A HISTORICAL SHIFT
Human activity has considerably increased certain greenhouse gases. Global concentrations of CO2 have gone from around 280 ppm (parts per million) in 1750 to 380 ppm in 2005. This growth is essentially the result of the use of fossil fuels (transportation and heating...) and of the change in the way land is used.
This increase leads to the prediction of an increase in the planet’s temperature. In one century, the average temperature has increased by 0.74°C. This may seem like a small change, but in comparison, a glaciations period corresponds to a decrease of only 2 to 4 degrees Celsius.
For years, various people have denied the reality of this phenomenon, its importance or man’s role in it. Certain of these skeptics have been financed by the oil industry and in particular, by Exxon. Today, the international scientific community has rallied around the IPCC (Intergovernmental Panel on Climate Change), which is responsible for synthesizing information about climate change. Several reports have been published, establishing a state of the art in this field and the IPCC was even awarded the Nobel Prize for Peace in 2007.
IPCC scientists have elaborated different scenarios for the possible evolution of the climate. These scenarios are organized into families, based on hypotheses about modes of development -- more or less global, more or less rapid, more or less energy consuming, etc. They evaluate the quantities of greenhouse gases that could be released into the atmosphere and the corresponding increases in temperatures.
If humanity does not change its way of functioning, the quantity of greenhouse gases will reach thresholds that will profoundly affect our climate: the standard threshold used is that of 450 ppm of CO2 in the atmosphere (see above). The average warming hypothesis that follows is on the order of +2°C in 2100, but much more pessimistic scenarios (+6°C or even more) are no longer being excluded.