Fred,
As I said in my first response to you, the main argument does not rely on the claim that climates will change anyway. Rather we want to limit the potential for harmful consequences of adverse climate events and contend better with disasters of all sorts after they occur. As a means to achieving that end, policies directly focused on those events are likely to be more efficient than a blunt, and likely ineffective (because developing countries will not go along with the program) tool to influence changes in the distributions of weather events.
If there is substantial uncertainty about the magnitude of the likely effects of CO2 on the distribution of weather events (as you suggest), that raises the risk of investing in CO2 control and makes such investments less desirable than the alternatives of directly handling the potential for harmful consequences of adverse climate events and contend better with disasters of all sorts after they occur.
If at least some of the consequences (weather or otherwise) of increased CO2 are beneficial, that also shifts the balance toward the policies I am advocating — accept the good and treat the bad.
If measures to cope with weather disasters are useful for things like earthquakes and terrorist attacks, that also shifts the balance toward such policies.