Figure 6.6: 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). For detailed explanations see Section
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, guided by the spread in the published values of the
forcing and physical understanding. 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� (LOSU) index
is accorded to each forcing, with H, M, L and VL denoting high, medium, low and
very low levels, respectively. This represents our subjective judgement about
the reliability of the forcing estimate, involving factors such as the assumptions
necessary to evaluate the forcing, the degree of our 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 CO2, CH4, N2O, and halocarbons (see Tables
6.1 and 6.11) shown; halocarbons refers to all
halogen-containing compounds listed in Table 6.1.
�FF� denotes fossil fuel burning while �BB� denotes biomass
burning aerosol. Fossil fuel burning is separated into the �black carbon�
(bc) and �organic carbon� (oc) 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 their 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 6.8d). 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.
