Global warming could leave Britain facing more severe flooding than existing models predict because they have failed to take account of the way that plants’ consumption of water will change with the atmosphere, scientists said yesterday.

Although computer projections already suggest that Britain will experience heavier winter rainfall as the climate warms, the picture may be even worse because rising carbon dioxide levels will cause plants to mop up less groundwater than at present, research has shown. Land that is saturated with water will not be able to absorb heavy rainfall, leading to more floods, scientists said.

Increased rainfall alone will boost the volume of river flows in Europe by 11 per cent when carbon dioxide in the atmosphere reaches double preindustrial levels, which is forecast by the middle of the century.

The lower uptake of water from plants, however, will increase this still further, adding two percentage points to swell rivers by 13 per cent compared with preindustrial levels, according to the study by the Met Office and the Centre for Ecology and Hydrology in Oxfordshire. Britain is likely to experience even greater rises in river volume than this, researchers said.

The figures do not discriminate between European regions, and while the Mediterranean is expected to become drier, northern Europe is likely to get wetter, particularly in winter.

The research, published in Nature, suggests that models on which Britain’s adaptation plans for global warming are based underestimate the extent to which flood risk will increase.

At the same time, the risk of drought in summer is likely to be lower than predicted, as the effect of carbon dioxide on plants leaves more water in rivers and reservoirs.

Richard Betts, of the Met Office’s Hadley Centre, who led the study, said: “It is a double-edged sword. It means that increases in drought due to climate change could be less severe as plants lose less water. On the other hand, if the land is saturated more often, you might expect that intense rainfall events are more likely to cause flooding, which is what happened this summer.”

The models used by the Environment Agency need to be adjusted in the light of the new data, which should also be used in considering issues such as the replacement of the Thames Barrier. Dr Betts added that he has informed the Environment Agency of his team’s findings. “They were quite cross,” he said.

While plant growth is expected to increase as carbon dioxide levels in the atmosphere rise, the research has shown that the same effect will lower the amount of water they take up from the ground and release into the air as vapour. This is because higher atmospheric concentrations of carbon dioxide shrink the size of microscopic holes in plant leaves called stomata, through which water escapes into the air. Smaller stomata mean that plants will absorb less groundwater, leaving more to saturate the ground and flow into rivers.

In some parts of the world this is likely to be beneficial, offsetting some of the increased tendency to drought that higher temperatures would bring. In regions that are already wet, however, it will raise the risk of flooding.

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