The climate of any region is largely determined by four geographic aspects:
Latitude, distance from the sea, direction of the prevailing winds and
Climate variation factors
Other factors influence the global climate system: atmosphere, oceans,
ice, land and the various forms of life.
Ultraviolet, visible and infra-red solar radiations are Earth's main
sources of energy. There
is an established balance between the incmoing solar energy and the telluric
infra-red radiation emitted by the Earth. Part of the Earth's radiation
is absorbed and reemitted by the 'greenhouse' effect and part is lost
in space. You'd be hard pressed to find
people anywhere that don't have an opinion on the 'greenhouse' effect
or solar radiations one way or the other.
Horizontal variation of solar energy
The balance is fragile and any variation in the factors that affect this
incoming and outgoing energy process or which modifies the energy repartition
will affect the world climate.
The climate changed during Earth's history. Ice ages alternating
with warm periods provide an example. Some changes were worlwide, while
others simply affected an area or a hemisphere. In addition, a number
of natural factors contribute to modify the Earth's climate during various
periods. It is important to understand these factors when seeking to detect
the influence of humanity
on the climate:
Variations of the solar energy emissions. The quantity of energy
emitted by the Sun is not constant. There are evidences revealing that
the Earth's temperature corresponds to a solar cycle. Long term changes
Modifications of the Earth's orbit. The orbit
of the Earth around the Sun changes slowly. This influences the quantity
of energy which is reflected and absorbed. It is thought that these variations
of Earth's orbit are one of the factors that triggered the ice ages.
Seasonal variations of the air temperature
The greenhouse effect. Approximately 1/3 of the energy emitted
by the Sun returns to space after penetrating Earth's atmosphere. A fraction
of what remains is then absorbed by the atmosphere, but the major part
is absorbed by the Earth's surface. The surface returns infra-red energy
and while part of this energy is lost in space, another part is absorbed
again and re-emitted by the clouds and gases like water vapor, carbon
dioxide, methane and oxide nitrous. This contributes to heat Earth's surface
and the troposphere to a temperature 33°C higher than what it would be
otherwise.It is the natural greenhouse effect which is essential for life.
Aerosols. These are very fine particles that remain in suspension
in the atmosphere during a very long time. They reflect the solar radiation
and also absorb it. By modifying the quantity of the aerosols in the atmosphere,
one modifies the quantity of the reflected and absorbed solar energy.
The greenhouse effect amplification. The greenhouse gases naturally
present in the atmosphere (e.g., water vapor, carbon dioxide, methane
and oxide nitrous) keep the Earth at a sufficiently high temperature so
that life is possible. Scientific studies reveal that various human activities,
whose combustion of fossile fuels for producing electrical energy, heating
and transport, produce greenhouse gases. By increasing concentrations
of these gases and by rejecting new greehouse gases such as chloroflurocarbures
(CFC), humans are likely to be contributing to increasing the average
temperatures of Earth.
Lan use evolution. By replacing forests with arable lands or
the natural vegetation by asphalt and concrete, humanity modifies the
way in which terrestrial surface reflects sunlight and releases heat.
All these changes can also modify the regional configurations of evaporation,
streaming and rains.
Atmospheric aerosols. Due to its agricultural and industrial
activities, humanity adds great quantities of fine particles called aerosols
to the atmosphere. Most of the aerosols are quickly falling due to gravity
and precipitations, but they do not less influence the atmosphere radiative
absorption. It is the quantity and the nature of these particles as well
as the nature of underneeth surface (land or water) that determine if
this have a heating effect of not. Nevertheless, the regional effects
can be important.