Driving home from Lake Tahoe, Leah Wills watched the column of ash-gray smoke from the Moonlight fire grow and grow – until finally she was under it.

Overhead, the sky that September afternoon in 2007 turned eerie pink. Orange-red flecks of burning bark streaked like missiles through the air. And the smoke – eye-watering and acrid – was inescapable.

"It was like a nuclear cloud," said Wills, 59, a policy analyst for the Plumas County Flood Control District who lives near the tiny hamlet of Genesee. "I've been to Denali and Kilimanjaro. I grew up with tornadoes. I've seen some big things. I never saw anything that big in my life."

Wildfire has marched across the West for centuries. But no longer are major conflagrations fueled simply by heavy brush and timber. Now climate change is stoking the flames higher and hotter, too.

That view, common among firefighters, is reflected in new studies that tie changing patterns of heat and moisture in the western United States to an unprecedented rash of costly and destructive wildfires.

Among other things, researchers have found the frequency of wildfire increased fourfold – and the terrain burned expanded sixfold – as summers grew longer and hotter over the past two decades.

The fire season now stretches out 78 days longer than it did during the 1970s and '80s. And, on average, large fires burn for more than a month, compared with just a week a generation ago.

Scientists also have discovered that in many places, nothing signals a bad fire year like a short winter and an early snowmelt. Overall, 72 percent of the land scorched across the West from 1987 to 2003 burned in early snowmelt years.

Across the Sierra, satellite imagery shows that today's wildfires are far more destructive than fires of the past, leaving larger portions of the burned landscape looking like nuclear blast zones. That searing intensity, in turn, is threatening water quality, wildlife habitat, rural and resort communities and firefighter lives.

As the climate warms, the ability of the region's mixed conifer forest ecosystem to recover from these destructive fires is in danger.

"We're getting into a place where we are almost having a perfect storm" for wildfire, said Jay Miller, a U.S. Forest Service researcher and lead author of a recent paper published in the scientific journal Ecosystems linking climate change to the more severe fires in the Sierra.

"We have increased fuels, but this changing climate is adding an additional stress on the whole situation," Miller said. "When things get bad, things will get much worse."

Longer, more intense fire seasons

That future may already have arrived. This year, the fire season got off to an early June start in the north state and only recently came to a close. Statewide, 1.4 million acres burned in 2008, just shy of last year's 1.5 million acres, the highest total in at least four decades.

"When I started fighting fire, the normal fire season was from the beginning of June to the end of September," said Pete Duncan, a fuels management officer for the Plumas National Forest. "Now we are bringing crews on in the middle of April and they are working into November and December."

"And we're seeing fires now burning in areas that normally we wouldn't consider a high-intensity burn situation."

Just a few weeks ago, Duncan heard about one such incident: the Panther fire on the Klamath National Forest near the Oregon border.

"It made an eight-mile run one afternoon, in late October. It burned through an area of fairly high elevation old-growth timber and at very high severity," Duncan said.

"I was kind of amazed," he added, "that something would have burned to that scale. To make a 40,000-acre run in an afternoon is significant for any time of year – but particularly for that time of year."

The Moonlight fire, which burned across the Plumas National Forest and timber industry land north of Quincy, was one of the most environmentally destructive in recent memory.

Call The Bee's Tom Knudson, (530) 582-5336.