Some gases are naturally present in the planet's atmosphere which contributes to the retention of heat close to the earth surface. These gases are called «Greenhouse gases» (GHG) and are essentially formed from water vapour (H 2O), carbon dioxide (CO 2), methane (CH 4), nitrous oxide (N 2O) and ozone (O 3).
A fraction of the solar radiation that reaches the earth surface is reflected in the form of infrared radiation or heat. This heat is retained in the lower part of the atmosphere by the greenhouse gases. This phenomenon is called «Greenhouse effect». This natural phenomenon has allowed the development and the support of life on earth by maintaining a mean temperature of 15oC. Without these greenhouse gases, the earth mean temperature would be in the order of -18oC, and life would not be possible.
Since the beginning of the industrial era, about 200 years ago, concentrations of greenhouse gases have increased. These concentrations were quite stable over the last few hundred years. Use of fossil fuels such as petroleum, coal or natural gas, or changes in land use such as deforestation, some agricultural practices and industrial processes have played a major role in the increase of the greenhouse gases. In fact, the energy sector is responsible for about a third of the GHG emissions.
Natural ecosystems, such as lakes, rivers, forests and peat are emitting GHG. In reservoir, we generally observe a rapid increase of the gross emissions of CO 2 and CH 4 after flooding. The maximum is reached within 4 years after flooding. After this period, there is a gradual decrease of the emissions and a return to emissions comparable to those of natural lakes within a 10 years period. During the first years following impoundment, a large part of the GHG emissions of the reservoir are related to the decomposition of the labile fraction of the flooded organic material.