6.13 Global Mean Radiative Forcings
6.13.1 Estimates
The global, annual mean radiative forcing estimates from 1750 to the present
(late 1990s; about 2000) for the different agents are plotted in Figure
6.6, based on the discussions in the foregoing sections. As in the SAR,
the height of the rectangular bar denotes a central or best estimate of the
forcing, while the vertical line about the bar is an estimate of the uncertainty
range, guided by the spread in the published results and physical understanding,
and with no statistical connotation. The uncertainty range, as employed in this
chapter, is not the product of systematic quantitative analyses of the various
factors associated with the forcing, and thus lacks a rigorous statistical basis.
The usage here is different from the manner “uncertainty range” is
defined and addressed elsewhere in this document. The SAR had also stated a
“confidence level” which represented a subjective judgement that the
actual forcing would lie within the specified uncertainty range. In order to
avoid the confusion over the use of the term “confidence level”, we
introduce in this assessment a “level of scientific understanding”
(LOSU) that represents, again, a subjective judgement and expresses somewhat
similar notions as in the SAR (refer also to IPCC, 1999). The LOSU index for
each forcing agent is based on an assessment of the nature of assumptions involved,
the uncertainties prevailing about the processes that govern the forcing, and
the resulting confidence in the numerical value of the estimate. The subjectivity
reflected in the LOSU index is unavoidable and is necessitated by the lack of
sufficient quantitative information on the uncertainties, especially for the
nonwellmixed greenhouse gas forcing mechanisms. In the case of some forcings,
this is in part due to a lack of enough investigations. Thus, the application
of rigorous statistical methods to quantify the uncertainties of all of the
forcing agents in a uniform manner is not possible at present.
The discussions below relate to the changes with respect to the SAR estimates.
In many respects, there is a similarity between the estimates, range and understanding
levels listed here, and those stated in the recent studies of Hansen et al.
(1998) and Shine and Forster (1999). Table 6.11 compares
the numerical values with the estimates in the SAR. Also, the Northern to Southern
Hemisphere ratio is shown for the present estimates (see also Section
6.14). Table 6.12 summarises the principal
aspects known regarding the forcings, along with a brief listing of the key
uncertainties in the processes which, in turn, lead to uncertainties in and
affect the reliability of the quantitative estimates.
The total forcing estimate for wellmixed greenhouse gases is slightly less
now, by about 1% (see Section 6.3) compared to the estimate
given in the SAR. The uncertainty range remains quite small and these estimates
retain a “high” LOSU. This forcing continues to enjoy the highest
confidence amongst the different natural and anthropogenic forcings.
The estimate for stratospheric O_{3} has increased in magnitude, owing mainly
to the inclusion of observed ozone depletions through mid1995 and beyond. It
is an encouraging feature that several different model calculations yield similar
estimates for the forcing. The uncertainty range remains similar to that given
in the SAR. These arguments suggest an elevation of the confidence in the forcing
estimate relative to the SAR. Accordingly, a “medium” LOSU is assigned
here. A still higher elevation of the rank is precluded because the O_{3} loss
profile near tropopause continues to be an uncertainty that is significant and
that has not been adequately resolved. Also, the global stratospheric temperature
change calculations involved in the forcing determination are not quantitatively
identical to the observed changes, which in turn, affects, the precision of
the forcing estimate.
The estimate for tropospheric O_{3} (0.35 ± 0.15 Wm^{2}) is on firmer grounds
now than in the SAR. Since that assessment, many different models have been
employed to compute the forcing, including one analysis constrained by observations.
These have resulted in a narrowing of the uncertainty range and increased the
confidence with regards to the central estimate. The preceding argument strongly
suggests an advancement of the confidence in this forcing estimate. Hence, a
“medium” LOSU is accorded for tropospheric O_{3} forcing. Key uncertainties
remain concerning the preindustrial distributions, the effects of stratospherictropospheric
exchange and the manner of its evolution over time, as well as the seasonal
cycle in some regions of the globe.
As the LOSU rankings are subjective and reflect qualitative considerations,
the fact that tropospheric and stratospheric O_{3} have the same ranks does not
imply that the degree of confidence in their respective estimates is identical.
In fact, from the observational standpoint, stratospheric O_{3} forcing, which
has occurred only since 1970s and is better documented, is on relatively firmer
ground. Nevertheless, both O_{3} components are less certain relative to the wellmixed
greenhouse gases, but more so compared with the agents discussed below.
The estimate for the direct sulphate aerosol forcing has also seen multiple
model investigations since the SAR, resulting in more estimates being available
for this assessment. It is striking that consideration of all of the estimates
available since 1996 lead to the same best estimate (–0.4 Wm^{2}) and uncertainty
(–0.2 to –0.8 Wm^{2}) range as in the previous assessment. As in the
case of O_{3}, that could be a motivation for elevating the status of knowledge
of this forcing to a higher confidence level. However, there remain critical
areas of uncertainty concerning the modelling of the geographical distribution
of sulphate aerosols, spatial cloud distributions, effects due to relative humidity
etc. Hence, we retain a “low” LOSU for this forcing.
The SAR stated a radiative forcing of +0.1 Wm^{2} for fossil fuel (FF) black
carbon aerosols with a range +0.03 to +0.3 Wm^{2}, and a “very low”
level of confidence. For biomass burning (BB) aerosols, the SAR stated a radiative
forcing of 0.2 Wm^{2} with a range 0.07 to 0.6 Wm^{2}, and a “very low”
level of confidence. In the present assessment, the radiative forcing of the
black carbon component from FF is estimated to be +0.2 Wm^{2} with a range from
+0.1 to +0.4 Wm^{2} based on studies since the SAR. A “very low” LOSU
is accorded in view of the differences in the estimates from the various models.
The organic carbon component from FF is estimated to yield a forcing of –0.1
Wm^{2} with a range from –0.03 to –0.30 Wm^{2}; this has a “very
low” LOSU. Note that extreme caution must be exercised in adding the uncertainties
of the organic and black carbon components to get the uncertainty for FF as
a whole. For BB aerosols, no attempt is made to separate into black and organic
carbon components, in view of considerable uncertainties. The central estimate
and range for BB aerosols remains the same as in the SAR; this has a “very
low” LOSU in view of the several uncertainties in the calculations (Section
6.7).
Mineral dust is a new component in the current assessment. The studies on the
“disturbed” soils suggest an anthropogenic influence, with a range
from +0.4 to 0.6 Wm^{2}. In general, the evaluation for dust aerosol is complicated
by the fact that the shortwave consists of a significant reflection and absorption
component, and the longwave also exerts a substantial contribution by way of
a trapping of the infrared radiation. Thus, the net radiative energy gained
or lost by the system is the difference between nonnegligible positive and
negative radiative flux changes operating simultaneously. Because of this complexity,
we refrain from giving a best estimate and accord this component a “very
low” LOSU.
As explained in Section 6.8, the “indirect”
forcing due to all tropospheric aerosols can be thought of as comprising two
effects. Only the first type of effect as applicable in the context of liquid
clouds is considered here. As in the SAR, no best estimate is given in view
of the large uncertainties prevailing in this problem (Section
6.8). The range (0 to 2 Wm^{2}) is based on published estimates and subjective
assessment of the uncertainties. Although several model studies suggest a nonzero,
negative value as the upper bound (about 0.3 Wm^{2}), substantial gaps in the
knowledge remain which affect the confidence in the model simulations of this
forcing (e.g., uncertainties in aerosol and cloud processes and their representations
in GCMs, the potentially incomplete knowledge of the radiative effect of black
carbon in clouds, and the possibility that the forcings for individual aerosol
types may not be additive), such that the possibility of a very small negative
value cannot be excluded; thus zero is retained as an upper bound as in the
SAR. In view of the large uncertainties in the processes and the quantification,
a “very low” LOSU is assigned to this forcing. Inclusion of the second
indirect effect (Chapter 5) is fraught with even more
uncertainties and, despite being conceptually valid as an anthropogenic perturbation,
raises the question of whether the model estimates todate can be unambiguously
characterised as an aerosol radiative forcing.
Aviation introduces two distinct types of perturbation (Section
6.8). Contrails produced by aircraft constitute an anthropogenic perturbation.
This is estimated to contribute 0.02 Wm^{2} with an uncertainty of a factor of
3 or 4 (IPCC, 1999); the uncertainty factor is assumed to be 3.5 in Figure
6.6. This has an extremely low level of confidence associated with it. Additionally,
aviationproduced cirrus is estimated by IPCC (1999) to yield a forcing of 0
to 0.04 Wm^{2}, but no central estimate or uncertainty range was estimated in
that report. Both components have a “very low” LOSU.
Volcanic aerosols that represent a transient forcing of the climate system
following an eruption are not plotted since they are episodic events and cannot
be categorised as a centuryscale secular forcing, unlike the others. However,
they can have substantial impacts on interannual to decadal scale temperature
changes and hence are important factors in the time evolution of the forcing
(see Section 6.15). Some studies (Hansen et al., 1998;
Shine and Forster, 1999) have attempted to scale the volcanic forcings in a
particular decade with respect to that in a quiescent decade.
Landuse change was dealt with in IPCC (1990) but was not considered in the
SAR. However, recent studies (e.g., Hansen et al., 1998) have raised the possibility
of a negative forcing due to deforestation and the ensuing effects of snowcovered
land albedo changes in midlatitudes. There are not many studies on this subject
and rigorous investigations are lacking such that this forcing has a “very
low” LOSU, with the range in the estimate being 0 to –0.4 Wm^{2} (central
estimate: 0.2 Wm^{2}). Note that the landuse forcing here is restricted to that
due to albedo change.
Solar forcing remains the same as in the SAR, in terms of best estimate, the
uncertainty range and the confidence level. Thus, the range is 0.1 to 0.5 Wm^{2}
with a best estimate of 0.3 Wm^{2}, and with a “very low” LOSU.
Table 6.11: Numerical values of the global and annual
mean forcings from 1850 to about the early 1990s as presented in the SAR,
and from 1750 to present (about 2000) as presented in this report. The estimate
for the wellmixed greenhouse gases is partitioned into the contributions
from CO_{2}, CH_{4},N_{2}O, and halocarbons. An approximate estimate of the Northern
Hemisphere (NH) to Southern Hemisphere (SH) ratio is also given for the
present report (see also Figure 6.6). The uncertainty
about the central estimate (if applicable) is listed in square brackets.
No uncertainty is estimated for the NH/SH ratio. 



Global mean radiative forcing (Wm^{2}) [Uncertainty]

NH/SH ratio




SAR

This Report

This Report


Wellmixed greenhouse gases
{Comprising CO_{2}, CH_{4}, N_{2}O, and halocarbons} 
+2.45 [15%]
{CO_{2} (1.56); CH_{4} (0.47);
N_{2}O (0.14); Halocarbons (0.28)}

+2.43 [10%]
{CO_{2} (1.46); CH_{4} (0.48);
N_{2}O (0.15); Halocarbons (0.34)}

1


Stratospheric O_{3} 
0.10 [2X]

0.15 [67%]

<1


Tropospheric O_{3} 
+0.40 [50%]

+0.35 [43%]

>1


Direct sulphate aerosols 
0.40 [2X]

0.40 [2X]

>>1


Direct biomass burning aerosols 
0.20 [3X]

0.20 [3X]

<1


Direct FF aerosols (BC) 
+0.10 [3X]

+0.20 [2X]

>>1


Direct FF aerosols (OC) 
*

0.10 [3X]

>>1


Direct mineral dust aerosols 
*

0.60 to +0.40

*


Indirect aerosol effect 
0 to –1.5
{sulphate aerosols}

0 to 2.0
{1^{st} effect only; all aerosols}

>1


Contrails 
*

0.02 [~3.5 X]

>>1

Aviationinduced cirrus 
*

0 to 0.04

*


Landuse (albedo) 
*

0.20 [100%]

>>1


Solar 
+0.30 [67%]

+0.30 [67%]

1


