Plants less thirsty in a warmer world

UK researchers have found that rising carbon dioxide (CO₂) levels could have potential benefits for drier parts of the world - because plants become less thirsty.

Richard Betts and his colleagues from the Hadley Centre in Exeter have found that a doubling of CO₂ concentrations compared with pre-industrial levels, which is predicted within the next 50-100 years, will result in 6% more water in rivers because of more runoff from the land.  This greater availability of water occurs because higher CO₂ levels cause plants to use less water.  The undersides of a plant's leaves are peppered with tiny pores called stomata, which can be opened to allow plants to soak up the CO₂ they need for growth from the atmosphere.  The CO₂ enters the leaf by dissolving in a layer of water spread over the surfaces inside each of the stomata.  But this water constantly evaporates into the air in a process called transpiration, which is why plants need a continuous supply from their roots.  Higher CO₂ levels, however, mean that plants don't need to open their stomata as wide or for as long to obtain the same amount of carbon dioxide, which cuts down their water usage.  And what the plants don't drink then ends up back in rivers.

The team reached their conclusions, which are published in this week's Nature, by using the same climate models which are used to predict day to day weather forecasts and which are therefore well tested and viewed as reasonably accurate. "This indicates that freshwater resources may be less limited than previously assumed," says Betts.