The Spectroscopic Foundation of Radiative Forcing of Climate by Carbon Dioxide

Torn, M. S., Lawrence Berkeley National Laboratory

Radiation Processes

Warm Boundary Layer Processes

Mlynczak MG, T Daniels, D Kratz, DR Feldman, WD Collins, EJ Mlawer, M Alvarado, J Lawler, LW Anderson, D Fahey, L Hunt, and J Mast. 2016. "The spectroscopic foundation of radiative forcing of climate by carbon dioxide." Geophysical Research Letters, 43(10), 10.1002/2016gl068837.


The figure illustrates the correlated nature of the fluxes, and hence the insensitivity of RF to uncertainty in line strengths.


The figure illustrates the correlated nature of the fluxes, and hence the insensitivity of RF to uncertainty in line strengths.

Science

We explored the state-of-the-science of how rising atmospheric CO2 concentrations warm the climate system through radiative forcing caused by spectroscopic absorption features. We found that uncertainties in the absorption line parameters and understanding of the shape of CO2 lines, both of which are important for CO2 radiative forcing, are much smaller than the actual forcing.

Impact

This work rigorously explores the current state of knowledge for how CO2 warms the climate system, showing that the scientific foundation on which climate change research is based is solid. It also provides a path towards the systematic evaluation of the state of the science of radiative forcing by other greenhouse gases such as methane.

Summary

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 lineshape is also examined. Overall, the spectroscopic uncertainty in present-day CO2 RF is less than one percent, indicating a robust foundation in our understanding of how rising CO2 warms the climate system.