Contrary to the belief that plants could help cool earth, scientists have found that they 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 contribute to global warming beyond what is caused by the ‘radiative’ effects of CO2, Prof Govindasamy Bala of Indian Institute of Science (IISc), Bangalore,one of the authors of the study, 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, who conducted the study jointly with Long Cao and Ken Caldeira of Carnegie Institution for Science at Stanford University, said. Explaining 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 plant transpiration and thus warming of the land surface,” he said.

“Plants do photosynthesise and remove CO2 from the atmosphere and thus could help in cooling 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 long wave radiation) and indirectly through its physiological effect (reducing transpiration of land plants), he said. “We compared 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 was published in the May 3-7 online edition of Proceedings of the US National Academy ofSciences. “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.

The greenhouse warming effect of CO2 has been known for a long time but Prof Bala and his colleagues were concerned that it is not as widely recognised 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 Cao said.

In their climate modelling 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 over the entire global land surface, the effects from physiological change account for 15 per cent of warming.

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