GCM greenhouse and high‐obliquity Solutions for early Proterozoic glaciation and middle Proterozoic warmth

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Abstract
The first definitive evidence of low‐latitude glaciation occurs in the early Proterozoic approximately 2.3 billion years ago. This period of glaciation occurs near the time for a significant rise in oxygen levels. A reduction in the methane producing population through the oxidation of Earth's atmosphere has been proposed as the cause for generating cold climatic conditions. Another alternative is that cold climatic conditions could have been generated by the migration of a landmass into low latitudes if Earth's obliquity was considerably higher than present. These two hypotheses are tested with global climate model simulations using present‐day and high‐obliquity (70°) boundary conditions. Further, the solar constant is reduced by 17% and 10 × CO2 is used. An idealized super‐continent is used and methane mixing ratios vary from 0.1 ppmv to 25 ppmv in the simulations. For the present‐day obliquity, low‐latitude glaciation occurs when CH4 concentrations are reduced from 10 to 0.1 ppmv. However, the same result is produced when the oceanic heat transport is turned off. In high obliquity simulations, low‐latitude glaciation occurs when the idealized super‐continent is moved from middle into low latitudes. Finally, high‐obliquity may explain the warm middle Proterozoic, as warm simulated climatic conditions occur even for a low latitude super‐continent using a solar constant representative of the Middle Proterozoic and reduced atmospheric methane mixing ratios.