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Global 'greening' mops up carbon dioxide – but for how long?

Researchers have attributed vegetation growth to the tune of 18 million square kilometres primarily to carbon dioxide. Phil Ritchie reports.

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Increased carbon dioxide levels in the atmosphere have boosted plant growth so much that it's as if a green continent twice the size of Australia has been added to the world in just three decades.

A study published in Nature Climate Change found plants soaked up around a quarter of the 10 billion tonnes of the greenhouse gas emitted by human activities each year. As a result, up to half of all vegetated areas of land have "greened" – that is, grown more leaves on plants and trees – while only 4% has become "browner" with fewer leaves.

The results "track well" with the Intergovernmental Panel on Climate Change's models that showed as atmospheric carbon dioxide rises, so too would plant activity, says University of Melbourne climate scientist Peter Rayner. But, he adds, it's only a matter of time before vegetation reaches its limit: "Nature is doing its best to clean up after us but it can't keep up with fossil fuel emissions."

Global warming isn't helping either. "There are clear doubts about whether these carbon storage benefits can persist under an increasingly warmer climate," says ecologist Rod Keenan, also from the University of Melbourne.

Photosynthetic plants produce sugars from sunlight, water and carbon dioxide. The powerhouses in leaves – chloroplasts – are filled with the green photosynthetic pigment chlorophyll. As long as plants have enough water, increased levels of carbon dioxide can crank up photosynthesis.

Zaichun Zhu from China’s Peking University and an international team of colleagues analysed 33 years of data taken by NASA and National Oceanic and Atmospheric Administration satellites which measured the amount of reflected green light on landmasses below. More green equals more chlorophyll and thus, more leaves.

And after plugging the satellite data into 10 climate models, they found carbon dioxide explained most of the greening, particularly in the tropics.

In the relatively sparsely vegetated, high-altitude regions, climate change had a big effect. For instance, on the Tibetan plateau, global warming enhanced photosynthesis and lengthened the growing season, while greening of the Sahel desert in Africa was driven mostly by more rainfall.

The team admits that their modelling isn't perfect. It doesn’t consider regional factors, such as forest management, grazing, irrigation, natural disasters and changes in cultivation practices.

Nevertheless, there's no doubt that climate change, thanks to carbon dioxide pumped into the atmosphere, is "affecting every corner of the globe and that humans are now a major force in shaping how the Earth is functioning", says Pep Canadell, a climate scientist at Australia's science organisation CSIRO and co-author of the paper. "Yes, we are indeed in the Anthropocene."

Just because carbon has been stored by plants today, it doesn't mean it'll stay that way. "As the climate warms further, extended droughts in some regions may result in rapid loss of vegetation cover and the release of stored carbon," Keenan says.

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Phil Ritchie is a Melbourne-based journalist.
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