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 SO4- particles, whether from oxidation of DMS or anthropogenic SO2, are not the only source of atmospheric aerosols. Other sources include windblown 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 SO4- 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 indirect. 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 composition and size distribution of the particles. However, the effect seems to be significant 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 SO4- aerosols from fossil fuel burning relative to 1750 (pre-industrial times) is -0.4 (range 0.2 to -0.8) Wm-2. Similarly the globally averaged figure for biomass burning over the same period is -0.2 (range -0.1 to -0.6) Wm-2. These numbers can be compared with radiative forcing attributed to greenhouse gas emissions since pre-industrial times of +2.4 (range +2.2 to +2.7) Wm-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 CO2 and similar gases. Thirdly, the range of the uncertainty on the estimates of the effect of aerosols is very large, for which the Intergovernmental Panel on Climate Change (IPCC 2001, see Section 7.5) classifies the level of scientific understanding as 'low' or 'very low'. Lastly, the spatial distribution of the radiative forcing due to anthropogenic aerosols is very patchy compared with that of the greenhouse gases. This last effect is due to the very different

Fig. 7.22 Modelled geographical distribution of annual direct radiative forcing (Wm-2) from anthropogenic sulphate aerosols in the troposphere. The negative forcing is largest over, or close to, regions of industrial activity. After IPCC (1995). With permission of the Intergovernmental Panel on Climate Change.

Fig. 7.22 Modelled geographical distribution of annual direct radiative forcing (Wm-2) from anthropogenic sulphate aerosols in the troposphere. The negative forcing is largest over, or close to, regions of industrial activity. After IPCC (1995). With permission of the Intergovernmental Panel on Climate Change.

residence times of SO4- and other particles in the atmosphere (typically a few days) compared with those of the major greenhouse gases, which remain in the atmosphere for periods of years. An example of this patchiness of the aerosol radiative forcing is shown in Fig. 7.22, which gives the distribution across the globe of the forcing due to anthropogenic SO4- aerosols. Not surprisingly in view of where most of the precursor SO2 is made, coupled with the short residence time of SO4- particles, the effect is most pronounced over the continents and especially in regions of high industrial activity.

Indirect effects of aerosols on climate arise from the fact that the particles act as nuclei on which cloud droplets form. In regions distant from land, the number density of SO4- particles is an important determinant of the extent and type of clouds. By contrast, over land there are generally plenty of particles for cloud formation from wind-blown soil dust and other sources. Since clouds reflect solar radiation back to space, the potential link to climate is clear. The effect is likely to be most sensitive over the oceans far from land and for snow-covered regions like Antarctica, where land sources of particles have least effect. In such areas a major source of aerosols is the DMS route to SO4- particles (Fig. 7.23). Thus, marine phytoplankton are not only the major source of atmospheric acidity but also the main source of cloud condensation nuclei (CCN) and so play an important role in determining cloudiness and hence climate.

Solar radiation

Albedo reflection

Cloud condensation nuclei

Albedo reflection

Cloud condensation nuclei

Fig. 7.23 Dimethyl sulphide-cloud condensation nuclei-climate cycle. After Fell and Liss (1993). Reprinted with permission of New Scientist.

The concept illustrated in Fig. 7.23 was proposed several years ago, and one group of proponents went further and suggested that the plankton actually played a role in regulating, in contrast to affecting, climate. The idea was that if a change occurred in the temperature of the atmosphere (e.g. due to altered levels of CO2

or change in solar radiation being received), then the DMS-producing plankton might respond in such a way as to reduce the change. For example, if the air temperature increased then the resulting warming of surface seawater would lead to increased production of DMS by the plankton. This in turn would increase the flux of DMS across the sea surface and so raise the number of CCN in the atmosphere. The resulting enhanced cloudiness would tend to cool the atmosphere, so opposing the warming which initiated the cycle. The process would work in reverse for an initial cooling. If correct, this feedback loop would imply that marine phytoplankton are able to regulate to some degree, at least, the temperature of the atmosphere and thus the Earth's climate.

This idea was tested by examining ice cores from Antarctica for their content of DMS atmospheric oxidation products (MSA and nss-SO^) over the last glacial cycle (as discussed earlier for CO2; Fig. 7.10). The results, shown in Fig. 7.24, clearly indicate that both MSA and nss-SO4- were at higher concentrations during the last glaciation than since its termination about 13 000 years ago. This is the opposite of what would be predicted if planktonic DMS production were reducing any temperature change. Although the results do not support the notion of plankton regulating climate, it is now widely accepted that without CCN formed from DMS the amount of cloudiness, and hence the climate, over large parts of the globe would be significantly different both now and in the past.

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