The spectroscopic foundation of radiative forcing of climate by carbon dioxide.

The radiative forcing (RF) of carbon dioxide (CO ₂ ) is the leading contribution to climate change from anthropogenic activities. Calculating CO ₂ RF requires detailed knowledge of spectral line parameters for thousands of infrared absorption lines. A reliable spectroscopic characterization of CO ₂ forcing is critical to scientific and policy assessments of present climate and climate change. Our results show that CO ₂ RF in a variety of atmospheres is remarkably insensitive to known uncertainties in the three main CO ₂ 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 CO ₂ absorption lines. RF computed with a Voigt line shape is also examined. Overall, the spectroscopic uncertainty in present-day CO ₂ RF is less than 1%, indicating a robust foundation in our understanding of how rising CO ₂ warms the climate system.