By SETH BORENSTEIN

WASHINGTON (AP) — The number of days when the weather gets hot, dry and windy — ideal to spark extreme wildfires — has nearly tripled in the past 45 years across the globe, with the trend increasing even higher in the Americas, a new study shows.

And more than half of that increase is caused by human-caused climate change, researchers calculated.

What this means is that as the world warms, more places across the globe are prone to go up in flames at the same time because of increasingly synchronous fire weather, which is when multiple places have the right conditions to go up in smoke. Countries may not have enough resources to put out all the fires popping up and help won’t be as likely to come from neighbors busy with their own flames, according to the authors of a study in Wednesday’s Science Advances.

In 1979 and for the next 15 years, the world averaged 22 synchronous fire weather days a year for flames that stayed within large global regions, the study found. In 2023 and 2024, it was up to more than 60 days a year.

“These sorts of changes that we have seen increase the likelihood in a lot of areas that there will be fires that are going to be very challenging to suppress,” said study co-author John Abatzoglou, a fire scientist at the University of California, Merced.

The researchers didn’t look at actual fires, but the weather conditions: warm, with strong winds and dry air and ground.

“It increases the likelihood of widespread fire outbreaks, but the weather is one dimension,” said study lead author Cong Yin, a fire researcher at University of California, Merced. The other big ingredients to fires are oxygen, fuel such as trees and brush, and ignition such as lightning or arson or human accidents.

This study is important because extreme fire weather is the primary — but not only — factor in increasing fire impacts across the globe, said fire scientist Mike Flannigan of Thompson Rivers University in Canada, who wasn’t part of the study. And it’s also important because regions that used to have fire seasons at different times and could share resources are now overlapping, he said.

Abatzoglou said: “And that’s where things begin to break.”

More than 60% of the global increase in synchronous fire weather days can be attributed to climate change from the burning of coal, oil and natural gas, Yin said. He and his colleagues know this because they used computer simulations to compare what’s happened in the last 45 years to a fictional world without the increased greenhouse gases from burning fossil fuels.

The continental United States, from 1979 to 1988, averaged 7.7 synchronous fire weather days a year. But in the last 10 years that average was up to 38 days a year, according to Yin.

But that is nothing compared to the southern half of South America. That region averaged 5.5 synchronous fire weather days a year from 1979 to 1988; over the last decade, that’s risen to 70.6 days a year, including 118 days in 2023.

Of 14 global regions, only Southeast Asia saw a decrease in synchronous fire weather, probably because it is getting more humid there, Yin said.

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