In some regions, this means that more than a quarter of the warming that results from increases in CO2 could be due to its direct impact on plants.

As they photosynthesise, plants take in CO2 through tiny pores in their leaves called stomata. At the same time, they release water vapour, a process called evapotranspiration that has the effect of cooling the plant down, in much the same way that perspiration cools our bodies. However, increasing levels of CO2 cause plants to close their stomata, which means less water is released and the plants don’t cool down as much. 

The researchers used a computer model to compare the direct effects of CO2 on temperature with those caused by its effect on plant evapotranspiration. They found that when they doubled the concentration of atmospheric CO2, the effect of suppressing evapotranspiration in plants accounted for an average of 16 per cent of warming over the global land surface. However, in some regions, the effect rose to 25 per cent.

‘These results show that how plants respond to carbon dioxide is very important for making good climate predictions,’ said one of the study’s authors, Ken Caldeira.

July 2010