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Polar bears are threatened by melting sea ice in the Arctic. 圖片放大

Nine ways in which the Earth could be tipped into a potentially dangerous state that could last for many centuries have been identified by scientists investigating how quickly global warming could run out of control.

http://www.guardian.co.uk/environment/2008/feb/05/climatechange

Ice boulders left behind after a flood caused by the overflowing of a lake in Greenland. Photograph: Uriel Sinai/Getty images

"There's a perception that global warming is something that will happen smoothly into the future, but some of these ecosystems go into an abrupt decline when warming reaches a certain threshold," said Tim Lenton, an environmental scientist at the University of East Anglia and lead author of the study. (read all...)
環境科學家 Tim Lenton 說：「必需領悟到全球暖化是重大事件，將會很順利發生進入未來（意即必定會發生），且當暖化達到一個確實的門檻時，這些生態系統將會突然地崩解！」

"If we know when the different tipping points are, we can use them to inform targets to limit global warming. It gives us something to aim for," he added.
又說：「如果我們知道這不同的翻越點（傾覆點、傾斜點）時，我們可以以它為執行目標，來限制全球暖化，它付予了我們有價值的目標去執行。」

Nine ways in which the Earth could be tipped into a potentially dangerous state that could last for many centuries have been identified by scientists investigating how quickly global warming could run out of control.

A major international investigation by dozens of leading climate scientists has found that the "tipping points" for all nine scenarios – such as the melting of the Arctic sea ice or the disappearance of the Amazon rainforest – could occur within the next 100 years.

The scientists warn that climate change is likely to result in sudden and dramatic changes to some of the major geophysical elements of the Earth if global average temperatures continue to rise as a result of the predicted increase in emissions of man-made greenhouse gases.

Most and probably all of the nine scenarios are likely to be irreversible on a human timescale once they pass a certain threshold of change, and the widespread effects of the transition to the new state will be felt for generations to come, the scientists said.

"Society may be lulled into a false sense of security by smooth projections of global change. Our synthesis of present knowledge suggests that a variety of tipping elements could reach their critical point within this century under anthropogenic [man-made] climate change," they report in the journal Proceedings of the National Academy of Sciences.

The study came out of a 2005 meeting of 36 leading climate scientists who drew on the expertise of a further 52 specialists. It is believed to be the first time that scientists have attempted to assess the risks of what they have termed "tipping elements" in the Earth's climate system.

The nine elements range from the melting of polar ice sheets to the collapse of the Indian and West African monsoons. The effects of the changes could be equally varied, from a dramatic rise in sea levels that flood coastal regions to widespread crop failures and famine. Some of the tipping points may be close at hand, such as the point at which the disappearance of the summer sea ice in the Arctic becomes inevitable, whereas others, such as the tipping point for the destruction of northern boreal forests, may take several more decades to be reached.

While scenarios such as the collapse of the Indian monsoon could occur within a few years, others, such as the melting of the Greenland ice cap or the West Antarctic ice sheet, may take several centuries to complete. "Our findings suggest that a variety of tipping elements could reach their critical point in this century under human-induced climate change," said Professor Timothy Lenton, of the University of East Anglia, who led the study.

A tipping point is defined as the point where a small increase in temperature or other change in the climate could trigger a disproportionately larger change in the future. Although there are many potential tipping points that could occur this century, it is still possible to avoid them with cuts in greenhouse gases, said Professor Lenton.

He added: "But we should be prepared to adapt ... and to design an early-warning system that alerts us to them in time."

Irreversible changes

* Arctic sea ice: some scientists believe that the tipping point for the total loss of summer sea ice is imminent.

* Greenland ice sheet: total melting could take 300 years or more but the tipping point that could see irreversible change might occur within 50 years.

* West Antarctic ice sheet: scientists believe it could unexpectedly collapse if it slips into the sea at its warming edges.

* Gulf Stream: few scientists believe it could be switched off completely this century but its collapse is a possibility.

* El Niño: the southern Pacific current may be affected by warmer seas, resulting in far-reaching climate change.

* Indian monsoon: relies on temperature difference between land and sea, which could be tipped off-balance by pollutants that cause localised cooling.

* West African monsoon: in the past it has changed, causing the greening of the Sahara, but in the future it could cause droughts.

* Amazon rainforest: a warmer world and further deforestation may cause a collapse of the rain supporting this ecosystem.

* Boreal forests: cold-adapted trees of Siberia and Canada are dying as temperatures rise.