbing our belly, chewing gum, and more, but for fire.”