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The Most Important Number for the West’s Hideous Fire Season

Updated at 7:07 p.m. ET on Sept. 15, 2020.

To understand the ravenous wildfire season in the American West this year, boil some ravioli. Put the heat on high. After about 10 minutes, the pasta will go limp and start to break apart. Keep boiling. When the pot holds a shallow puddle of water and a pile of soggy debris, keep going. Don’t turn down the heat until the last bubbles of water sizzle and vanish. Then—and only then—the lump of ravioli will start to singe and burn and smoke.

Water, when heated, “wants” to evaporate; it will turn to gas before allowing most solids suspended in it to heat beyond the boiling point. This principle, readily observable in the kitchen, has recently doomed forests stretching from California to Washington State. One of the hottest, driest summers on record has led to a barrage of “megafires” that have killed at least 35 people, burned nearly 5 million acres, and destroyed thousands of homes and buildings.

The expansive forests of the West, in other words, spent months boiling off. Now they are burning.

[Read: The West has never felt so small]

In the past few months, one in every 33 acres of California has burned. This year is already the most destructive wildfire season, in terms of acreage affected, in state history. In 2018, during California’s last annus horribilis, I noted that six of the 10 largest wildfires in state history had happened since 2008. That list has since been completely rewritten. Today, six of California’s 10 largest wildfires have happened since 2018—and five of them have happened this year.

If you’re having trouble following this year’s western fire season, you are not alone: The fire scientists are too. “There are two dozen fires burning right now that singularly would have been the top story on the national news 10 or 20 years ago,” Daniel Swain, a climate scientist at UCLA and the National Center for Atmospheric Research, told me. A few days ago, he said, he learned of the Slater Fire, which has killed two people. The Slater Fire is burning near the site of the Happy Camp Complex Fire, which was itself one of the worst blazes in state history when it raged in 2014. Yet though the Slater Fire, having merged with another blaze, is larger than the Happy Camp fire ever was, the Slater Fire does not rank among the five biggest fires raging today in the state.

“There’s almost no importance in talking about record-breaking events anymore, when talking about fires in California,” Swain said, “because we’ve broken all the records so many times that … what do they even mean anymore?”

[Read: Photos of California’s suffocating smoke]

It is the same story in the Northwest. More than 1 million acres have burned in Oregon, where tens of thousands of residents are under an evacuation order and officials have warned of a “mass fatality incident.” And across the West, those not living in the path of fires have had to contend with a cloud of toxic fog that stretches from the Inland Empire to Idaho.

California and the West have always burned. Their plants and ecosystems are evolved to endure and thrive in seasonal fires. But this regional chaos is something different, Swain said, caused by a “perfect firestorm” of elements. A windstorm whipped California and Oregon earlier this month, turning valleys into blowtorches. Many western forests are crowded with fuel after a century in which authorities fought every fire, no matter how remote. And a rare lightning storm last month provided an enthusiastic source of ignition for fires. All of those factors may explain aspects of why there are so many fires right now.

But they do not capture the unusual ferocity of this fire season. “What we’re seeing right now is that every fire is becoming a super-intense fire,” Swain said. “Even if you assume we need more fire on the landscape, we probably don’t need more of this kind of fire.” To explain the severity, you have to go back to the conditions that preceded August. This has been “one of the hottest and driest years on record in this part of the country,” he said. “And surprise, surprise, now there are hot fires.”

The primary driver of the fires this year, he said, is California’s rising air temperature. Over the past century, climate change has warmed California by about 3 degrees Fahrenheit. This warming has now started to affect the behavior of water stored in vegetation across the state. In hotter, drier air, liquid water is more willing to become a gas.

[Read: Forest fires are setting Chernobyl’s radiation free]

In other words, water responds to the sponginess—the thirstiness—of the atmosphere. And this summer, the atmosphere across the West had a lot of thirst. Climate scientists measure this trait using a statistic called the vapor-pressure deficit, or VPD.

“In the summer, in California and across most of the West, as long as there’s fuel to burn, then the climate variable that tends to matter most is the vapor-pressure deficit,” Park Williams, a research professor at Columbia University, told me. “In California this year, the vapor-pressure deficit has been record-breakingly high.”

The vapor-pressure deficit indexes two other measurements: the air temperature and the relative humidity. Both measurements affect the air’s sponginess. Hotter air is more likely to bump water into a gas state, while drier air can hold more water vapor overall. The vapor-pressure deficit measures the overlap. “It’s the difference between the amount of water vapor that’s in the air and the amount of water vapor that the air can possibly hold,” Williams said.

When the vapor-pressure deficit is high, it means the atmosphere has become an immense, six-mile-high sponge. The arid air will induce water to evaporate from wherever it’s hiding—the soil, the wooden boards of houses, the limbs and leaves of trees and underbrush.

The vapor-pressure deficit in August in California, as calculated by Park Williams

In August, even before the state saw a searing Labor Day heat wave, the vapor-pressure deficit reached its all-time peak in California. “So it was drying out these forests. They were already primed,” Williams said. The vapor-pressure deficit was extremely high across the West, he said.

That explains part of what’s been so dangerous about this month’s fires: They have grown explosively. Several have swelled to a size of 100,000 acres—that is, more than 150 square miles—in the first 12 hours of their existence, Swain told me. “That statistic is so astonishing that I’m having trouble putting it into words,” he said.

“[These fires] just poofed into existence—they were nothing and then they were megafires,” he said. The North Complex Fire, which has killed at least 15 people, “burned essentially 200,000 acres in a day—that alone would be one of the largest fires in California history, but it did that on the same day that eight fires made enormous runs.” And although the North Complex Fire was pushed by high winds, many other fires swelled to 50,000 or 100,000 acres in more stagnant weather. “In some ways, a 50,000-acre run in the forest with no wind at all is even more alarming.”

[Read: Two disasters are exponentially worse than one]

The horror of this year may seem, in retrospect, like an outlier. But the onslaught of fires in recent years should teach us something about climate change and western wildfire, the researchers said: Thanks to VPD, every additional amount of warming leads to exponentially more fire than the year before it. This is because VPD measures the absolute saturation of the atmosphere, which increases even if the atmosphere stays just as humid.

Swain sketched out an example for me. (To make it easier for me to follow, he used simplified units, not those recorded by scientists.) If the air used to be able to hold 100 “units” of humidity, but it held 50 units of humidity on average, then the vapor-pressure deficit would be the difference between the two: 50. If the temperature rose one degree Celsius, then it would be able to hold 107 units of humidity. Even if relative humidity stayed the same at 50 percent—that is, even if average humidity rose to 53.7 units of humidity—then VPD would still rise. That’s because the VPD in the new climate would be 53.7, which is larger than the old VPD of 50.

“The higher you are on the scale, the faster the rate of increase,” Swain said. VPD turns out to, much like unchecked coronavirus, grow exponentially.

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