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New estimtes of raditive forcing due to well mixed greenhouse gases

Authors:
  • Center for International Climate and Environmental Research (CICERO)

Abstract

We have performed new calculations of the radiative forcing due to changes in the concentrations of the most important well mixed greenhouse gases (WMGG) since pre-industrial time. Three radiative transfer models are used. The radiative forcing due to CO2, including shortwave absorption, is 15% lower than the previous IPCC estimate. The radiative forcing due to all the WMGG is calculated to 2.25 Wm−2, which we estimate to be accurate to within about 5%. The importance of the CFCs is increased by about 20% relative to the total effect of all WMGG compared to previous estimates. We present updates to simple forcing-concentration relationships previously used by IPCC.
... We find that the CO 2 should be 2.1 ± 0.1 W/m 2 per CO 2 doubling. This should be compared with the 3.7 W/m 2 per CO 2 doubling obtained from radiative transfer models, 55 suggesting perhaps that the CO 2 has a radiative forcing that is smaller than expected, or that there are unaccounted systematic errors. The amplitude and period of the secondary period modulation of the fast oscillation (presumably due to radial epicyclic motion of the solar system in the galaxy). ...
... It is, therefore, customary to define the greenhouse effect as the change in radiative forcing associated with a doubling in the concentration of greenhouse gases. For CO 2 , the standard estimate for the radiative forcing associated with a doubling of CO 2 is ∆F ×2 ≈ 3.7 W/m 2 .55 Other greenhouse molecules that have made a contribution over geological time scales are methane and ammonia. ...
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... For both models, IRF4×CO2 increases from ~5 W m -2 when IRF4×CO2 is computed in a pre-industrial climate to ~8 W m -2 when it is computed in an elevated-CO2 climate. This challenges the widely held assumption that the IRF2×CO2 is constant (19,20). To the contrary, it demonstrates that the CO2 IRF is a dynamic quantity that changes substantially as the climate changes. ...
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