In the last 20 years, California’s northern forests have experienced a stark increase in lands burned by fire. Now scientists have a better idea why.

The culprit is a familiar one — human-caused climate change, driven primarily by the burning of fossil fuels, according to findings published on Monday in Proceedings of the National Academy of Sciences.

But other aspects are new. The paper presents for the first time a portrait of fires in an alternate California in which human-caused climate change hadn’t happened. 

And by comparing that world to our own, it offers a sobering warning for any ecosystem — notably Canada and the Western U.S.  — in which temperature, not the availability of trees, is the primary factor limiting the size of fires.

Fossil fuel burning expanded vulnerable forests

In those regions, lead author Mario Turco told The Hill, the impact of a century of burning fossil fuels has vastly expanded the amount of forest vulnerable to fire.

That relationship portends a grim future, he noted. California’s coming fires could burn up to 50 percent more land than the fires of today.

Those specific prescriptions focus on the pine forests of Northern and Central California. But Turco emphasized that similar dynamics were at work in the record Canadian wildfires that clogged the air of the U.S. Northeast last week.

In Canada, the amount of land burned by fire thus far this season is now 13 times the national average — only a couple of weeks into what was traditionally considered to be the fire season.

“What is happening in Canada this month is something strange because there are not so many analogs in the past,” Turco told The Hill.

California, by contrast, offered the scientists a golden setting for a natural experiment. In addition to its long history of big blazes — in particular, the enormous fires in the summers of 2018, 2020 and 2021 — California has unusually good data around fires.

Unlike many other fire-prone areas, California’s state agencies have been collecting rigorous data on the extent of fire in the state for decades. 

That data shows that the 10 largest wildfires in California history also happened in the last 20 years, with those fires getting bigger and more powerful later in that period. (Half occurred in 2020 alone, and eight since 2017.)

But while the increased scale of the problem is clear — as is the fact that it has come alongside an implacable rise in both global temperatures and the burning of fossil fuels — it has historically been difficult to untangle how much human-caused climate change, specifically, has contributed to California’s current fire problem.

Simulation shows scope of problem

To answer that question, the team simulated an alternate California that had experienced only natural climate variation since 1996.

While the scientists didn’t make this point explicitly, this experiment amounted to a rough simulation of an alternate past: one in which people rapidly stopped burning fossil fuels once their role in climate change was discovered. 

Off-ramps from the current crisis into such an alternate world appear early. As NASA notes, the Swedish scientist who first predicted the relationship between rising levels of carbon dioxide — the major byproduct of burning — and rising temperatures first published in 1896.

That relationship was confirmed as early as 1938, when British engineer Guy Callendar linked the world’s use of fossil fuels to a slight — but already statistically significant — rise in temperatures; by 1956 U.S. Office of Naval Research was already promulgating “The Carbon Dioxide Theory of Climatic Change.”

But even in a world where such scientific work had led to an early, widespread and total move away from fossil fuels, climate change would not stop. The sun’s power would still fluctuate, as would the distance — thanks to an orbital wobble — between it and Earth, each of which would impact how much heat reached the surface.

And down on Earth, changes in terrestrial processes — like volcanic eruptions and the spread or contraction of forests — would also have led to broader changes in the climate. 

Humans caused threefold rise in burned lands

But on that alternate Earth, natural variations in the climate wouldn’t be enough to produce the dramatic rise in burned areas that California has experienced over the past 30 years, the PNAS paper found.

In fact, the scientists wrote, our world was accelerating away from that nature-only paradigm. The historical record shows that California has had nearly twice as big an increase in burned land — 1.7 times, to be exact — between 1971 and 2021 as it would have under purely natural cycles. 

And the magnitude of that difference was increasing, they wrote. Human-caused climate change has led to a “remarkable” threefold increase in burned lands between the latter half of that period — 1996 and 2021 — and the equivalent period over on nature-only Earth.

That has been a pivotal period for climate change — not only in terms of its rise as political issue, but also in the acceleration of the forces driving it. (More than half of the carbon discharged into the atmosphere since the dawn of the industrial revolution came from fires lit — in fireplaces, power plants and internal combustion engines — since 1990.) 

What role does forest management play?

One major question rests outside the scope of the paper: the role of “forest management” — or mismanagement — on the fire problem. 

Since the 2018 fire that leveled Paradise, Calif., state and federal officials have faced harsh criticism for their decades-long policy of suppressing low-grade wildfires that might otherwise have cleaned fuels from the forest — sowing the ground for the unstoppable fires of the present.

Conservative politicians have appealed to similar management concerns to explain away the impacts of climate change, as in Donald Trump’s 2018 claim that California faced such bad fires because — unlike the forest nation of Finland — the state didn’t “rake” its forests.

To Trump’s defense — and despite the bemusement those remarks caused on both sides of the Atlantic — the Finnish forestry sector really does use giant mechanized rakes to pull flammable tree residues out of its forests after they have been cut.

That real geographic difference, however, conceals an even more important historical one. Such tree residue — which is now burned in district-level power-plants — could once have been safely left to decompose in place, before climate change heated up the forests. 

“Trump has a point,” fire ecologist Matthew Hurteau wrote in a 2018 op-ed in The Washington Post. “The U.S.could get away with poor management, until global warming.”

Reinforcing that idea, the PNAS study didn’t find any historic change in forest management that was big enough to explain the size of the jump in the amount of California that now burns, Turco said.

The one place that scientists did find such a connection was Europe — where they found that forest policy had led to a striking decrease in the amount of land burned, even as human-caused climate change pushed in the opposite direction.

That might not be entirely good news, Turco said. “It means that the tool of fire management — where basically they try to suppress all fires — is working. But that doesn’t mean it will work in the future climate.”

On the one hand, because the climate itself was heating up and drying out European forests, leading to a greater capacity for fire. On the other hand, because with each wildfire stopped too early, “we are creating more fuel to be burned in the future.”

That rising climatic pressure towards bigger wildfires makes it more important “to focus not only on suppression, but also on preparedness, proactive action, proactive management,” he added.

That’s something the Biden administration Forest Service has staked out as a priority. The agency sees the decades of forest mismanagement of forest fuels as a serious and systemic problem they are spending billions to fix.

Turco said those steps are important, even if blunting the speed of human-caused climate change is the main imperative.

“It’s quite dangerous to say ‘climate change is responsible for all the problems of the forest fires in California — so it’s not important to have a good plan,’” he added. “Because this is not true.”