Cast an eye over the many forests' worth of scientific literature on global warming and it quickly becomes clear that working out what a temperature rise of a few degrees will mean for life anywhere on the planet is far from straightforward. Vast ice sheets may melt, sea levels will rise, and faced with a new climate, species must adapt, move or perish. Yet the precise details of how any of it will happen are, frankly, unknown.
Now it seems the future has become even more uncertain. Climate scientists say they have identified several weak links around the world, regions where global warming could bring about the catastrophic collapse of vital ecosystems. The consequences will be felt far and wide.
Surely civilisation will have enough time to protect itself against the consequences, while ecosystems could gradually adapt? Not so, say scientists studying the world's weakest links.
John Schellnhuber, research director at the Tyndall Centre for Climate Change Research in Norwich, played a key role in identifying the systems where global warming could produce dramatic environmental damage. He calls them the "tipping points", the Achilles heels of the planet. Here, we present a list of some tipping points and what might happen if they are triggered.
Sahara desert
Occupying some 3.5m square miles of northern Africa, the Sahara desert is expected to shrink with global warming as more plentiful rain brings a flourish of vegetation to its southernmost reaches. For those on the edge of the desert, the fertile land will undoubtedly be a boon, but the Sahara plays a broader role in the health of the planet. The dry dust that is whipped up from the desert by strong prevailing winds contains crucial nutrients that seed the Atlantic and may even help fertilise the Amazon.
As the Sahara turns from brown to green, the flux of nutrients into the ocean is expected to drop, restricting food available for plankton, the smallest of links in the marine food chain. As the number of plankton falls, so does food for aquatic creatures further up the food chain.
That's not the only knock-on effect. Plankton lock up the greenhouse gas CO2 from the atmosphere, and so help counter global warming. With fewer plankton, the oceans will take less of the gas from the Earth's atmosphere.
Amazon forest
The size of western Europe, the Amazon forest is one of the most biodiverse regions on Earth. Models suggest that with global warming will come a drop in Amazonian rainfall, leading to the gradual death of the forest and subsequent collapse of the myriad ecosystems it supports.
The extinction of species is only one consequence of a warmer planet. As the trees die off, they will fall and rot, releasing carbon dioxide. In the worst-case scenario, the quantities of CO2 emitted could be of the same order of magnitude as from the 20th century's total fossil fuel output.
Greenland ice sheet
The Greenland ice sheet holds about 2.6 cubic kilometres of fresh water, which is some 6% of the planet's supply. It is imperative that this water remains frozen. If global warming sees temperatures rise by more than about 3C, Greenland is likely to begin a slow thaw, steadily releasing all that water ? currently resting on land ? into the north Atlantic Ocean.
Climate models suggest that a more drastic temperature increase of some 8C could see the Greenland ice sheet disappear almost entirely, a thaw that would see the seven seas rise by seven metres. Such a dramatic rise in sea level would cause flooding that is bound to have a devastating impact on people living on unprotected shorelines around the globe.
North Atlantic current
The North Atlantic current is one of the strongest ocean currents in the world, of which the Gulf stream is the precursor.
It works like a conveyer belt. Surface water in the North Atlantic is first cooled by westerly winds from North America, making the water more dense and salty so it sinks to the ocean floor before moving towards the equator. Driven by winds and replacing the cold water moving south, warm water from the Gulf of Mexico moves upward into the Atlantic.
The effect of the current on climate is dramatic. It brings to Europe the equivalent of 100,000 large power stations' worth of free heating, propping up temperatures by in excess of 10C in some parts.
Global warming could change all that, though not very quickly. Computer models predict that as global warming increases, so will rainfall in the North Atlantic. Gradually, the heavier rains will dilute the sea water and make it less likely to sink, a process that could bring the whole conveyer to a gradual halt.
Total shutdown of the current would lead to a rapid regional sea level rise of about a metre along the UK coast.
The monsoon
During March and April the Indian subcontinent begins to heat up, reaching some of the highest surface temperatures of the year by May. The hot land produces a sharp temperature gradient between the land and sea which causes an abrupt reversal of the winds from seaward to landward.
As the winds strike the Himalayas and are deflected upwards, they create a low pressure system, forcing rainclouds to release their stores of water. While the monsoon season can cause incredible flood damage, local populations are largely adapted and to some extent reliant on the weather.
If global warming has the expected effect of heating India even more, the monsoon season could become far more severe. What happens will be influenced by the level of pollution in the region. Sulphur dioxide and even dust make rain droplets smaller and so diminish overall rainfall. These substances also increase the reflectivity of clouds, which prevents the ground from heating up so much.
Both of these factors would weaken the monsoon, causing havoc for Indian agriculture, with serious consequences for food production.
source: http://www.guardian.co.uk/guardianweekly/story/0,12674,1333874,00.html 24oct04
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