What causes climate change?
Historical studies of our climate show that both atmospheric carbon dioxide levels and the Earth's temperature have varied over time. However, since the beginning of the industrial age, the concentration of Greenhouse gases* has increased very rapidly, by 50%. Each year, human activity emits 6 billion metric tons of carbon into the atmosphere and about 3.3 billion are in excess and stored in the atmosphere. This is just a small amount in relation to the 150 billion metric tons exchanged naturally between the atmosphere, plants and the oceans, but it is enough to increase the greenhouse effect and disrupt the climate's delicate balance.

The greenhouse effect
The greenhouse effect is a natural, vital phenomenon. Without it, the average temperature on the Earth's surface would be -18°C rather than the current +15°C. This natural phenomenon allows water to remain liquid and creates the conditions necessary for life. The Earth's surface absorbs roughly half of the energy it receives from the Sun. This energy is then radiated back as heat (infrared rays), a portion of which is absorbed by clouds and atmospheric gases. Together, they act like a lid that reflects this thermal energy back to Earth, thus heating the lower atmosphere. This balanced process-called the greenhouse effect-ensures relatively stable temperatures on the Earth's surface.

While the greenhouse effect is crucial for maintaining a livable temperature on Earth,
an enhanced effect can alter the climate's delicate balance.

Solar energy absorbed by the earth is radiated back as heat (infrared rays),
a portion of which is absorbed by the atmosphere and returned to Earth.
This is what is known as the greenhouse effect.

Greenhouse gases
Greenhouse gases* are minor components of the atmosphere but their role is crucial. These molecules are able to trap infrared rays, thereby helping the Earth to retain part of the heat provided by solar radiation.
Water vapor is the major greenhouse gas, even though it represents less than 1% of total gas volume in the atmosphere. It accounts for nearly 60% of the greenhouse effect and its role is easily observed, as on winter nights, which are much colder if the sky is cloudless. The scarcity of water vapor also explains why deserts are very hot during the day and freezing at night.
Other trace gases play an important role because they have a very powerful greenhouse effect. These include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), tropospheric ozone (O3, created by the emission of nitrogen oxides and volatile organic compounds) and man-made gases such as CFCs. These molecules' contribution to greenhouse forcing depends on their optical properties and their lifespan in the atmosphere.

Principal Greenhouse Gases