Can deserts slow global warming?

From the vast expanse of the Sahara Desert to the scrublands of the United States, arid and semi-arid lands account for nearly half the Earth’s land surface. Together, they form one of the largest ecosystems on the planet.

A novel experiment in the Mojave Desert, led by Dave Evans, professor of biological sciences, indicates these arid areas are a major sink for atmospheric carbon dioxide (CO2).

The long-term research project exposed plots of land to high levels of atmospheric carbon dioxide 24 hours a day for 10 years. The findings, published in the journal Nature Climate Change earlier this month, suggest that arid lands may increase their carbon uptake enough in the future to account for 15 to 28 percent of the amount of CO2 currently being absorbed by land surfaces.

The results point to the global importance of arid lands, said Evans. “As CO2 levels go up, they will increase their uptake of CO2 from the atmosphere. They can’t take it all up, but they’ll help.”

Simulating rising CO2

Forest soils have more organic matter and, square foot for square foot, hold much more carbon. But because arid soils cover so much area, they can have an outsized role in the earth’s carbon budget and in how much the earth warms as heat-trapping gases accumulate in the atmosphere.

In 1997, researchers marked off nine octagonal plots about 75 feet in diameter on the Nevada National Security Site, a large swath of uninhabited desert located 65 miles north of Las Vegas, and surrounded them with a series of vertical PVC pipes. For the next decade, air was continuously pumped through the pipes and dispersed over the plots to simulate different atmospheric conditions. Three plots were exposed to air with carbon dioxide concentrations at the current level of 380 parts per million and three others received air with CO2 concentrations of 550 parts per million, the level expected in 2050. The last three plots served as controls and received only ambient air.

The idea for the experiment originated with scientists at universities in Reno and Las Vegas and the Desert Research Institute. Evans, a specialist in ecology and global change, was brought in for his expertise in deserts and nutrient cycling.

At the end of the experiment, researchers removed the soil and plants down to a meter deep and measured how much carbon has been absorbed. The CO2 used in the simulation included a specific chemical fingerprint that could be detected when the material was evaluated. Benjamin Harlow in the WSU Stable Isotope Core Laboratory led the analysis.

While forest ecosystems tend to store carbon in plant matter, the Mojave researchers found most carbon was being taken up by increased activity in the rhizosphere, a microorganism-rich area around the roots.

“I was surprised at the magnitude of the carbon gain and that we were able to detect it after 10 years,” said Evans, “because 10 years isn’t very long in the life of an ecosystem.”

The effect on climate change

From an optimistic point of view, the research suggests that, come 2050, arid ecosystems will be doing more than their fair share of taking earth-warming carbon out of the atmosphere. But a potential cause for concern is what happens to these ecosystems as the planet’s population grows and people look for places to develop and live.

“Land is extremely valuable,” said Evans. “A lot of growth may occur in these areas that are fairly arid and we don’t know what tha’’s going to do then to the carbon budget of these systems.”