What is the greenhouse effect?
Versión en español English version
Mars the red planet of our solar system presents a thin atmosphere for what the greenhouse effect becomes weak and its surface is much colder than our planet surface. The average temperature in Mars is of -63° C with a maximum temperature of 20° C and a minimum of - 140° C.
Venus, the second planet from the Sun, the planet brother of the Earth is called both since they are very similar in size and mass. But there finish the similarities. The atmosphere of Venus constituted mainly by carbon dioxide, almost any vapor of water, and drops of sulfuric acid cause an greenhouse effect so intense that higher temperatures than 450° C are reached. Its surface is hotter than Mercury surface, planet more near at Sun. The high density of the atmosphere produces a pressure in the surface of 90 times that of the Earth, similar to the one that is to 900 meters deep in the Earth oceans.
The sun operates as the nuclear reactor that gives energy to the whole planetary system of which is part the Earth. The nuclear explosions that happen in their surface emit radiant energy that travels through the space and it arrives to the external part of our atmosphere with an approximate flow of 1.4 * 103 W/m2 .
This energy flow means that to the atmospheric external layer of unit surface (1m2), perpendicular to the line that unites the Earth with the Sun, it receives something less than 1 400 J each second. When multiplying this value for all area of the Earth circle and to divide this product for the whole surface of the planet it is obtained the average quantity of energy that arrives at the planet, known as sun average constant, equal to 342 W/m2.
The capacity to reflect this incident radiation for different dispersive phenomenon is around 30%. That is to say something more than 100 W/m2 is returned to the space by the different atmospheric components and the constituents of the terrestrial surface. This reflection doesn't take place in an uniform way in all the regions of the planet. As neither 70% of the incoming radiation is absorbed equally in the equatorial areas and in the poles, neither in the superior layers of the atmosphere it is absorbed in the same measure that in the terrestrial surface. These differences promote the thermal convective currents and vaporization - condensation phenomenon, responsible on high degree of the weather behaviour.
But it interests us to put attention in the destination of the heterogeneous electromagnetic radiation that is absorbed. The wave longitude of this radiation is understood in an interval from the ultraviolet one near until the infrared (200 nm - 4 000nm) .
Good part of the ultraviolet radiation, especially the acquaintance as radiation hard of more energy content and for consequence of more penetration power it is absorbed by the stratospheric ozone layer and it is affirmed that only 9% of the radiation ultraviolet incident, the next to the visible one, is the one that arrives to the planet surface.
The infrared radiation is absorbed by the molecules of certain gases in the different atmospheric layers. Some gases, as oxygen and nitrogen are transparent to almost all radiations, while others as water vapour, carbon dioxide, methane and nitrogen oxides are transparent to the radiations of short longitude wave (ultraviolet and visible), while they absorb the long radiations (infrared) .Particularly the molecules of water vapour and of carbon dioxide absorb intensely to certain frequencies of this radiation type. In quantitative terms it is considered that the energy corresponding to the infrared region that arrives at the sea level can be a 49% of which arrives to the external atmosphere.
The visible radiation suffers diverse dispersive phenomenons among those that are responsible for the blue colour of our sky vault.
In sum, around
half of the solar energy that enters to the planet reaches its
surface. The rest is reflected to the space or it is absorbed by the
component gases of the atmosphere.
The greenhouse effect maintains the temperature average on the surface of the Earth in 15 °C. If greenhouse gases didn't exist the temperature of the planet it could end up reaching - 18 °C, too cold for the majority of the alive organisms. In such a supposition most of the heat irradiated from the surface of the planet could get lost directly toward the external space.
Until here we have examined shortly as this effect it has conditioned a average temperature in the surface of our planet that favoured the origin of the life. It is not necessary to forget that the proteins (proteus, the first thing according to the Greeks) they are denaturalized in the interval between 45 - 60 ºC.
If we integrate to this effect the curtain protector of the radiation hard ultraviolet played by the ozone layer, it arrives to the surface of the Earth 9% of the radiation UV presents in the solar rays, it is understood the special conditions that were given in our atmosphere to promote the development of the life.
Echerri Luis (2002): Tema 10 Cambio Climático y Efecto Invernadero. Ciencias de la Tierra y el Medio Ambiente. Libro Electrónico.
U.S.EPA (2005): What is the problem? EPA'S Global Warming Site.