The idea that ‘Climate science is settled’ runs through today’s popular and policy discussions. Unfortunately, that claim is misguided. It has not only distorted our public and policy debates on issues related to energy, greenhouse-gas emissions and the environment. … The crucial scientific question for policy isn’t whether the climate is changing. That is a settled matter: The climate has always changed and always will. […] Nor is the crucial question whether humans are influencing the climate. That is no hoax: There is little doubt in the scientific community that continually growing amounts of greenhouse gases in the atmosphere, due largely to carbon-dioxide emissions from the conventional use of fossil fuels, are influencing the climate. There is also little doubt that the carbon dioxide will persist in the atmosphere for several centuries. The impact today of human activity appears to be comparable to the intrinsic, natural variability of the climate system itself Rather, the crucial, unsettled scientific question for policy is, ‘How will the climate change over the next century under both natural and human influences?’ … Even though human influences could have serious consequences for the climate, they are physically small in relation to the climate system as a whole. … We often hear that there is a ‘scientific consensus’ about climate change. But as far as the computer models go, there isn’t a useful consensus at the level of detail relevant to assessing human influences. … Policy makers and the public may wish for the comfort of certainty in their climate science. But I fear that rigidly promulgating the idea that climate science is “settled” (or is a “hoax”) demeans and chills the scientific enterprise, retarding its progress in these important matters.