seasons), with the remaining 70% from fossil fuel sources (mainly in Europe,
including the UK). In winter essentially all the sulphur is from this latter source.
This is a good example of the utility of isotope measurements in environmental
sciences, because it is possible to attribute sulphur to its sources without the need
to know the strengths of those sources and without recourse to an atmospheric
dispersion and deposition model. This avoids the considerable uncertainties asso-
ciated with estimating these parameters.
7.3.3 The sulphur cycle and climate
In the previous section we examined aerosol particles as sources of acidity in the
atmosphere; here we look at their role in controlling climate. First we should
note that SO 42 - particles, whether from oxidation of DMS or anthropogenic SO 2 ,
are not the only source of atmospheric aerosols. Other sources include wind-
blown dust from soils, smoke from combustion of biomass and industrial
processes, and sea-salt particles produced by bubble bursting at the sea surface.
However, most study to date has been on SO 42 - aerosols and, for this reason and
also because they seem more important in a global context than other types, we
will concentrate on them here.
The role of aerosols in climate can be divided into two types: direct and indi-
rect. In the direct effect the particles absorb and scatter energy coming from the
sun back to space. This tends to cool the atmosphere since solar radiation, which,
in the absence of the aerosols, would warm the air, is now partially absorbed by
the particles or reflected upwards out of the atmosphere.
It is difficult to estimate the size of the effect since it depends not only on the
total aerosol mass loading in the atmosphere, but also on the chemical composi-
tion and size distribution of the particles. However, the effect seems to be sig-
nificant in terms of climate changes induced by human consumption of fossil
fuels. Data in the 2001 report on ‘Radiative Forcing of Climate Change’ by the
Intergovernmental Panel on Climate Change (IPCC) are instructive here. The
globally averaged assessment of direct radiative forcing effect of SO 42 - aerosols
from fossil fuel burning relative to 1750 (pre-industrial times) is -0.4 (range 0.2
to -0.8) W m-^2. Similarly the globally averaged figure for biomass burning over
the same period is -0.2 (range -0.1 to -0.6) W m-^2. These numbers can be com-
pared with radiative forcing attributed to greenhouse gas emissions since pre-
industrial times of +2.4 (range +2.2 to +2.7) W m-^2. Four important things should
be noted from the comparison. Firstly, the direct effect of aerosols on radiative
forcing is smaller globally than that due to greenhouse gases, but is by no means
insignificant. Secondly, the sign of the forcing is opposite to that for greenhouse
gases, so that the effect of rising aerosol loadings is to reduce to some extent the
warming effect of CO 2 and similar gases. Thirdly, the range of the uncertainty
on the estimates of the effect of aerosols is very large, for which the Intergov-
ernmental Panel on Climate Change (IPCC 2001, see Section 7.5) classifies the
level of scientific understanding as ‘low’ or ‘very low’. Lastly, the spatial distrib-
ution of the radiative forcing due to anthropogenic aerosols is very patchy com-
pared with that of the greenhouse gases. This last effect is due to the very different
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