The data on global climate chance from 1950 to the present leads to some pretty solid conclusions - the Earth's climate is warming and human greenhouse gas emission is at least one of the leading causes. What everyone really wants to know, however, is not what has happened but what will happen to our climate if we keep doing what we're doing. That's a much tougher question because it relies almost totally on computer models that use hundreds of parameterizations, simplifications, and approximations, some of which are poorly understood. However the models aren't stupid - when one feeds in historical data one generally reproduces historical results - and climate scientists generally know what the uncertainty on each of these estimates are.
Based on these models the IPCC estimates that there will be a 1.8 to 4 degree Celsius rise in average global temperature by the year 2100, however with all the uncertainties some people might be inclined to say that there's a good chance it really won't be that bad. They're right - turns out it might be worse.
From NASA's ever-useful Earth Observatory comes the results of running a suite of climate models thousands of times tweaking various parameters randomly (but in proportion to their uncertainty) on each run. By doing this scientists get a feel for just how much what they don't know about their models can impact the results. We call the result a probability distribution function, which shows the relative likelihood of the global average temperature increase by 2100. Here's the result:
Clearly the highest probability is for an increase of between 1.8 and 4 degrees, as the IPCC claimed, but according to the models there is almost no chance of less than 1.8 degrees of warming while there is a decent chance of between 4 and 6 degrees of warming, and a slim chance of as much as 10 to 12 degrees of warming.