Nearly 90 large fires are raging across the United States, intensifying this year’s fire season. The forecast shows no sign of letting up.

A fire cycle that goes from years of high fire activity to lower activity isn’t entirely unusual, Dr. Swain said. But what is notable, he added, is that a warming climate makes the high-activity years more intense.

After a few mild years, “some natural upward kick” would be expected, he said. But, he added, “the climate change piece of this makes this a much bigger kick.”