permaculture

["Lawrence F. London, Jr." <lflj@intrex.net>]

<http://news.bbc.co.uk/2/hi/science/nature/6969122.stm>
Last Updated: Wednesday, 29 August 2007, 17:00 GMT 18:00 UK
Climate flooding risk 'misjudged'
Flooding in Geneva, Illinois, US Image: Jeff Haynes/AFP/Getty Images
The flooding risk may have been underestimated, says the report
Climate change may carry a higher risk of flooding than was previously 
thought, the journal Nature reports.

Researchers say efforts to calculate flooding risk from climate change do not take into account the effect carbon 
dioxide (CO2) has on vegetation.

Higher atmospheric levels of this greenhouse gas reduce the ability of plants to suck water out of the ground and 
"breathe" out the excess.

Plants expel excess water through tiny pores, or stomata, in their leaves.

Their reduced ability to release water back into the atmosphere will result 
in the ground becoming saturated.

Areas with higher predicted rainfall have a greater risk of flooding. But 
this effect also reduces the severity of droughts.

The findings suggest computer models of future climate change may need to be 
revised in order to plan for coming decades.

Soil saturation

Plants perform two functions that are of key importance in climate change.

They absorb CO2 from the atmosphere and convert it to oxygen as a by-product 
of their energy generation process.

Stomata, Met Office
Plants open stomata less widely when CO2 is high, losing less water
Plants also absorb water through their roots, and release it back into the 
atmosphere through their stomata.

CO2 enters plants through the stomata; water evaporates through the same 
holes.

The higher the level of atmospheric CO2, the more the pores tighten up or 
open for short periods.

As a result, less water passes through the plant and into the air in the form of evaporation. In turn, this means that 
more water stays on the land, eventually running off into rivers when the soil becomes saturated.

A team led by Dr Richard Betts from the Met Office has modelled how this will 
affect climate change predictions.

"It's a double-edged sword," said Dr Betts, "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."

CLIMATE CHANGE

Factory graphic Animated guide
Find out how the greenhouse effect works and more...

IPCC ASSESSMENT
Climate change 'can be tackled'
Climate curbs: Who will buy?
Billions face climate change risk
Mapping climate change
Stark picture of warming world
Through the climate window

GLOBAL POLITICS
Politicians sign new climate pact
China building more power plants

EU/UK POLITICS
UK emissions on the rise
EU agrees on carbon dioxide cuts

FEATURES
Climate action 'needs devolution'
Power station harnesses Sun
Scrutinising climate economics

BACKGROUND
Q&A: Climate change
The evidence
Models 'key to climate forecasts'
Earth - melting in the heat?

INTERACTIVE
Rich pay poor to cut carbon?

FROM ACROSS THE BBC
Climate change portal


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