Abstract
Abstract. During the Archaean, the Sun's luminosity was 18 to 25% lower than the present day. One-dimensional radiative convective models (RCM) generally infer that high concentrations of greenhouse gases (CO2, CH4) are required to prevent the early Earth's surface temperature from dropping below the freezing point of liquid water and satisfying the faint young Sun paradox (FYSP, an Earth temperature at least as warm as today). Using a one-dimensional (1-D) model, it was proposed in 2010 that the association of a reduced albedo and less reflective clouds may have been responsible for the maintenance of a warm climate during the Archaean without requiring high concentrations of atmospheric CO2 (pCO2). More recently, 3-D climate simulations have been performed using atmospheric general circulation models (AGCM) and Earth system models of intermediate complexity (EMIC). These studies were able to solve the FYSP through a large range of carbon dioxide concentrations, from 0.6 bar with an EMIC to several millibars with AGCMs. To better understand this wide range in pCO2, we investigated the early Earth climate using an atmospheric GCM coupled to a slab ocean. Our simulations include the ice-albedo feedback and specific Archaean climatic factors such as a faster Earth rotation rate, high atmospheric concentrations of CO2 and/or CH4, a reduced continental surface, a saltier ocean, and different cloudiness. We estimated full glaciation thresholds for the early Archaean and quantified positive radiative forcing required to solve the FYSP. We also demonstrated why RCM and EMIC tend to overestimate greenhouse gas concentrations required to avoid full glaciations or solve the FYSP. Carbon cycle–climate interplays and conditions for sustaining pCO2 will be discussed in a companion paper.
Highlights
Sagan and Mullen (1972) argued from solar models that, in the early Archaean, the young Sun’s luminosity was 25 % lower compared to
To understand why atmospheric general circulation models (AGCM) (Charney et al, 2013; Wolf and Toon, 2013; Kunze et al, 2014), Earth system models of intermediate complexity (EMIC) (Kienert et al, 2012, 2013) and radiative convective climate model (RCM) (Rosing et al, 2010; Kasting, 1984) differ in simulating the early Earth climate, we investigated the Precambrian climate using an atmospheric general circulation model coupled to a slab ocean FOAM
Fig. 3. 3-D climate simulations from 3.5 to 1 Ga with a greenhouse radiative forcing by CO2 and CH4 fixed to 19 W m−2 ( Ftrop; see Fig. 1). (a) Radiative deficit required for the onset of a pan-glaciation with RCM, FOAM runs (* modern clouds and cloud-free runs and ** large-droplet clouds (20 μm))
Summary
Sagan and Mullen (1972) argued from solar models that, in the early Archaean, the young Sun’s luminosity was 25 % lower compared to now. Using a vertical one-dimensional radiative convective climate model (RCM), Kasting (1984, 2010) calculated that an atmospheric carbon dioxide partial pressure (pCO2) of 0.3 bar or ∼ 1000 times the preindustrial atmospheric level (PAL) (i.e. 0.28 mbar or 280 ppm) was needed to maintain a mean global surface temperature of 15 ◦C for a Sun 75 % dimmer than today. Using Archaean boundary conditions including a reduced incoming solar radiation, a faster Earth rotation and a reduced continental surface, CLIMBER simulates a present-day climate for a carbon dioxide partial pressure fixed at 0.6 bar. The authors explained their results by the ice-albedo feedback, which is enhanced during the Archaean due to a faster Earth rotation rate. EMIC and AGCMs, and discuss how these models differ in representing the ice-albedo feedback mechanism
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