climatic change: climate and greenhouse effect

A. To understand the greenhouse effect

The greenhouse effect is a natural phenomenon, indispensable to life on Earth and that assures a middle temperature of +15°C about instead of -18 °C. In fact, a temperature of -18°C would have the oceans frozen, what would increase their albédo considerably (reflective power) making fall the temperatures around -100°C...

The Earth receives the major part of his/her/its energy of the sun (mainly as visible light), a part is directly reflexive, another absorbed and a last radiated as infrareds by our planet. The infrared radiance given out by the Earth is intercepted in part by gases to greenhouse effect of the terrestrial atmosphere while the rest is distributed toward the space.
Thus, the steam of water, methane, the dioxide of carbon and the monoxide of nitrogen, that are the main gases to greenhouse effect (GES) contribute to trap the sent back energy, increasing the middle temperature of the Earth. Indeed, these are gases to structure polyatomique (at least 3 atoms) that keep the infrared radiance unlike the diatomic molecules (99% of the atmosphere) that have a too simple structure.

The clouds have a double role in the greenhouse effect: opposite the solar radiance, the clouds act mainly like a sunshade that sends back toward the space a big part of the rays of the Sun. The reflecting power, or albédo, of the low-altitude thick clouds, is thus very elevated, of the order of 80%. on the other hand, the cirruses that are constituted clouds of altitude of ice crystals, have an effect very weak sunshade since they are transparent but participate strongly to the greenhouse effect. The middle temperatures of the globe (measured to 2 m above soil under shelter) are of: +15,1 °C on average (polar regions: -20°C, moderate +11°C, equatorial,: +26°C).
On March where the atmosphere is held and therefore the absent greenhouse effect, the middle temperature is of -50°C. On Venus, where the atmosphere is charged very in carbon dioxide, the middle temperature is of +420°C. We understand therefore that the concentrations in gas to greenhouse effect on Earth permitted the apparition of the shapes of life that we know that are sensitive to the temperatures.

Carbon dioxide, methane, nitrous oxide and other gases capture and hold heat in the earth’s atmosphere instead of letting it escape into space. The trapped heat warms the planet like a greenhouse. The gases occur naturally, but human activities since the Industrial Revolution are believed to have greatly boosted their concentrations. Some greenhouse gases can linger hundreds of years in the atmosphere and vary in their ability to trap heat. Nitrous oxide traps heat 296 times better then CO2, while methane is 23 times more potent.

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