1971-2000 2041-2070 Very large fires per year More frequent White areas have no data 1971-2000 2041-2070 Very large fires per year More frequent White areas have no data 1971-2000 2041-2070 Very large fires per year More frequent White areas have no data Very large fires per year More frequent White areas have no data 1971-2000 2041-2070 Very large fires per year Fewer More White areas have no data 1971-2000 2041-2070 1971-2000 2041-2070 Very large fires per year Fewer More White areas have no data Source: International Journal of Wildland Fire

A warmer world makes for a more combustible country. That’s the conclusion in the most comprehensive assessment of the effects of climate change on the United States, released by the Trump administration just weeks after the deadliest and most destructive wildfire in California history.

The report says the continued release of greenhouse gases from cars, factories and other sources will make fires more frequent, including very large fires that burn more than 12,400 acres. And wildfire risk in the United States won’t just be a Western problem.

“One of the big warnings there is about the potential for increased fire in the southeast,” said Andrew Light, a contributor to the report and a senior fellow at the World Resources Institute.

Warming Has Already Led to More Burning

Human-caused warming has increased the area burned by wildfire in the Western United States, according to the report, “particularly by drying forests and making them more susceptible to burning.”

A recent study cited by the report estimated the total acres burned in western forests under current climate conditions and in a model without human-caused warming. It found that half as much forest area would have burned between 1984 and 2015 in a world not warmed by climate change.

In the West, Forest Fires Burn More Acres In a World Warmed by Climate Change Actual area burned 25 million acres 25 million acres 20 20 15 15 Cumulative acres burned 10 10 Estimated area burned without climate change Estimated area burned with climate change 5 5 1985 1990 1995 2000 2005 2010 2015 1985 1990 1995 2000 2005 2010 2015 Actual area burned 25 million acres 25 million acres 20 20 15 15 Cumulative acres burned 10 10 5 5 Estimated area burned without climate change Estimated area burned with climate change 1985 1995 2005 2015 1985 1995 2005 2015 Actual area burned 25 million acres 20 15 10 Estimated area burned without climate change 5 1985 1995 2005 2015 25 million acres 20 15 Cumulative acres burned 10 Estimated area burned with climate change 5 1985 1995 2005 2015 Source: Proceedings of the National Academy of Sciences ; data updated through 2017 by A. Park Williams

Climate change is not the only factor determining the size and destructiveness of a fire. Humans are increasingly intruding into wildland areas to build communities, increasing both the likelihood of fires and their devastation. Historical wildland management that focused on fire suppression has created some areas that are ripe for burning.

“Wildfire is an essential part of many forest ecosystems, but two major factors have produced the catastrophic fires we’re seeing in the western U.S. — old policies and human-caused climate change,” said Patrick Gonzalez, a forest ecologist at the University of California, Berkeley and a contributor to the climate report.

There is evidence “that area burned is more closely related to climate,” Dr. Gonzalez said.

Flames Where They Once Were Rare

The report notes that warmer winters in the Northwest have reduced snowpack, the thick layers of snow that would form in the mountains over winter and melt through spring and summer. The decline in snowpack has decreased the amount of water available in summer, increasing wildfire risk.

Forest fires in the region “are expected to increase as temperatures increase and as summer droughts deepen,” the report says.

Warmer temperatures and drought are also expected to increase wildfires in the Southeast, which already experiences more billion dollar disasters than any other region in the United States.

In November 2016, wildfires near Gatlinburg, Tenn., caused widespread damage and the mandatory evacuation of at least 14,000 residents. Jessica Tezak/Knoxville News Sentinel, via Associated Press

Wildfires Could Worsen Climate Change

Extreme wildfires are not only a consequence of climate change, they also can contribute to the rise in carbon emissions. “Two-thirds of the carbon emitted by California ecosystems from 2001 to 2010 came from six percent of the land area that burned,” said Dr. Gonzalez, referring to his published research.

Dr. Gonzalez said the result was that California’s ecosystem, including its grasslands and forests, actually emitted more greenhouse gases than it took in, becoming a net contributor to the planet’s warming.

And increasingly, there’s concern about what happens after a fire strikes. Over the past 15 or 20 years, the number of forests rebounding after a wildfire has declined.