The Spectroscopic Foundation of Radiative Forcing by Carbon Dioxide*

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 line 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 pure Voigt line shape is also examined. Overall, the spectroscopic uncertainty in present-day CO₂ RF is less than 1% (global average), indicating a robust foundation in our understanding of how rising CO₂ warms the climate system.