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Climatologists give waterworld warning for Earth
As the world gets warmer, it is getting wetter. And one of the main conclusions reached at Europe's largest ever
earth sciences conference was that we are less prepared for it than ever.
While some delegates were still reeling from the catastrophic floods that hit the continent in August 2002, others
warned that the risk of future flooding has been vastly underestimated. And studies of past episodes of climate
change suggest that a wetter world may be not only a consequence of global warming but a trigger for further,
more dramatic temperature rises.
The first task was to take stock, in a session devoted to analysing last summer's floods. Most agreed the event was
a freak of nature - an unfortunate and unpredictable convergence of events. A cyclone disappeared and then
reappeared over central Europe, taking everyone by surprise.
It was followed a couple of days later by a second more powerful cyclone that was halted by a region of high
pressure, causing it to dump its huge load of rain over a relatively small area. "It was like a perfect storm," says
Jirí Stehlík from Prague's Czech Hydrometeorological Institute.
The floods hit hardest in the Czech Republic, where 15 people died and 220,000 were evacuated. In Prague a flood
this severe would normally be expected only once every 500 years; in the south of the country it was a once-in1000-year event, and some areas received half their expected annual rainfall in just four days.
Freak of nature or not, the disaster was an ominous warning of the kind of events likely to be triggered as global
temperatures rise. And they could happen more often than we thought, according to Richard Betts from Britain's
Hadley Centre for Climate Prediction and Research in Berkshire. He warned colleagues at the meeting that they
have been underestimating the risk of future flooding.
Current models of how climate change will affect average rainfall only take account of the ability of air to hold more
water as it gets warmer. This means there will be more evaporation in a warmer world, and therefore more rainfall.
This alone would increase river flows worldwide by about one per cent by 2100, making rivers more likely to burst
their banks.
But that picture ignores the effects of greenhouse gases on plants, Betts pointed out. In response to high levels of
carbon dioxide, plants shrink their stomata - the holes in the surface of their leaves through which gases pass in
and out. This drastically reduces water loss from the plants, leaving more water in the soil.
When Betts included these changes in his models of groundwater levels, he found the effect could increase
groundwater by 10 per cent over the next century - 10 times as much as global warming alone. A region of central
Africa covering part of the Democratic Republic of Congo was one of the worst-affected locations, with soils in the
area dealing with an extra six centimetres of water a year.
When wetlands ruled the world
Delegates in sessions devoted to past episodes of climate change also talked about the implications of rising rainfall
- and whether it could be a trigger for rapid warming, not just a consequence. One of the liveliest debates
concerned the most dramatic changes in the Earth's climate: steep jumps in temperature that can occur in a
matter of decades as ice ages draw to a close. Understanding how and why these swings happened is critical to
working out whether we are now at risk of triggering something similar.
One popular explanation is that slight initial temperature changes cause a sudden release of vast amounts of the
greenhouse gas methane held frozen beneath the seabed in a form known as a gas hydrate. But Mark Maslin from
University College London shook things up by suggesting that wetlands were to blame - at least for the most recent
rapid warming event, after the last ice age about 18,000 years ago.
His theory came about as a way to settle an argument between oceanographers and land-based palaeontologists.
From computer models of plant growth, land-based researchers calculate that about 1000 gigatonnes of carbon has
been added to land since the last ice age. This all comes from CO 2 in the sea, transferred via the atmosphere.
But oceanographers looking at marine sediments reckon only 500 gigatonnes of carbon has been lost from the
ocean. They work this out by measuring the relative amounts of carbon-12 and carbon-13 isotopes in the water
over time. Plants preferentially use carbon-12, so the more carbon they take up, the higher the proportion of