Abstract/Summary

The radiative forcing (RF) of climate change due to increases in carbon dioxide (CO2) concentration is primarily in the wavenumber region 500–850 cm−1 (wavelengths of approximately 12 to 20 μm). It originates from absorption and emission of infrared radiation due to vibrational–rotational transitions of the CO2 molecule. While this RF has been the subject of intense and detailed study, to date, the contribution of different vibrational transitions to this forcing has not been explored. This article presents an analysis of radiative transfer calculations that quantify the role of different vibrational transitions and illustrates that while the fundamental bending mode contributes nearly 90% of the total infrared intensity, it contributes less than half of the RF at present-day CO2 concentrations; this is because the absorption at the centre of this fundamental band is so intense that the effect of additional CO2 is strongly muted. By successively adding in additional CO2 bands to the calculations, it is demonstrated that a key spectroscopic phenomenon, known as Fermi Resonance (an interaction between excited states of the bending and the symmetric stretching modes of CO2) leads to a significant spreading of the infrared intensity to both higher and lower wavenumbers, where the fundamental bending mode is less important. The Fermi Resonance transitions contribute only about 4% of the total infrared intensity in this spectral region but cause more than half of the present-day RF. The less-abundant isotopologues of CO2 have little impact on the spectrally integrated RF, but this small contribution results from a compensation between more significant positive and negative contributions to the spectral RF. This work does not alter the results of detailed RF calculations available in the literature; rather, it helps explain the physical basis of that forcing.