Throughout its history, Earth has experienced vastly different climates, including “snowball Earth” episodes, during which the planet is believed to have been entirely covered in ice, and hothouse periods, during which prehistoric alligators may have roamed the Arctic. On shorter timescales we can consider Earth to roughly be in energy balance. The climate system operates as a heat engine, which drives the wind and sea currents; however it is an extremely dissipative, irreversible system. I will describe the irreversibility of the water cycle under subsaturated conditions in the atmosphere, and the constraints this poses on the general circulation and on smaller circulations like hurricanes. What constituents play a similar role on other planets? We will then quickly survey applications of variational approaches to climate and geophysical flows. Finally, I will discuss the controversial Maximum Entropy Production Principle.