Scientists contend that if money were redirected from firefighting into projects like fireproofing homes, those communities could actually be made safer. But the politics of the shift would be difficult, at best.

Climate change complicates the picture. It is making wildfires more likely, essentially punching through the human effort to suppress fires. That may, in the short term, help achieve the scientific goal of having more fire on the landscape. But longer term, it could lead to profound changes in forests, potentially driving some creatures to extinction.

The question coming into focus is simple, but answering it in the age of global warming will be a lifetime challenge for a rising generation of forest managers: How much fire is enough?

A History of Fires

Scientists are still trying to figure out how regularly forests burned in what is now the United States in the centuries before European settlement, but reams of evidence suggest the acreage that burned was more than is allowed to burn today — possibly 20 million or 30 million acres in a typical year. Today, closer to four million or five million acres burn every year.

Scientists say that returning forests to a more natural condition would require allowing 10 million or 15 million acres to burn every year, at least.

“From an ecological standpoint, everything I’ve learned teaches me this is a good idea: Stop putting out fires,” said Jennifer R. Marlon, a geographer at Yale who was among the first to use the term “fire deficit” to describe the situation. “These forests are made to have fire.”