Big changes are in store for the nation's forests as global warming increases wildfires and insect infestations, and generates more frequent floods and droughts, the U.S. Department of Agriculture warns in a new report.

The study released Tuesday is part of the National Climate Assessment and will serve as a roadmap for managing national forests across the country in coming years.

It says the area burned by wildfires is expected to at least double over the next 25 years, and insect infestations often will affect more land per year than fires.

Dave Cleaves, climate adviser to the chief of the U.S. Forest Service, said climate change has become the primary driver for managing national forests, because it poses a major threat to their ability to store carbon and provide clean water and wildlife habitat.

"One of the big findings of this report is we are in the process of managing multiple risks to the forest," Cleaves said on a conference call on the report. "Climate revs up those stressors and couples them. We have to do a much better job of applying climate smartness ... to how we do forestry."

The federal government has spent about $1 billion a year in recent years combating wildfires. Last year was the warmest on record in the lower 48 states and saw 9.2 million acres burned, the third-highest on record, according to the National Oceanic and Atmospheric Administration website.

Insect infestations widely blamed on warming temperatures have killed tens of millions of acres of trees.