资源环境科技发展态势分析平台

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.

Plain Language Summary Generally, the Earth's surface and atmosphere maintain energy balance: they emit more heat as they get warmer. However, there are a few cases where this breaks down. For some areas in the Earth's oceanic tropics, the top-of-atmosphere thermal emission either does not change or decreases as the surface warms. This unstable energy balance regime is called the super-greenhouse effect (SGE). Here we use a mix of satellite observations, radiative transfer modeling, and theory to show how the SGE occurs; we conclude that the SGE acts as a transition between two stable atmospheric states, dry and moist. The driving mechanism is the moistening of the upper atmosphere via deep convection. We show that a key feedback is the boundary layer becoming optically thick, such that all heat emitted from the surface is reabsorbed and the surface is unable to radiate directly to space. This drives the rapid surface warming necessary to initiate deep convection. We study how this feedback functions in both clear-sky and cloudy conditions. Investigating the mechanisms causing this imbalance is important for understanding how deep convection initiates, how circulation dissipates this excess energy, and how this might change under a warming climate.