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2013-01-04. Hydrogen-Nitrogen Greenhouse Warming in Earth's Early Atmosphere

posted Jan 5, 2013, 10:44 PM by Alan Gould
| Robin Wordsworth and Raymond Pierrehumbert, Science Vol. 339 no. 6115 pp. 64-67. Relevant to GSS Life and Climate chapter 2. Excerpt: One of the most durable questions about Earth’s early climate arises from the faint young Sun effect: Because progressive accumulation of He in a star’s core causes its luminosity to increase with age, the solar energy incident on Earth was significantly lower [∼75% of present-day values 3.8 billion years ago (Ga)] during the Hadean and Archean eras. Because geological evidence shows that Earth was not in a globally glaciated, snowball state throughout this time, additional mechanisms must have been present to warm the climate. Previous explanations for this altered climate have included increased atmospheric ammonia or CH4, a decreased surface albedo, and changes in the distribution of clouds. However, all of these mechanisms have subsequently been shown to suffer important defects. …  on the pre-biotic/early Archean Earth, the atmospheric N2 content was around two to three times the present-day value. Hydrogen, the most abundant gas in the solar system, has previously been ignored in the Archean climate budget, presumably because it was long thought to be a minor constituent even in the early atmosphere. …It was long believed that the escape of H2 in the Archean was rapid, … However, recent numerical calculations imply that the rate of hydrodynamic H2 escape on the early Earth was more strongly constrained by the adiabatic cooling of the escaping gas, given a limited extreme ultraviolet (XUV) energy input. As a result, H2 could have been a major constituent (up to ∼30% by volume) of the Archean atmosphere unless surface or ocean biogeochemistry continuously removed it. …molecular hydrogen interacts strongly with infrared radiation via collision-induced absorption (CIA), the strength of which scales with the product of the densities of the two interacting gases. CIA has been well studied for the gas giant planets and Titan, where it dominates radiative transfer in the middle and lower portions of the atmosphere. On early Earth, N2 and H2 may both have been abundant in the atmosphere, so interacting pairs of N2-N2, H2-N2, and H2-H2 should all be considered as potential contributors to greenhouse warming. …CH4 greenhouse was believed necessary to solve the faint young Sun problem. In contrast, our results show that an early climate dominated by abiotic H2-N2 and CO2 warming is consistent with both observational and theoretical limits on atmospheric CO2 levels…. H2-N2 warming is also likely to be important in the search for biosignatures on super-Earth exoplanets, whose higher masses imply lower energy-limited hydrogen escape rates and larger typical atmospheric N2 inventories. Because incident XUV flux is a function of orbital distance, H2-N2 warming may be of particular importance to the habitability of terrestrial exoplanets that are far from their host stars.   …. Read the full article: http://www.sciencemag.org/content/339/6115/64.full