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  PlanetArk 29 Aug 07
Predictions Off for Global Warming Flood Risk - Study
Story by Michael Kahn

Yahoo News 29 Aug 07
Flooding risk from global warming badly under-estimated: study
by Richard Ingham

Global warming may carry a higher risk of flooding than previously thought, according to a study released on Wednesday by the British science journal Nature.

It says efforts to calculate flooding risk from climate change do not take into account the effect that carbon dioxide (CO2) -- the principal greenhouse gas -- has on vegetation.

Plants suck water out of the ground and "breathe" out the excess through tiny pores, called stomata, in their leaves. Stomata are highly sensitive to CO2.

The higher the level of atmospheric CO2, the more the pores tighten up or open for shorter periods. As a result, less water passes through the plant and into the air in the form of evaporation. And, in turn, this means that more water stays on the land, eventually running off into rivers when the soil becomes saturated.

In a paper published in February 2006, British scientists said the CO2-stomata link explained a long-standing anomaly.

Over the last 100 years the flow of the world's big continental rivers has increased by around four percent, even though global temperatures rose by some 0.8 degrees Celsius ( 1.35 degrees Fahrenheit) during this period.

Today, as a result of the unbridled burning of oil, gas and coal, levels of CO2 in the atmosphere are around a third more than in pre-industrial times in the middle of the 18 th century.

The new study takes the 2006 discovery an important step further by projecting what could happen to water runoff in the future. If CO2 levels double compared with pre-industrial concentrations -- a common scenario in climate simulations -- the effect on plants alone would lead to an increase of six percent in global runoff, it says.

Until now, scientists have generally estimated an increase in runoff of between five and 17 percent compared with the pre-industrial era. But this is based only on one yardstick, called radiative forcing.

In other words, it only measures the warming effect that greenhouse gases have on the water cycle and not the indirect impact that CO2, the biggest culprit, has on vegetation.

The "radiative forcing" yardstick also predicts that higher temperatures will increase evaporation, causing greater water stress and longer droughts. Both forecasts are offbeam, says the new paper.

By widening the picture to include the CO2-stomata factor, the likelihood is that the risk of flooding will be worse than thought, but the risk of drought rather less so.

"The risks of rain and river flooding may increase more than has been previously anticipated, because intense precipitation events would be more likely to occur over saturated ground," it says.

"In contrast, the risks of hydrological drought may not increase as much as expected on the basis of meteorological changes alone."

Flooding is a major problem, especially in poor countries that do not have the money to upgrade drainage systems to cope with runoff from saturated soils.

Since June, nearly 3,200 people in South Asia have been killed by heavy monsoon rains and snow melt. More than 20 million people have been affected in the eastern Indian state of Bihar alone.

The authors of the Nature paper are led by Richard Betts of the Hadley Centre for Climate Prediction and Research, part of Britain's Met Office.

PlanetArk 29 Aug 07
Predictions Off for Global Warming Flood Risk - Study
Story by Michael Kahn

LONDON - Current predictions for global warming underestimate the risk of floods and overestimate the impact of droughts by not taking into account the role plants play in absorbing carbon dioxide, researchers said on Wednesday.

They found higher levels of the greenhouse gas predicted for the end of the century will lead to an increase in the amount of water that plants hold in the soil, said Richard Betts, a meteorologist at Britain's Met Office who led the study.

This means areas expected to see increased rainfall might have more severe flooding while droughts in other regions may not be as bad, he said in a telephone interview.

"People may be underestimating flood risks because they do not expect the soil to be as saturated as it might be," Betts said. "We also suggest the conservation of water by plants would partly offset the scarcity during a drought."

The findings underscore the need to take a wider view of climate change to better understand and predict the impact of rising temperatures, he added.

Using global climate models linked to data on vegetation and soil content, the team of British researchers measured the effect of carbon dioxide levels expected to rise dramatically by the end of the century.

During photosynthesis -- the process through which plants absorb energy and produce oxygen -- carbon dioxide enters plants through tiny pores called stomata. Water eventually evaporates back into the atmosphere through these stomata.

But higher levels of carbon dioxide in the air cause these tiny holes to not open as widely, leading to reduced water loss from the plant and leaving more water in the soil, Betts said.

Greenhouse gases such as carbon dioxide are widely blamed for global warming. Scientists say average temperatures will rise by between 2-6 degrees Celsius by the end of the century, causing droughts, floods and violent storms.

"Climate change is more than just a change in the meteorological conditions. It is also a change in the whole ecology" Betts said. "We need to study this to get the whole picture because this hasn't been looked at before."

With plants extracting less water from the soil, the surplus water will drain into rivers and increase global flows another 6 percent on top of the 11 percent rise already predicted due to global warming, Betts said.

The study did not indicate which areas might experience the greatest change but Betts said this was the next step for his team.

"We will need to quantify things and look at things like water availability and the details of how intense rainfalls may turn into flash floods," he said.
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