Figure 9: Global, annual-mean
radiative forcings (Wm^{-2}) due to a number of agents for the period
from pre-industrial (1750) to present (late 1990s; about 2000) (numerical values
are also listed in Table 6.11 of Chapter
6). For detailed explanations, see Chapter 6.13. The
height of the rectangular bar denotes a central or best estimate value, while
its absence denotes no best estimate is possible. The vertical line about the
rectangular bar with “x” delimiters indicates an estimate of the uncertainty
range, for the most part guided by the spread in the published values of the forcing.
A vertical line without a rectangular bar and with “o” delimiters denotes
a forcing for which no central estimate can be given owing to large uncertainties.
The uncertainty range specified here has no statistical basis and therefore differs
from the use of the term elsewhere in this document. A “level of scientific
understanding” index is accorded to each forcing, with high, medium, low
and very low levels, respectively. This represents the subjective 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 physical/chemical
mechanisms determining the forcing, and the uncertainties surrounding the quantitative
estimate of the forcing (see Table 6.12). The well-mixed
greenhouse gases are grouped together into a single rectangular bar with the individual
mean contributions due to CO_{2}, CH_{4}, N_{2}O and halocarbons
shown (see Tables 6.1 and 6.11).
Fossil fuel burning is separated into the “black carbon” and “organic
carbon” components with its separate best estimate and range. The sign of
the effects due to mineral dust is itself an uncertainty. The indirect forcing
due to tropospheric aerosols is poorly understood. The same is true for the forcing
due to aviation via its effects on contrails and cirrus clouds. Only the “first”
type of indirect effect due to aerosols as applicable in the context of liquid
clouds is considered here. The “second” type of effect is conceptually
important, but there exists very little confidence in the simulated quantitative
estimates. The forcing associated with stratospheric aerosols from volcanic eruptions
is highly variable over the period and is not considered for this plot (however,
see Figure 6.8). All the forcings shown have distinct
spatial and seasonal features (Figure 6.7) such that
the global, annual means appearing on this plot do not yield a complete picture
of the radiative perturbation. They are only intended to give, in a relative sense,
a first-order perspective on a global, annual mean scale and cannot be readily
employed to obtain the climate response to the total natural and/or anthropogenic
forcings. As in the SAR, it is emphasised that the positive and negative global
mean forcings cannot be added up and viewed a priori as providing offsets in terms
of the complete global climate impact. [Based on Figure
6.6]