Geoscience Reference
In-Depth Information
glacial and interglacial climates. The Sun, which has a high temperature
in the order of 6,000 K emits radiation in the band X and visible
spectrum whereas the Earth, whose average temperature is 255 K,
mainly emits radiation in infrared wavelengths following
Stefan-Boltzmann's law of blackbody radiation [TRE 92]. To say that
the Earth is in a radiative equilibrium means that the energy received
from the Sun is equal to the energy emitted into space, which leads to
an average “radiative” temperature of 255 K (or -18°C). Indeed, the
average temperature observed is close to 15°C, or 33°C more than the
radiative temperature. This gap is due to the presence of gas in very
small quantities (trace gases) whose interaction with different
radiative wavelengths significantly alters the energy balance, in
quantity according to the wavelength. Of these gases, greenhouse gases
in particular react with infrared wavelengths.
The main greenhouse gas is water vapor. Water, whether it be in
vapor, solid or liquid form, is indeed the main contributor to the
warming of atmospheric conditions (more than 60%). Water vapor
evidently contributes to the character and evolution of the atmosphere;
its distribution is very heterogeneous, its renewal is rapid and
the course of its cycle plays an active role in the equations that
describe the atmosphere. Other significant contributions to the
greenhouse effect are brought about by trace gases with a long life
(CO
2
, CH
4
, N
2
O, etc.) just as by more reactive components such as
ozone. These contributions vary according to the properties of
radiative absorption of the gas considered and according to their
concentration.
2.2.4.
The role of greenhouse gases - GHG
7
The evolution of human activity, through the exploitation of soils,
agriculture, industry and transport, is built upon an intensive
exploitation of natural resources; the production of energy, initially
drawn from coal, has progressively moved toward the exploitation of
other forms of fossil hydrocarbons. The consumption of fossil energy,
deforestation and intensive agriculture have produced a lot of trace
7 GHG: greenhouse gas.
