Using line-by-line Monte Carlo to compute the Earth’s outgoing longwave radiation and CO2’s radiative forcing

The Earth’s radiation budget is a crucial part of climate and its evolution. Being part of this budget, the outgoing longwave radiation (OLR) has been extensively studied, especially in the context of climate-change and anthropogenic greenhouse gases emissions modifying the Earth’s radiative equilibrium.

In this study we present a new line-by-line radiative code RadForcE, we have developed to compute the global OLR and radiative forcing over a 10-year period. Based on a backward longwave Monte Carlo method, RadForcE uses line-by-line spectroscopic data for several molecular gases (CO₂, H₂O, CH₄ and O₃) from high-resolution databases GEISA and HITRAN, as well as different continua. The clouds’ vertical distributions are taken into account with a vertical overlap subgrid parameterization that is sampled "on the fly" for each optical path along vertical atmospheric profiles. Those profiles are sampled over a 10-year period all over the globe, either from GCM outputs or from ERA5 reanalysis, to compute the unbiased global OLR at a very small computational cost (~10 minutes on a laptop).

We this new method we are also able to directly compute any greenhouse gas radiative forcing, and present estimates of the radiative forcing for a doubling of CO₂. The Monte Carlo approach allows us to identify, for each outgoing optical path at the top of the atmosphere, the emitting species as well as the altitude of emission. By doing so, we can visualize the profile of altitude of emission for each species, as well as how some gases can screen other species’ emission or the surface’s emission. We can also visualize, for a doubling of CO₂, the increase of stratospheric emission by CO₂, and its screening of the surface’s emission and water vapor tropospheric emission.