I also wanted to address your point about the large ranges that scientists are predicting. Obviously rates of temp change are dependent on how much or how little green house gases are emitted. China, as we all know, have become by far the biggest producer. If they were to reduce their emissions on a large scale in the next 2 decades them alone would change the estimates by quite a degree. if they don't and in fact continue to increase their emissions as they have in the last decade that would also effect the stats to the higher end.
Again, I'm far from an expert, I'm just looking at this logically. It's impossible to give an exact figure when the aspects of what is causing the temp rises are so open to change in the next few decades. The pendulum could really swing either way. If we take this seriously and make changes then we could be on the lower end. if we don't take heed then we are going to be in the high range.