Research Highlights

Warming from decline in “sweating” rate of plants may not abate in the future: Climate Modeling study

- Govindasamy Bala, CAOS

Bangalore: Plants remove carbon dioxide from the atmosphere via photosynthesis and thus could help to cool down the warming planet. They also directly warm the land surface when the atmospheric CO2 content increases. This later warming, caused by reduced canopy transpiration, is associated with reduced stomatal opening and increased water efficiency under elevated CO2 levels. According to a new study by scientists at the Indian Institute of Science, published online on 14 November 2011 in the journal Environmental Research Letters, this physiology-related warming may not abate in the future.

"Though photosynthetic rate saturates at higher CO2 levels, the rate of reduction in transpiration by plants remains nearly constant" says Prof. G. Bala of Divecha Center for Climate Change, who directed the research. He further adds "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 is receiving more scrutiny only recently. "Up to a third of the warming from increased carbon dioxide could be due to its impact on plant physiology in some regions" says Prof. Bala who was an author in earlier 2009 and 2010 papers on this topic.

The present work, for the first time, investigates the decreases in canopy transpiration as a function of CO2 concentrations using a global-scale land surface model. Other previous global modelling studies have looked at such effects for only a doubling of CO2 levels. The authors studied the canopy transpiration behaviour using a contemporary global land-model for the CO2 range from 100ppmv to 1200ppmv. Current atmospheric CO2 concentration is around 400 ppmv. The continued decline in canopy transpiration at elevated CO2 levels indicates that the warming from physiological effect of CO2 will not abate at higher CO2 concentrations.

Canopy transpiration is a major component of the water cycle over land and it is important to understand its variability and trends because changes in canopy transpiration could alter the rainfall and steamflow. "Changes to the global water cycle due to climate change such as more frequent floods are major concerns. Apart from changes in water cycle from CO2-greenhouse effect, there could be important long term consequences from the CO2 physiological effect." says Prof. Bala.

The land surface model simulations for this study were performed on a supercomputer supported by Supercomputer Education and Research Center at the Indian Institute of Science. Scientists from Carnegie Institution and NASA were collaborators in this study.


Percent changes in photosynthesis and canopy transpiration for varying levels of CO2 concentrations. For instance, "400-200" denotes the percentage changes for an increase in CO2 concentrations from 200 to 400 ppmv. Hatched areas are regions where changes are not statistically significant at the 99% confidence level. For changes from 600 to 800 ppmv and 800 to 1000 ppmv, the change in photosynthesis is minimal (i.e. it has saturated). However, canopy transpiration continues to decline all over the globe for these increases in CO2 concentrations.