The growth of population is a major factor behind climate change today. Human-caused climate change is fundamentally an imbalance of scale, as people release heat-trapping gases into Earth’s atmosphere faster than the oceans and living things can remove them. This imbalance stems from both the explosion of technologies made possible through the combustion of fossil fuels since the late 1700s and the more than sevenfold increase in human numbers since that time.The size of today’s population and its continued growth also put at risk the social and institutional resilience needed to adapt successfully to the impacts of climate change, ranging from sea-level rise to more extreme weather events. Slower population growth followed by a gradual decline in population size would facilitate future reductions in greenhouse gas emissions and help societies adapt to the changes in climate that are now all but inevitable, since (due to the long lag times built into the climate system) they will be the product of emissions that we failed to cut in past years and decades.Despite its key contribution to climate change, population plays little role in current discussions on how to address this serious challenge, particularly at the governmental level. Although many policymakers would welcome slower population growth, there is a concern that policies to slow growth will violate the right of couples to determine their own family size. Moreover, population is associated with sensitive issues including sexuality, contraception, abortion, migration, and religion. As a result, the debate on climate change tends to focus on the role of human technologies and their economic foundations, rather than on critical human numbers and behaviors.