I'd say the worst case is we spend billions of dollars (that could have been spent elsewhere, say improving human health) on something that makes no difference at all.
The other problem is that "makes no difference at all" is a good outcome. A bad outcome is that we reduce carbon usage drastically, which reduces economic output drastically, which causes many deaths. (This is not hyperbole. Take China, for instance. They're polluting like crazy, but even with all the pollution, people are better off. They're moving to the cities to get out of rural poverty. That poverty shortens life spans more than the pollution does.)