The Southwest has already experienced such sudden shocks. One of the most dramatic occurred just a few miles from McDowell's outdoor lab. There, in 2011, an aspen tree fell onto a power line, sparking a fire stoked by hot, dry weather and drought-seasoned fuels that burned 43,000 acres in its first 14 hours. The Las Conchas blaze raged through pine and fir canopies on the Jemez Mountains' eastern flank, killing entire stands. Some are unlikely to regenerate, ever, and are already being replaced by oak and locust shrubs. The worst-hit and driest areas have yet to sprout much of anything at all.

"That is the land manager's worst nightmare," Stephenson says of the Las Conchas Fire. "The biological potential has been lost, there's going to be soil loss, erosion, the trees' seed source has been killed off. That was not an easy transition."

If the warming trend continues – as it surely will without heroic intervention – Stephenson hopes land managers can slow the pace of change and influence its outcome for key forests in the Sierra. The Giant Forest in Sequoia National Park, for instance, contains the most massive tree on the planet – known the world over as the General Sherman. "It's a place of high social value," Stephenson says. To help protect the Giant Forest from sudden death in an insect outbreak or a big wildfire, managers can thin trees and set small, controlled fires to reduce competition and increase the resilience of individual trees. If research begins to show that certain species can't survive the future climate, Stephenson says, managers may decide to let those trees go, assisting their migration to more hospitable terrain and perhaps planting new species in their place.

So far, though mortality among many of the area's tree species is increasing, the giant sequoias seem unchanged. There's too little information to draw strong conclusions about the whole population, but preliminary studies suggest that, in their prime habitat, the trees are actually thriving, benefiting perhaps from an extended growing season. Coast redwoods also appear to be growing vigorously, perhaps basking in extra sunlight as coastal fog declines.

Of course, this trend could change. These two iconic giants slurp more water than any other trees on earth, and future changes in water supply could hurt. Desperate measures to save them – like installing sprinkler systems – are already being discussed. "Mortality of big trees is a one-way street," says Stephenson. "You can't replace them once they're gone."

Models are useful in planning for the future, but we needn't wait for them to be perfected in order to start grappling with the effects of climate change on forests. The mechanisms and trends scientists like Nate McDowell, Park Williams and Nate Stephenson are uncovering are already in motion – and gaining momentum. The future is all around us, plain to see.

That's especially true in the Jemez Mountains. USGS research ecologist Craig Allen has spent his career in this landscape, never growing bored. Allen is a whip-smart man with boyish, straight-cut bangs whom McDowell calls "one of the godfathers of tree mortality." Like Stephenson, a colleague and friend, he's studied the same place for 30-plus years.

Change itself does not surprise him. But some of the changes he's seeing now are painful. "Because trees live longer than we do, we tend to view them as timeless," Allen says. "It's unsettling when these landscapes flip overnight." Asked about his favorite tree species, Allen deflects the "unfair question," but acknowledges that he loves old trees the most. The longer a tree lives, the more visible its history becomes – in gnarled bark, fire scars, and in the case of conifers, flattened tops. "You can feel this sense of endurance," Allen remarks. "In human terms, we would call it wisdom."

Allen has spent a lot of time thinking – and publishing papers – about the global significance of the rapid changes in the Jemez. "I don't want to overstate the lessons of the Southwest for the rest of the world," he says. "But it's a preview of what could happen." Drought won't kill all of the world's trees; some forests may get wetter and grow better. Still, the expected increase in global temperature is so extreme that it could easily convince most trees that they've moved to a new planet, and outweigh the potential upsides of climate change for plants, such as more carbon dioxide to consume.

Forests' widespread vulnerability is already evident. A hot, dry spell in Europe in the mid-2000s wiped out oak, fir, spruce, beech and pine. Drought has picked off aspen, jack pine, and black and white spruce in Western Canada's boreal forest – at both high and low elevations. A once-in-a-century drought in 2005, followed by another five years later, killed vast numbers of trees in the Amazon rainforest.

"One thing we are going to lose, and it might be in most places later this century, is old trees," Allen says. "Even if the system can still grow abundant vegetation, the historically dominant old trees are dominant because they're tuned to that historic climate window – which is already not the climate that we're in."

As grim a prospect as that is, it is also an opportunity to make the forests we still have more resilient – and to start doing so now. It is possible, even in the Southwest. The region goes through natural wet and dry cycles, and within the next 10 years, says Allen, the current dry spell is likely to let up. That will ease the pressure on trees from wildfires, beetles and the weather itself, and allow land managers to more safely thin forests with low-intensity fires. Combined with landscape-scale mechanical thinning, these measures could soften the blow of the next drought, and reduce the risk of future catastrophic fires or insect outbreaks. If managers want to plant trees – perhaps more drought-hardy species from other places – the wetter cycle will give seedlings an opportunity to establish themselves.

Most of the old trees we love may still perish, but there are better and worse ways for that to happen. When landscapes change incrementally, they are more likely to maintain species diversity, soil health, and basic functions like erosion control. Past droughts that killed trees in what seemed like apocalyptic fashion, Allen says, in fact caused a gradual reshuffling of the landscape. The 1950s drought, for instance, killed most ponderosa at the dry end of the tree's range, but not all. It didn't leave a treeless landscape, prone to the kind of erosion and soil loss that follows severe wildfire. "It didn't leave a desert," Allen says. Piñon and juniper replaced the dead ponderosa. Going forward, change of this sort may represent a best-case scenario. The question is: How fast can new trees colonize landscapes as the old trees die?

If his daughter or his twin boys have children, Allen wonders, what will they want to know about the forests he knew? He is brainstorming a new project: documenting the size, age, diversity and three-dimensional structure of trees in old forests, archiving tree-ring samples and the histories they hold, and recording the sounds of birds and wind blowing through the canopy. He also hopes to document what ancient forests mean to people by involving artists, poets, ethnographers and the elders of cultures for whom forests are important. Such a project, he is starting to think, might be just as important as scientific research. Someday, even if only through a virtual experience, his grandchildren could still walk through the Jemez Mountains' ponderosa or the Olympic Peninsula's rainforest, and hear the whisper of the breeze through the treetops.