I was strolling through Muir Woods National Park a couple of months ago. The giant redwoods are an amazing sight, and, no offense to our Colorado woodlands, but they make one of the most beautiful forests imaginable.

The overstory is vast and shady. The understory is lush. In between, these giants rise up, massive and mighty. And ancient. It's humbling.

But one of the most astonishing sights in the forest isn't the trees. It's a sign that warns of the fate these behemoths are facing from global warming. They've lived for millennia along the California coast, but rising temperatures are messing with the environment they rely on.

Those trees dwarf humans below, but the changes we have brought to the planet are more powerful than even these giants. They can survive wildfire after wildfire, but they can't stand a warming globe.

They're not alone. National parks across the country are facing landscape-altering changes at the hands of global warming. Most famous is the melting ice at Glacier National Park in Montana. The question at hand is: How long will it be until that name is a misnomer? How long until those beautiful glaciers are gone for good? Not long. Photos of those glaciers now already reveal a shadow of the ones that stood there decades ago.

But it's not just Glacier. A new report by the Natural Resources Defense Council calls climate change the “greatest threat” to the country's national parks. The report lists 25 parks most at risk due to melting ice, drought, flooding, disappearing wildlife and other factors. Many of those most threatened are here in the West, where droughts have become the rule, and rain comes more often as destructive torrents and dangerous flooding. Animals and plants are vanishing.

“This is not just a concern for the future,” says the report, produced by the Rocky Mountain Climate Organization for the NRDC. “The national parks that we Americans so cherish are already being harmed by a changing climate.”

Colorado scores two threatened national parks on the list, and others that could be candidates. At Rocky Mountain National Park, researchers say, the snowpack is already shrinking. That doesn't just mean fewer scenic snowcapped peaks. It means less water for the critters that fill the park's lush west side, and for the Colorado River that flows downhill from there.

Many believe the devastating outbreak of the mountain pine beetle is connected to global warming. That epidemic has swept across the national park, just as it has swept across the state and is sweeping across the Roaring Fork Valley. Sudden aspen tree die-off — the peculiar, widespread death of entire stands of aspen trees across the West — could also be a result of climate change. (Question to ponder: What becomes of Aspen when there are no aspens?)

The sensitive tundra ecosystem of Rocky Mountain National Park, as with the rest of the high country, is disappearing. Wildflowers could diminish under global warming. The park's bighorn sheep could find themselves run out of the mountains. Those little rodent-like pika critters could disappear. Fish are dwindling. Ozone levels in the air are rising.

Down at Mesa Verde, beetles have ravaged the piñon forests. Hotter, drier weather seems to have driven away the Mexican spotted owl. But despite the droughts, downpours are on the upswing, threatening to erode the ancient dwellings that have stood at Mesa Verde for centuries.

Hotter weather is harming fish in the Black Canyon of the Gunnison. Deluges threaten desert country at Dinosaur National Park.

Similar threats face national parks across the country. Yellowstone is losing its whitebark pines, and that could threaten the park's iconic grizzlies, which depend on them for food. Hot summer temperatures are searing trout in their streams. Zion is losing its woodlands. And yes, Glacier is losing its glaciers, as well as the waterfalls and streams that flow from them.

Rising seas could drown the Everglades and California's coastal parks. Yosemite is already being choked with air pollution.

“If we continue heedlessly adding heat-trapping pollution to the atmosphere, we could lose whole national parks for the first time,” the report says.

It's no accident that this report comes out on the heels of Ken Burns' series, “The National Parks: America's Best Idea.” Burns sought to stir the nation's consciousness with the history of our national parks. It's a rich heritage, but heritage isn't enough.

If nothing else reminds us of the real threats of climate change, a visit to the national parks, while they're still around, ought to.

Reach David Frey on his Web site, www.davidfrey.me.