Water vapour, a harmless molecule, is the most abundant greenhouse effect gas (or GEG). Human activities only affect the amount present in the atmosphere slightly. They therefore only play a secondary role in current global warming. However, the concentration of other greenhouse gases has significantly increased since the Industrial Revolution. These gases are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and a group of rare gases including halogen gases.
Paradoxically, CO2 is one of the weakest greenhouse gases. A sulfur hexafluoride (SF6) molecule, for example, « warms » several thousand times more than a CO2 molecule. It also has a « life span » in the atmosphere of about 3200 years compared to about 100 years for CO2. By combining these two parameters, researchers have calculated that SF6 has a Global Warming Potential (GWP) that is 22 800 times higher over a century than CO2 ’s! Luckily, only very small amounts of SF6 are emitted into the atmosphere. In comparison, every year, man emits about 30 billion tons of CO2.
The relative importance of different gases can be compared. In 2004, carbon dioxide represented 77 % of greenhouse gas emissions linked to human activity, methane represented 14.3 %, nitrogen oxide, 7.9 % and halogen gases, 1.1 %. CO2 mainly comes from the combustion of fossil energy and deforestation while methane is mainly produced by agriculture and the use of natural gas. Nitrous oxide comes from fertilization. Halogen gases are used in industry. Some are used as refrigerants (and therefore also weaken the ozone layer).
Each gas is emitted into the atmosphere by different processes. To limit these emissions, specific systems – economic, technological and political – must therefore be developed for each case.