Four model outputs with varying initial water inventories denoted by blue shading. These outputs use a mass-dependent deep-water cycle, an albedo (α) of 0.3, and escape processes are included. (A) Mass of all surface water reservoirs over the planet’s 4.5 Gyr evolution. (B) Surface temperature. (C) Precipitation calculated from wind-driven evaporation. (D) Mass of all surface CO₂ including atmospheric CO₂ and dissolved carbon. The ingassing fluxes of CO₂ (E) and H₂O (F) for various initial water inventories are denoted by purple shading. Outgassing fluxes of CO₂ (E) and H₂O (F) are denoted by dotted green shading. (G) Net CO₂ flux into the interior. (H) Net H₂O flux into the interior. In model runs with larger (10%, 100% of Earth’s oceans) water inventories, surface temperatures are temperate through geologic time, but model runs with low initial water show runaway warming. Smaller (0.1%, 0.01% of Earth’s oceans) initial water inventories have an unbalanced carbon cycle, increasing the flux of surface water into the interior while suppressing H₂O outgassing. With less surface water, precipitation rates decrease (C), and silicate weathering fluxes become runoff limited, allowing volcanic outgassing to dominate (E). This results in surface warming (B) and increased concentrations of atmospheric CO₂ (D).