2014MNRAS.440.1435G

Exoplanet habitability studies are often performed using simple energy balance models (EBMs), which rely on an understanding of a planet's radiative properties. However, knowledge of these properties is currently limited. Here, I use Earth as a testbed for examining habitability and model uncertainty in a diffusive latitudinal EBM. Using an empirical approach, I parametrize Earth's radiative properties - top of atmosphere albedo and outgoing infrared radiation - as a function of surface temperature, using measurements from the NASA Clouds and Earth's Radiant Energy System instruments. When using surface temperature to construct these functions, a bias analysis shows only small global biases of 3 and - 0.3W/m2 are introduced for the albedo and infrared functions, respectively. I also show there is a very fine balance between Earth's incoming solar and outgoing infrared radiation in the observational data, which is substantially smaller than previously assumed in this model. Using observationally derived radiative functions for an Earth-like planet, the snowball transition can be determined within a semimajor axis of ∼ 3 percent, with a structural uncertainty of ∼ 2 percent in the radiative function construction. This work shows that diffusive EBMs can be successfully used to identify habitable exo-Earths within ∼ 3 percent if the radiative properties are known.