The world can burn only a quarter of proven reserves of oil, gas and coal to be confident of staying within safer climate limits, unless untested carbon fixes work, experts said.
In two studies published in the journal Nature, scientists honed the basis for urgent climate action as the world tries to map by the end of this year agreement on a new treaty to replace the Kyoto Protocol.
People could burn no more oil, gas and coal in their homes, cars and factories after 2024, at current rates, to limit to one in five the chance of exceeding 2 degrees Celsius warming world-wide, one article found.
"It really casts doubt on whether any investment into more fossil fuel exploration is really a good investment," said the Potsdam Institute's Malte Meinshausen, who led the study.
"It makes a bet on technologies which at the moment are pretty uncertain still, or otherwise just leads us on a path to 3 or 4 degrees (Celsius) warming," he said, referring to untested climate fixes which trap carbon from the flue gas of power plants or else sucks it out of the air.
Both articles tallied total global emissions with warming by a certain date, in a novel "carbon budget" approach that could aid UN-led climate talks by showing negotiators the climate impacts of any agreement in Copenhagen in December.
A key concern was uncertainty - meeting even the most ambitious carbon-cutting targets could still result in climate change which inflicted water shortages on billions of people and risked coastal flooding for millions more.
"We won't know, won't resolve that uncertainty for sure until after we've made major reductions in emissions," said Oxford University's Myles Allen, who led the second study. Carbon emissions are rising at about 3 percent a year, although that rate is expected to fall temporarily as a result of the recession.
Without urgent action the climate problem could get out of hand, becoming very difficult or impossible to contain within safer limits, both studies found.
Even at 2 degrees warming above pre-industrial levels there is a risk of serious impacts, for example that the Greenland ice sheet melts over centuries, adding 7 metres sea level rise, and worse droughts ensue in dry areas such as Australia, southern Europe and south-west United States.
Fighting climate change will be expensive, for example to replace cheap fossil fuels with more costly renewable energy. Both Nature studies recommended that global emissions peak by 2020 or sooner. Japan advocated that they peak "in the next ten to twenty years".
"Late and rapid reductions are risky, expensive and disruptive, and hence potentially politically infeasible," both researchers said in a joint commentary in Nature.
The second study said that people could only emit another 500 billion tonnes of carbon for all time, which they would do in the next 40 years at current rates. That implied a 50 percent chance of exceeding 2 degrees. A safer target may be to limit emissions to 200 billion tonnes from 2009-2050, the first study estimated, for a one in four chance of exceeding 2 degrees.
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