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...