a) Incident radiative spectrum of M-dwarf Gliese 581 at the top of the atmosphere (black line) and on the surface of Gl 581d for the dry condition with P_s = 5 bar (CO₂ condensation is not taken into account). The red and blue lines indicate surface incident radiative spectra with and without Rayleigh scattering, respectively. b) Red line: percentage of incident radiation absorbed by CO₂ as a function of P_s, and blue line: percentage of incident radiation absorbed and scattered by CO₂.