Water Vapor Spectroscopy and Thermodynamics Constrain Earth’s Tropopause Temperature

As Earth warms, the tropopause is expected to rise, but predictions of its temperature change are less certain. Longstanding theories tie the tropopause temperature to outgoing longwave radiation (OLR), but this contradicts recent work in which simulations exhibit a Fixed Tropopause Temperature (FiTT) even as OLR increases. The FiTT is thought to result from the interaction between upper tropospheric moisture and radiation, but a predictive theory for FiTT has not yet been formulated. Here, we build on a recent explanation for the temperature of anvil clouds and argue that tropopause temperature, defined by where radiative cooling becomes negligible, is set by water vapor's maximum spectroscopic absorption and Clausius-Clapeyron scaling. This "thermospectric constraint'' makes quantitative predictions for tropopause temperature that are borne out in single column and general circulation model experiments where the spectroscopy is modified and both the radiative and lapse-rate tropopause change in response. This constraint provides a theoretical foundation for the FiTT hypothesis, shows how tropopause temperature can decouple from OLR, and suggests a way to relate the temperatures of anvil clouds and the tropopause.