Examining the role of varying surface pressure in the climate of early Earth

Abstract. During the Archean Eon in 2.7 billion years ago, solar luminosity was about 75 % of the present-day level, but the surface temperature was suggested to similar to or even higher than modern. What mechanisms act to maintain the temperate climate of early Earth is not clearly known yet. Recent studies suggested that surface air pressure was different from the present level. How does varying surface air pressure influence the climate? Using an atmospheric general circulation model coupled to a slab ocean with specified oceanic heat transport, we show that decreasing (increasing) surface pressure acts to cool (warm) the surface mainly because the greenhouse effect of pressure broadening becomes weaker (stronger). The effect of halfing or doubling the surface pressure on the global-mean surface temperature is about 10 K or even larger when ice albedo feedback or water vapor feedback is strong. If the surface pressure was 0.5 bar, a combination of a CO₂ partial pressure of about 0.04 bar and an oceanic heat transport of twice the present-day level or a combination of a CO₂ partial pressure of about 0.10 bar and an oceanic heat transport of half the present-day level is required to maintain a climate similar to modern, under a given CH₄ partial pressure of 1 mbar. Future work with fully coupled atmosphere-ocean models is required to explore the strength of oceanic heat transport and with cloud resolving models to examine the strength of cloud radiative effect under different surface air pressures.