A Simple Spectral Model for Earth’s Albedo

It is well-known that Earth's planetary albedo is about 0.3. Less clear is how this value might change in different climates. Here we propose a simple conceptual model for Earth's albedo. Our main insight is that, for a clear-sky N₂-H₂O atmosphere, the atmosphere can be approximated in the shortwave spectrum as either perfectly absorbing (due to water vapor absorption) or perfectly scattering (due to Rayleigh scattering); in contrast, clouds are approximately perfect scatterers throughout the shortwave spectrum. We use these approximations to derive analytic albedo expressions from the two-stream equations, which we validate against line-by-line model calculations.

Our results indicate that, for a clear-sky atmosphere, as surface temperature rises from 200 K to 500 K, Earth’s planetary albedo initially decreases with warming until around 350 K due to enhanced water vapor absorption, and then increases due to intensified Rayleigh scattering. Turning to idealized high and low cloud scenarios, cloudy atmospheres have significantly higher albedos than a clear-sky atmosphere at low temperatures. However, for an atmosphere with low clouds, albedo generally decreases with warming due to increased water vapor absorption above the clouds. In contrast, an atmosphere with high clouds exhibits nearly constant albedo with temperature, as high clouds mask the influence of the underlying atmosphere. These findings suggest that Earth’s clear-sky shortwave feedback is positive below 350 K and negative above 350 K. As for cloudy scenarios, low clouds induce a strong positive shortwave feedback at low temperatures, while high clouds don’t. Our simple model improves understanding of Earth’s planetary albedo and the role of shortwave feedback for the runaway greenhouse. Furthermore, our work suggests low clouds generally tend to destabilize Earth’s climate, which has potential implications for future climate change adaptation.