Temps de lecture :4 minutes
The greenhouse effect was discovered in the XIXth century: Joseph Fourier described it in 1824. It is the process by which certain atmospheric gases absorb some of the energy of sunlight reflected by the Earth. Without this, they would go back into space. This energy heats the planet. And the more gases there are, the more they heat the planet.
This is a natural mechanism. Thanks to it, the Earth’s average temperature is compatible with life. Without it, the Earth’s average temperature would go down to -18°C and even much lower. But the greenhouse effect is not always favourable to life as on Venus, it is the cause of its unbearable surface temperature: about 460°C.
The recent greenhouse gas concentration increase on Earth has caused global warming. The phenomenon has not surprised scientists: in 1896, the Swedish researcher Arrhenius calculated that if the CO2 in the atmosphere doubled, the average temperature would go up by 4°C. (See the Global warming summary).
Different greenhouse gases
There are different greenhouse gases. The Intergovernmental Panel on Climate Change (IPCC) counted over forty. The most common is water vapour and the best-known is carbon dioxide (CO2). Both are naturally present in the atmosphere. There is also methane (CH4), nitrous oxide (N2O), ozone (O3) as well as gases created by Man, like hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulphur hexafluoride (SF6), for example.
Global warming power
All greenhouse gases do not have the same effect on the climate. Some are several thousand times stronger than CO2 (an effect measured by a technical parameter called radiative forcing). Others will act over much longer periods of time than CO2: whilst it has an average lifetime of a century in the atmosphere, some last for several tens of thousands of years. This is why a comparative parameter is used: the global warming potential (GWP) with a reference value 1 for CO2. Over a century, the methane GWP is 25 and the SF6 GWP is about 22800.
Since the beginning of the industrial era, around 1750, greenhouse gas concentration has increased. Carbon dioxide (CO2) increased by 30 % and went from 280 ppm (part per million) to 381.2 ppm in 2006 and methane (CH4) increased by 145 % and went from 715 ppb (part per billion) to 1 782 ppb.
Emissions of different gases can be brought down to one unit, metric tonnes of CO2 equivalent, by using the GWP. When this is done, it can be noticed that emissions have increased more over the past decades. Between 1970 and 2004, they increased by +80 % and reached 30 billion tons of CO2 equivalent in 2006.
Not all greenhouse gases remain in the atmosphere. Some are absorbed. Out of the (approximate) 7 billion tons of carbon emitted every year (about 30 billion tons of CO2), it is generally estimated that about 3 billion are absorbed by oceans and 1 billion tons are absorbed by forests and soil (it is hard to get a precise estimate). Of course, to increase this absorption would mean fighting global warming. This is the issue that is at the centre of a lot of research. Conversely, it is possible that the level of absorption would go down in certain conditions and this would worsen the phenomenon (See the Global warming summary).
If there were about 700 Gt (gigatons or 1 billion tons) of carbon in the atmosphere, oceans would contain almost 40 000 Gt of it. On Earth, the biosphere (vegetation in particular) stocks about 550 Gt and soils stock about 1 500 Gt.
Emissions per activity sector
According to the experts from the Intergovernmental Panel on Climate Change, in 2006, industry represented 19.4 % of the world’s emissions, followed by forest activities (deforestation) with 17.4 %, transport 13.1 %, agriculture 13.5 % and housing 7.9 %. Aviation’s share of pollution is between 3 and 5 % and the European Union is responsible for about half the CO2 emissions from the international air transport of developed countries. The latter increased by 73 % between 1990 and 2003.
Distribution per country
In 2007, developed countries were responsible for 70 % of emissions of which 22 % were produced by the United States and 15 % by Europe.
Since 2007, China has emitted the most greenhouse gas, followed by the United States and Indonesia which moved into third place following the destruction of the forest. The United States still emits the most greenhouse gas and has a rate that is five times higher than China’s per inhabitant.
To compensate greenhouse gas emissions, the association Goodplanet created the ‘‘ program. Visit their site to calculate your GG emissions for a journey and to find out about their voluntary compensation projects.
Voici la liste des principaux gaz à effet de serre.
H2O (vapeur d’eau) : 55 % des émissions totales de GES ; d’origine naturelle et anthropique
CO2 (dioxyde de carbone) : 39 % des émissions totales dont près de 70 % d’origine anthropique ; issu de la combustion des énergies fossiles (pétrole, charbon, gaz) et de la biomasse
CH4 (méthane) : 2 % des émissions totales et 13 % d’origine anthropique ; issu de l’agriculture (rizières, élevages), de l’extraction des combustibles fossiles, de leur combustion et des décharges N2O (protoxyde d’azote) : 2 % des émissions totales et 16 % d’origine anthropique ; issu de l’agriculture, de la combustion de la biomasse et des produits chimiques comme l’acide nitrique
Les gaz fluorés (HFC: Hyfluocarbures, PFC : Perfluocarbures, SF6 : Hexafluorure de soufre) : 2 % des émissions, d’origine anthropique ; utilisés dans les systèmes de réfrigération, les aérosols, les mousses isolantes, l’industrie des semi-conducteurs… ; pouvoir de réchauffement 1 300 à 24 000 fois supérieur à celui du CO2 ; durée de vie très longue.
O3 (ozone): 2 % des émissions totales et part relative dans l’effet de serre additionnel comprise entre 10 et 20 % (Ce gaz n’est pas pris en compte dans le Protocole de Kyoto).