The Carbon cycle

a) Carbon Dioxide

As the complicated fate of other organic compounds is the subject of several other chapters (plant emissions, urban emissions, tropospheric chemistry), we will look at the carbon cycle here, which was a subject of lively debates at the climate summits in Kyoto, Den Haag and Bonn.

From the analysis of ice drill cores, it is known that the carbon dioxide concentration in the air varied between approximately 180 ppm (parts per million) during the ice ages and 280 ppm during the interglacial periods during the last 400,000 years. Only since the industrial revolution, the concentration raised clearly above this level up to approximately 370 ppm nowadays.

This chapter shall not only give an overview about this development and its sources like power production, forest- and savannah fires, but also give an outlook into the future. In addition, the involved cycles are described. Carbon dioxide is not removed chemically from the air, but it is taken up by plants and the oceans. In the biosphere, it is converted to biomass by photosynthesis. In the water of the oceans, it is dissolved and transported from the surface layers to the depth of the oceans by global circulation. As a rough estimate, only half of the amount of the carbon dioxide that was produced in the 1990ies stayed in the atmosphere in the end. The rest was taken up by oceans and the biosphere. The latter lead to the idea that the planting of forests should be accounted for carbon dioxide reduction, which was discussed during the climate summits. Uncertainties in model experiments that lead to this idea are reported, pointing to the fact, that oceans as well as the biosphere might be neither a permanent nor an inexhaustible sink.

The amount of emitted carbon dioxide belongs to the most reliable climatological data and quantities will be reported in this chapter. Even today, but definitively in the coming future, obvious regional shifts can be expected, which are linked to the economical development in for example Southeast Asia.

b) Methane

The simplest reduced carbon gas methane is also a very important greenhouse gas. It has significant anthropogenic, but also natural sources. Methane emissions are linked closely to the population growth because for example ruminant (e.g. cattle farming) like also cultivation of rice are significant sources. Methane as digester gas develops naturally in marshland and swampland. Large quantities are probably accumulated in the enormous permafrost areas of the Northern Hemisphere (approx. 25% of the land surface). In the case of thawing, they could become a significant source of methane.

Methane chemistry is closely linked to OH, ozone and carbon monoxide (CO) chemistry via oxidation. Oxidation of methane by OH is the main removal process. This is explained in short.

c) The Sulphur cycle

Sulphurous compounds particularly play a role in the atmosphere in the form of sulphate / sulphuric acid. While acidic rain by sulphur oxides were drastically decreased in Europe by filters in the power stations, such emissions are e.g. in Southeast Asia an explosive subject and especially power stations are still a significant source.

A natural source of sulphuric compounds is especially plankton in the oceans, emitting dimethylsulphide, which is oxidised step by step in the air. Sulphuric acid in combination with water is a main source for aerosols and therefore plays an important role in cloud development and cloud chemistry. These facts are illustrated including some simple chemical processes.

Volcanic eruptions are temporal sources of sulphur. Their meaning will be described in brief, as well as the sulphates of sea salt aerosols, which are explained mainly in the chapter aerosols.

Back to the table.