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[BEIJING] Global warming could cause plants in northern regions to lose carbon to the atmosphere rather than sequester it, according to a new international study.

The research, published in Nature yesterday (3 January), looked at atmospheric carbon dioxide concentrations and carbon dioxide held in ecosystems such as forests in the Northern Hemisphere in the past 20 years.

It is widely believed that the warming trend in the past two decades — with spring starting earlier and winter later, increasing the growing season — has increased plants' productivity, enabling them to absorb more carbon from the atmosphere (see Climate change 'boosts plant health in China').

But lead author of the research, Piao Shilong from the Laboratory of Climate and Environmental Sciences at France's National Scientific Research Centre, says this theory has not fully considered seasonal variation.

"In warmer autumns, the more prosperous terrestiral ecosystems may lose more carbon," Piao told SciDev.Net.

This is because in autumn, plant respiration — the process by which plants produce energy and release carbon dioxide — occurs more than photosynthesis, the process by which plants absorb carbon for growth.

The scientists found that in response to autumn warming, northern ecosystems are currently losing 90 per cent of the increased carbon dioxide that plants take up during spring.

"If future autumn warming occurs at a faster rate than in spring, the ability of northern ecosystems to sequester carbon may be diminished earlier than previously suggested," write the scientists.

According to Piao — who has since moved to the Beijing-based Peking University — while both respiration and photosynthesis could be boosted by warmer temperatures, the lower intensity of sunshine, drier weather and plants' low sensitivity to temperatures could all contribute to respiration outstripping photosynthesis during autumn.

Zhang Chengyi, a leading scientist at the China Meteorology Administration's National Climate Centre, says the study has offered an interesting new angle on analyses of the global carbon cycling.

"But to reach the conclusion [of net carbon loss in autumn] needs more consideration. In a gradual warming trend, the plants may adapt to the situation, taking up more carbon in autumn too," Zhang told SciDev.Net.

In an accompanying comment in the same issue of Nature, John B. Miller from the University of Colorado suggests that more observations in the tropics and other poorly-observed regions in the ocean and on land are needed to further verify the conclusions.

Link to full paper in Nature

Reference: Nature 415, 49 (2008)