What is the greenhouse effect ?

As a result of human  activities, the level of carbon dioxide (CO₂) in the atmosphere has increased by 30 % in the course of the last two centuries.

In France, it is estimated that between 70 % and 90 % of CO₂ emissions result from the combustion of fossil fuels and about 10 % are linked to agriculture and deforestation. CO₂ is a slightly acid gas, which has a tendency to disappear naturally from the atmosphere. Its life is nevertheless of the order of a hundred years (see picture 1).

Methane (CH4) is present in the atmosphere in very low quantities. Until 1850, its concentration was 0,8 ppmv (parts per million in volume) and had hardly changed for 400 000 years. However, its role cannot be ignored since its capacity to absorb infrared radiation is 20 to 50 times greater than that of CO₂. Furthermore, since the beginning of the Industrial Age, its concentration in the atmosphere has not stopped increasing : 1 ppmv in 1900, 1,1 ppmv in 1950 and 1,8 ppmv today. This increase has  a number of causes, essentially linked to human activities.

Methane is a gas produced during anaerobic (in the absence of oxygen) fermentation. Such reactions take place :
- in rice fields (175 to 350 million tonnes p.a.) and in wetlands (150 to 300 million tonnes p.a.) : the microorganisms in sodden soils or associated with the roots in paddyfields give off CH4 in the course of their metabolic activity ;
- in the rumen of ruminating (cud chewing) animals (75 to 150 million tonnes p.a.) : the digestive system of ruminants (cows, sheep, etc.) has a first compartment, the paunch or rumen, in which the bacterial digestion of the fodder takes place.  CH4 is produced during this phase and eructed by the animals.

Agricultural activities are, as a consequence, at the origin of 57 % of the discharges of CH4 into the atmosphere. The expansion of this type of activity, linked to the growth in world population, results in an increase in methane concentration. In addition, the use of fossil fuels and the associated discharges contribute to the increase. Each year, the concentration of CH4 rises by 0.5 %. Nonetheless, there is one positive aspect, methane is easily oxidised : its life in the atmosphere is only 12 years.

Agricultural activities, particularly the use of nitrogen fertilisers, are the cause of a rise  in the concentration of nitrous oxide (N2O). N2O is  present naturally in the atmosphere in minute quantities, but its concentration has increased by 15 % since the start of the Industrial Age. Most importantly, its capacity to absorb infrared radiation is 310 times that of CO₂. and its life in the atmosphere is relatively long (120 years).

The chlorofluorocarbons (CFC) are synthesised gases which appeared recently in the atmosphere. They have been banned since 1990. They were used in aerosols (shaving creams, deodorants, etc.) and as refrigerating and propellant gas. The increase in their concentration during the last century was 4 %. Given that their power of infrared radiation absorption  is very high (16 000 times higher than that of CO₂ !), the CFC’s contributed significantly to the intensification of the greenhouse effect (see picture 2).

The addition of this « anthropogenic »  greenhouse effect to the natural greenhouse effect has direct consequences for the Earth’s temperature and climate. This is why, during the 2nd Earth Summit in Rio de Janeiro in June 1992, the international community launched a programme aimed at combating global warming.