David Keith is a professor of applied physics at Harvard University and a leading thinker on energy and geoengineering. In this episode, Keith explains why we should be thinking seriously about injecting sulfur or other materials into the atmosphere to reduce the amount of solar radiation that hits the surface of the Earth, why the science of geoengineering needs more funding, and some of the issues that will have to be resolved to allow more robust experimentation with the technology. (Recorded September 28, 2022.)