Illustration of the change in cloud cover for the Archean Earth. The left panel shows the current Earth with global mean insolation at top of the atmosphere and reflected radiation (yellow arrows), latent heat flux (red arrow) and cloud cover. The fluxes in percent of the global mean insolation are given in brackets. The right panel shows the Archean Earth, with the insolation at 3 Ga, no ozone and enhanced CO2 (i.e. 60mbar) for the same global mean surface temperature as current Earth (287.9 K). Red + indicate the warming of high latitudes and blue – indicate the cooling of equatorial regions and the upper troposphere compared to the current Earth. The Archean Earth has less lower clouds and more upper clouds. Values are taken from Wolf and Toon .
Stellar evolution models predict that the solar luminosity was lower in the past, typically 20-25 % lower during the Archean (3.8-2.5 Ga). Despite the fainter Sun, there is strong evidence for the presence of liquid water on Earth’s surface at that time.
This “faint young Sun problem” is a fundamental question in paleoclimatology, with important implications for the habitability of the early Earth, early Mars and exoplanets. Many solutions have been proposed based on the effects of greenhouse gases, atmospheric pressure, clouds, land distribution and Earth’s rotation rate.
Here we review the faint young Sun problem for Earth, highlighting the latest geological and geochemical constraints on the early Earth’s atmosphere, and recent results from 3D global climate models and carbon cycle models. Based on these works, we argue that the faint young Sun problem for Earth has essentially been solved. Unfrozen Archean oceans were likely maintained by higher concentrations of CO2, consistent with the latest geological proxies, potentially helped by additional warming processes.
This reinforces the expected key role of the carbon cycle for maintaining the habitability of terrestrial planets. Additional constraints on the Archean atmosphere and 3D fully coupled atmosphere-ocean models are required to validate this conclusion.
Benjamin Charnay, Eric T. Wolf, Bernard Marty, François Forget
Comments: 22 pages, 7 figures, 1 table. Accepted for publication in Space Science Reviews. Part of ISSI special collection on Diversity of Atmospheres
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2006.06265 [astro-ph.EP] (or arXiv:2006.06265v1 [astro-ph.EP] for this version)
From: Benjamin Charnay
[v1] Thu, 11 Jun 2020 09:13:10 UTC (1,287 KB)