Radiative forcing continues to be a useful tool to estimate, to a first
order, the relative climate impacts (viz., relative global mean surface temperature
responses) due to radiatively induced perturbations. The practical appeal
of the radiative forcing concept is due, in the main, to the assumption that
there exists a general relationship between the global mean forcing and the
global mean equilibrium surface temperature response (i.e., the global mean
climate sensitivity parameter, )
which is similar for all the different types of forcings. Model investigations
of responses to many of the relevant forcings indicate an approximate near
invariance of l (to about 25%). There is some evidence from model studies,
however, that can be substantially
different for certain forcing types. Reiterating the IPCC WGI Second Assessment
Report (IPCC, 1996a) (hereafter SAR), the global mean forcing estimates are
not necessarily indicators of the detailed aspects of the potential climate
responses (e.g., regional climate change).
The simple formulae used by the IPCC to calculate the radiative forcing
due to well-mixed greenhouse gases have been improved, leading to a slight
change in the forcing estimates. Compared to the use of the earlier expressions,
the improved formulae, for fixed changes in gas concentrations, decrease the
carbon dioxide (CO2) and nitrous oxide (N2O) radiative
forcing by 15%, increase the CFC-11 and CFC-12 radiative forcing by 10 to
15%, while yielding no change in the case of methane (CH4). Using
the new expressions, the radiative forcing due to the increases in the well-mixed
greenhouse gases from the pre-industrial (1750) to present time (1998) is
now estimated to be +2.43 Wm-2 (comprising CO2 (1.46 Wm-2),
CH4 (0.48 Wm-2), N2O (0.15 Wm-2)
and halocarbons (halogen-containing compounds) (0.34 Wm-2)), with
an uncertainty1 of 10% and a high
level of scientific understanding (LOSU).
The forcing due to the loss of stratospheric ozone (O3) between
1979 and 1997 is estimated to be -0.15 Wm-2 (range: -0.05 to -0.25
Wm-2). The magnitude is slightly larger than in the SAR owing to
the longer period now considered. Incomplete knowledge of the O3
losses near the tropopause continues to be the main source of uncertainty.
The LOSU of this forcing is assigned a medium rank.
The global average radiative forcing due to increases in tropospheric O3
since pre-industrial times is estimated to be +0.35 ± 0.15 Wm-2.
This estimate is consistent with the SAR estimate, but is based on a much
wider range of model studies and a single analysis that is constrained by
observations; there are uncertainties because of the inter-model differences,
the limited information on pre-industrial O3 distributions, and
the limited data that are available to evaluate the model trends for modern
(post-1960) conditions. A rank of medium is assigned for the LOSU of this
forcing.
The changes in tropospheric O3 are mainly driven by increased
emissions of CH4, carbon monoxide (CO), non-methane hydrocarbons
(NMHCs) and nitrogen oxides (NOx), but the specific contributions
of each are not yet well quantified. Tropospheric and stratospheric photochemical
processes lead to other indirect radiative forcings through, for instance,
changes in the hydroxyl radical (OH) distribution and increase in stratospheric
water vapour concentrations.
Models have been used to estimate the direct radiative forcing for five
distinct aerosol species of anthropogenic origin. The global, annual mean
radiative forcing is estimated as -0.4 Wm-2 (–0.2 to –0.8
Wm-2) for sulphate aerosols; -0.2 Wm-2 (–0.07 to
–0.6 Wm-2) for biomass burning aerosols; -0.10 Wm-2
(–0.03 to –0.30 Wm-2) for fossil fuel organic carbon
aerosols; +0.2 Wm-2 (+0.1 to +0.4 Wm-2) for fossil fuel
black carbon aerosols; and in the range -0.6 to +0.4 Wm-2 for mineral
dust aerosols. The LOSU for sulphate aerosols is low while for biomass burning,
fossil fuel organic carbon, fossil fuel black carbon, and mineral dust aerosols
the LOSU is very low.
Models have been used to estimate the “first” indirect effect
of anthropogenic sulphate and carbonaceous aerosols (namely, a reduction in
the cloud droplet size at constant liquid water content) as applicable in
the context of liquid clouds, yielding global mean radiative forcings ranging
from -0.3 to -1.8 Wm-2. Because of the large uncertainties in aerosol
and cloud processes and their parametrizations in general circulation models
(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, a range of radiative forcing from 0 to -2 Wm-2
is adopted considering all aerosol types, with no best estimate. The LOSU
for this forcing is very low.
The “second” indirect effect of aerosols (a decrease in the precipitation
efficiency, increase in cloud water content and cloud lifetime) is another
potentially important mechanism for climate change. It is difficult to define
and quantify in the context of current radiative forcing of climate change
evaluations and current model simulations. No estimate is therefore given.
Nevertheless, present GCM calculations suggest that the radiative flux perturbation
associated with the second aerosol indirect effect is of the same sign and
could be of similar magnitude compared to the first effect.
Aerosol levels in the stratosphere have now fallen to well below the peak
values seen in 1991 to 1993 in the wake of the Mt. Pinatubo eruption, and
are comparable to the low values seen in about 1979, a quiescent period for
volcanic activity. Although episodic in nature and transient in duration,
stratospheric aerosols from explosive volcanic eruptions can exert a significant
influence on the time history of radiative forcing of climate.
Owing to an increase in land-surface albedo during snow cover in deforested
mid-latitude areas, changes in land use are estimated to yield a forcing of
-0.2 Wm-2 (range: 0 to -0.4 Wm-2). However, the LOSU
is very low and there have been much less intensive investigations compared
with other anthropogenic forcings.
Radiative forcing due to changes in total solar irradiance (TSI) is estimated
to be +0.3 ± 0.2 Wm-2 for the period 1750 to the present.
The wide range given, and the very low LOSU, are largely due to uncertainties
in past values of TSI. Satellite observations, which now extend for two decades,
are of sufficient precision to show variations in TSI over the solar 11-year
activity cycle of about 0.08%. Variations over longer periods may have been
larger but the techniques used to reconstruct historical values of TSI from
proxy observations (e.g., sunspots) have not been adequately verified. Solar
radiation varies more substantially in the ultraviolet region and GCM studies
suggest that inclusion of spectrally resolved solar irradiance variations
and solar-induced stratospheric O3 changes may improve the realism
of model simulations of the impact of solar variability on climate. Other
mechanisms for the amplification of solar effects on climate, such as enhancement
of the Earth’s electric field causing electrofreezing of cloud particles,
may exist but do not yet have a rigorous theoretical or observational basis.
Radiative forcings and Global Warming Potentials (GWPs) are presented for
an expanded set of gases. New categories of gases in the radiative forcing
set include fluorinated organic molecules, many of which are ethers that may
be considered as halocarbon substitutes. Some of the GWPs have larger uncertainties
than others, particularly for those gases where detailed laboratory data on
lifetimes are not yet available. The direct GWPs have been calculated relative
to CO2 using an improved calculation of the CO2 radiative
forcing, the SAR response function for a CO2 pulse, and new estimates
for the radiative forcing and lifetimes for a number of gases. As a consequence
of changes in the radiative forcing for CO2 and CFC-11, the revised
GWPs are typically 20% higher than listed in the SAR. Indirect GWPs are also
discussed for some new gases, including CO. The direct GWPs for those species
whose lifetimes are well characterised are estimated to be accurate (relative
to one another) to within ±35%, but the indirect GWPs are less certain.
The geographical distributions of each of the forcing mechanisms vary considerably.
While well-mixed greenhouse gases exert a significant radiative forcing everywhere
on the globe, the forcings due to the short-lived species (e.g., direct and
indirect aerosol effects, tropospheric and stratospheric O3) are
not global in extent and can be highly spatially inhomogeneous. Furthermore,
different radiative forcing mechanisms lead to differences in the partitioning
of the perturbation between the atmosphere and surface. While the Northern
to Southern Hemisphere ratio of the solar and well-mixed greenhouse gas forcings
is very nearly 1, that for the fossil fuel generated sulphate and carbonaceous
aerosols and tropospheric O3 is substantially greater than 1 (i.e.,
primarily in the Northern Hemisphere), and that for stratospheric O3
and biomass burning aerosol is less than 1 (i.e., primarily in the Southern
Hemisphere).
The global mean radiative forcing evolution comprises of a steadily increasing
contribution due to the well-mixed greenhouse gases. Other greenhouse gas
contributions are due to stratospheric O3 from the late 1970s to
the present, and tropospheric O3 whose precise evolution over the
past century is uncertain. The evolution of the direct aerosol forcing due
to sulphates parallels approximately the secular changes in the sulphur emissions,
but it is more difficult to estimate the temporal evolution due to the other
aerosol components, while estimates for the indirect forcings are even more
problematic. The temporal evolution estimates indicate that the net natural
forcing (solar plus stratospheric aerosols from volcanic eruptions) has been
negative over the past two and possibly even the past four decades. In contrast,
the positive forcing by well-mixed greenhouse gases has increased rapidly
over the past four decades.
Estimates of the global mean radiative forcing due to different future scenarios
(up to 2100) of the emissions of trace gases and aerosols have been performed
(Nakic´enovic´ et al., 2000; see also Chapters
3, 4 and 5). Although there
is a large variation in the estimates from the different scenarios, the results
indicate that the forcing (evaluated relative to pre-industrial times, 1750)
due to the trace gases taken together is projected to increase, with the fraction
of the total due to CO2 becoming even greater than for the present
day. The direct aerosol (sulphate, black and organic carbon components taken
together) radiative forcing (evaluated relative to the present day, 2000)
varies in sign for the different scenarios. The direct aerosol effects are
estimated to be substantially smaller in magnitude than that of CO2.
No estimates are made for the spatial aspects of the future forcings. Relative
to 2000, the change in the direct plus indirect aerosol radiative forcing
is projected to be smaller in magnitude than that of CO2.