WITHIN a century, Europe could be in the grip of a big freeze. The Amazon
basin may be desert, the Arctic devoid of ice and the Sahara covered in
forests.
These dramatic possibilities were endorsed by 1800 climate scientists from
100 countries who met in Amsterdam last week. Their warning comes on the eve of
new talks on the Kyoto Protocol to stem global warming, and is targeted at
politicians wavering over the need to halt the accelerating greenhouse
effect.
The latest findings include the first hint of a slowdown in the Atlantic
currents that keep Europe warm. They warn that climate change will have
previously unimagined effects. Many of the predictions of standard climate
models, including those published last week by the Intergovernmental Panel on
Climate Change, could turn out to be wrong, they say.
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The problem is that the Earth is prone to sudden and drastic flips in climate
and ecology. This means that predictions from climate models, which assume
steady warming and gradual responses from the ecosystem, could be well wide of
the mark. In reality, the scientists say, the Earth takes up different stable
states鈥攁nd can change suddenly from one to another. That will play havoc
with both human life and nature.
Take the Sahara, for example. Martin Claussen of the Potsdam Institute for
Climate Impact Research in Germany says the region was covered in forest and
bush around 6000 years ago, when the world was last as warm as it is now. But
within a few decades it switched to desert. It could flip back to forest again
during the coming century, says Claussen鈥檚 colleague Victor Brovkin.
Brovkin says that the Sahara is on a knife-edge because a small increase in
rainfall could trigger a self-perpetuating spread of vegetation. Plants absorb
moisture that eventually evaporates, which helps clouds to form. This in turn
triggers more rain, helping more plants to grow. 鈥淭he Sahara has two potentially
stable states: desert and heavily vegetated. It could flip between the two, and
only a little increase in rainfall could do it,鈥� he says. Climate models
predicting that the Sahara will spread could turn out to be completely
wrong.
Meanwhile the Amazon rainforest risks flipping the opposite way. Heat and
drought could turn it into a desert sometime in the middle of the century. New
evidence revealed by Peter Cox of Britain鈥檚 Meteorological Office in Bracknell
shows that as the forests die they would release billions of tonnes of carbon
dioxide into the air, accelerating global warming. According to his colleague
John Mitchell, if other forests went the same way they would push the upper
limit of likely global warming in the coming century from 6 掳C to 8 掳C
or more.
Many features of the Earth鈥檚 atmosphere and ecology are now operating at
temperatures not seen for hundreds of thousands of years, and changing at
unprecedented rates. So human activities could at any time 鈥渢rigger changes with
severe consequences for Earth鈥檚 environment and its inhabitants鈥�, the scientists
warn in an end of conference declaration.
For instance, Stefan Rahmstorf, also from Potsdam Institute for Climate
Impact Research, says that the first signs of a slowing in the North Atlantic
circulation have now been seen. That could herald a collapse of the Gulf Stream
that keeps Europe up to 10 掳C warmer than other areas on the same latitude.
Without the warm Gulf Stream, most of the continent will cool drastically.
The North Atlantic circulation, of which the Gulf Stream is a part, is driven
by ice formation in the far north of the ocean. As the sea freezes, it leaves
behind very dense, saline water that sinks to the ocean floor and is replaced by
warm water from the tropics.
Rahmstorf predicts that as the world warms in coming decades, this
鈥渄eep-water formation鈥� will rapidly decline as less ice forms and more fresh
water flows into the Arctic from rivers. 鈥淯ntil three weeks ago I would have had
to admit that this was a purely theoretical calculation,鈥� he says. 鈥淏ut now we
have received data showing that the current has decreased by 20 per cent since
1950.鈥�
Other dramatic changes forecast by scientists last week include a
proliferation of fires in both the boreal forests of the north and tropical
rainforests, where the fire season is set to lengthen by at least a month.
Runaway melting of the Arctic ice cap and permafrost are also on the cards, they
say. In Siberia, temperatures are already rising by 0.5 掳C per decade.
快猫短视频s also warned of ways in which global warning could spiral out of
control. Besides the death and burning of forests, they include the release of
large amounts of the greenhouse gas methane from melting permafrost, and a
collapse of the oceans鈥� ability to absorb carbon dioxide as global warming makes
ocean circulation ever more sluggish.
