Earth’s greenhouse effect is manifested as the difference between thermal infrared radiation emitted at the Earth surface and that emitted to space at the top of the atmosphere. This difference is due mainly to absorption and downward emission of radiant energy by atmospheric trace gases. The greenhouse effect is an essential feature of Earth’s climate system that results in global mean surface temperature about 32 K greater what it would otherwise be for the same planetary absorption of solar radiation. Increases in atmospheric carbon dioxide, methane, nitrous oxide, and chlorofluorocarbons due to human activities over the past 200 years have increased the greenhouse effect by about 1% relative to the radiative fluxes that drive the climate system. The resultant increase in global temperature and other changes in climate are of great societal concern. This article introduces the physics of the greenhouse effect and more broadly of Earth’s climate system and of climate change and provides resources for further study. It reviews the processes responsible for the greenhouse effect, the anthropogenic increase in the greenhouse effect, and the response of the climate system to this increase. Developing prognostic capability to determine this response to an accuracy that would be useful to inform policymaking is the major challenge facing climate scientists today.