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Greenhouse gas.

Greenhouse gases absorb infrared radiation emitted by the earth's surface and thus contribute to the greenhouse effect.

The increase in their concentration in the Earth's atmosphere is one of the factors causing global warming.

A gas can only absorb infrared radiation from three atoms per molecule, or from two if they are two different atoms.

What is a greenhouse gas ?

By greenhouse gas, we mean a gas present in the atmosphere that retains part of the heat received by solar energy in the atmosphere.

The increase in the concentration of greenhouse gases in the atmosphere results in a rise in its temperature.

Some gases are produced by human activities, in particular fluorinated gases.

What is the greenhouse effect ?

Thanks to greenhouse gases (GHGs) naturally present in the atmosphere, the Earth and its atmosphere absorb part of the energy it receives from the Sun, the rest being returned to space.

This natural phenomenon, called the greenhouse effect, makes life possible on Earth, because without it, the average temperature of the planet would indeed be around -18°C.

These greenhouse gases, in particular their concentration in the atmosphere, therefore play an important role in regulating the climate.

Natural emissions of water vapor and carbon dioxide are compensated by rain for water vapor, photosynthesis or their dissolution in the oceans for CO2, which allows a stable concentration of greenhouse gases in the atmosphere.

Do greenhouse gases have the same warming power?

The capacity of each gas to return energy to the ground is evaluated in comparison with CO₂.

This is the global warming power or potential (GWP).

The GWP of a gas depends on its ability to intercept and reflect solar radiation as well as its lifetime.

The value of the GWP is determined according to a time scale, generally 100 years.

GWP is measured relative to CO2 and is expressed in tonnes of CO2 equivalent.

To express greenhouse gas emissions in tonnes of CO2 equivalent, the emissions of each gas are weighted by a coefficient depending on its GWP.

This coefficient is:

1 for CO₂.
25 for CH₄.
298 for N₂O.
22,800 for SF₆.
17,200 for NF₃.

It varies from:

140 to 11,700 for HFC.
6,500 to 9,200 for PFC.

The greenhouse gas inventories, when they relate to all emissions, are given according to a global warming potential at 100 years, consistent with the data disseminated within the framework of international agreements and global conventions.

The United Nations Framework Convention on Climate Change includes provisions relating to the communication of information on emissions into the air, namely, emissions of direct greenhouse gases (CO2, CH4, N2O, HFCs, PFC, SF6, NF3) and, with a marginal impact, indirect greenhouse gases (NOx, CO, NMVOC, SO2).

Anthropogenic water vapor emissions are marginal, do not contribute significantly to global warming and are excluded from greenhouse gas emission inventories.