436 GREENHOUSE GASES EFFECTS
1900 1950 2000 2050
–1
0
1
2
3
4
∆T
°K
(a)(c)(b)FIGURE 7 Changes in the globally-averaged mean surface temper-
atures relative to the mean for 1850–1920; dotted curve—observed
change since 1880; dashed curve—model computations of the effects
of increasing greenhouse gases from 1850–1990 and extrapolated to
2050 AD; solid curve—model predictions of changes caused by both
greenhouse gases and aerosols from 1850–2040.They also absorb and emit long-wave radiation but usually
with small effect because their opacity decreases at longer
wavelengths and they are most abundant in the lower tropo-
sphere where the air temperature, which governs emissions,
is close to the surface temperature. Aerosols also serve as
cloud condensation nuclei and therefore have the potential
to alter the microphysical, optical and radiative properties
of clouds.
The larger aerosol particles of d 0.1 m, if produced in
large quantities from local sources such as forest fires, volca-
noes and desert storms, may significantly influence the radia-
tion balance on local and regional scales, both by scattering
and by absorption and emission, especially if they contain
carbon particles. However, such particles are rapidly removed
from the troposphere by precipitation and are not normally
carried long distances. On the global scale, smaller particles
of d 0.1 m are more important, their dominant effect being
to cool the atmosphere by scattering solar radiation to space.
Some recent calculations by Charlson et al. (1990) of
the impact of anthropogenic sulphate particles on the short-
wave radiation balance in cloud-free regions conclude that,
at current levels, they reduce the radiative forcing over the
Northern Hemisphere by about 1 W/m^2 with an uncertainty
factor of two. A rather more sophisticated treatment by
Kiehl and Briegel (1993) calculated the annually-averaged
reductions in radiative forcing due to back-scattering of solar
radiation by both natural and anthropogenic sulphate aero-
sols to be 0.72 W/m^2 in the N. Hemisphere, 0.38 W/m^2 in the
southern hemisphere the global value of 0.54 W/m^2 being
about half of that calculated by Charlson. However, the
high aerosol concentrations over the heavily industrialised
regions of the eastern USA, central Europe and South-East
Asia produced reduction of 2 W/m^2 that are comparable
to the cumulative increases produced by greenhouse gases
emitted since the industrial revolution.
In addition to the direct effect on climate, anthropogenic
sulphate aerosols may exert an indirect influence by acting
as an additional source of effective cloud condensation
nuclei, thereby producing higher concentrations of smaller
cloud droplets leading to increased reflectivity (albedo) of
clouds, especially of low clouds, for solar radiation, which is
sensitive to the ‘effective’ droplet radiusrWNeffa( )^13 where W is the liquid–water concentration of the cloud
(in g/m^3 ) and N is the number concentration of the droplets.
The first calculations of this indirect effect on climate
have been made to the UKMO by Jones et al. (1994), using
their climate model that predicts cloud liquid water and ice
content and parameterizes r eff linking it to cloud type, water
content and aerosol concentration. The concentration and
size distribution of the aerosol, and its spatial distribution
are calculated in the same manner as in Kiehl and Briegel but
the particles are assumed to consist of ammonium sulphate
as being characteristic of aerosol produced in industrially
polluted air.The calculations indicate that the enhanced back-scatter
of solar radiation, mainly from low-level clouds in the atmo-
spheric boundary layer, produces an annually-averaged global
cooling of 1.3 W/m^2 but that over the highly industrialized
regions, where r eff may be reduced by as much as 3 m, the
cooling may exceed 3 W/m^2. However, it must again be empha-
sized that these calculations contain major uncertainties, prob-
ably even larger than those for the direct effect.
Taking them at face value, the calculations of the direct
and indirect effects combined, suggest an average global
negative forcing of 1.5–2 W/m^2 that may have largely
offset the positive forcing of 2.3 W/m^2 by greenhouse gases
to-date, and this may be at least part of the reasons for failure
to detect a strong greenhouse signal.
The first results of introducing sulphate aerosols into a
coupled atmosphere-ocean model come from the UKMO
(Mitchell et al. 1995). The model, starting from an initial
state determined by surface observations in 1860, was run
forward to 1990 with no man-made greenhouse gases or
aerosols as a control experiment. The model’s average global
surface temperatures showed realistic inter-annual varia-
tions but no overall rise over this period. In the perturbation
experiment greenhouse gases were gradually increased from
1860 to reach a 39% equivalent increase in CO 2 by 1990;
this resulted in a temperature rise of 1C compared with an
observed rise of only 0.5C, (Figure 7). The next step was to
compute the effects of sulphate aerosols with best estimates
of concentration and geographical distribution. The direct
effects of increasing the back-scatter of solar radiation was
to reduce the warming between 1860 and 1990 to only
0.5C, very close to the observed, but over and downwind of
the highly industrialized regions of North America, Europe
and Southern Asia, the aerosols largely nullify the warming
caused by the greenhouse gases.C007_002_r03.indd 436C007_002_r03.indd 436 11/18/2005 10:28:02 AM11/18/2005 10:28:02 AM