The climate of earth is changing at a rate unprecedented in human history. The heat content of the ocean is increasing, the Arctic Ocean is losing its ice cover, sea level is rising, and weather is becoming increasingly extreme. Emissions of CO2 associated with combustion of coal, oil, and natural gas are primarily responsible for these changes. Minimizing the disruptive impact of current practice for future climate requires a revolutionary change in our energy system. The course discusses current understanding of human induced climate change and the actions that must be promoted to ensure a more sustainable global future. The recorded lectures are from the Harvard Faculty of Arts and Sciences course Science of the Physical Universe 25.