In the early 1990s, I worked on stratospheric photochemistry, i.e., ozone depletion. I even wrote a book about it (makes a great gift for that person who has everything!). By the mid-1990s, it was clear to me that the science of stratospheric ozone was truly settled. We understood just about everything. You know what I did? I decided to switch fields and study the climate system.
A constant argument in the policy debate over climate change is whether "the science of climate change is settled." It is clearly not. If it were, then I would have switched to another field, just as I did in the mid-1990s. No respectable scientist wants to do research on a system that's well understood.
However, everyone needs to recognize that we don't need perfect knowledge in order to take action. We make important decisions in the face of uncertainty all the time (e.g., should we invade Iraq?). In the case of climate change, we know enough to know that climate change carries a very real risk of severe, even catastrophic impacts over the next century. We can argue about the exact value of the risk: is it 20%, 50%, or 80%? I don't know the exact value, but it's not zero.
The decision about what to do about the uncertain risk of climate change is a policy decision and not a scientific result. How risk averse are we? Are we willing to trade some economic growth to reduce a risk of catastrophic climate change? This is something we have to decide as a society, and this is where the public debate needs to be.