Plants can't help cool earth if CO2 is in excess: Study

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 said.

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".

"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.

"But stomata opens less widely and the canopy sweats less when CO2 is increased which causes a decline in planttranspiration 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.