A natural system of trapping the Earth’s heat. Solar rays which penetrate the atmosphere to reach the Earth’s surface are then partly re-radiated by this suface. Certain gases present in the atmosphere absorb these ascending rays and reflect them back to the surface as heat, which allows the Earth to be at a temperature of 15°C. Human activity contributes to the increase in content of these gases (carbon dioxide CO2, methane CH4 and chlorofluorocarbons CFCs) in the atmosphere, which in turn, raises the Earth’s temperature.
Groundwater table
The first water encountered when digging a well. The table is generally free, i.e. its surface is at atmospheric pressure. It can also be under pressure if the cover material is not very permeable. It circulates, when it is free to do so, in an aquifer with an unsaturated zone near ground level.