A Life of Science: A Series by New Scientists
Chaos into clarity. We are measuring the desert breathe.
I’m harnessed to the top of a 20-foot weather station, swaying as a river of wind carrying the acrid scent of creosote washes over the Chihuahuan Desert. I hold my breath and reach to clean the dusty lens of a sensor that scans the desert air for traces of greenhouse gases. Nervous breathing would taint the data with spikes of carbon dioxide and water vapor. Scattered below are blackened ocotillo limbs—leaves shed, thorns bared, waiting patiently for the impossible promise of rain.
The steady wind is broken by pulses of turbulence that swirl like eddies in a stream. This is why I am here. I am part of a team at the University of Arizona that asks how climate change impacts carbon and water cycling in dry ecosystems. Despite the unrelenting heat, desert shrubs mine water in the soil and transport it to leaves to be split by sunlight and used by photosynthetic machinery to make sugar from thin air. By extracting carbon dioxide and releasing trails of vapor, growing plants leave fingerprints on the wind that our sensors detect by flashing air with pulses of ultrasonic sound and infrared light ten times each second. The sensors send signals to a computer that translates turbulence into meaning. Chaos into clarity. We are measuring the desert breathe.
I pause on the tower and note a subtle rise in humidity. This morning we left Tucson under a sky of unbroken blue. The wind has veered southerly and now carries vapor wicked from the Sea of Cortez. Clouds fray the azure sky, leaning from their moorings on ragged peaks below. Strong winds aloft. Auspicious.
I grew up with eyes turned skyward and then attended San José State University to study meteorology. Because California’s drowsy weather contrasted with the storms in textbooks, my department organized a summer field course on the North American monsoon—a seasonal shift in winds when land and air conspire to raise storms from heat and vapor. In summer 2012 my class packed into two 12-passenger vans and drove 700 miles to Flagstaff, Arizona. We pored over weather charts and satellite scans to forecast where storms would fire off, then raced toward red blotches on radar, stopping to take snapshots of the atmosphere with weather balloons. The air above the Colorado Plateau carried just enough moisture to fuel storms without marring the sky in cloud. Gray curtains bound by blue.
Before graduating I took a class on the carbon cycle and found that biology tarnished the clarity of a physically driven atmosphere. In contrast to meteorology textbook depictions of earth’s surface as a monochrome void, I learned that plants and soil in ecosystems warm and humidify the atmosphere from the ground up, both driving and responding to weather and climate.
From there I started graduate school at Iowa State University to find out if a biofuel crop called sorghum could outgrow corn during drought. Unlike California, Iowa is not irrigated, and farmers can’t turn on the water if it stops raining. Instead they count on the good chance it will rain, and it often does. Landlocked air can be surprisingly humid. On the sprawling lawn that is the Midwest, endless rows of transpiring corn pump prairie air full of water until the seams split and it rains. We ran the experiment for two wet years but never got a drought.
I came to Arizona expecting drought. The tower I’m working on stands over land with a reputation for dryness, which led to its selection by the U.S. Department of Agriculture in 1953 as the sole research watershed in the Southwestern United States’s semiarid rangeland. Since then, scientists have studied Walnut Gulch Experimental Watershed, near Tombstone, to sharpen our understanding of how land management and climate impact the hydrology and health of desert rangelands.
Walnut Gulch averages just 12 inches of annual rainfall and is rigged to track it all. A 2008 study1 published in Water Resources Research counted 88 rain gauges to complement the watershed’s countless soil moisture probes, several flumes, and two towers. It drains a mosaic of grass, shrub, and bare soil characteristic of dryland ecosystems—the expansive arid and semiarid regions that cover over 40 percent of Earth’s land and are home to two billion people, according to the Food and Agriculture Organization of the United Nations2.
Drylands must be among the most anxious of Earth’s biomes. Plants in drylands exist in a perpetual game of chance, jolted between rainy years and drought. Strong swings in plant growth tied to rainfall act like a control knob on global climate because growing plants soak up carbon dioxide, helping offset rising emissions.
While we have known that drought causes these ecosystems to soak up less carbon, we are learning that plants have a new thing to worry about: the atmosphere is becoming thirstier because climate warming increases the evaporative power of air. And plants are picky—when the air gets too dry, they slow growth to save water. This response to hot, dry air indicates that in a warmer future, plants may struggle to use the limited rain they get, which could reduce the amount of carbon dryland ecosystems remove from the air.
I step down from the tower and help the research technician, Ross, load our truck. The tower is ready to measure but the landscape looks dead and I feel it all: sun, heat, stagnant dopamine. Some hazy resentment of the desert that comes on when the June sun at apogee bleaches color from the void.
I find myself remembering the verdant cornfields of Iowa, where soaking rains and fertile soils raise some of the most productive crops in the world. But instead I’m here—south of the prairie, west of Eden, driving back to Tucson. Ross says he has felt this way before. He points to the San Pedro River, that impossible ribbon of green in the shimmering basin. He says drylands aren’t the most productive but they cover the globe. We talk about how their stories may hold lessons for ecosystem health in the hotter, drier climate of tomorrow.
I turn right to drive north on Highway 90 as a thunderstorm erupts off the Whetstone Mountains that parallel the road. The windshield blurs to sea-glass. Vapor driven from soil and leaves has collected, condensed, now fallen. An apparition materialized. Days later the ocotillos awaken, flush green streaks, and the desert breathes again.
Matthew Roby is a Ph.D. student in the School of Natural Resources and the Environment at the University of Arizona. His research asks how heat and drought alter the flow of carbon and water between arid ecosystems and the atmosphere.
Header photo by Matthew Roby. Photo of Matthew Roby by Nick Ohde.
