Collecting such voluminous amounts of detailed data requires a unique toolbox. The plane itself is geared toward special mission work with its high-payload capacity and short takeoff and landing capabilities. An imaging spectrometer, resting atop a hole cut in the belly of the plane, absorbs light across the spectrum, from ultraviolet to short-wave infrared. It allows the CAO to measure 23 different chemicals in the trees, including water, nitrogen, and sugar content. To work properly, internal sensors within the imaging spectrometer are kept at -132 Celsius, atomically cold temperatures.

A laser system next to the imaging spectrometer fires a pair of lasers from the bottom of the plane 500,000 times per second, creating a three-dimensional image of the terrain below, and every tree on it. A second spectrometer, this one with an enhanced zoom capacity, allows the team to take measurements of individual branches on a tree—from 12,000 feet up. Finally, a piece of equipment known as an Internal Measurement Unit records the X,Y, and Z axes as well as pitch, roll, and yaw of the plane to ensure that its positioning in the air doesn’t compromise the accuracy of the data it collects from the ground. “This unit is the same technology as what’s in the nose of a cruise missile,” Asner explained. “Because of that, the State Department has a say in what countries we visit.” The CAO studies forests all over the world—Peru, Malaysia, Panama, South Africa, Hawaii.

Once airborne, we dismissed the sprawl of the Central Valley for the coastal mountains. To the naked eye, the Shasta-Trinity National Forest looked splendorous, 2.2 million acres of rivers and mountains. Mount Shasta, a 14,179-foot active volcano, was still holding on to a handsome cap of snow and the landscape was vibrant and green. Asner’s spectrometer shared a different story. “Visual assessment doesn’t tell you much,” he said. On his computer screen, the green trees below were all reading red. They were dead. We just couldn’t see it yet. “A lot of this was not here last year,” he said with the clinical efficiency of a doctor diagnosing a cancer patient. CAO’s statewide findings suggest tens of millions of trees might not survive another dry winter.

Sugar pine (Pinus lambertiana), a species that grows in large, contiguous groves and can live 500 years, has been hit the hardest, accounting for some 70 percent of the mortality, but cedar, fir, and oak are all suffering as well. It’s not just the lack of precipitation that’s killing these trees; it’s the cascading effect of climate change. Water-stressed trees make easier targets for mountain pine beetles (Dendroctonus ponderosae), which lay their eggs in the trunk and eat the trees.

Asner shared a map of the Giant Forest. The sequoias were a cool, comforting shade of blue, demonstrating high water content. Water seeks its low point, Asner explained, and the Giant Forest sits in a plateau cup. “It’s an oasis, a refugio. Right now, those trees are of least concern.” It was bittersweet news, like celebrating the last house standing after a tornado.

“Drought is a cumulative process,” Asner explained as the plane made a long bank off the western slope of Shasta. “Forests have biological inertia. We don’t know where the physiological tipping point is. Currently, we’re losing carbon from the forest.”

Forests are supposed to absorb carbon, so I wasn’t sure if I’d heard Asner correctly over the communication system. I tapped my headphones to make sure they were still working. “I’m sorry, did you just say the forests are releasing carbon into the atmosphere?” Automobiles, coal-fired power plants, cattle production—those are all carbon sources. But the mighty forests of California?

“That’s my guess,” he said. “It’s hard to imagine the forests are still carbon sinks.”