What is global warming?

Global warming is the enhance in the temperature of the planet due to accumulation of some gases in the atmosphere, especially gases that retain the solar energy reflected by the planet surface. The major gas that causes the global warming is carbon dioxide, CO2, but other gases act as "warming gases" too, like methane, CH4, and nitrous oxide, N2O. The exaggerated enhance of carbon dioxide in the atmosphere has been caused by the burning of fossil fuels (mostly oil and coal) in industrial and urban societies and by forest fires. (It is significant to note that the natural warming given by gases of the atmosphere is fundamental for the maintenance of the planet temperature.)