The concentration of greenhouse gases in the atmosphere - from our fossil-fuel dependent economies - is on the rise. The warming of the Earth and these gases, most scientists believe, are linked.
What scientists don't know is what happens next - by how much the Earth might carry on getting warmer and, if it does, how fast.
Predictions vary widely - from between 1.5 degrees Centigrade to nearly six degrees Centigrade over the coming century.
The trouble is, that even with the world's largest supercomputers, climate modellers simply don't have the computing muscle to produce more reliable long term forecasts. But there is a way round it.
If a golfer driving down a fairway were to take a swing over and over again, after a while a pattern would emerge - a cluster of results - the more times he does it, the clearer that pattern becomes.
Scientists want to do a similar thing to bring down the margin of error in today's climate computer models. Again, the more times they run their models the clearer the pattern - and the less the uncertainty about what's likely to happen in the decades to come.
The plan is to persuade as many people as possible to download onto their PCs software to run their own climate model.
Tens of thousands of these separate models - running in parallel - will tell scientists at Oxford University more about our future climate than even the world's most powerful supercomputers can manage.
Everyone who signs up gets to run their own model with its own built-in uncertainties - things scientists themselves don't know for sure - like how fast trees take up carbon dioxide, or how much sunlight clouds reflect.
The model uses spare capacity whenever your computer's sitting idle. It charts temperature changes from 1920 to today, to check how well it's doing, then beyond - to forecast up to 2080.
The software comes with a promise it won't damage your computer during the three months or so it takes to run the full model. And apparently you can leave it running overnight without worrying about wasting energy because you waste less than running the same model on supercomputers, so long as you switch off your monitor.
First results are due in May. The Oxford team hopes to give some regional breakdown - not city by city - but likely temperature and rainfall changes for maybe six blocks covering the UK.
But this is first and foremost a global climate forecast, and if enough people take part, it could be the most powerful yet - perhaps one that's hard for anyone to ignore.
For more information about this experiment click here
Susan Watts' report can be seen on Newsnight on Tuesday, 14 February, 2006 on BBC TWO.