The Fixed Tropopause Temperature Hypothesis: FiTT for Purpose?

The Fixed Tropopause Temperature (FiTT) hypothesis says that Earth's radiative tropopause, the top of the troposphere, remains at a fixed temperature even as the surface warms. FiTT currently lacks a physical explanation and evidence across models. Here, we derive the tropopause temperature from first principles and find that the tropopause temperature should be fixed because it depends on water vapor's atmospheric distribution and its spectroscopy. We then show that a fixed tropopuase temperature is a robust feature of Earth's atmosphere by testing FiTT across a hierarchy of climate models. Our work gives a renewed appreciation for the role of thermodynamics and spectroscopy in determining a major feature of Earth's general circulation.