The theoretical triggers are the apocalyptic side to global warming, giving the lie to the common perception of it as an incremental threat that will rise predictably, like a straight line on a graph.

A widespread view of climate change is that the Earth's surface temperature will gently rise as more and more carbon gas is spewed out by fossil fuels, trapping heat from the Sun.

The change would be progressive, which means humans would have enough time to respond to the crisis and plants and animals have a better chance of adapting to its effects.

But scientists at a conference here on global warming say there is also the risk of sudden, catastrophic, irreversible and uncontrollable climate change that could be triggered in as-yet unknown conditions.

"There's still a great deal we don't know about these rapid non-linear events," British scientist Sir John Houghton, a leading member of the UN's top panel on global warming, said on Tuesday.

One scenario centres on the future of the Gulf Stream, the current that brings warm water to the northeastern Atlantic from the tropics and gives Western Europe a climate that is balmy for its northern latitude.

What would happen to this oceanic conveyor belt if cold fresh water were dumped on it from melting polar ice and changed rainfall patterns, the result of warm weather?

When this idea was first put forward in the late 1990s, some doomsters predicted the Gulf Stream would stop, pitching Britain, Ireland and much of coastal western Europe back into an Ice Age.

But two computer models, put forward Tuesday, show how far scientists fail to agree on the probability of this event and on its likely impact.

University of Illinois professor Mike Schlesinger told AFP that he had modelled a "business as usual" simulation in which the world continued with uncontrolled emissions of the carbon gases that cause global warming.

"I was surprised to find out that it's 70-percent likely that there will be a shutdown in this circulation over a 200-year timeline," he said.

"Over Europe, the shutdown would cause a cooling of perhaps one or two degrees [C, 2-4 F], superposed on [several degrees of] warming," he said.

"So what you get is a smaller warming in Europe, you don't get an Ice Age out of that."

Just as remarkable was this discovery: the shutdown caused such a disruption in global weather patterns that Alaska became a lot warmer in winter.

"This is serious news for the permafrost," he said.

In contrast, Richard Wood, of Britain's Hadley Centre for Climate Centre and Research, was far more cautious.

"Little can currently be said about the probability, except that it is subjectively considered low during the 21st century," Wood's study said.

His simulation -- entirely hypothetical -- of the Gulf Stream shutdown suggests that parts of Britain would be far colder than the so-called Little Ice Age of the 17th and 18th centuries, when winter "Frost Fairs" were held on the frozen River Thames.

Worst hit would not be Alaska, but central America, where farm production would fall by 106 percent, according to this calculation.

Another doomsday worry is about the future of carbon which is already stored in the soil in the form of decayed leaves and rotting vegetation, and in the capacity of the sea to go on absorbing man-made carbon pollution.

Scientists at the conference agreed that if temperatures go beyond a threshold, this stored carbon in the soil will be released into the air. And at some point, the sea, which has already absorbed 48 percent of the carbon dioxide emitted by burning oil, gas and coal, will no longer be able to absorb any more pollution.

That means vast amounts of gas will be dumped into the air, amplifying the global warming crisis at a stroke.

But carbon storage in such vast and complex mechanisms is a complex and little-understood phenomenon.

"The precise point at which the land biosphere will start to provide a positive feedback [i.e. release CO2 into the air instead of storing it] cannot yet be predicted with certainty," says Peter Cox of Britain's Centre for Ecology and Hydrology.

"This depends on a number of poorly understood processes, such as the long-term response of photosynthesis and soil respiration to increased temperatures and the possible acclimation of photosynthesis to high CO2."

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