‘Double hazards’ map points to a hidden geography of wildfire risk

Nearly 20 different regions across the Western U.S. face rising danger from wildfires, a new study finds, demonstrating the challenge the Biden administration faces as it starts a 10-year, multibillion-dollar investment in reducing the country’s fire risk.

The areas, where zones of particularly vulnerable vegetation meet those of especially fierce drought, spread across huge swaths of the country.

“Each plant is different, each species is different and the geography of a place defines how a plant’s moisture level responds to different environmental conditions,” study lead author Krishna Rao said in a statement.

When some of these differences are projected across the American West, they reveal a previously hidden geography of risk. 

Many of the hot spots Rao’s team found are remote — sparsely populated parts of central Wyoming or the rugged Utah-Nevada border near Dinosaur Valley National Monument.

But others contain large cities, which are disproportionately spilling into the vulnerable, fire-prone wildlands around their edges — as with the land that burned over New Year’s in Colorado’s most expensive wildfire on record.

In that second category, one danger zone stretches to encompass the entire Los Angeles Basin; another reaches from Houston into the pine forests of east Texas. Others surround Las Vegas and Reno, Nev.; Denver; Albuquerque, N.M.; and Medford, Ore.

One enormous zone of particular risk looks on the map like a person standing in central Nevada, their torso taking up virtually all of eastern Oregon; another juts up from Texas’s Big Bend region east of the New Mexico border.

The study comes out against the shadow of two back-to-back record-setting years that exploded the notion of the discrete fire season, as blazes broke out throughout 2020 and 2021. 

Last year alone, 7.7 million acres — an area about the size of Maryland — burned, with 29,000 fires having already started before the season’s traditional beginning in June as overstocked forests full of trees weakened or killed by drought went up in flames.

In January, the Biden administration committed to a 10-year “fuel treatments” program to reduce wildfire risk on state, tribal, private and federal landscapes across the West — a project that could ultimately cost $50 billion.

But with officials unclear where that money will come from, it’s important to identify the areas of highest risk.

That’s what the Stanford team has done using an innovation in remote sensing that allowed scientists to look with new detail at remote landscapes.

In particular, it’s given them new insight into the big disparity between how vulnerable different ones can be to the same drought. 

The range should be familiar to anyone who owns houseplants.

“You go on vacation, the plants don’t get watered, they’re all experiencing the same drought. But you come back to your office, and they will have responded differently — maybe one is dead, one is brown but alive, one is green,” said co-author Noah Diffenbaugh of the Stanford Woods Institute for the Environment.

Mapping the diverse vulnerability across entire ecosystems or regions has traditionally been a difficult, labor-intensive process, senior author Alexandra Konings noted in a statement.

But by using satellite-mounted microwave sensors and machine learning, the scientists were able to map areas of most vulnerable vegetation across the entire Western U.S.

Then researchers compared those vulnerable areas with rise in a metric called vapor pressure deficit, which measures how “thirsty” the air is — or the difference between how much water the air holds versus how much water it can.

By putting these two measures together, the researchers identified the 18 zones that represent “double hazards” — as their unusually vulnerable trees and brush wither faster in the unusually dry, hot and absorbent air, leading to greater risk of fire.

That risk grows even higher when another factor is added, the researchers noted: people. The Biden wildfire prevention plan pays particular focus to the zone known as the wildland-urban interface (WUI), where cities, towns and sprawl merge into forest, grasslands and shrub — and where 1 in 4 Californians live.

Development in this area is particularly dangerous, said co-author Noah Diffenbaugh.

“The presence of people in the WUI both puts people in harm’s way and promotes ignitions, because the vast majority of ignitions that threaten homes are human caused,” he sad.

Risk isn’t spread evenly through the WUI: certain urban-abutting areas have more sensitive vegetation, which corresponds to higher risk of fire. 

It also corresponds to having higher numbers of people move there: The most drought-sensitive ecosystems in the WUI have experienced 50 percent faster population growth than the WUI as a whole.

“The areas of WUI that are most sensitive — that have the highest plant water sensitivity, the highest highest wildfire risk — those areas have grown disproportionately,” Diffenbaugh said.