Infrared Forcing by Greenhouse Gases W. A. van Wijngaarden1 and W. Happer 1 Department
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of Physics and Astronomy, York University, Canada of Physics, Princeton University, USA
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June 18, 2019 Abstract We review how the atmospheric temperatures and the concentrations of Earth’s five most important, naturally-occurring greenhouse gases, H2 O, CO2 , O3 , N2 O and CH4 control the cloud-free, thermal radiative fluxes from the Earth to outer space. Computations based on the line intensities HITRAN data base alone, with no absorption continuums, are used to evaluate fluxes and intensities for greenhouse-gas concentrations similar to those of the year 2019. Calculated top-of-the atmosphere spectral intensities are in excellent quantitative agreement with satellite measurements at various latitudes. Also calculated are per-molecule forcings in a hypothetical, optically thin atmosphere, where there is negligible saturation of the absorption bands, or interference of one type of greenhouse gas with others. Then the per-molecule forcings are of order 10−22 W for H2 O, CO2 , O3 , N2 O and CH4 . For current atmospheric concentrations, the per-molecule forcings of the abundant greenhouse gases H2 O and CO2 are suppressed by four orders of magnitude from optically-thin values because of saturation of the strong absorption bands and interference from other greenhouse gases. The forcings of the less abundant greenhouse gases, O3 , N2 O and CH4 , are also suppressed, but much less so. For current concentrations, the per-molecule forcings are two to three orders of magnitude greater for O3 , N2 O and CH4 , than those of H2 O or CO2 . Doubling the current concentrations of CO2 , N2 O or CH4 only increases the forcings by a few per cent.
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