Seen from above, the mountains of central Colorado are a snow-covered mosaic of meadows, aspens and lodgepole pines. Some of those pines are green, their branches holding new-fallen snow. Others are red fading to brown - the telltale signs of trees killed by an epidemic of bark beetles that have wiped out millions of acres across the West.
Scientists say those trees are also a clear indication of global warming, one of several indelible marks a warming planet has left on the West.
"We have beautiful forests. We have to protect them," said Al Gore, the former vice president who has become a climate change crusader.
Gore appeared Friday at the symposium Forests at Risk: Climate Change & the Future of the American West. The conference was hosted by For the Forest, a local group that formed after residents became concerned about the spread of pine beetles into the mountains that surround the ski town, but whose focus has widened as scientists increasingly pointed to a link between beetles and a warming globe.
"It is a moral issue," Gore told the crowd gathered at the Aspen Institute, whose campus sits below hillsides turning red from beetle kill, "and we have to be as a generation willing to stand up and do the right thing."
It wasn't his first visit here to the White River National Forest. In the summer of 1971, Gore, newly-released from the Army after serving in Vietnam, packed up his Chevy Impala and left his Tennessee home to camp in the Colorado mountains. The next year he did the same.
"I have my own relationship with the forest here," said Gore.
Those forests have changed in those 40 years. They've changed in the past five years. Widening swaths of pines have been killed by beetles. Other insects have killed spruce and pinons. Sudden aspen decline has wiped out entire stands of the West's most iconic tree. Wildfires, hotter and fiercer than ever before, have scorched thousands of acres.
Each phenomenon has its own causes, but scientists at the symposium agreed they all share climate change in common.
"The linkage that these scientists have found over and over again to global warming I'm sure some people resist but it's a fact," Gore said. "It's unprecedented and we have to face up to it."
While Gore was the most famous name at the symposium, his was a rare voice of activism in a conference heavy on science.
"The planet's getting warmer. It's getting warmer rapidly and it's projected to continue to do so," said Craig Allen, a researcher for the U.S. Geologic Survey based at Bandelier National Monument in New Mexico who has studied the impact of climate change on trees around the world.
"We see it from the Amazon to the boreal forests in Siberia and everywhere in between," Allen said.
Scientists recited a litany of attacks on the West's forests linked to global warming. Bark beetles have devastated 2 million acres of trees across the West, particularly in the Greater Yellowstone Ecosystem. They have spread higher and farther north than ever before and are threatening to reach unprecedented areas east of the Continental Divide.
More than 1 million acres of aspens in Colorado and Wyoming have been affected by sudden aspen decline, and aspens are expected to disappear altogether from at least two-thirds of the areas they recently thrived.
It's not just epidemic die-offs. Individual trees are also dying at higher and higher rates across the West.
Wildfires are burning hotter and spreading farther, and wildfire seasons are lasting longer than ever.
The common denominator is global warming, researchers said, which has left trees weakened by drought while higher temperatures have helped insects, disease and fire to thrive.
Global warming change the look of forests forever, researchers said, but exactly what they will look like remains to be seen.
"We're heading into uncharted territory," said University of Arizona researcher Tom Sweetnam, who has studied the worsening of wildfires due to climate change.
For Gore, the forests are only a part of a picture of global impacts from rising temperatures. Last year, the hottest recorded on earth, saw unprecedented catastrophic wildfires and floods around the world, Gore said, including his hometown of Nashville, Tenn.
"This is a challenge to the existence of our civilization as we know it," he said. "The things we love, like the forests, are at risk and there are things that can be done to mitigate the damage, but in order to solve the problem we have to solve the root cause. We're putting 90 million tons (of pollution) into the earth's atmosphere ever 24 hours."
But what can be done? That question ran through the conference, but had few answers. Land managers said they were struggling to help create ecosystems that would be more resilient to climate change.
Sen. Mark Udall. D-Colo., called for Congress to put "a price on carbon," either through cap and trade, a carbon tax or a similar mechanism.
'I hope we get to that point before we've lost the opportunity to lead," Udall said. "While we dither and while we debate, the Chinese are acting, the Germans are acting, the Spaniards are acting, many other countries are acting."
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