Even moderate emissions could warm the Earth by as much as 3C by 2050, leaders of a huge climate simulation project have warned.
Scientists who harnessed the power of thousands of volunteers' home computers forecast a faster rate of warming than has been predicted before.
The models showed that average world temperatures are on course to rise by between 1.4C and 3C given mid-range greenhouse gas emissions.
According to the findings, the world is very likely to cross the critical "two degrees barrier" at some point this century if emissions continue unabated.
Experts believe warming of 2C above pre-industrial levels could trigger runaway climate change that cannot be reversed.
Almost 10,000 climate simulations were run using volunteers' home computers. The project, climateprediction.net, was part of the BBC Climate Change Experiment.
Scientists selected the most realistic projections after comparing them with regional temperature changes over the past 50 years. None passing the quality control test showed less than 1C warming by 2050. But a surprisingly high fraction of simulations, around 15%, predicted warming by as much as 3C.
The authors concluded that a 3C rise by 2050, compared with the average for 1960 to 1990, marked the upper end of the "likely range" of global warming. The research is published in the journal Nature Geoscience.
Lead author Dr Dan Rowlands, from Oxford University's Department of Physics, said: "It's only by running such a large number of simulations - with model versions deliberately chosen to display a range of behaviour - that you can get a handle on the uncertainty present in a complex system such as our climate.
"Our work was only possible because thousands of people donated their home computer time to run these simulations."
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