The spectroscopic foundation of CO2 climate forcing
Thursday, 17 December 2015
Poster Hall (Moscone South)
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.