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 says, 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 0.74 degrees Celsius 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 2 degrees Celsius, with midwinter mercury spiking even higher. And "eight of the last 10 summers have been extreme wildfire seasons in Siberia," American researcher Amber Soja says.

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," says the University of Northern British Columbia researcher. "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 2.6 million hectares of forest.

The insect has struck even more devastatingly to the north, in British Columbia, where clouds of beetles have laid waste to 14 million hectares - twice the size 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 400,000 hectares of woodland, and even more in neighboring Alaska, in a 15-year-old epidemic unmatched in its longevity and extent.

Climate change

"It's a fingerprint of climate change," Aynslie Ogden, senior researcher for the Yukon Forest Management Branch, says 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 7.5-millimeter insect does its damage.

"Usually the female bores into the tree first, followed by the male, and then they mate and they both excavate a main egg gallery which runs parallel to the wood grain," he says.

The hatched larvae, just beneath the outer bark, then feed via perpendicular galleries they bore around the tree, cutting off the flow of nutrients and killing the plant.

Winter temperatures of -40 degrees Celsius 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, have weakened the spruces' ability to extrude sticky pitch to trap and expel beetles.

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, says Legare.

As he speaks, smoke from dozens of fires, some nearby in the Yukon, some in distant Alaska, wafted over a landscape already bleak with dead forest.

In a 2007 assessment, the Intergovernmental Panel on Climate Change (IPCC), the United Nations-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 US West could increase by 50 percent by the 2050s, even under the best-case warming scenario projected by the IPCC.

In Siberia, "fire has been increasing, and there's an earlier fire season," Soja, of the US National Institute of Aerospace, reported from the Sukachev Institute of Forestry in Krasnoyarsk. "For most of Siberia, temperatures are increasing more than in North America."

In Canada, the 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.

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 into the skies, feeding an unstoppable cycle.

In smoky gray southwest Yukon, the 1,400 native Champagne-Aishihik people feel it most. The stricken forest's fallen trees are keeping them from traditional fur-trapping rounds, the streams seem warmer and the fishing is off.

Over the top of the world in Siberia, they're preparing for an upsurge in the highly destructive Siberian moth, a caterpillar that devours forests of pine, spruce, fir and larch.

As the Yukon warms and burns, its foresters hope for at least an early warning of one immediate threat, the mountain pine beetle. They have set traps at the British Columbia border to alert them if the non-native insect moves northward.

"The Yukon pines probably don't have natural defenses. They may be uniquely susceptible to this pest," says Green. "Then you'll have the potential for fires in large areas of dead trees. With the needles still on them, they literally explode with fire."