What is the greenhouse effect ?

The Earth was formed 4.6 billion years ago. At that time, the primeval atmosphere consisted almost entirely of helium and hydrogen. Solar winds gradually hounded these light gases out of the atmosphere, whereas they are still to be found in the atmosphere of the gaseous giant planets (Jupiter, Saturn, Uranus, Neptune), which consist, not of rock like the Earth or Mars, but largely of hydrogen in its liquid and solid states.

Jupiter (click to enlarge)
© Nasa
The atmosphere of Jupiter consists of hydrogen and helium.

4 billion years ago, strong volcanic et meteoritic activity led to the extraction of gases from the Earth’s crust in the form of emissions of carbon dioxide (CO₂), water vapour (H2O), ammonia (NH3), nitrogen (N2), etc. The secondary atmosphere so formed, very rich in carbon dioxide, was comparable to the present atmosphere of Venus. A reduction in temperature and an increase in pressure resulted in torrential rains, which were at the origin of the primeval oceans. These latter absorbed some of the CO₂ in the atmosphere, which became proportionally richer in N2, a gas that is a lot less soluble in water.

Venus (click to enlarge)
© Nasa
The atmosphere of Venus is composed of 95.5 % carbon dioxide and 4.5 % nitrogen. The colouring of this image enables many details to be seen in the clouds of sulphur dioxide which cover the planet.

Chlorophyll and photosynthesis made their appearance. Oxygen was given off by the marine plants in the oceans. This element was responsible for heavy oxidation activity, leading, for  example, to the  formation of iron ores, which are exploited today in Mauritania and elsewhere.

Phytoplankton in massive proliferation in the  Atlantic ocean (click to enlarge)
© Esa
Phytoplankton consists of microscopic plant life. The chlorophyll contained in the cells of these plants enables them to use sunlight as a «food » to build their structure. This vegetal synthesis, produced with the aid of sunlight, is called photosynthesis.

1.7 billion years ago, a tertiary atmosphere started very slowly to become enriched in oxygen ; the present oxygen level was not attained until the middle of the  Cretaceous Period, that is to say 100 million years ago. For its part, the CO₂ in the atmosphere continued to diminish, the corresponding carbon being stored in sedimentary and carboniferous rocks (oil, coal).

The enrichment of oxygen in the atmosphere allowed the process of breathing in the air to develop. It is also at the origin of the formation of an atmospheric layer indispensable for the conquest of the terrestrial environment by organisms as we know them : the ozone layer (O3). The ozonosphere was created 1 billion years ago. Situated in the stratosphere, this layer reduces penetration of the ultraviolet component of the sun’s radiation and of cosmic rays, which, in the absence of the ozone layer, would be lethal for life on Earth. The rays are high-energy radiation, capable of disrupting the structure of organic molecules.