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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 Well-mixed Greenhouse Gases

The well-mixed 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 (Jacquinet-Husson 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 CFC-11. Ballard et al. (2000) performed an intercomparison of

laboratory data from five groups and found the range in the measured absorption cross-section of HCFC-22 to be about

10%.

Several previous studies of radiative forcing due to well-mixed greenhouse gases have been performed using single, mostly

global mean, vertical profiles. Myhre and Stordal(1997) investigated the effects ofspatial and temporal averaging on the

globally and annually averaged radiative forcing due to the well-mixed 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 latitudinalresolution 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).

Not all greenhouse gases are well mixed vertically and horizontally in the troposphere. Freckleton et al. (1998) have

investigated the effects of inhomogeneities in the concentrations of the greenhouse gases on the radiative forcing. For CH4(a

well-mixed greenhouse gas), the assumption that it is well-mixed horizontally in the troposphere introduces an error much less

than 1% relative to a calculation in which a chemistry-transport model predicted distribution of CH4 was used. For most

halocarbons, and to a lesser extent for CH4 and N2O, the mixing ratio decays with altitude in the stratosphere. For CH4 and

N2O, this implies a reduction in the radiative forcing of up to about 3% (Freckleton et al., 1998; Myhre et al., 1998b). For

most halocarbons, this implies a reduction in the radiative forcing up to about 10% (Christidis et al., 1997; Hansen et al.,

1997a; Minschwaner et al., 1998; Myhre et al., 1998b) while it is found to be up to 40% for a short-lived component found

in Jain et al. (2000).

Trapping of the long-wave radiation due to the presence of clouds reduces the radiative forcing of the greenhouse gases

compared to the clear-sky forcing. However, the magnitude of the effect due to clouds varies for different greenhouse gases.

Relative to clear skies, clouds reduce the global mean radiative forcing due to CO2 by about 15% (Pinnock et al., 1995;

Myhre and Stordal, 1997), that due to CH4 and N2O is reduced by about 20% (derived from Myhre et al., 1998b), and that

due to the halocarbons is reduced by up to 30% (Pinnock et al., 1995; Christidis et al., 1997; Myhre et al., 1998b).

The effect ofstratospheric temperature adjustment also differs between the various well-mixed greenhouse gases, owing to

different gas optical depths, spectral overlap with other gases, and the vertical profiles in the stratosphere. The stratospheric

temperature adjustment reduces the radiative forcing due to CO2 by about 15% (Hansen et al., 1997a). CH4 and N2O

estimates are slightly modified by the stratospheric temperature adjustment, whereas the radiative forcing due to halocarbons

can increase by up to 10% depending on the spectral overlap with O3(IPCC, 1994).

Radiative transfer calculations are performed with different types of radiative transfer schemes ranging from line-by-line

models to band models (IPCC, 1994). Evans and Puckrin (1999) have performed surface measurements of downward

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models to band models (IPCC, 1994). Evans and Puckrin (1999) have performed surface measurements of downward

spectralradiances which reveal the optical characteristics of individual greenhouse gases. These measurements are compared

with line-by-line calculations. The agreement between the surface measurements and the line-by-line model is within 10% for

the most important of the greenhouse gases:CO2, CH4, N2O, CFC-11 and CFC-12. This is not a direct test of the

irradiance change at the tropopause and thus of the radiative forcing, but the good agreement does offer verification of

fundamentalradiative transfer knowledge as represented by the line-by-line (LBL) model. This aspect concerning the LBL

calculation is reassuring as severalradiative forcing determinations which employ coarser spectralresolution models use the

LBL as a benchmark tool(Freckleton et al., 1996; Christidis et al., 1997; Minschwaner et al., 1998; Myhre et al., 1998b;

Shira et al., 2001). Satellite observations can also be useful in estimates of radiative forcing and in the intercomparison of

radiative transfer codes (Chazette et al., 1998).

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