Solar radiation, as it passes through the Earth's atmosphere is partly absorbed by the aqueous vapour, ozone and dust that make up the atmosphere itself (25%); partly it is reflected back into space by clouds, atmospheric dust and the Earth's surface (30%); the remaining share reaches Earth's atmosphere and is absorbed by the materials and organisms that make it up.

The energy thus received is re-emitted by the earth's surface but at a greater wavelength of the sun's. Earth's radiation, in fact, falls prevalently in the infrared. Great part of this energy is reabsorbed by the so-called greenhouse gases (CO2 carbon dioxide, CH4 methane, aqueous vapour, N2O nitrogen protoxide, etc.) to later be newly re-emitted in all directions. A sizeable part returns to the surface contributing to its warming, a part is newly absorbed by the atmosphere and a small part is finally lost in space (around 6%). Owing to the atmosphere's presence the average temperature on Earth is therefore of  about 15° C compared to the around -19 °C there would otherwise be without it.

The differing behaviour of the atmosphere to solar radiation and terrestrial one derives from its chemical composition. Greenhouse gases on the basis of their molecular structure reflect terrestrial radiation (infrared), but they are transparent to solar radiation (visible wavelength). This capacity is a specific characteristic of each molecule: thus composites present in compositions that are radically lower than CO2 or with high permanence times can significantly contribute to the phenomenon. To be able to compare the contribution of each greenhouse gas the Global Warming Potential (GWP) is calculated. This indicates the relationship between the atmosphere warming power of any gas with respect to that of the CO2 valued over a specific temporal horizon. Changes in the chemical composition of the atmosphere, and particularly in the concentration and presence of greenhouse gases, can therefore affect its energetic balance and cause variations in Earth's temperature.

According to the Intergovernmental Panel on Climate Change report, the concentration of CO2 form the beginning of the industrial revolution has grown of 31%. This increase is owed mainly to the emissions caused by the use of fossil fuels in industry, in the sector of energy production and in transportation.

In the same period, because of industrial and agricultural activities, CH4 concentrations have doubled (GWP = 21) even if the quantity is in any case considerably inferior to CO2. Starting form the sixties other powerful greenhouse gases were emitted into the atmosphere: CFCs (CFC11 - GWP = 4000; CFC12 - GWP = 8500) and Halons (Halon1310 - GWP = 5800). They are also the greatest culprits for the destruction of the stratospheric layer of ozone and they were progressively substituted with other gases which are less harmful to the ozonosphere but in any case still with a high GWP (HCFC 22 =1700; HCFC123 = 93; HFC 125 = 3200; HFC 134 = 1300; HFC 227 = 3300; perfluoridemethane = 6300 etc.)

On the other hand temperature rise could be slowed down by the cooling effect of the aerosol of atmosphere pollutants (particularly SO2) and by dust generated, for example, by volcanic eruptions that slow down the penetration of solar radiation. IPCC, considering both the greenhouse gas trend, and the aerosol emission sceneries, based on a consistent number of climate models has estimated a rise in temperature between 1,4 and 5,8 °C from 1990 to 2100.

In this phenomenon's evaluation, we must in any case keep in mind that a time scale of one-two centuries is considered. On a geological scale, global temperature was characterised by oscillations of much greater size. An example would be the glaciations, on the causes of which differing hypotheses were elaborated.
 
Global temperature rising can bring about very complex unbalances on the environment. Briefly, the possible effects can be: rise of evaporation and the quantity of water in the atmosphere, determining a minor frequency but greater intensity in rains, with floods and erosions, as well as of snowing in the colder regions; reduction of ground moisture and agricultural productivity in regions already exposed to the risk of desertification; decrease of the glacial surfaces in all continents; sea level rising (in the past 100 it has risen between 10 and 25 cm); increase of extreme climate events connected to "El Nino"; increase of infectious and contagious diseases directly and indirectly tied in with draught and flooding and increase of illnesses and victims caused by heat waves in temperate regions.

November.2001