Past Earth warmed by tidal resonance-induced organization of clouds under a shorter day

Solar heating causes the periodic expansion and contraction of Earth’s atmosphere known as the atmospheric tide. This is observed at the surface as a semidiurnal pressure oscillation that appears to influence convection and rainfall. Roughly 0.5 to 1.0 billion years ago, when day length was roughly 21–22 hours, the tide would have been resonant, or close in frequency, with atmospheric Lamb waves of 10.5–11.0 hour periods. This ‘Lamb resonance’ would have amplified the pressure oscillation, perhaps strongly enough to affect the global or tropical climate. Here we run a general circulation model at different rotation rates to model the resonance and its impact on climate. The resonance exerts a dominant control on tropical cloud cover, convection and rainfall: sunrise and sunset are cloudy and rainy, whereas midday and midnight are clear and dry. Generally clear skies at noon lower the albedo, contributing 2–4 K warming in the global average, which would have helped counter the 10% fainter Sun. The hydrological cycle becomes more active, and the atmosphere moister. Our work highlights the role of tidally induced adiabatic expansion in controlling tropical precipitation, helping explain modern-day observations of a semidiurnal rainfall pattern.