Published online 19 August 2010 | Nature | doi:10.1038/news.2010.418

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Satellite data indicate that carbon storage by plants is decreasing despite climate warming.

Is one of the world's largest carbon sinks filling up? ymgerman/iStockphoto

The capacity of plants to act as a carbon sink could be on the decline.

As global temperatures have risen in recent decades, the amount of atmospheric carbon being converted into plant biomass has increased in step. However, in a paper published today in Science, ecologists Maosheng Zhao and Steve Running at the University of Montana in Missoula report a surprising reversal of this trend over the last decade, despite its having been the warmest on record1.

In 2003, a study on which Running was a co-author, led by Ramakrishna Nemani, who is also at the University of Montana, reported an increase in plant productivity between 1982 and 1999. The researchers attributed that trend to a warmer climate and increased solar radiation2. Zhao and Running expected to find a similar increase for 2000-2009 — an expectation that was not met.

Along with the oceans, plants are doing us a great service by taking up about half of all fossil-fuel carbon dioxide emissions from the atmosphere, says Running. "This is the first indication that it might be slipping."

The duo analysed visible and infra-red spectrum data from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite to distinguish different plant ecosystems and to measure the density of the vegetation. They then examined factors that influence plant growth, such as water availability and day length, to estimate the amount of atmospheric carbon accumulated as plant biomass — the Net Primary Production (NPP).

Click for a larger version of this image. Maosheng Zhao / Steven Running / University of Montana

The results show that carbon uptake by plants did increase in some areas — primarily in the Northern Hemisphere — including parts of North America, western Europe, India and China. But in areas where carbon uptake decreased, the drop was sharp. In the Southern Hemisphere, 70% of plant-covered land, including regions of South America, Africa and Australia, showed a decrease in NPP.

"On balance," Running says of carbon uptake by plants, "when you add up all the pluses and minuses, there is a decrease".

Over the limit

The researchers say that this fall in global carbon uptake can be attributed to regional droughts, such as the severe drought in the Amazon in 2005, and a general drying trend in the Southern Hemisphere that has worsened with global warming.

Why the Northern and Southern Hemispheres have responded differently to warmer climates and increased drought is unclear. Running suggests that the variations could be attributable to the different constraints faced by plants on opposite sides of the Equator. In the Northern Hemisphere, the limiting factor for plant growth tends to be temperature and the length of the growing season — both of which have increased with global warming. The main limitation on plant growth in the Southern Hemisphere, meanwhile, is water availability — which is why droughts have had a greater impact there.

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Michael Crimmins, a climatologist at the University of Arizona in Tucson, says that "this paper does a nice job" of highlighting areas of the planet where plant growth is limited by the availability of water, rather than by temperature. "I'm glad to finally see some global-scale evidence that a warmer world is not necessarily a greener world," he says.

However, Bill Munger, an ecologist at Harvard University in Cambridge, Massachusetts, is critical of the methods used in the paper. "The type of modelling they used is good at highlighting spatial patterns in vegetation processes, but very dependent on assumed influence of moisture and temperature," he says.

Running agrees that they had to make some assumptions. "I fully acknowledge that when you're making global-level calculations you're seeing only a very small part of the activity of the ecosystem and inferring the rest," he says.

"Until we see another 10–20 years of data, it would be premature to guarantee that this is a permanent trend, but it certainly means that we'd better be watching this really carefully," he adds.