Whatever the outcome of the UN climate summit in Copenhagen, Nature may have some extremely nasty surprises up its sleeve, say scientists.

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.