Fred Pearce has a useful post on the uncertainties of climate predictions, including speculation that the next IPCC report may report greater uncertainty than in the past:
We are all — authors and readers of IPCC reports alike — going to have to get used to greater caution in IPCC reports and greater uncertainty in imagining exactly how climate change will play out. This is probably healthy. It is certainly more honest. But it in no way undermines the case that we are already observing ample evidence that the world is on the threshold of profound and potentially catastrophic warming. And it in no way undermines the urgent need to do something to halt the forces behind the warming.
Some argue that scientific uncertainty should make us refrain from action to slow climate change. The more rational response, given the scale of what we could face, is the precise opposite.
So far, I have had no luck in getting climate skeptics to grasp why uncertainty is not a reason for inaction. Let me try just one more time. Suppose you have some symptoms that could be a fatal disease or could be something minor. You’re not certain which it is. Is that a good reason for ignoring the problem? Really?