Greenhouse gases: Definition, Importance and Examples

Greenhouse gases are often talked about. But what is it exactly? Lets learn about greenhouse gases.

Greenhouse gases (GHGs) are characterized by their ability to absorb infrared radiation, which gives them a role in the Earth’s radiation balance. The most important naturally occurring gases are water vapour, carbon dioxide and methane.

Greenhouse gases: Carbon dioxide, methane, ozone…

Greenhouse gas emissions due to human activity concern exclusively the following gases:

• Carbon dioxide (CO2): the modification of the natural cycle of carbon dioxide comes mainly from the increase in emissions by fossil fuels (6.4 Gt/year) and from the modification of the biosphere by combustion wood and soil depletion (2 Gt/year).

Currently, the mixing ratio of this gas is 370 ppm. The residence time of this carbon dioxide in the atmosphere is very variable: it oscillates between 15 and 120 years.

• Methane (CH4) comes from ruminant farming, rice cultivation, landfills, oil, gas and coal mining. Its warming power is greater than that of carbon, but the mixing ratio in the atmosphere is much lower at 1.7 ppm. Its lifetime in the atmosphere is nine years.

• Nitrous oxide (N2O) is emitted both biogenic and anthropogenic. Fossil combustion and biomass combustion emit on average 30 to 40 billion tonnes per year; nitrogen fertilizers fix atmospheric nitrogen (80 Mt per year), but a large fraction is re-emitted into the atmosphere, in the form of reactive compounds. The mixing ratio of nitrous oxide is 310 ppb, and its lifespan is 150 years.

• Ozone (O3) is mainly emitted by human industrial activity. This gas contributes 6% of the total greenhouse effect, but its role is complex, since it has both radiative and reactive properties. In the troposphere, the mixing ratio varies greatly, between 10 and 500 ppb. It does not stay more than two months in the atmosphere.

• Chlorofluorocarbon gases (HCFCs, CFCs) are aerosol can propellants, refrigerant gases or come from various industries. They can remain in the atmosphere for several years, and their mixing ratio is 3.8 ppb.

• Sulfur hexafluoride (SF6) is a leak detector gas also used for electrical insulation.

Nitrogen and oxygen, essential constituents of the atmosphere, are not involved in the greenhouse effect.

How do human activities Affect the greenhouse effect?

Human activities (combustion of fossil fuels, use of fertilizers, industrial processes, livestock farming, change in land use, etc.) are also the source of greenhouse gas emissions into the atmosphere.

Since the pre-industrial era, the global concentrations of greenhouse gases emitted by human activities have grown significantly. They exceed the pre-industrial values determined from ice cores that may cover several thousand years.

The greenhouse effect is amplified and with it the part of solar energy absorbed by the planet. The increase in the energy stored by the Earth has impacts on the major balances that govern the current climate (increase in the temperature of the atmosphere and the oceans, etc.).

The consequences are numerous: modification of rainfall patterns, changes in the number of summer days, extreme climatic events, etc.

Do greenhouse gases have the same global warming power?

In order to be able to compare the greenhouse gases with each other, the ability of each gas to return energy to the ground is evaluated in comparison with CO2.

This is referred to as global warming potential 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 factor 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 HFCs;
• 6,500 to 9,200 for PFCs.

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 the 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.