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]