Forests not only serve as refuges from city life, but could also be among the last fortresses between a livable planet and an increasingly hostile one.

Forests can pull carbon from the air and store it in roots and leaves, locking it out of the atmosphere. Through complex markets, nations can bank trees to lower their greenhouse gas emissions while continuing to use fossil fuels, the root cause of climate change.

But those forests are burning up. Global carbon emissions from forest fires have increased 60 percent since 2001, according to a new study published Thursday.

“We had to check the calculations because it’s such a big number,” said Matthew Jones, the lead author of the report and a physical geographer at the University of East Anglia in England. “It’s revealed something quite staggering.”

The study, published in Science, used machine learning to group the world’s forest ecosystems into 12 categories. Each kind of forest reacted differently to a combination of drivers that can influence how fires start and how severe they become, including global warming, along with changes in land use and vegetation growth.

Burning boreal forests, largely in the colder climes of Canada and Siberia, were by far the biggest contributors. Researchers revealed that one type of boreal forest almost tripled its annual carbon emissions between 2001 and 2023.

Last century, emissions from tropical forests like the Amazon and Congo rainforests were dominant as trees were cleared for logging, farming and other uses. But, under climate change, fire-favorable weather has increased across other regions.

Global warming has brought more extreme hot and dry conditions and more lightning storms that can set trees ablaze. So much of the boreal forests burned in these conditions that they offset the tropics’ slightly declining emissions during the same time period.

“The problem is these regions are not used to the climate of today,” Dr. Jones said, “so they didn’t evolve to deal with fires that could have the potential to kill them.”

Those fires aren’t just burning more carbon, but they’re also burning more severely, threatening a forest’s resilience. Extreme fires eat through more organic material like vegetation and soil as these ecosystems become drier. They can lead to a higher tree mortality, according to the study, which can slow a forest’s rebound for biodiversity and continued carbon storage.

Last year, wildfires in the boreal forests of Canada produced more carbon emissions than the burning of fossil fuels in all but the three largest polluters, China, India and the United States, according to a study published in August in the journal Nature. In an email, Dr. Jones said those emissions were the best example yet of the staggering global changes.

Marc-André Parisien, a research scientist at the Canadian Forest Service, said, “It’s our most extreme fire season in Canada’s recorded history.” The smoke from those fires was so widespread beyond Canada’s borders, he added, “you probably ate some of that smoke in 2023.”

While carbon emissions grow from forest fires, countries are struggling to successfully reduce emissions from burning fossil fuels.

The Paris climate agreement that nations signed onto in 2015 aimed to limit global warming to 1.5 degrees Celsius, or 2.7 degrees Fahrenheit, above preindustrial levels. But now that target is “hanging by a thread,” according to Secretary General António Guterres of the United Nations, as the world keeps getting hotter. Last year was the hottest year on record, a record that could be broken in 2024.

During less active fire years, less populated but highly forested countries like Canada can act like a carbon sink for the world’s emissions. But during a year with frequent fires, like 2023, those fires can transform Canada into a source of carbon emissions. Increased fire activity in North America’s boreal forests alone could result in carbon losses causing a temperature rise above the Paris agreement limits by midcentury.

“In a lot of countries, national plans for dealing with climate change are relying more and more on the forests and reforestation schemes,” Dr. Jones said.

“Research that’s pointing to more severe or intense fires are a threat to a lot of plans to deliver net-zero” emissions, he added. “All this research is about raising an alarm now that perhaps we can’t rely on forests as much as some of these plans are.”

Best practices, like mechanical thinning or controlled burns of forests to make them less dense, could help reduce the severe burning in boreal forests. But those woodlands are so extensive, it would be impossible to manage them all. Instead, experts said, along with better fire management practices, reducing emissions from fossil fuels remains the best path forward.

“We’re always millions of acres behind,” said Lisa Allyn Dale, a climate lecturer at the Columbia Climate School. “There’s no aspiration to treat every acre of forest.”

And forests still need fires to stay healthy.

“We’ll never eradicate fire,” Dr. Dale said. “We’re trying to reduce risk from unwanted fires.”