The researchers also found strong links between the extreme heat and wildfire activity in 2021. That year, 21% of the land burned in North America was scorched by fires that started during and within the heat dome, with that figure rising to 34% when taking into account fires that started within 10 days, the researchers found.

The size of the heat dome made it particularly troubling because it resulted in what the study authors called widespread synchronous burning, with many disparate areas igniting at the same time. That posed a challenge to fire agencies because they tend to ask for help from other places when they don’t have enough resources locally.

“If other areas are also experiencing the same resource strain, you can reach a bottleneck at some point,” Jain said.

When there aren’t enough resources to attack fires when they first start, blazes that might have otherwise been snuffed out when they were small grow large and difficult to contain, resulting in the need for yet more resources, said John Abatzoglou, a professor of climatology at UC Merced who also worked on the study. If this type of synchronous activity persists in future years, it could force fire managers to reevaluate the reliability of resource-sharing arrangements, he said.

The study did not look specifically at how the heat dome, which extended into Northern California, affected the state’s fire season. That summer, the 963,000-acre Dixie fire, which started July 13, became the first to burn from one side of the Sierra Nevada to the other, followed in short order by the 221,000-acre Caldor fire.

In general, it’s difficult to completely attribute a fire to any individual factor, because flames are often fueled by a complex interplay of conditions — anything from overstocked forests to wind, Abatzoglou said. Still, in 2021, California had its hottest June to July in the observational period, and researchers have established a strong relationship between warm, dry summers and area burned in the state’s forests, he said.

“It’s obviously difficult to say how much the heat dome itself was responsible for those fires,” Abatzoglou said. “But based on the hellaciously warm temperatures in that month, the significant heat wave events, we can say that those conditions certainly helped enable fuels to become incredibly available and provided less resistance to fire, once a fire started.”

The findings add to the understanding of how climate change can affect extreme weather events — and the potential role these events can play in fire activity.

“This is the latest in a growing body of evidence about the causes of wildfire conditions globally but in particular in western North America,” said Noah Diffenbaugh, a climate scientist at Stanford University who was not involved in the study. “I think this, in particular, is an advance in linking record-breaking fire weather conditions to the specific atmospheric conditions for a specific event.”

It’s important to untangle climate change’s influence on extreme weather events like the heat dome, which are increasing in frequency and intensity, Diffenbaugh said. A lot of infrastructure and risk management systems are built around assumptions about how these events will play out, so if that changes, those systems become stressed, he said.

“The linking, through this careful analysis, of the contribution of climate change to the record-breaking fire weather conditions associated with the heat dome is a really great example of the kind of research that we need to accurately quantify the risk of climate change,” Diffenbaugh said. “Both the climate change that we’re already living with, and the climate change that we can expect to happen in the future, even if the world’s ambitious global warming goals are achieved.”

Studies that attempt to quantify the role of climate change in individual events can also help calculate the health costs and financial toll of planetary heating from carbon emissions, which have been cited by a growing number of lawsuits seeking damages.

Learning under what conditions these events occur can also help people understand how a warming climate can lead to more extremes in the future, Jain said.

And all indications suggest that that future is approaching quickly. Since the study was written, Canada’s 2021 wildfire season was dwarfed by that of 2023, which saw more than 45 million acres burn. Jain now has a preprint examining how heat waves played a role. Although there was no single event as extreme as 2021’s heat dome, some regions of Canada saw many more heat events than on average, he said.

“So 2023 was not dominated by a single event, but when you looked overall at the number of these events that occurred, it was a very extreme year in terms of heat waves,” he said. “And, of course, 2023 was the warmest year on record globally.”

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