Abstract: Why does climate change? How does climate change? This chapter describes flows of energy in the climate system. Climate can change when there are changes to energy flows between the different components of the climate system—internal changes. Climate can also change as a response to a change in total input or output energy—external changes. The energy budget of the planet is critical for understanding how the climate system may change over time, because on long timescales, climate is governed largely by the total amount of energy in the system and how that energy is stored and moves. Examples of interactions and internal feedbacks from past changes to the climate are described, and where the energy may go in the future is discussed.