Plants Could Saturate in CO2

New satellite data shows that carbon storage by plants is decreasing, in spite of the climate warming, say ecologists Maosheng Zhao and Steve Running at the University of Montana in Missoula, Montana, in a paper published today in Science.

Normally, as temperatures rose during the last decades, so did the amount of carbon that is transformed into plant biomass, but this new study shows that this tendency might have began to reverse.

Plants and oceans usually capture about half of the carbon dioxide emissions from the atmosphere, caused by fossil fuels and this new satellite data “is the first indication that it might be slipping,” Running said.

Before drawing conclusions, the two ecologists analyzed visible and infra-red spectrum data from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite, for identifying different ecosystems and measuring the density of the vegetation.

After considering factors that influence plant growth like day length and water availability, they estimated the amount of atmospheric carbon turned into plant biomass, called the Net Primary Production (NPP).

They announced that carbon absorption by plants has grown in certain areas of the globe like in parts of North America, western Europe, India and China - mostly in the Northern Hemisphere, but in the areas where carbon uptake decreased, it is rather serious.

In regions of south Africa, Africa and Australia, 70 percent of the plant-covered land presented a decrease in NPP, and as Running says, “on balance, when you add up all the pluses and minuses, there is a decrease.”

Running also took part in a 2003 study led by University of Montana colleague Ramakrishna Nemani, that reported an increasing plant productivity between 1982 and 1999, and scientists thought that the tendency was due to a warmer climate and high solar radiation.

This is why, this time Zhao and Running expected to see a similar increase for 2000-2009, in vain.

They think that a possible cause for this decrease in carbon absorption is caused by regional droughts, the the one in the Amazon in 2005 and the general tendency in the Southern Hemisphere.

Michael Crimmins, a climatologist at the University of Arizona in Tucson, thinks that it is a good thing that this paper emphasizes the ares of the globe affected by lack 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 said.

On the other hand, Bill Munger, an ecologist at Harvard University in Cambridge, Massachusetts, is not so happy about the method that has been used in the paper.

He says that “the type of modeling they used is good at highlighting spatial patterns in vegetation processes, but very dependent on assumed influence of moisture and temperature.”

Running admits that some suppositions had to be made as “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.”

“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,” Nature News quotes.

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