Haines Junction, Yukon Territory ? A veil of smoke settled over the forest in the shadow of the St. Elias Mountains, in a wilderness whose spruce trees stood tall and gray, a deathly gray even in the greenest heart of a Yukon summer.

“As far as the eye can see, it’s all infested,” forester Rob Legare said, looking out over the thick woods of the Alsek River valley.

Beetles and fire, twin plagues, are consuming northern forests in what scientists say is a preview of the future, in a century growing warmer, as the land grows drier, trees grow weaker and pests, abetted by milder winters, grow stronger.

Dying, burning forests would then only add to the warming.

It’s here in the sub-Arctic and Arctic — in Alaska, across Siberia, in northernmost Europe, and in the Yukon and elsewhere in northern Canada — that Earth’s climate is changing most rapidly. While average temperatures globally rose 1.3 degrees Fahrenheit in the past century, the far north experienced warming at twice that rate or greater.

In Russia’s frigid east, some average temperatures have risen more than 3.6 degrees Fahrenheit, with midwinter mercury spiking even higher. And “eight of the last 10 summers have been extreme wildfire seasons in Siberia,” American researcher Amber J. Soja pointed out by telephone from central Siberia.

Releasing carbon dioxide

Along with shrinking the polar ice cap and thawing permafrost, scientists say, the warming of the Arctic threatens to turn boreal forest — the vast cover of spruce, pine and other conifers blanketing these high latitudes — into less of a crucial “sink” absorbing carbon dioxide and more of a source, as megatons of that greenhouse gas rise from dead, burning and decaying wood.

American forest ecologist Scott Green worries about a “domino effect.”

“These things may occur simultaneously,” said the researcher from the University of Northern British Columbia. “If the bark beetles kill the trees, you’ll have lots of dead, dry wood that will create a really, really hot fire, and then sometimes you don’t get trees regenerating on the site.”

Dominoes may already be falling in western North America.

From Colorado to Washington state, an unprecedented, years-long epidemic of mountain pine beetle has killed 6.5 million acres of forest. The insect has struck even more devastatingly to the north, in British Columbia, where clouds of beetles have laid waste to 35 million acres — twice the area of Ireland. It is expected to kill 80 percent of the Canadian province’s lodgepole pines before it’s finished.

Farther north, in the Yukon, the pine beetle isn’t endemic — yet. Here it’s the spruce bark beetle that has eaten its way through 1 million acres of woodland, and even more in neighboring Alaska, in a 15-year-old epidemic unmatched in its longevity and extent.

“It’s a fingerprint of climate change,” Aynslie Ogden, senior researcher for the Yukon Forest Management Branch, said in Whitehorse, the territorial capital. “The intensity and severity and magnitude of the infestation is outside the normal.”

Hiking through the wild and beetle-ravaged Alsek valley, Legare, the Yukon agency’s forest health expert, explained how the quarter-inch insect does its damage.

The hatched larvae, just beneath the outer bark, then feed via perpendicular galleries they bore around the tree, cutting off nutrients moving through the phloem and killing the plant. Its needles turn reddish, later gray, and eventually wind topples the dead wood.

Beetles are thriving

Winter spells of minus-40-degree temperatures once killed off larvae, but those deep freezes now occur less often. And warmer summers enable some beetles to complete their reproductive cycle in one year instead of two, speeding up population growth.

Years of summer drought, meanwhile, weakened the spruces’ ability to extrude sticky pitch, to trap and expel beetles. Because the snow-streaked peaks of the 15,000-foot-high St. Elias range block moisture from the Pacific, a mere 10 inches of precipitation falls each year. Even a slight shortfall stresses the trees.

The Yukon has experienced smaller, briefer beetle outbreaks in the past, fed by patches of fallen trees left by road construction. But “what makes this infestation different” is that climate change is a primary cause, Legare said.

In an authoritative 2007 assessment, the Intergovernmental Panel on Climate Change (IPCC), the U.N.-sponsored scientific network, cited multiple studies linking the spread of wildfires to warmer, drier conditions.

This June, in the latest such study, as early flames flared in California’s wildfire season, Harvard scientists said the area burned in the western United States could increase by 50 percent by the 2050s, even under the best-case warming scenario projected by the IPCC.

In Canada, area burned is double what it was in the 1970s, despite greater firefighting capacity and some recent favorable weather, said Mike Flannigan, a fire researcher for the Canadian Forest Service.

He cited three key reasons: warmer temperatures are drying the forests, lengthening the fire season and generating more lightning, cause of the worst wilderness fires.

Flannigan worries, too, that future fires smoldering through the carbon-heavy peatlands that undergird much of the boreal region would pour unparalleled amounts of carbon dioxide, the main global-warming gas, into the skies, feeding an unstoppable cycle.