You are drawing a false analogy between smoking, the potential for nuclear war and climate change. The issues have nothing in common, and that should be obvious. Besides, what does smoking have to do with this doomsday clock? Did they include that, too?
Interestingly, German scientists in the 1930's were among the first to indicate a strong correlation between smoking and lung cancer. Mainstream scientists outside paid no attention to that work because smoking was accepted as a normal activity. Who knows, maybe some of them ignored the evidence because of the political regime in Germany at that time. Maybe it was too much of a case of who was saying it rather than what was being said.
With respect to climate change, the debate continues because there is a debate. Period. The science that goes into the IPCC isn't settled, either. The IPCC is a panel with a specific agenda that selects evidence to support its assertions. The document it produces is not peer-reviewed. Inside it, the scientific summary is chock full of scientists stating uncertainties, pointing out where more research needs to be done, and the like. Scientific critics have argued that not only are those uncertainties important, but that other criticisms and contrary evidence are being neglected. This does not always settle too nicely among those for whom environmentalism is just a political stance free of supporting information.
