For an introduction to the 5 phase experiment, click here.

Sulphur dioxide (SO₂) is emitted by fossil fuel combustion and can react with other molecules (OH or hydrogen peroxide), particularly in clouds, to form what is known as sulphate aerosol (particles suspended in the air). This means that the amount of sulphate in the atmosphere is related to the amount of SO₂ emissions, but also to the availability of these other molecules, the number of cloud droplets in the air etc.

Since in this experiment we use a model has an interactive sulphur cycle, we can identify how the total amount of sulphate in the atmosphere is affected by changing emissions of sulphur dioxide and/or the concentrations of carbon dioxide (CO₂) in the atmosphere. In the sulphur cycle experiment we double CO₂ (phase 3 - top map), change SO₂ emissions to those expected for 2050 (phase 4 - middle map) and do a combined experiment of doubled CO₂ and 2050 SO₂ emissions together (phase 5 - bottom map) to identify the effects of a warmer atmosphere on future sulphate concentrations in the atmosphere.

Maps (a), (b) and (c) show the total amount of sulphate in the whole depth of the atmosphere (the 'column mean') (in mg m-2) for the doubled CO₂ phase (phase 3), the 2050 emissions phase (phase 4) and for the combined 2050 sulphate emissions and doubled CO₂ phase (phase 5), averaged over the last 8 years of each phase. The emissions used follow the IPCC A2 scenario (which is the same as 'world 3' in the simplified discussion here) which suggests a very heterogeneous development of the world in to the future, which assumes self reliance and preservation of local identities i.e. a business as usual scenario where developing countries are allowed to develop and developed countries slowly reduce emissions of SO₂ with no spread of technologies (see here). The A2 scenario was used as it provides one of the highest sulphur emissions scenarios available.

The first interesting point to look at is in comparing (a) and (b). You can see, that when 2050 SO₂ emissions are used instead of present day emissions, the amount of sulphate over the USA and Western Europe is expected to be lower. Emissions of SO₂ are expected to decrease in these regions and the figures reflect that.

Emissions increase in the sub-continent and southeast Asia in response to technological development and cause the total amount of sulphate to increase in these regions (and throughout the sub-tropics see fig (b)). There are also increases in the amount of sulphate over South America and Africa, again associated with technological development and increased emissions there. Sulphate particles scatter solar radiation back to space and so reduce the amount of solar radiation reaching the surface of the Earth. Also, as sulphate is highly soluble in water, it makes clouds more reflective, again reducing the the amount of solar radiation reaching the Earth's surface. Both these mechanisms mean that sulphate has a cooling effect. When CO₂ is doubled along with the increases in SO₂ emissions (phase 5) there is little change in the total amount of sulphate compared to the other phases, indicating that the amount of sulphate the atmosphere is not particularly affected by CO₂ increases. This result suggests that in the future, sulphate may still be efficient at reducing some of the warming associated with higher CO₂ levels.