By Alex Kirby
BBC News Online environment correspondent
A big online attempt to predict how the global climate will change this century wants to hear ordinary people's views.
The experiment hopes to predict the next century's climate
climateprediction.net, a British initiative, is asking people to log on and say which of four possible worlds they think will be most likely by 2100.
Participants are asked to say how much carbon dioxide they expect the world to emit over the next hundred years.
The scientists say they want to know what people think as their priorities now will affect what really happens.
The climateprediction.net website explains that carbon dioxide (CO2) is a greenhouse gas released into the atmosphere from the burning of fossil fuels.
It says: "In the atmosphere, it insulates the Earth - the more there is, the warmer the Earth. How much we emit... will determine how the Earth's climate changes.
"Tell us how much CO2 you think people all over the world will emit over the next 100 years. To help you, we will give you the choice of four possible future worlds.
"Each develops in a different way, so that by 2100 they are very different in terms of how many people there are, how wealthy they are, how much technology has advanced and, crucially, what kind of fuel they use."
The four worlds are derived from the work of the Intergovernmental Panel on Climate Change.
Two suppose a rapid rise in global wealth, one depending mainly on fossil fuels and the other using mainly renewable energy.
Vote and win
The two other scenarios suggest a slow rise in wealth, again either with or without a significant increase in fossil fuel use.
There are prizes for the best answers to the question "What do you think will happen to carbon dioxide emissions?" Voting has opened and will close at 2359 GMT on 21 March.
climateprediction.net is a 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.
Cloud visualisation from the experiment
Any computer user can take part by visiting the experiment's website. Each then downloads and runs a unique version of the Met Office's climate model, simulating several decades of global climate at a time.
Dr Sylvia Knight of climateprediction.net told BBC News Online: "Our preliminary results should be available in the next couple of months.
"We've had 47,000 people registering on the site, about 20,000 results so far, and 10,000 or so currently running. I think some people are worried about security, though we haven't infected anyone.
"So it's by far the largest full-scale climate model ensemble ever undertaken. We've used 4,000 years of computer time already, and it will last till the end of 2006.
"We'll take into account the results of the vote when we design the final phase of the experiment, producing a probability-based forecast of 21st-Century climate."
The on-line vote is part of National Science Week, and will be launched in a debate on BBC Radio Five Live from 1130 to 1200 GMT on 12 March.