Relative integrated IR absorption in the atmospheric window is not the same as relative radiative efficiency.

In their paper “Design strategies to minimize the radiative efficiency of global warming molecules,” Bera et al. (1) mistakenly equate the relative magnitudes of integrated absorption within the atmospheric window (800–1,400 cm) with relative radiative efficiencies of different molecules. Radiative efficiency is defined as the change in net radiation at the tropopause caused by a given change in greenhouse gas concentration or mass and has units of W·m·ppb (2, 3). Radiative efficiency is calculated using radiative transfer models of the atmosphere and depends on the strength and spectral position of a compound’s absorption bands, atmospheric structure, surface temperature, and presence or absence of clouds (2, 3). The atmospheric window is a region in the infrared where the Earth’s atmosphere is relatively transparent, through which blackbody radiation from Earth’s surface can escape to space, thereby cooling the planet. Species with long atmospheric lifetimes (years) and that absorb strongly in the atmospheric window have a particularly pronounced contribution to radiative forcing of climate change (Fig. 1). It is well-known that, because of the Planck function and absorption by CO2, O3, and H2O, the effectiveness of absorption varies substantially with frequency and that it is important to account for these factors in estimations of radiative efficiency of different molecules (2, 3). By introducing a weak absorber with uniform absorption at all wavelengths in a narrow-band radiative transfer model of the global average mean atmosphere, Pinnock et al. (4) estimated the radiative forcing per unit cross-section for a 1 ppb increase in mixing ratio. Fig. 2 shows the results from Pinnock et al. (4) for the 800to 1,400-cm region considered by Bera et al. (1). As seen from Fig. 2, there are large differences in the efficiency of absorption at different frequencies. For example, at 1,250–1,400 cm, the absorption is ∼10 times less effective than at 800–1,100 cm. It is incorrect to equate the relative magnitudes of integrated absorption within the atmospheric window (800–1,400 cm) with relative radiative efficiencies. The statement by Bera et al. (1) that “CF3CF = CF2 is shown to have a smaller radiative efficiency relative to CF3CF3, 1,421 km·mol -1 compared to 1,529 km·mol” is inconsistent with the accepted definition of radiative efficiency. As indicated in Figs. 1 and 2, radiative efficiency depends on both the intensity and the frequency of the absorption within the atmospheric window. Finally, designing fluorinated compounds with short atmospheric lifetimes is the most effective method to ensure low global warming potentials (GWP) and is a strategy adopted by industry; for example, CF3CF = CH2 has a reactive >C = C< double bond, a lifetime of ∼11 d, and a GWP of 4 (5).