Evidence for Radiative-Convective Bistability in Tropical Atmospheres
Earth’s energy balance requires that energy absorbed and emitted at the top of the atmosphere be equal; to first order this balance is maintained via the Planck feedback: outgoing longwave radiation increases as surface temperature increases. Failure of the Planck feedback to stabilize the climate is described by three generally independent phenomena: the super-greenhouse effect, the runaway greenhouse, and multiple equilibria of radiative-convective atmospheres. Here we use satellite observations and models to show that the existence of the super-greenhouse gives rise to a radiative-convective instability which is relevant to Earth’s tropics. The super-greenhouse is caused by the low troposphere becoming optically thick, causing a positive feedback on near surface temperature and moisture, driving deep convection, column moistening, and reduced outgoing longwave radiation. Aspects of the runaway greenhouse physics are implicated, but a local runaway greenhouse is avoided. These results have implications for understanding the response of the tropics to a warming world.