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Climate Change 2001:

Working Group I: The Scientific Basis

Chapter 6. Radiative Forcing of Climate Change Go

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6.3 Well­mixed Greenhouse Gases

The well­mixed greenhouse gases have lifetimes long enough to be relatively

homogeneously mixed in the troposphere. In contrast, O3(Section 6.5) and the

NMHCs (Section 6.6) are gases with relatively short lifetimes and are therefore

not homo­geneously distributed in the troposphere.

Spectroscopic data on the gaseous species have been improved with successive

versions of the HITRAN (Rothman et al., 1992, 1998) and GEISA databases

(Jacquinet­Husson et al., 1999). Pinnock and Shine (1998) investigated the

effect of the additional hundred thousands of new lines in the 1996 edition of the

HITRAN database (relative to the 1986 and the 1992 editions) on the infrared

radiative forcing due to CO2, CH4, N2O and O3. They found a rather small

effect due to the additional lines, less than a 5% effect for the radiative forcing

of the cited gases and less than 1.5% for a doubling of CO2. For the

chlorofluorocarbons (CFCs) and their replacements, the uncertainties in the

spectroscopic data are much larger than for CO2, CH4, N2O and O3, and differ

more among the various laboratory studies. Christidis et al. (1997) found a range

of 20% between ten different spectroscopic studies of CFC­11. Ballard et al.

(2000) performed an intercomparison of laboratory data from five groups and

found the range in the measured absorption cross­section of HCFC­22 to be

about 10%.

Several previous studies of radiative forcing due to well­mixed greenhouse

gases have been performed using single, mostly global mean, vertical profiles.

Myhre and Stordal (1997) investigated the effects of spatial and temporal

averaging on the globally and annually averaged radiative forcing due to the

well­mixed greenhouse gases. The use of a single global mean vertical profile to

represent the global domain, instead of the more rigorous latitudinally varying

profiles, can lead to errors of about 5 to 10%; errors arising due to the temporal

averaging process are much less (~1%). Freckleton et al. (1998) found similar

effects and suggested three vertical profiles which could represent global

atmospheric conditions satisfactorily in radiative transfer calculations. In the

above two studies as well as in Forster et al. (1997), it is the dependence of the

radiative forcing on the tropopause height and thereby also the vertical

temperature profile, that constitutes the main reason for the need of a latitudinal

resolution in radiative forcing calculations. The radiative forcing due to

halocarbons depends on the tropopause height more than is the case for CO2

(Forster et al., 1997; Myhre and Stordal, 1997).
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(Forster et al., 1997; Myhre and Stordal, 1997).