Sulphur compounds in the atmosphere tend to promote cooling of the earth because, unlike CO2, they reflect radiation back into space before it can reach the earth's surface. Over the sea, they can also act as cloud condensation nuclei attracting water vapour, and forming stratus and stratocumulus clouds. These clouds reflect sunlight so that they again reduce the radiation reaching the ground. However, clouds also stop infrared radiation escaping from the earth back into space. Recent work with satellites has indicated that on balance, in a worldwide context, these clouds cool more than they warm the earth.
The major source of the cloud-forming sulphur compounds in the atmosphere is dimethylsulphide (DMS) gas, which is produced by phytoplankton (see Section 4.3.3). Thus phytoplankton may help to cool the climate although it is not known how important this effect is. Volcanic eruptions such as that of Mount Pinatubo in the Philippines in 1991 also release large quantities of sulphur in the form of sulphur dioxide into the atmosphere.
Some scientists have suggested that global warming could be reduced by 'fertilizing' the open ocean with large amounts of iron. Such experiments have been aimed at increasing productivity since lack of iron is one of the limiting factors for phytoplankton growth (see Section 9.6.4). As described in Section 10.2.1, increased productivity can theoretically help to reduce atmospheric CO2 levels as the phytoplankton utilize CO2 in their photosynthesis.
The various systems described in Sections 10.2.1 to 10.2.5, including atmospheric CO2, ozone, and sulphur compounds, interact in complex ways and make it very difficult for scientists and climate specialists to predict the rate and extent of possible climatic changes. There is, however, a general agreement that increased levels of CO2 in the atmosphere are resulting in a trend towards global warming.
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