California wildfires are five times bigger than they used to be

Days after wildfire smoke from Canada turned skies orange along the US Eastern seaboard, the study is further confirmation of past research showing that higher temperatures and drier conditions in many parts of the world make wildfires more likely.

The peer-reviewed research, published Monday in the Proceedings of the National Academy of Sciences, finds that California wildfires scorch the most area when temperatures are high and less area when it’s cooler.

Marco Turco, a climate researcher at the University of Murcia in Spain, and colleagues designed the study to try to identify how much of the increase in the burned area of California fires was due to climate change, and how much to natural variability. They conducted a statistical analysis of temperature and forest-fire data for California summers in the period 1971 to 2021. They then drew on modeling that shows how the last several decades might have evolved without human-caused greenhouse gas emissions.

The result: Burnt area grew 172% more than it would have without climate change. Human-made effects began to overwhelm what would be expected without greenhouse gas pollution after 2001, the researchers concluded.

Many published studies have linked climate change and California forest fires. A key factor cited in them is the dryness of the air, which scientists call “vapor pressure deficit” — the difference between how much moisture is in the air and how much it can potentially hold at a certain temperature. But forest fires are complicated events, and other factors figure in, too, including trends in rain, spring snowpack, the frequency of extreme heat and poor forest management that leaves dried or otherwise combustible wood on the ground ready for a spark.

What Turco found was that one of those variables was far and away the most useful explanation of the data: temperature, or more specifically, the monthly average of daily peak temperature.

“This was quite surprising to me,” Turco said. “I ran a lot of tests in order to be sure that this really simple model works. For me, it was too easy, and too simple to have this strong results. But ultimately, he said, “the relationship is very strong”.

“It’s another excellent paper that shows the relationship between anthropogenic climate change and drying the climate,” said Glen MacDonald, a climate scientist and distinguished professor at the University of California at Los Angeles, who was not a part of the study. “This is true in California, but it is true in the western United States in general and [is] increasing fires.”

Wildfires worsened by greenhouse gases tore through Australia in 2019 and 2020 and Siberia in 2020.