In 2023 and 2024, the hottest years on record, more than 78 million acres of forests burned around the globe. The fires sent veils of smoke and several billion tons of carbon dioxide into the atmosphere, subjecting millions of people to poor air quality.

Extreme forest-fire years are becoming more common because of climate change, new research suggests.

“Climate change is loading the dice for extreme fire seasons like we’ve seen,” said John Abatzoglou, a climate scientist at the University of California Merced. “There are going to be more fires like this.”

The area of forest canopy lost to fire during 2023 and 2024 was at least two times greater than in the previous nearly two decades, according to a new study published Monday in the journal Proceedings of the National Academy of Sciences. The researchers used imagery from the LANDSAT satellite network to determine how tree cover had changed from 2002 to 2024, and compared that with satellite detections of fire activity to see how much canopy loss was because of fire.

Globally, the area of land burned by wildfires has decreased in recent decades, mostly because humans are transforming savannas and grasslands into less flammable landscapes. But the area of forests burned has gone up.

Boreal forests lost more than two times the canopy area in 2023-24 compared with the period between 2002 and 2024, the study found. Tropical forests saw three times as much loss, and North American forests lost nearly four times as much canopy, mostly because of Canada’s wildfires.