Tetsuo Taki, Hiroyuki Kurokawa, Yuka Fujii, Kosuke Aoki

Although non-greenhouse gases can vary substantially in abundance in Earth-like atmospheres, their climatic influences remain insufficiently understood. To investigate how such gases regulate climate, we vary the abundance of N$_2$ as a representative non-greenhouse component in one-dimensional N$_2$--CO$_2$--H$_2$O model atmospheres. Beyond pressure broadening of absorption lines and Rayleigh scattering emphasized in previous studies, our results show that changes in background N$_2$ pressure influence climate by modifying the amount of atmospheric H$_2$O, producing two effects: altering the thermodynamic lapse rate (H$_2$O-dilute warming) and changing the radiative contribution of H$_2$O to the greenhouse effect (H$_2$O-load warming). The resulting climate response to increasing N$_2$ depends on the CO$_2$ abundance. Under low CO$_2$ conditions, dilution of atmospheric H$_2$O leads to warming, whereas under high CO$_2$ conditions, increased H$_2$O loading also produces warming. At sufficiently high N$_2$ abundances, Rayleigh scattering induces cooling, an effect further amplified by the accompanying decrease in atmospheric H$_2$O. Under high CO$_2$ conditions, however, enhanced H$_2$O loading increases the absorption of stellar radiation and overwhelms the contribution of Rayleigh scattering, causing the cooling response to disappear. These results reveal multiple physical pathways through which non-greenhouse gases influence climate and provide a framework for understanding climate responses and habitability in diverse Earth-like atmospheres.