Contrary to the belief that plants could help cool land, scientists have found that plants will directly warm the land surface when there is excess carbon dioxide in the atmosphere.

A recent global scale model study points to an emerging consensus that the physiological effects of increasing atmospheric CO₂ on plants on land will increase global warming beyond that caused by the 'radiative' effects of CO₂, Prof Govindasamy Bala, one of the authors of the study from Indian Institute of Science (IISc), Bangalore told PTI.

Carbon dioxide warms the earth because it is a greenhouse gas. However, elevated CO₂ in the atmosphere causes plants to transpire less and provide less 'evaporative cooling', he said.

"For scientists trying to predict global climate change in the coming century, the study underscores the importance of including plant biology in their climate models," Bala, jointly with Long Cao and Ken Caldeira, of the Carnegie Institution for Science at Stanford University, said.

Explaining the plant physiology, Bala said, "The CO₂-physiological effect arises from a change in plant transpiration rate under elevated atmospheric CO₂".
"But stomata opens less widely and the canopy sweats less when CO₂ is increased which causes a decline in plant transpiration and thus warming of the land surface," he said.

"Plants do photosynthesise and remove CO₂ from the atmosphere and thus could help to cool down the warming planet," Bala said.

Plants have a very complex and diverse influence on the climate system, Prof Ken Caldeira from Stanford said, adding "plants take CO₂ out of the atmosphere, but they also have other effects, such as changing the amount of evaporation from land surface. It's impossible to make good climate predictions without taking all of these factors into account."

An increase in atmospheric CO₂ concentration influences climate both directly through its radiative effect (trapping longwave radiation) and indirectly through its physiological effect (reducing transpiration of land plants), he said.

"We compare the climate response to radiative and physiological effects of increased CO₂ using the National Center for Atmospheric Research (NCAR) coupled Community Land and Community Atmosphere Model," he said.

The paper is published in the latest edition of May 3-7 online edition of Proceedings of the US National Academy of Sciences.

"The scale is important when we deal with global climate change. For average land, plants transpire about 25 cm of water each year. With doubling of CO₂, this amount goes down to 20 cm. This change of 5 cm is about the same magnitude as decrease of evaporation from deforestation or annual global water extraction by humans for irrigation and other consumptive use" the scientists said. .

"On a hot day, we sweat more, release more water through pores in our skin and cool ourselves. Similarly, while doing photosynthesis (food production process in plants using photons from sun), plants cool the environment by releasing water through the pores called stomata on the surface of leaves," Bala said.

The greenhouse warming effect of CO₂ has been known for a long time but Prof Bala and his colleagues at Stanford were concerned that it is not as widely recognized that CO₂ also warms planet by its physiological effects on plants.

"There is no longer any doubt that CO₂ decreases evaporative cooling by plants and that decreased cooling adds to global warming. This effect would cause significant warming even if CO₂ were not a greenhouse gas," lead author Dr Cao said.

In their climate modeling study, scientists doubled the concentration of atmospheric CO₂ and recorded the magnitude and geographic pattern of warming from the greenhouse and physiological effects. They found that averaged over the entire global land surface the effects from physiological change account for 15 per cent of warming, with greenhouse effects accounting for the rest.

The scientists also found larger runoff from the land surface in most areas for elevated CO₂, as more water from precipitation bypasses the plant cooling system and flows directly to rivers and streams.

Earlier models based on greenhouse effects of CO₂ had also predicted higher runoff, but the new research predicts that changes in 'evapotranspiration' due to high CO₂ will have an even stronger impact on water resources.

For a doubling of CO₂, this study finds 65 per cent of the runoff increase coming from the physiological effect and the rest from greenhouse effect.

"We find that the impact of physiological effect is stronger than greenhouse effect on land surface hydrology" Bala added.