California faces another year of drought and elevated wildfire risk due to climate change, expert says

SAN FRANCISCO — For the nation’s most populous state, drought and extreme wildfires remain in the forecast.

Like much of the West, California endured extreme drought conditions and another grueling fire season in 2021. The rainy season, however, got off to a promising start, with above-normal precipitation in October and December. But hopes of an end to the drought gripping most of the Western U.S., which scientists have linked to climate change, quickly evaporated as record dry conditions took hold in California in January and February.

“I think the big picture is that this year is almost certainly not going to be enough to really change the long-term drought conditions,” UCLA climate scientist Daniel Swain told Yahoo News. “California is still going to be in a severe drought this summer unless something extraordinary happens in March. And while March probably will be wetter than either January or February, that’s not a very high bar since it was record dry during those months in California.”

How dry was the start of 2022 in the state? An index of weather stations located near reservoirs in Northern California reported just 1.74 inches of precipitation in January and February, according to the San Jose Mercury News, just 10 percent of the average amount and the lowest tallied since the state started keeping records in 1921.

“The long-term drought conditions will likely persist into summer, and we’ll likely see a drought-influenced summer wildfire setup this year,” Swain said. “I think the even more relevant thing is that you have medium-term drought in California that’s still going to be there. We can’t say exactly how severe it will be this summer because it depends on what happens over the next couple of months, but it will still be there in some form. And then there’s the long-term trend toward warmer temperatures, greater vapor pressure deficits and drier landscapes.”

While it’s easy enough to understand why less rain in an environment helps dry out vegetation and increase the chance for wildfires, an emerging body of research has also linked the amount of moisture in plants to vapor pressure deficits (VPD). A recent paper published in Science Advances defines VPD as “the difference between the water vapor pressure at saturation and the actual water vapor pressure for a given temperature, [and] is an important driver of atmospheric water demand for plants.”

Vapor pressure deficits help explain why the risk of fire danger continues to grow in some parts of the world even though it’s also true that for every 1°C of warming, the Earth’s atmosphere holds 7 percent more moisture. Along with the rise in global temperatures, that vapor pressure deficit has been experiencing “a sharp increase” since the 1990s.

“It’s mainly a paradoxical effect of that humidity increase, actually. Essentially, the main reason why climate change is influencing wildfire extent/size/severity/risk — really, any metric you can measure — is through vegetation aridity,” Swain said, adding, “Literally the dryness level of vegetation. In general, what’s going on, and this is true across all regions, including the Arctic, the tropics, the midlatitudes, the deserts, wherever, is that when conditions are favorable for wildfires, the background conditions are drier. So you get vegetation that burns more readily.”

On its website, the National Oceanic and Atmospheric Administration (NOAA) highlighted a recent study linking vapor pressure deficits with increased wildfire risk.

“The leading cause of the rapid increase of wildfires over the Western U.S. is the rapid increase of surface air vapor pressure deficit, or VPD, a measure of how thirsty the atmosphere is.” NOAA stated on its website. “When VPD is higher, the air is hot and dry and draws more moisture from soil and plants, which not only increases biofuel flammability, but also reduces river flows and reservoir levels.”

While the causal mechanism between vapor pressure deficits and the overall health of plants is tricky to understand, Swain noted that it is just one more way that climate change “presents more hazards for humans.”

What this means in places like California is that rain is not the only factor when it comes to wildfire danger.

“The long-term trend is toward longer and more severe fire seasons with larger and more intense fires,” Swain said. “That’s certainly been reflected in the real-world observations in the past couple of years in California.”

A report released last week by the United Nations Environment Program found that climate change will increase wildfires globally by 50 percent by the year 2100. In part, that’s due to the overall health of plants.

“Wildfires are burning longer and hotter in places they have always occurred, and are flaring up in unexpected places too, in drying peatlands and on thawing permafrost,” the report states.

On the bright side, Swain noted that the extreme amount of precipitation in some portions of the West will help alleviate the historic drought.

“There’s been improvement in the Northwest, especially Washington state, which was super-wet this year, but in the Southwestern broader mega-drought region, this winter is not helping anybody out in that regard. There’s been some decent snow in the central and northern Rockies, but in general, the Colorado River watershed is still experiencing this extremely severe long-term multidecade drought,” Swain said.

And as is well established, with continued drought comes elevated risk of wildfire.

“Most of the regions that have seen really severe fire conditions the last few years due to drought, there’s a good chance that we’ll see those same background conditions this year,” Swain said. “Will we get luckier with the wind events and the conditions? We can always hope.”