PARIS — If Earth heats by four degrees Celsius -- some seven degrees Fahrenheit -- the planet we call home would become a very unwelcoming place.
Even as some world leaders tamp down expectations for the December 7-18 UN climate conference, experts say the threat of a 4C (7.2 F) warming over pre-industrial times is all too plausible.
Once that threshold is crossed, what might a four-degree world look like?
Oceans have risen by at least a metre (3.25 feet), drowning several island nations and driving hundreds of millions of people in Bangladesh, Thailand and Vietnam and other delta nations to scramble for higher ground.
Polar bears are a folk memory, starved to extinction in an Arctic where temperatures have soared by 15 C (27 F), nearly four-fold the global average.
Australia is routinely swept by white-hot fires of the kind that claimed 170 lives last February.
A third -- perhaps more -- of the Amazon forest has been reduced to desolate shrubland, its treasure chest of flora and fauna decimated.
Asia's eternal fountain, the Himalayan glaciers, are running dry.
South Asia's precious monsoon, once reliable as clockwork, has become fickle, dumping too little or too much rain.
A quarter of the planet's mammals are on a downward spiral toward extinction.
"A 4.0 C increase in global mean temperatures has the potential to threaten human security and quality of life in a manner unprecedented in recent history," says Arizona State University professor Pamela McElwee.
Francois Gemmene, a researcher at France's Institute for Sustainable Development and International Relations (IDDRI), adds: "At 4.0 C, climate-driven migration redraws the map of population distribution across the surface of the globe."
Science fiction? If only.
On November 16, an international team of scientists, the Global Carbon Project, said carbon emissions had surged by 29 percent from 2000 to 2008.
This places Earth on track with the worst-case warming scenario put forward by the UN's Nobel-winning Intergovernmental Panel on Climate Change (IPCC), it said.
Under its so-called "business-as-usual" forecast, voracious use of coal and other fossil fuels would see planetary warming of 4.0-to-6.4 C (7.2-to-11.5 F) by 2100 compared with 2000.
To that, add another 0.74 C of warming that has occurred since the Industrial Revolution.
This would spell disaster for Earth's population, 6.7 billion today, on course for nine billion in 2050.
"The carrying capacity of the planet could fall to one billion people or less," said John Schellnhuber, director of the Potsdam Institute for Climate Impact Research in Germany.
In September, Britain's Met Office, a leading centre on climate change, said the 4.0 degree rendezvous could come as early as 2060 -- in time for you or your children to experience it first-hand.
Crossing that threshold, they conclude, would send the misery index into unchartered territory.
In 2080, three billion people would struggle to find adequate water.
Yields of the major crops that feed most of the planet today would shrink, some dramatically, resulting in chronic hunger for tens, possibly hundreds, of millions. Africa, where so many teeter today at or below subsistence levels, would be hit especially hard.
For biodiversity, already on the cusp of the sixth major extinction in Earth's history, "a four degree world would be mayhem," said Pavan Sukhdev, a leading expert on the economics of ecosystems.
"Even two degrees, if you break it down, would create regional catastrophes in many places," he told AFP.
The loss of coral reefs -- which may be a foregone conclusion even at current levels of warming -- will leave half a billion people without livelihood.
The good news, virtually all of these experts said, is that there is still time to halt the slide if greenhouse-gas emissions peak soon enough and fall thereafter.
But the window of opportunity is narrowing rapidly.
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