Temperatures around the world could rise by as much as 11C, according to one of the largest climate prediction projects ever run. This figure is twice the level that previous studies have suggested.
Scientists behind the project, called climateprediction.net, say it shows that a “safe” upper limit for carbon dioxide is impossible to define.
The results of the study, which used PCs around the world to produce data, are published in the journal Nature.
Climateprediction.net is run from Oxford University, and is a distributed computing project; rather than using a supercomputer to run climate models, people can download software to their own PCs, which run the programs during downtime.
More than 95,000 people have registered, from more than 150 countries; their PCs have between them run more than 60,000 simulations of future climate.
Each PC runs a slightly different computer simulation examining what happens to the global climate if levels of carbon dioxide in the atmosphere double from pre-industrial levels – which may happen by the middle of the century.
What vary most between the simulations are the precise nature of physical processes like the extent of convection within tropical clouds – a process which drives the transport of heat around the world.
So no two simulations will produce exactly the same results; overall, the project produces a picture of the possible range of outcomes given the present state of scientific knowledge.
The lowest rise which climateprediction.net finds possible is 2C, ranging up to 11C.
The timescale would depend on how quickly the doubling of CO2 was reached, but large rises would be on a scale of a century at least from now.
“I think these results suggest that our need to do something about climate change is perhaps even more urgent,” the climateprediction.net chief scientist David Stainforth told BBC News.
“However, with our current state of knowledge, we can’t yet define a safe level in the atmosphere.”
On Monday, the International Climate Change Taskforce, co-chaired by the British MP Stephen Byers, claimed it had shown that a carbon dioxide concentration of over 400 ppm (parts per million) would be ‘dangerous’.
The current concentration is around 378 ppm, rising at roughly 2ppm per year.
Source: BBC News/Richard Black