They say Earth's biosphere has numerous "tipping points" -- triggers that cause global warming and its impacts to lurch up a gear or two, rather than occur than in a smooth, incremental way.

In other words, the planet itself would become the main driver of warming, making the crisis far more difficult to manage.

Many of the tipping points have only been discovered within the last decade or so, and experts admit to many unknowns as to how and when they could occur.

Here is a summary of the main triggers, outlined by the UN's Intergovernmental Panel on Climate Change (IPCC) and in studies published in peer-reviewed journals:

ARCTIC SEA ICE LOSS

The Arctic ice cap, which in winter covers some 15 million square kilometres (5.8 million square miles), is shrinking. Whether the region's first ice-free summer happens in five years or 50 is only a matter of 'when', not 'if', many scientists say.

As the ice disappears, so too does a massive mirror that reflects sunlight back into space. The dark ocean left uncovered soaks up the Sun's radiative force, and the warming helps to melt neighbouring patches of ice.

This vicious cycle of warming is what scientist's call a "positive feedback loop" -- less ice means more heat absorbed, and more heat absorbed means less ice.

Sea ice floats on water, so its melting does not add to ocean levels, although its loss would have an impact on biodiversity.

GREENLAND AND WEST ANTARCTIC ICE SHEETS

Greenland is covered with a blanket of frozen water thick enough to lift global sea levels by seven metres (23 feet).

Less than a decade ago, there seemed no risk that the ice sheet would be lost except over a geological timescale, measurable in terms of thousands of years.

Since 2000, though, Greenland has lost 1,500 billion tonnes of ice, contributing 0.75 mm (0.03 inch) annually to sea levels, and some scientists fear it could collapse within a couple of centuries.

A global average temperature increase of 3.0 degrees Celsius (5.4 degrees Fahrenheit) -- an unfortunately plausible end point for this century -- would mean a 9.0-to-11.0 C (16.2-to-19.8 F) jump in the Arctic region, enough to tip the balance, according to the IPCC.

The West Antarctic ice sheet was likewise considered relatively immune to global warming over the short term, but the recent dramatic collapse of ice shelves points to a more imminent danger.

Were the whole ice sheet to give way, it would add another five metres (16 feet) to the ocean watermark.

A big concern is that these masses of ice are so huge that, once the melting starts, seas will continue to rise for decades or centuries.

Even a one-metre (3.25-feet) rise -- a widely-shared forecast for 2100 -- would wipe several island states off the map and disrupt hundreds of millions of lives in low-lying deltas, especially in Asia and Africa.

PERMAFROST TIME BOMB

Locked inside permafrost, covering a fifth of Earth's land surface, are billions of tonnes of carbon in the form of methane, a greenhouse gas 30 times more potent than CO2.

The top three metres (10 feet) of this frozen landscape -- up to a kilometre (half-mile) thick -- contain as much carbon as Earth's atmosphere.

As temperatures rise, more and more methane is freed and enters the atmosphere, adding to the greenhouse effect.

The leakage is not only occurring on land. Methane ice formations called clathrates, in the Arctic and Antarctic oceans, are also bleeding the heat-trapping gas.

It was probably the same process that sparked runaway global warming some 635 million years ago, ending the longest ice age Earth has ever known.

IPCC chairman Rajendra Pachauri called this year for a special task force to study whether thawing permafrost could cause "abrupt, irreversible climate change."

SAGGING SINKS

More than half the CO2 humans generate is absorbed, in roughly equal measure, by forests and oceans.

Earth's plant life is so far keeping pace with emissions despite tropical deforestation. But oceans are showing signs of fatigue, according to a study released last week by the Global Carbon Project (GCP), an international consortium of climate scientists.

Over the last half century, the percentage of CO2 pumped into the atmosphere that stays there has gone up from 40 to 45 percent, fuelling the greenhouse effect.

Part of the decrease may be due to carbon saturation and rising emissions. But rising temperatures also cause ocean acidification, hampering the ability of marine organisms -- plankton, algae, coral -- to transform CO2 into calcium-rich shells that help to lock away carbon for millennia.

earlier related report
Sarkozy hails US, Chinese climate proposals
Manaus, Brazil (AFP) Nov 26, 2009 - French President Nicolas Sarkozy, in Brazil Thursday for a one-day meeting on climate change and Amazon forest conservation, hailed new US and Chinese proposals on combating global warming as "extremely encouraging."

At a press conference, Sarkozy praised US President Barack Obama's "courage" for setting goals that would reduce US carbon emissions by 17 percent by 2020, while also offering positive words for China's proposed moves to reduce carbon emissions.

"The latest statements by Barack Obama and China's leaders are extremely encouraging in making Copenhagen a success," said Sarkozy, who is attending the meeting because France's overseas department of French Guiana is in the region.

The other governments in attendance were Bolivia, Brazil, Colombia, Ecuador, Guyana, Peru, Venezuela and Surinam, all nations that straddle the ecologically-imperiled Amazon river basin.

The meeting was called by Brazilian President Luiz Inacio Lula da Silva to draw up a "common stance" on saving the Amazon jungle.

Officials said the meeting aims to adopt tough measures to combat global warming and preserve rainforests, ahead of the December 7-18 climate change conference in the Danish capital Copenhagen.

"We are determined to protect the Amazon and use its resources in a sustainable way," participant nations said in a final declaration Thursday as the meeting concluded.

Beijing has vowed to cut carbon intensity, measured per unit of GDP, by 40-to-45 percent from 2005 levels within a decade, putting its first-ever emissions targets on the table.

The new proposals by the world's two biggest carbon emitters for curbing pollution may have breathed life into UN climate talks, but fall short of what scientists say is needed to avert serious global warming.

And the impact of Thursday's Amazon meeting was thrown into doubt by the notable absence of two of the region's big hitters -- Venezuelan President Hugo Chavez and his Colombian counterpart Alvaro Uribe, at loggerheads over an agreement granting US access to Colombian military bases.

Beyond Lula and Sarkozy, the only other head of state was Guyana President Bharrat Jagdeo, with other countries dispatching senior officials to what had been billed as a leaders summit.

The centerpiece of the meeting was a Brazilian proposal to fight rampant deforestation throughout the Amazon basin with financial help from rich nations.

"Let no gringo (foreigner) ask us to let an Amazonian starve to death under a tree," Lula said in a speech before the countries met in the planet's largest rainforest.

"We want to preserve (the forest), but they (other countries) have to pay for that preservation."

Lula's chief adviser Marco Aurelio Garcia explained that Brazil was seeking an agreement from Amazon basin countries "because in Europe, everyone thinks the Amazon is a zoo, a botanical garden and does not realize that it is more complex, there are 30 million people living here."

Greenpeace's Amazon official Paulo Adario told AFP that the Lula-Sarkozy alliance was significant "because France has an important leadership role in the European Union and Brazil is also showing growing leadership on the international stage."

The two leaders met two weeks ago in Paris to plan for the summit.

The clearing of wide swathes of jungle for farming and livestock, especially in Brazil, is reducing the planet's capacity to absorb greenhouse gases -- chiefly carbon dioxide -- that contribute largely to global warming and climate change, Greenpeace warned ahead of the summit.

As the fourth-largest greenhouse gas producer, Brazil has promised to cut its CO2 emissions by 36-39 percent by 2020. Half that effort will come from reducing deforestation in the Amazon jungle by 80 percent.

Brazil this year has managed to curb deforestation to its lowest level in 20 years, but 7,000 square kilometers (2,700 square miles) of rainforest still disappeared.

Unusually, non-Commonwealth leaders Sarkozy, UN Secretary General Ban Ki-moon and Danish Prime Minister Lars Loekke Rasmussen are to address a Commonwealth summit Friday as part of an effort to influence the Copenhagen climate talks.

The 192-nation talks backed by the United Nations aim to craft a post-2012 pact for curbing the heat-trapping gases that drive global warming.

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