Abstract
Abstract. At the beginning of the Archean eon (ca. 3.8 billion years ago), the Earth's climate state was significantly different from today due to the lower solar luminosity, smaller continental fraction, higher rotation rate and, presumably, significantly larger greenhouse gas concentrations. All these aspects play a role in solutions to the "faint young Sun paradox" which must explain why the ocean surface was not fully frozen at that time. Here, we present 3-D model simulations of climate states that are consistent with early Archean boundary conditions and have different CO2 concentrations, aiming at an understanding of the fundamental characteristics of the early Archean climate system. In order to do so, we have appropriately modified an intermediate complexity climate model that couples a statistical-dynamical atmosphere model (involving parameterizations of the dynamics) to an ocean general circulation model and a thermodynamic-dynamic sea-ice model. We focus on three states: one of them is ice-free, one has the same mean surface air temperature of 288 K as today's Earth and the third one is the coldest stable state in which there is still an area with liquid surface water (i.e. the critical state at the transition to a "snowball Earth"). We find a reduction in meridional heat transport compared to today, which leads to a steeper latitudinal temperature profile and has atmospheric as well as oceanic contributions. Ocean surface velocities are largely zonal, and the strength of the atmospheric meridional circulation is significantly reduced in all three states. These aspects contribute to the observed relation between global mean temperature and albedo, which we suggest as a parameterization of the ice-albedo feedback for 1-D model simulations of the early Archean and thus the faint young Sun problem.
Highlights
Hydrology and During the Archean eon (E3.8abritllhionSyyeasrstetom2.5 billion years ago), the Sun’s luminosity incrSeacseied nfrcomesabout 75 % to about 82 % of its present-day value (Bahcall et al, 2001)
Under pre-industrial conditions, the radiative forcing corresponding to a removal of the clouds is 28.8 W m−2. This is in stark contrast to 17.2 W m−2 in the case of a climate state with a solar insolation reduced to 75 %, present-day topography and a similar global mean SAT of 288 K which can be reached when pCO2 = 0.48 bar is applied
In a separate set of experiments, we have found the critical CO2 partial pressures corresponding to the uncertainty ra7ngeDoifsceuarslsyi-oAnrchean rotation rates to be 0.36 and 0.43 bar
Summary
Hydrology and During the Archean eon (E3.8abritllhionSyyeasrstetom2.5 billion years ago), the Sun’s luminosity incrSeacseied nfrcomesabout 75 % to about 82 % of its present-day value (Bahcall et al, 2001). In this light, indicators of liquid surface water (Lowe, 1980; Walker, 1982) raise the question regarding which mechanisms counteracted thisOlocweeralunmSinocsiiety,nacperoblem that was first explicitly phrased by Sagan and Mullen (1972) and is known as the “faint young Sun problem” (Feulner, 2012). Most solutions that have been suggested involve larger greenhouse gas concentrations Their warming impacts on global temperature and glaciation have mainly been studied with 1-D radiative-coSnvoelcitdiveEmaordtehls Due to a limited number of simulations, no critical CO2 partial pressure required to prevent the Earth’s surface from fully freezing was explicitly identified in these studies
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