A massive worldwide online effort to predict how the global climate will change this century is being launched in the UK.
By Alex Kirby
BBC News Online environment correspondent
Computer users anywhere on Earth can join by downloading a climate model from a website.
A temperature simulation from the experiment
The organisers say it will be the world's largest climate prediction experiment.
They hope it will result in a much more robust picture of the probable future climate.
The experiment is being launched on 12 September at the Science Museum in London and at the British Association science festival in Salford.
It is the fruit of collaboration between the universities of Oxford and Reading, the Met Office, the Open University, the Rutherford Appleton Laboratory, and a software company, Tessella Support Services.
They expect it to generate "the world's most comprehensive probability-based forecast of 21st-century climate".
Any computer user can take part by visiting the climateprediction website. Each will then run a unique version of the Met Office's climate model, simulating several decades of global climate at a time.
The site says: "To take part in the challenge of the experiment you need to download some software which runs on your PC (whenever you are not using the processor for other jobs).
Home computers are now powerful enough for the experiment
"The software is a Windows-based version of a state-of-the-art climate model (courtesy of the Hadley Centre)."
Dr Sylvia Knight is at the atmospheric, oceanic and planetary physics department at Oxford.
She told BBC News Online: "Each model is slightly different in its starting conditions (the weather on the day the model is started), the fields it is forced with (for instance, different scenarios for how carbon dioxide develops this century), and most crucially in the approximations in the physics.
"All climate models have to make approximations, because the climate is so complicated."
The model will run as a background process on ordinary desktops without affecting other computing tasks. The results will go back to the organisers on the internet when the experiment ends.
The simulations of present climate and past changes will be used to test different versions of the model, and the most realistic will be used to predict the century's climate.
Dr Myles Allen, of the University of Oxford, said: "Thanks to chaos theory we can't predict which versions of the model will be any good without running these simulations, and there are far too many for us to run them ourselves.
"Together, participants' results will give us an overall picture of how much human influence has contributed to recent climate change, and of the range of possible changes in the future."
David Stainforth, the experiment's chief scientist, said: "While many model studies in the past have made plausible predictions of climate change, it hasn't been possible to quantify our confidence in these predictions.
"We hope to be able to say, for the first time, what the climate probably will and, more importantly, probably won't do in the future."
The experiment is possible only because the climate models designed a few years ago to run on the world's best supercomputers can now work happily on any up-to-date personal computer.
Asked whether potential participants might need reassurance, Dr Knight said: "We are very confident that it is as impossible as these things ever are to get a virus or a worm by downloading the software.
"But convincing the public will be hard - all we can do is say that it is 'industry standard'."