Research addresses big unknown of global warming

A national team of researchers, including an Arizona State University scientist, has discovered that arid lands, among the most expansive ecosystems on the planet, take up an unexpectedly large amount of carbon as levels of carbon dioxide increase in the atmosphere. Download Full Image

The study’s findings give scientists a better handle on the earth’s carbon budget – how much carbon remains in the atmosphere as carbon dioxide contributes to global warming, and how much gets stored in lands or oceans in other carbon-containing forms.

“This study has pointed out the importance of these arid ecosystems,” said R. Dave Evans, a Washington State University professor of biological sciences and lead investigator on the project. “They are a major sink for atmospheric carbon dioxide, so as CO2 levels go up, they’ll increase their uptake of CO2 from the atmosphere. They’ll help take up some of that excess CO2 going into the atmosphere. They can’t take it all up, but they’ll help.”

The findings, published in the journal Nature Climate Change, come after a novel 10-year experiment in which researchers exposed plots in the Mojave Desert to elevated carbon-dioxide levels similar to those expected in 2050. The researchers then removed soil and plants down to a meter deep and measured how much carbon was absorbed.

Given the complexities of the datasets generated over 10 years, Kiona Ogle, an associate professor with ASU’s School of Life Sciences in the College of Liberal Arts and Sciences, was brought into the study for her expertise in analyzing such data, and for her knowledge of desert and plant ecology.

“I found it surprising that after 10 years of exposure to elevated CO2, which would be expected to stimulate plant photosynthesis and growth, and hence plant carbon storage, the carbon contained in vegetation was not significantly affected,” said Ogle.

Ogle added, “The major effects of elevated CO2 levels were seen in the amount of carbon stored in soils, which was about 20 percent higher under elevated CO2. Thus, desert soils, but not necessarily desert vegetation, are expected to sequester more carbon under a future CO2 environment. But, the soil carbon must come from some place, and it is likely that plants are 'pumping' this carbon from the atmosphere into the soils.”

The idea for the experiment originated with scientists at Nevada’s universities in Reno and Las Vegas, and the Desert Research Institute. Researchers from the University of Idaho, Northern Arizona University and Colorado State University also contributed to the project.

The study addresses one of the big unknowns of global warming: the degree to which land-based ecosystems absorb or release carbon dioxide as it increases in the atmosphere.

Receiving less than 10 inches of rain a year, arid lands run in a wide band at 30 degrees north and south latitude. Along with semi-arid areas, which receive less than 20 inches of rain a year, they account for nearly half the earth’s land surface.

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

The study suggests that arid lands may increase their carbon uptake enough in the future to account for 15 to 28 percent of the amount currently being absorbed by land surfaces.

Overall, said Evans, rising CO2 levels may increase the uptake by arid lands enough to account for 4 to 8 percent of current emissions. The experiment did not account for other possible changes stemming from climate change, such as varying precipitation and warming temperatures.

The research suggests that come 2050, arid ecosystems will be doing more than their fair share of taking earth-warming carbon dioxide 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 that’s going to do then to the carbon budget of these systems.”

The U.S. Department of Energy’s Terrestrial Carbon Processes Program and the National Science Foundation’s Ecosystem Studies Program funded the research.