Earth’s atmosphere behaves like a planetary-scale heat engine: it absorbs solar radiation at the warm tropical surface and emits terrestrial radiation in the cooler troposphere, producing work, in the form of atmospheric circulations, in the process. On smaller scales, moist convection itself can be conceptualised as a heat engine, with the engine’s work output providing a measure of the energy of convective motions. This heat-engine perspective provides a view of the atmosphere grounded in the second law of thermodynamics and highlights the role played by irreversible processes in the climate system.
In this talk, I will review the heat-engine perspective of Earth’s climate and describe how it provides constraints on the behaviour of atmospheric circulations from moist convective clouds to the general circulation. I will show how the second law can help in the understanding of phenomena such as the weakening of the tropical circulation under warming, and convective self-aggregation. Finally, I will discuss theories of moist convective updraft velocities, and how understanding the atmospheric heat engine was key to their development.