Tropospheric moistening processes allow for additional stable climate states on Earth-like planets

Over Earth's history Earth's climate has gone through substantial changes such as from total or partial glaciation to warm moist climates and vice versa. While simple one-dimensional energy balance models (1D-EBM) cannot explain transitions between these states, 1D-EBMs allow for mapping out the possible distinct stable climate states on Earth and Earth-like planets. Several studies have shown that, including an ice-albedo feedback, 1D-EBMs allow for the establishment of three distinct climate states: total or intermediate glaciation and ice-free conditions. Here, we add an empirically-derived tropospheric moistening process to these classical studies and replace approximations for the radiative fluxes with vertically- and spectrally-resolved radiative transfer calculations. Similar to the iceline separating different glacial climate states, the moistening line then allows classifying additional warm-moist climate states. We identify a maximum of six physically-distinct stable climate states: total glaciation, intermediate glaciation with dry or warm-moist tropics, ice-free conditions with dry or warm-moist tropics, and the runaway greenhouse climate. We present how the establishment of these climate states depends on different atmospheric compositions (N₂ ranging from 0.1 to 10 bar and CO₂ ranging from 10^-8 to 1 bar partial surface pressure), and different meridional energy transports. We find that the water vapour mixing ratio of the saturated atmosphere is the key factor in determining how many climate states can establish for specific atmospheric compositions. Finally, we generalize the analysis to a range of different values of solar irradiance and orbital parameters such as the obliquity and eccentricity. We determine that all stable climate states can establish in the same parameter space, suggesting the possible occurrence of multiple non-linear transitions between states in the climate evolution of Earth and Earth-like planets.