The spectroscopic foundation of CO2 climate forcing

Abstract:

The radiative forcing (RF) of carbon dioxide (CO₂) is the leading contribution to climate change from anthropogenic activities. Calculating CO₂ RF requires detailed knowledge of spectral line parameters and lineshape functions for thousands of infrared absorption lines. A reliable spectroscopic characterization of CO₂ forcing is therefore a critical input to scientific and policy-oriented assessments of present climate and future climate change. Our study is partly motivated by a recent assertion that CO₂ RF values, and hence predictions of climate sensitivity to elevated CO₂, have a significant high bias because the CO₂ spectroscopic parameters being used are incorrect. Our results show that CO₂ RF in a variety of atmospheres is remarkably insensitive to known uncertainties in the three main CO₂ spectroscopic parameters: the line strengths, half widths, and line shapes. We demonstrate that this is due largely to the definition of CO₂ RF, which is the difference between the CO₂ longwave net flux at the tropopause for doubled CO₂ concentrations from the preindustrial era. We also assess the effects of sub-Lorentzian wings of CO₂ lines and find that the computed RF is largely insensitive to the spectral lineshape function. Overall, the spectroscopic uncertainty in present-day CO₂ RF is less than a few percent. Our study highlights the basics and subtleties of RF calculations, addressing interests of the expert and non-expert.