Plants lose their cool as carbon dioxide rises

Scientists have added a new phenomenon to the burgeoning list of global warming culprits.

Primary driver of climate change carbon dioxide, known for its “heat-trapping” property, has another deadly impact – it also lowers the cooling effect that plants produce through transpiration, scientists at the Indian Institute of Science (IISc.), the Carnegie Institution for Science, and NASA Ames Research Center, California, have found.

Their research paper, published in the latest online edition of the U.S. journal, Proceedings of the National Academy of Sciences, shows how plants, which thrive on carbon dioxide and release oxygen during photosynthesis, dramatically decrease their ability to transpire moisture through leaves when exposed to high levels of the greenhouse gas.

As much as a third of global warming will soon be due to its impact on plant physiology (and the rest from the greenhouse effect of the gas) in some parts of the world, including parts of Asia and South America, says the paper, co-authored by Govindasamy Bala of the Center for Atmospheric and Oceanic Sciences, and Ramakrishna Nemani of the Divecha Centre for Climate Change of the IISc.

Map shows percentage of warming in different parts of the world due to carbon dioxide's physiological effect on plants. - DIVYA GANDHI

During photosynthesis, plants cool the environment by releasing water through pores, or ‘stomata,’ on the surface of leaves. However when CO2 is increased, stomata open up less and the canopy transpires less, in a phenomenon called “CO2 physiological forcing,” Professor Bala says. By increasing the CO2 levels (to twice the pre-industrial levels), this diminished transpiration of vegetation could account for 15 per cent of warming around the globe, their simulation models show.

CO2 is expected to be double that of its pre-industrial levels by the year 2050 or 2100, Professor Bala says. In India, these physiological changes could account for 10 per cent of warming, he adds. In East Asia, in the Amazon in South America and in parts of Europe and North America, it could account for 30 per cent of CO2 induced warming.

Scientists also found greater runoff from land in most areas with increased CO2, as more water from precipitation bypasses the plant cooling system and flows directly into rivers and streams.



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