What is the greenhouse effect?
It is a natural phenomenon! Earth receives its energy from the Sun in the form of heat and light. A part of the energy entering the atmosphere is absorbed by the atmosphere and the remainder heats the Earth. The Earth does not retain this energy and sends it back towards the exterior in the form of infrared radiation. The natural greenhouse gases, steam and carbon dioxide (CO2), retain part of this radiation, leaving the rest to continue its journey into space. The infrared radiation warms the atmosphere: if neither steam nor CO2 existed in the atmosphere, the average temperature on Earth would be –18°C, whereas thanks to those gases, it is +15 °C. To accelerate the growth of plants in a greenhouse, horticulturists increase the ambient temperature by saturating the air with steam.
Why has the greenhouse effect become a problem?
If the quantity of CO2 in the atmosphere increases, then more infrared radiation is retained and the temperature increases. On Venus, which does not receive very much more solar energy than the Earth, the atmosphere contains virtually only CO2, and its average surface temperature is +420 °C! By burning very large quantities of fossil energy (coal, oil, gas), man is gradually increasing the concentration of CO2 in the atmosphere. And his activities discharge other gases with an important greenhouse effect:
- methane, CH4, produced by waste tips, rice cultivation and the breaking of wind from cows. Its length of life in the atmosphere is 12 years, and its greenhouse effect 21 times stronger than CO2.
- nitrous oxide, N2O (laughing gas), discharged by nitrogen fertilisers. Length of life in the atmosphere 150 years; greenhouse effect 310 times stronger than that of CO2.
- gases of the fluorine family like the CFCs, used as cooling agents in air conditioners and propellants in aerosol canisters. Length of life in the atmosphere 120 years; greenhouse effect 16000 times stronger than that of CO2.
- Sulphur hexafluoride, SF6, used as a leak detector and as an insulator. Length of life in the atmosphere 50 000 years; greenhouse effect 24 000 times stronger than CO2.
Who is responsible?
Principally the developed industrial countries. The recent arrival of emerging countries amongst the emitters of greenhouse gases only heightens the problem. An American emits on average about 20 tons of CO2 per year, a Russian half of that, a Frenchman one third, a Chinese 10 times less, and an Indian 20 times less.
What are the activities producing greenhouse gases? If we take France as an example:
- road transport accounts for 25% of the total;
- industry in general for 20%;
- agriculture 19%;
- living accommodation, offices, businesses and state administration 18%;
- energy processing 13%;
- waste discharge in tips 3%;
- transport other than by road (excluding international air transport) 2%.
What are the solutions?
The only truly realistic solution is a major and rapid diminution in CO2 emissions. The theoretical level for stabilisation of the quantity of CO2 in the atmosphere corresponds to an emission of 0.5 tons of CO2 per year per inhabitant, that is to say about half of that generated today by an Indian, or by a Paris to New York return flight, or a 1500 km trip in an SUV! The Kyoto Protocol is a first step in the direction of reducing greenhouse gas emissions.
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