Worldwide slowdown in plant carbon uptake

Deep and extended droughts are responsible for a recent slowdown in the amount of carbon dioxide that land plants pulled from the atmosphere as they grew, a new study suggests.

Satellite data suggest that between 1982 and 1999, the world’s net primary production — the amount of carbon pulled from the air as CO2 and stored in living plants each year — rose about 6 percent, says Maosheng Zhao, an ecologist at the University of Montana in Missoula. A rise in carbon storage matches what many scientists expected in a warming world with higher atmospheric concentrations of CO2, he notes. But new analyses by Zhao and Montana colleague Steven Running, reported in the Aug. 20 Science, indicate that the amount of carbon pulled from the atmosphere by the growth of terrestrial vegetation dropped about 1 percent during the first decade of this century.

Changes in carbon storage during the decade varied by region, analyses of satellite images reveal. In general, warmer temperatures boosted growth in high-latitude regions and at higher elevations. But in the Amazon, which accounted for about two-thirds of the change in carbon storage, increased warmth boosted evaporation and induced water stress, thereby trimming carbon storage. In 2005, an intense drought in the Amazon caused many trees to die, says Zhao.

And while the loss of trees due to wildfires, diseases, insects and human activity may have contributed to the overall decline in carbon storage, the primary cause of the decline in natural carbon sequestration outside the Amazon is probably the substantial drought that many regions suffered, the researchers speculate.

Rates of carbon storage continued to increase in the Northern Hemisphere during the 2000s, but a strong drying trend in the Southern Hemisphere more than canceled those gains. Australia, for instance, suffered droughts in 2002, 2005, and from 2007 through 2009. Overall, the researchers estimate the world’s land plants stored about 550 million metric tons less carbon during the 2000s than they did during the previous decade.

Previous studies have hinted that drought is a major contributor to declines in plant productivity, says Inez Fung, an atmospheric scientist at the University of California, Berkeley. But, she adds, “what’s really cool about this paper is the global time series of satellite observations” — a set of data that can’t be reproduced by simply extrapolating from occasional studies of widely scattered plots of forest.

The new study also indicates that the CO2 fertilization effect on vegetation, thought by a few scientists to be a possible solution to the ever-increasing concentration of atmospheric carbon dioxide, isn’t automatic, Fung says. “Water is a major, major thing,” she notes.

The implications of last decade’s downturn in carbon storage aren’t clear. The team’s data don’t indicate whether the spate of droughts during the last decade — the hottest in the instrumental record, Zhao and Running note — is simply a normal variation in climate or whether the long-term increase in average global temperature has substantially changed the world’s hydrological cycle.

“Climate models are unanimous that temperatures will go up” in the future, says Running. “What’s unknown is whether precipitation will go up as well,” he adds. “We don’t have enough fundamental understanding of how climate is going to unfold.”