How a Stable Greenhouse Effect on Earth Is Maintained Under Global Warming

In a warming climate, greenhouse gases modulate thermal cooling to space from the surface and atmosphere, which is a fundamental feedback process that affects climate sensitivity. Recent studies have found that when relative humidity (RH) is constant with global warming, Earth's clear-sky longwave feedback would be dominated by surface cooling to space. Using a millennium-length coupled general circulation model and accurate line-by-line radiative transfer calculations, here we show that the atmospheric cooling to space accounts for 12%-50% of the feedback parameter from poles to tropics. A simple yet comprehensive model is proposed here for explaining the atmospheric feedback process. It is found that when RH is held constant, the atmospheric feedback stabilizes the climate because (a) water vapor spectral lines are weakened by the collision-broadening effect between water vapor and radiatively inert background gases, and (b) thermal emissions from other greenhouse gases increases due to enhanced Planck emission which is proportional to the surface warming. Each mechanism is responsible for half of the atmospheric feedback. We further elucidate that in hotter climates, the atmospheric feedback is more stabilizing because of (a) greater tropospheric opacity, and (b) more dramatic changes in air temperature with respect to transmission, owing to the pseudo-adiabatic expansion of air with surface warming. The sum of surface and atmospheric feedback, the clear-sky longwave feedback is accurately predicted by the simple model from the climate base state. Our study provides a theoretical way for assessing Earth's clear-sky longwave feedback, with important implications for Earth-like planets.