Global Warming

50 The greenhousegases

Level of scientific understanding (LOSU)

− 2

− 1

0

1

2

3

Radiative forcing (W m

−^2

)

Global and annual mean radiative forcing (1750 to present)

H M M L VL VL VL VL VL VL

CO 2

VL

CH 4

N 2 O

Halocarbons

Strat. O 3

Trop. O 3

Sulphate

FF

(bc)

FF

(oc) BB

dust

Contrails

Solar

Mineral

Trop. aerosol

indirect effect

(1st type)

Land-

use

(albedo)

Cirrus

VL

Aviation-induced

Figure 3.8Global, annual mean radiative forcings (W m−^2 )due to a number of

agents for the period from pre-industrial (1750) to 2000. The height of the

rectangular bar denotes a best estimate value while its absence denotes no best

estimate is possible because of large uncertainties. The vertical lines with ‘x’ or

‘o’ delimiters indicates estimates of the uncertainty ranges. A ‘level of scientific

understanding (LOSU)’ index is accorded to each forcing, with H, M, L and VL

denoting high, medium, low and very low levels respectively. This represents a

judgement about the reliability of the forcing estimate involving factors such as

the assumptions necessary to evaluate the forcing, the degree of knowledge of

the mechanisms determining the forcing and the uncertainties surrounding the

quantitative estimate of the forcing. The well-mixed greenhouse gases are

grouped together into a single rectangular bar with the individual contributions

shown. The second and third bars apply to stratospheric and tropospheric

ozone. The next bars denote the direct effect of aerosols. FF denotes aerosol

from fossil fuel burning and is separated into black carbon (bc) and organic

carbon (oc) components. BB denotes aerosols from biomass burning. The sign

of the effects due to mineral dust is itself an uncertainty. Only the first indirect

aerosol effect is estimated as little quantitative evidence exists regarding the

second. All the forcings have distinct spatial and seasonal variations (see Figure

3.7) so that they cannot be added up and viewed a priori as providing offsets in

terms of complete global climate impact.