AGW should be treated as any other insurance scenario. Nobody expects their house to burn down but I bet almost everybody out there (and everyone with a mortgage) has home insurance against several highly unlikely scenarios.
Now we're talking about our whole planet, the only place we can call home. It's just a little bit more important than a few walls made of wood and brick yet we are currently playing with matches in a home with no insurance.
Even if the uncertainty level regarding AGW was much higher than what it is now, we should still be all doing every damned thing possible to stop any type of GHG emissions.
The risk of doing nothing is enormous. The gain even if AGW was not real is having a totally transformed energy economy with countless new jobs that will support it.
There are literally no negatives.