Appendix D
Greenhouse Gas Spectral Overlaps and Their Significance

One of the difficulties in measuring the GWP of GHGs is that GHGs absorb infrared radiation at a variety of wavelengths [91] [92] [93] [94] [95] [96] [97] [98] [99] [100]. Some GHGs have common absorption bands. Table D1 shows how the GHGs absorption bands overlap. Water is the sole absorber in the windows from 0.5 micrometers (µm) to 2.0 µm and from 5.0 µm to 7.0 µm, ozone in the 8.0-µm to 10.0-µm window, carbon dioxide in the 14.7-µm to 16.5-µm window, and nitrous oxide in the 16.5-µm to 46.0-µm window. However, in some regions absorption frequencies of various GHGs overlap; water, carbon dioxide, and carbon monoxide absorption bands overlap in the 2.0-µm to 3.0-µm region; water and methane absorption bands overlap in the 3.0-µm to 4.0-µm region; carbon dioxide and carbon monoxide absorption bands overlap in the 4.0-µm to 5.0-µm window; nitrous oxide and methane absorption bands overlap in the 7.0-µm to 8.0-µm region; and carbon dioxide, ozone, and methane absorption bands overlap in the 13.7-µm to 14.7-µm region. Methane does not have a separate and distinct absorption window for itself like other GHGs.

Table D1. Overlap of Absorption Bands of Greenhouse Gases (Micrometers)
Greenhouse Gases	Greenhouse Gases' Absorption Bands
H2O	0.5 - 2.0
H2O, CO, and CO2	2.0 - 3.0
H2O and CH4	3.0 - 4.0
CO and CO2	4.0 - 5.0
H2O	5.0 - 7.0
N2O and CH4	7.0 - 8.0
O3	8.0 - 10.0
CO2, CH4, and O3	13.7 - 14.7
CO2	14.7 - 16.5
N2O	16.5 - 46.0
CH4 = Methane. CO = Carbon monoxide. CO2 = Carbon dioxide. H2O = Water vapor. N2O = Nitrous oxide. O3 = Ozone. Notes: Carbon dioxide absorbs infrared radiation at wavelengths of 2.69 micrometers (µm), 2.76 µm, 4.25 µm, 14 µm, and 15 µm. Carbon monoxide absorbs at 2.3 µm, and 4.7 µm. Water vapor absorbs at 0.6 µm, 0.72 µm, 0.82 µm, 0.94 µm, 1.10 µm, 1.38 µm, 1.87 µm, 2.70 µm, 3.20 µm, and 6.30 µm. Methane absorbs at 3.4 µm, 7.4 µm, 7.58 µm, and 7.87 µm. Nitrous oxide absorbs at 7.83 µm, 16.98 µm, and 44.9 µm. Ozone absorbs at 9.0 µm, 9.6 µm, and 14.2 µm. Sources: Snell-Ettre, Encyclopedia of Industrial Chemical Analysis, Vol. 8, p. 252; and N.V. Sidgewick, The Chemical Elements and Their Compounds, Vol. 11 (Oxford, United Kingdom: Clarendon Press, 1950), pp. 859-863.

Current global warming calculations and some climate models include infrared absorption characteristics of GHGs to a moderate extent [101], [102] [103]. The infrared absorption spectrum of atmospheric greenhouse gases is very complex. The monochromaticity (radiation of one wavelength) of most of its absorption bands of individual GHGs is lost due to pressure, temperature, aggregation, emission, and other factors. Mathematical and statistical models are used to deconvolute the polychromatic (radiation of more than one wavelength) infrared absorption spectrum of atmospheric GHGs, and this brings a certain amount of uncertainty into the data. In addition, uncertainty in the proper treatment of the water vapor continuum in the infrared spectrum of atmospheric GHGs still poses a challenge for line-by-line models to provide an absolute reference for evaluating less detailed model calculations [104]. The infrared absorption spectrum of atmospheric GHGs may also depend on latitude/longitude and altitude locations since GHG distributions other than carbon dioxide are not uniform.

Partly because the infrared absorption bands of the various components of the atmosphere overlap, the contributions from individual absorbers do not add linearly. Clouds trap only 14 percent of the radiation with all other major species present, but would trap 50 percent if all other absorbers were removed [105] (Table D2 and Figure D1). Carbon dioxide adds 12 percent to radiation trapping, which is less than the contribution from either water vapor or clouds. By itself, however, carbon dioxide is capable of trapping three times as much radiation as it actually does in the Earth's atmosphere. Freidenreich and colleagues [106] have reported the overlap of carbon dioxide and water absorption bands in the infrared region. Given the present composition of the atmosphere, the contribution to the total heating rate in the troposphere is around 5 percent from carbon dioxide and around 95 percent from water vapor. In the stratosphere, the contribution is about 80 percent from carbon dioxide and about 20 percent from water vapor. It is important to remember, however, that it is currently believed that the impact of water vapor produced from surface sources such as fuel combustion on the atmospheric water vapor concentrations is minimal.

Table D2. Efficiency of Heat Trapping by Greenhouse Gases and Clouds
Species Removed	Percentage Heat Trapped	Percentage Heat Not Trapped
Alla	0	100
H2O, CO2, O3	50	50
H2O	64	36
Clouds	86	14
CO2	88	12
O3	97	3
None	100	0
CO2 = Carbon dioxide. H2O = Water vapor. O3 = Ozone. aIncludes clouds. Source: V. Ramanathan and J.A. Coakley, Jr., “Climate Modeling Through Radiative-Convective Models,” Review of Geophysics & Space Physics 16 (1978):465.

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