Spectroscopic Foundation of Radiative Forcing of Climate by Carbon Dioxide

Scientists determine that overall spectroscopic uncertainty in carbon dioxide (CO2) radiative forcing is less than 1 percent.

The Science

A recent study explored the state-of-the-science of how rising atmospheric CO2 concentrations warm the climate system through radiative forcing caused by spectroscopic absorption features. They 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.

The 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 toward the systematic evaluation of the state-of-the-science of radiative forcing by other greenhouse gases such as methane.


The radiative forcing of CO2 is the leading contribution to climate change from anthropogenic activities. Calculating CO2 radiative forcing 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. The results of this study show that CO2 radiative forcing in a variety of atmospheres is remarkably insensitive to known uncertainties in the three main CO2 spectroscopic parameters: line shapes, line strengths, and half widths. Uncertainties in radiative forcing due to line mixing were specifically examined as this process is critical in determining line shapes in the far wings of CO2 absorption lines. Radiative forcing computed with a Voigt line shape also was examined. Overall, the spectroscopic uncertainty in present-day CO2 radiative forcing is less than 1 percent, indicating a robust foundation in current understanding of how rising CO2 warms the climate system.

Principal Investigator(s)

Margaret Torn
Lawrence Berkeley National Laboratory


This work was supported by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research, Atmospheric System Research (ASR) program and Atmospheric Radiation Measurement (ARM) program under contract number DE-AC02-05CH11231; andNational Aeronautics and Space Administration Climate Absolute Radiance and Refractivity Observatory (CLARREO) project at Langley Research Center.


Mlynczak, M., T. Daniels, D. Kratz, D. Feldman, W. Collins, E. Mlawer, M. Alvarado, J. Lawler, D. Fahey, L. Hunt, and J. Mast. 2016. “The Spectroscopic Foundation of Radiative Forcing of Climate by Carbon Dioxide,” Geophysical Research Letters 43(10), 5318-25. DOI: 10.1002/2016GL068837.