Andy Dessler does a terrific job in this piece of explaining CO2 basics:

So we know that adding carbon dioxide is going to warm the planet. This leads us to the real question: How much warming are we going to get?

Carbon dioxide by itself will only provide somewhere around 1 degree C warming over the next century. In order to get really large warnings over the 21st century, there needs to be strong positive feedbacks to amplify the initial warming from carbon dioxide.