Reducing human emissions of carbon dioxide by 80% by 2100 requires more than technological innovation. Historical rates of emissions decline due to such innovation of about 0.7% are insufficient to offset the 3% growth in emissions that stems from population and per capita income growth. Existing scientific and political debates are dominated by a “technophilic optimism” that projects emission reductions from technological improvement that are not supported by the evidence. If we fail to develop policies proactively to constrain population, affluence, and consumption while respecting other human values, we will almost certainly face impacts from climate change that constrain population, affluence, and consumption for us.