Here is a really sensible post on global warming from someone who is a phd physicist, but only an informed layperson on the specific subject of climate science.
We live with uncertainty on the total amount of global warming induced by a given increment of greenhouse gases. The truth is best expressed as a bell curve. The value with the highest probability is near the center of the curve, but there is some much lower probability that a value close to the left or right sides of the bell curve might end up being the truth. After studying the matter for some time, the author believes that the center of the bell curve is probably about where the IPCC says it is. However, his version of the curve is much flatter, because it is human nature to be more sure of a conclusion when you are the one doing the science.