Tranquil ecosystems may explain wild swings in carbon dioxide stashing

Placid prairies and austere scrublands may be key ecosystems for explaining mysterious year-to-year swings in the amount of carbon dioxide sucked out of the atmosphere.

The unassuming landscapes are responsible for up to 50 percent of the yearly variation in how much of the greenhouse gas is stashed on land, researchers report in the May 22 Science. Generally, land and oceans each absorb about a quarter to a third of annual CO₂ emissions, helping to buffer global warming. But from year to year, land-based “carbon sinks” have seemed inexplicably capricious, amassing large amounts of CO₂ one year and little to none the next.

“There’s quite significant variability,” says ecosystem scientist Anders Ahlström of Lund University in Sweden. Previous studies tracking CO₂ absorption on land have focused on lush tropical forests, which breathe in the largest share of the greenhouse gas for photosynthesis. Yet the new data suggest that semiarid ecosystems, including grasslands and shrub-spotted fields, play a key role in determining whether a little or a lot of humans’ yearly emissions are tucked away or left afloat.

Ahlström and colleagues categorized Earth’s land into six classes of land cover, including cropland and tropical forests. Using global ecosystem simulations, the researchers estimated the CO₂ absorbed or released by each land class every year from 1982 to 2011, comparing those estimates with global patterns of CO₂ fluxes.

The researchers found that year-to-year variations and increases in the CO₂ sucked in by land were largely due to semiarid ecosystems. These landscapes spring to life in particularly rainy wet seasons, swelling into large carbon sinks, and shrivel in hot, dry times. Those fluctuations account for about 40 to 50 percent of the year-to-year global swings in CO₂ uptake — the largest source of variation among all land-cover classes.

Until now, the source of variability has been largely unknown, says earth systems analyst Sibyll Schaphoff of the Potsdam Institute for Climate Impact Research in Germany. Ecologist A. David McGuire of the University of Alaska Fairbanks adds that the study raises new questions. He says it would be interesting to know how factors like fires and vegetation changes affect semiarid lands’ CO₂ hoarding.