The faint young Sun problem revisited with a 3-D climate–carbon model – Part 1

摘要

During the Archaean, theSun's luminosity was 18 to 25% lower than the present day. One-dimensionalradiative convective models (RCM) generally infer that high concentrations ofgreenhouse gases (CO₂, CH₄) are required to prevent the early Earth'ssurface temperature from dropping below the freezing point of liquid waterand satisfying the faint young Sun paradox (FYSP, an Earth temperature atleast as warm as today). Using a one-dimensional (1-D) model, it was proposedin 2010 that the association of a reduced albedo and less reflective cloudsmay have been responsible for the maintenance of a warm climate during theArchaean without requiring high concentrations of atmospheric CO₂(pCO₂). More recently, 3-D climate simulations have been performed usingatmospheric general circulation models (AGCM) and Earth system models ofintermediate complexity (EMIC). These studies were able to solve the FYSPthrough a large range of carbon dioxide concentrations, from 0.6 bar with anEMIC to several millibars with AGCMs. To better understand this wide range inpCO₂, we investigated the early Earth climate using an atmospheric GCMcoupled to a slab ocean. Our simulations include the ice-albedo feedback andspecific Archaean climatic factors such as a faster Earth rotation rate, highatmospheric concentrations of CO₂ and/or CH₄, a reduced continentalsurface, a saltier ocean, and different cloudiness. We estimated fullglaciation thresholds for the early Archaean and quantified positiveradiative forcing required to solve the FYSP. We also demonstrated why RCMand EMIC tend to overestimate greenhouse gas concentrations required to avoidfull glaciations or solve the FYSP. Carbon cycle–climate interplays andconditions for sustaining pCO₂ will be discussed in a companionpaper.