Rising levels of carbon dioxide in the atmosphere will lead to higher river levels and an increase in the risk of summertime flooding because plants will be less able to soak up rainwater from waterlogged soil, the Met Office has found.

The research reveals a significant link between rising CO2 levels caused by burning fossil fuels and the ability of plants to absorb water through their roots – a major factor that prevents saturation of the soil during wet summers. Met Office scientists said that a computer analysis of how the effect will influence river levels around the world has found that there will be costs as well as benefits. Droughts may be less severe than anticipated in some regions, but the risk of flooding in high-rainfall areas could increase significantly.

"It's a double-edged sword. It means that increases in drought due to climate change could be less severe as plants lose less water," said Richard Betts, a computer modeller at the Met Office Hadley Centre in Exeter, who led the study. "On the other hand, if the land is saturated more often you might expect that intense rainfall events are more likely to cause flooding."

Plants lose less water by evaporation when CO2 rises because the tiny pores, or stomata, on leaves, which absorb CO2 for photosynthesis, become narrower when CO2 concentrations are lower. Water evaporates from a plant more easily when the stomata are wide open, but less so when they are partially closed, which is why plants in high CO2 environments need less water, Dr Betts said. The effect is well documented in laboratory and field studies and last year the Met Office showed that the phenomenon is likely to explain at least in part why there has been an increase in the amount of rainwater run-off from rivers around the world over the past century. Now its scientists have quantified the effect.

Dr Betts and his colleagues used computer models to assess the effect of a CO2 concentration of 550 parts per million (ppm), double pre-industrial levels (current levels are about 381 ppm), on the ability of plants to take up water from the ground. Writing in Nature they estimate that global river flows will increase by about 6 per cent due to this effect, compared with an 11 per cent increase on average river flows due to the extra rainfall expected in a warmer world. He said that the findings show the importance of treating individual greenhouse gases such as CO2 and methane differently in terms of the effect they have on the Earth's climate.
(By Steve Connor, The Independent, 30/08/2007)