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. 2016 May 28;43(10):5318-5325.
doi: 10.1002/2016GL068837. Epub 2016 May 24.

The spectroscopic foundation of radiative forcing of climate by carbon dioxide

Martin G Mlynczak  1 , Taumi S Daniels  1 , David P Kratz  1 , Daniel R Feldman  2 , William D Collins  2 , Eli J Mlawer  3 , Matthew J Alvarado  3 , James E Lawler  4 , L W Anderson  4 , David W Fahey  5 , Linda A Hunt  6 , Jeffrey C Mast  6
Affiliations

The spectroscopic foundation of radiative forcing of climate by carbon dioxide

Martin G Mlynczak et al. Geophys Res Lett. .

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 for thousands of infrared absorption lines. A reliable spectroscopic characterization of CO2 forcing is critical to scientific and policy assessments of present climate and climate change. 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 shapes, line strengths, and half widths. We specifically examine uncertainty in RF due to line mixing as this process is critical in determining line shapes in the far wings of CO2 absorption lines. RF computed with a Voigt line shape is also examined. Overall, the spectroscopic uncertainty in present-day CO2 RF is less than 1%, indicating a robust foundation in our understanding of how rising CO2 warms the climate system.

Keywords: carbon dioxide; line shape function; radiative forcing; spectroscopy.

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Figures

Figure 1
Figure 1
Spectra of instantaneous RF for doubled CO2 (280 ppm to 560 ppm) for five different standard atmospheres. Percentiles of forcing integrating from 480 cm−1 are indicated as a function of wave number at the top of each panel. The integrated percentile to 1100 cm−1 is also indicated.
Figure 2
Figure 2
Spectra of RF differences (LBLRTM baseline minus perturbed) due to a 20% perturbation in the CO2 line mixing coefficients for the MLS and SAW atmospheres. While substantial spectral structure is evident, the integrated forcing difference is less than 0.022 W/m2 in both cases. Spectra of RF differences for the other three atmospheres are in Figure S2 in the supporting information.
Figure 3
Figure 3
Upwelling and downwelling fluxes, and RF values obtained by varying all line strengths by ±20% in steps of 1%. The figure illustrates the correlated nature of the fluxes and hence the insensitivity of RF to uncertainty in line strengths.
Figure 4
Figure 4
CO2 RF computed with the MRTA code as a function of Voigt truncation offset from line center for the five atmospheres. Diamonds indicate RF values computed with LBLRTM and are shown for comparison with the Voigt calculations at 50 cm−1 offset.
See this image and copyright information in PMC

References

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