Greenhouse’ gases

Carbon dioxide normally comprises ≈0.04% by volume of the Earth’s atmosphere, from which it is removed and returned according to the carbon cycle:

    The balance is a delicate one, and the increase in combustion of fossil fuels and decomposition of limestone for cement manufacture in recent years have given rise to fears that a consequent increase in the CO₂ content of the atmosphere will lead to an ‘enhanced greenhouse effect’, raising the temperature of the atmosphere. This arises because the sunlight that reaches the Earth’s surface has its maximum energy in the visible region of the spectrum where the atmosphere is transparent. However, the energy maximum of the Earth’s thermal radiation is in the infrared, where CO₂ absorbs strongly.

  Even a small increase in the CO₂ component of the atmosphere might have serious effects because of its effects on the extent of the polar ice caps and glaciers, and because of the sensitivity of reaction rates to even small temperature changes. The danger is enhanced by the cutting down and burning of tropical rain forests which would otherwise reduce the CO₂ content of the atmosphere by photosynthesis.  The second major ‘greenhouse’ gas is CH₄ which is produced by the anaerobic decomposition of organic material; the old name of ‘marsh gas’ came about because bubbles of CH₄ escape from marshes. Flooded areas such as rice paddy fields produce large amounts of CH₄, and ruminants (e.g. cows, sheep and goats) also expel sizeable quantities of CH₄. Although the latter is a natural process, recent increases in the numbers of domestic animals around the world are naturally leading to increased release of CH₄ into the atmosphere. Industrialized countries that signed the 1997 Kyoto Protocol are committed to reducing their ‘greenhouse’ gas emissions. Taking 1990 emission levels as a baseline, a target of ≈ 5% reduction must be achieved by 2008– 2012. This target is an average over all participating countries.