The piñons died during what Breshears dubbed a "global-change-type-drought." It's impossible to blame any particular weather event on climate change. Still, the drought was a glimpse of the future, when droughts are predicted to be hotter and drier. Breshears and his colleagues found that it took 15 months in extremely dry soils to kill the piñons around McDowell's office. The heat, they believed, had increased the overall death toll by siphoning more water from soil and plants, though they couldn't yet prove it.

Dramatic changes in Southwestern forests had been expected – eventually. Desert edges are already marginal tree habitat, and were predicted to become especially vulnerable to the future's hotter, more intense droughts. Still, the amount of dead wood around Los Alamos was startling. Piñons didn't die only at the ecological boundary between woodland and grassland, the dry end of their range where Breshears and others believed climate change impacts would first become visible. Instead, piñons died almost everywhere they grew.

No community can comfortably afford to lose its forests. Besides being nice places to hike and ski, forests provide food and shelter for birds and wildlife. Leaves scrub the air of pollutants humans saturate it with. And forests shelter winter snow, the source of most Westerners' water supply, filtering it to rivers and streams in spring.

More important from a global perspective is the fact that forests ingest an estimated quarter to a third of the carbon dioxide released by fossil fuels, effectively keeping the earth's burner turned down. When trees die, they not only stop absorbing CO2, but they also decompose, gradually releasing the carbon stockpiled in their wood. If enough forests collapse, the flame on the planetary heating element could turn from "low" to "high." Instead of slowing global warming, forests could start to make it worse.

Computer models either don't account for future tree death caused by climate change, or they do so simplistically. These shortcomings worry scientists, and with good reason: The most troubling thing it could mean is that the dramatic forecasts the models currently produce – the ones predicting not only a warmer climate, but also the fundamental transformation of life on earth – are understated.

Before scientists can more accurately predict our future climate, they have to complete a simpler task – at least, one that sounds simpler. They need to understand, in mechanistic detail, how trees meet their end.

After Nate McDowell spent a few years studying the inner lives of junipers, his attitude toward the trees softened. What junipers lack in majestic height and open, shady understories, they make up for in pluck and perseverance. McDowell, a spry, 41-year-old former endurance runner, began to appreciate these qualities. "They're just so tough," he says. "You have to respect someone who's tough."

Juniper doesn't cower in the face of drought. Even when extremely short on water, it doesn't close its stomata – the tiny pores on its needles that regulate the tree's basic bodily functions. Stomata allow trees to consume carbon dioxide and photosynthesize. They also let water escape, creating the tension that pulls water upward through the tree's circulatory system. If there's too little water in the soil, a tree's pipes can fill with air and break.

To prevent this, many trees close their stomata during droughts. Juniper, with its deep roots and sturdy build, doesn't. When extremely stressed, it begins severing the water supply to entire limbs – reducing the amount of water the whole tree needs to survive. This is why smooth, naked branches – the desert's version of driftwood – often protrude from living junipers otherwise covered in stringy bark and sharp needles.

Piñon is more cautious, slamming its stomata shut during drought. Perusing data Breshears and another colleague collected during the drought, McDowell had an epiphany: For a year, the piñons that died endured a level of water stress that should have kept their stomata shut. Photosynthesis is to trees what cooking is to people. It's how they eat. In trying to protect themselves from dying of thirst, he thought, maybe piñons had starved to death instead.