"Though photosynthetic rate saturates at higher CO2 levels, the rate of reduction in transpiration by plants remains nearly constant," says team leader Govindsamy Bala of the Divecha Center for Climate Change. "Unlike photosynthesis, canopy transpiration continues to decline at about 5% for every 100 ppmv increase in CO2 levels."
The greenhouse-warming effect of CO2 has been known for a long time but the physiological effect has only recently received more scrutiny. "Up to a third of the warming from increased CO2 could be due to its impact on plant physiology in some regions," says Bala.
The present work investigates for the first time the decrease in canopy transpiration for CO2 concentrations between 100 and 1200 ppmv using a global-scale land-surface model. Previous studies have looked at such effects only for a doubling of CO2 levels. "The continued decline in canopy transpiration at elevated CO2 levels indicates that the warming from the physiological effect of CO2 will not abate at higher CO2 concentrations," says Ranjith Gopalakrishnan, who is lead author of the paper.
Canopy transpiration is a major component of the water cycle over land. It is important to understand its variability and trends because they could alter rainfall and streamflow. "Changes to the global water cycle due to climate change, such as more frequent floods, are major concerns," adds Bala. "Apart from changes in the water cycle from CO2's greenhouse effect, there could be important long-term consequences from the CO2 physiological effect."
Scientists from the Carnegie Institution and NASA, both in the US, were collaborators in this study.