Carbon storage by plants may be decreasing despite climate warming

By Rhiannon Smith
Missoula, MT (August 20, 2010)- 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 record.

The study, entitled “Drought-Induced Reduction in Global Terrestrial Net Primary Production from 2000 Through 2009,” was conducted by Maosheng Zhao and Steven Running of the Department of Ecosystem and Conservation Sciences at the University of Montana, Missoula. Their Abstract states:

Terrestrial net primary production (NPP) quantifies the amount of atmospheric carbon fixed by plants and accumulated as biomass. Previous studies have shown that climate constraints were relaxing with increasing temperature and solar radiation, allowing an upward trend in NPP from 1982 through 1999. The past decade (2000 to 2009) has been the warmest since instrumental measurements began, which could imply continued increases in NPP; however, our estimates suggest a reduction in the global NPP of 0.55 petagrams of carbon. Large-scale droughts have reduced regional NPP, and a drying trend in the Southern Hemisphere has decreased NPP in that area, counteracting the increased NPP over the Northern Hemisphere. A continued decline in NPP would not only weaken the terrestrial carbon sink, but it would also intensify future competition between food demand and proposed biofuel production.

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).
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”.
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.
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.
Related Resources:
Nature- Earth’s green carbon sink on the wane
Science- Drought-Induced Reduction in Global Terrestrial Net Primary Production from 2000 Through 2009