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[BANGALORE] The global warming potential of carbon dioxide (CO2), a key greenhouse gas, is underestimated as climate change studies do not take into account the crucial fact that plants 'sweat' less when surrounded by more CO2, new research shows.
Climate modelling studies by a team from India and the United States show that plants lose less water when exposed to higher levels of CO2 — making the surrounding air warmer. 
Higher CO2 levels make stomata, tiny pores on plant leaves, open less widely and lose less water through a process called ‘transpiration’ that cools the surrounding air.
Less transpiration means less cooling and this impacts global climate change models, the team from Indian Institute of Science (IISc), Bangalore; Carnegie Institution, and National Aeronautics and Space Administration Ames Research Centre reported last month (7 May) in the Proceedings of the NationalAcademy of Sciences.
Worldwide, plants lose an estimated 25 centimetres of water every year through transpiration. But this could reduce to 20 centimetres when CO2 levels are doubled, Govindasamy Bala, professor at the centre for atmospheric and ocean sciences at IISc, and one of the study authors, told SciDev.Net.
The findings unsettle research findings and beliefs that plants reduce warming by photosynthesis since they add to warming by not losing enough water. The research also highlights the need to include plant biology in climate model studies, the scientists reported.
"The global warming potential of CO2 is underestimated," Bala said.
Carbon dioxide warms the earth in two ways. One is the 'radiative' effect in which it absorbs the sun's heat radiated back by the earth. The second is the ‘physiological’ effect in which it affects the way plants function, in this case, through reduced transpiration.
"This physiological factor contributes about 15 per cent of the total warming potential of CO2," the team reported. 

Reduced loss of water through plants increases water runoff on the ground by about eight per cent; and contributes up to 65 per cent of total increased runoff caused by doubling CO2 levels, the team reported.

This increased run-off is particularly pronounced in the thickly-vegetated Amazon and Central and Southern Africa, the scientists reported. It also cuts the amount of moisture in the air by seven per cent over parts of Amazon and Central Africa.

*Full paper available to users in developing countries