I recently heard a good analogy about climate science from Bill Chameides, chief scientist for Environmental Defense.
The state of science of climate change today is like a partially completed jigsaw puzzle. There are lots of pieces in place --- things we know quite well (like the water vapor feedback) --- and lots of areas where there are no pieces --- things we don't know particularly well (like cloud feedbacks). The point here is that you don't have to have every piece in place in order to know what the picture looks like. Consider the image above. While many pieces are missing, one can easily observe that it's an image of fruit.
Climate science is like that. While there's a lot we don't know, the big picture is still clear. We know the climate is warming, humans are contributing, and there's a risk of significant warming over the next century. The missing pieces don't change any of that.
The "uncertainty" argument would have you believe that, if even one piece is not in place, that we have no idea what the big picture is. That's simply not true.