Greenhouse gases - GHG: what do you need to know?

Greenhouse gases - GHG: what do you need to know?

Greenhouse Gases (GHGs) are gases that are naturally present in the atmosphere for the most part, but whose concentration is increasing due to emissions induced by human activities. What are these GHGs, what are their main sources of emissions and how do they contribute to global warming?

Matthieu Duault

Matthieu Duault

Climate Copywriter

Update :

What is the greenhouse effect?

The greenhouse effect is a natural phenomenon resulting from the influence of the atmosphere on the Earth's various heat flows and contributing to the balance of temperatures at the Earth's surface.

Most of the thermal energy received by the earth comes from the sun and its radiation. One third of the sun's rays that reach the Earth are reflected back into space. The remaining two-thirds are absorbed by the atmosphere, soil and oceans. The earth returns some of the energy it has received by radiating it back in the form of infrared heat radiation. This infrared radiation is in turn absorbed by the gases in the lower atmosphere (around 15 km above the ground), preventing part of the returned heat from escaping into space by keeping it at the earth's surface.

The greenhouse effect has "historically" kept the Earth's average temperature at around 15°C. If the greenhouse effect did not exist, the average temperature on Earth would be -18°C. The natural greenhouse effect has notably enabled the emergence of life on Earth, by creating temperature conditions suited to the development of organisms.

What is a GHG - Greenhouse Gas?

Greenhouse Gases (GHGs) are gaseous components naturally present in the air, resulting, for example, from volcanic activity, the decomposition of organic matter or simply from life on earth via respiration. These atmospheric gases absorb, among other things, the infrared radiation emitted by the earth and are therefore called GHGs because they contribute to the existence of the "greenhouse effect" phenomenon on the surface of the globe.

What's the link between GHGs and global warming?

Human activities, or anthropogenic emissions, lead to an accumulation of GHGs in the atmosphere. The GHGs emitted by human activities are added to the GHGs naturally present in the atmosphere, generating an increase in the concentration of GHGs in the atmosphere. This increase in GHG concentration generates an additional greenhouse effect that tends to raise temperatures at the earth's surface and in the lower layers of the atmosphere.

Despite the partial absorption of GHGs by natural sinks (forests, soils and oceans), the earth is no longer able to balance its GHG concentration, reinforcing the additional greenhouse effect and associated global warming from year to year.

What is the main source of GHG emissions?

GHGs are naturally present in the atmosphere due to their emission by natural phenomena. The famous carbon dioxide (CO2) is emitted during volcanic eruptions and forest fires. Methane (CH4) is naturally emitted by fermentation processes and the micro-organisms that populate wetlands.

The main source of non-natural GHG emissions over the last two centuries has been an increase in energy consumption, which has multiplied by ~10 in 100 years. Fossil fuel consumption in particular exploded over the last century, and these energies are major GHG emitters. Today, they still represent ~ 80% of the energy mix.

What are the main GHGs?

Carbon dioxide - CO2 

The best-known GHG is carbon dioxide, also known as carbon dioxide. Carbon dioxide is an inorganic compound with the chemical formula CO2.

CO2 has been present in the atmosphere since long before the appearance of man. In fact, it is created by the natural decomposition of animal and plant matter. It is absorbed by plants during the process of photosynthesis: the transformation of CO₂ and water into sugars under the effect of sunlight.

Human activities are unfortunately a source of excessive emissions of this gas into the atmosphere. One of the activities that emit CO2 of human origin is deforestation and the turning of soils. Turned-over soils release some of the organic carbon they store, and the removal of vegetation limits the natural capture of CO2, increasing its concentration in the atmosphere.

Industrial processes such as cement and lime production, as well as activities in the petrochemical and iron and steel industries, are major emitters. All activities involving the combustion of fossil fuels such as oil, coal or natural gas generate high levels of carbon dioxide emissions.

In Europe, CO2 accounted for 79% of greenhouse gas emissions in 2019. CO2 is now considered to be the main contributor to the additional greenhouse effect.

Methane - CH4

Methane, also known as "marsh gas", is a greenhouse gas with the chemical formula CH4. It is considered a hydrocarbon and the main component of natural gas.

There are relatively few sources of natural methane emissions. They include swamps, marshes, termites and oceans.

Anthropogenic methane emissions account for just over half of total methane emissions. Agriculture plays a particularly important role. According to the United Nations Environment Programme, "emissions generated by livestock, from manure and gastrointestinal discharges, account for around 32% ofman-made methane emissions".

Agricultural methane production can also come from rice paddies, which have to be flooded in order to grow. This creates conditions conducive to the growth of methane-emitting bacteria by limiting oxygen in the soil. The treatment of household waste, such as composting, and landfill sites also contribute to the release of methane into the atmosphere. Finally, natural gas production, distribution and consumption regularly result in significant methane leaks into the atmosphere.

Methane accounts for 12% of greenhouse gas emissions in Europe in 2019. Europe is the only region where CH4 emissions have decreased over the last 20 years.

Nitrous oxide - N2O

Nitrous oxide, also known as nitrous oxide or "laughing gas", is an oxygen compound of nitrogen. Its chemical formula is N2O. It is considered the third most important greenhouse gas, after CO2 and CH4.

Nitrous oxide has several emission sources. Microbial processes in cultivated soils, such as nitrification and denitrification, which are linked to the use of mineral nitrogen fertilizers, are the main source of nitrous oxide emissions into the atmosphere.

Certain industrial processes, such as the manufacture of glyoxal, adipic, glyoxylic and nitric acids, or the production of combustion equipment, are sources of N2O emissions. Finally, fossil fuels for domestic heating and transport, and biomass combustion, are also sources of nitrous oxide emissions.

N2O emissions in Europe in 2019 accounted for 6.8% of total greenhouse gas emissions. Nitrous oxide also contributes to the destruction of the ozone layer.

Water vapor - H2O

Water vapor, with the molecule H2O, is the gaseous state of water. It is the most important GHG in terms of the natural greenhouse effect. Evaporation of water from the Earth's surface and evapotranspiration by plants produce most of the water vapour in the atmosphere.

Nevertheless, human activity also plays a role in the concentration of this gas in the atmosphere. The cooling towers of nuclear power plants or the drags of aircraft flying for a few hours in the stratosphere are examples of additional H2O emissions generated by human activity. The quantity of water vapour induced by human activities is negligible compared with that induced by natural evaporation, and therefore makes only a minor contribution to the additional H2O greenhouse effect.

According to ADEME, water vapor is the most abundant greenhouse gas, occupying between 0.4% and 4% of atmospheric volume, whereas all other greenhouse gases occupy less than 0.1% of this volume.

Climate change, caused by rising concentrations of other greenhouse gases in the atmosphere, is having an impact on the water cycle. Rising average temperatures lead to a change in the saturation point at which water condenses, resulting in increased precipitation in some parts of the world and drought in others.

Fluorinated gases - HFCs, PFCs, NF3, SH6

Fluorinated gases are called "artificial" or "industrial" because their presence in the atmosphere is due to human activities, and they are not naturally present in the atmosphere, as is the case for the other GHGs presented above.

There are four different fluorinated GHGs:

  • Hydrofluorocarbons (HFCs) are organic compounds made up of carbon, hydrogen and fluorine.
  • Perfluorocarbons (PFCs) are also organic compounds, consisting solely of carbon and fluorine.
  • Nitrogen trifluoride (NF₃) is an inorganic compound with the empirical formula NF3. It is composed of nitrogen-fluorine.
  • Sulfur hexafluoride (SF6), composed of fluorine and sulfur.

Fluorinated gas emissions come from a variety of sources. Refrigerants used in cold chain equipment and air conditioning systems are responsible for gas emissions throughout their life cycle.

The foams used for insulation in the building and civil engineering sector produce the majority of fluorinated gas emissions, particularly during the manufacturing process.

Fluids emitted by fire-fighting systems, when in use or due to leaks, are sources of fluorinated gas emissions.

Aerosol devices release fluorinated GHGs both during use and during production.

The main sources of nitrogen trifluoride - NF3 emissions are linked to its use in the electronics industry, in particular for the production of solar panels, flat screens, semiconductors and light-emitting diodes. However, it should be noted that significant emissions come from the destruction of end-of-life electronic products, as this gas is often used for cleaning operations on electronic equipment prior to recycling or disposal.

Fluorinated gases (F-Gas) will account for around 2.5% of Europe's greenhouse gas emissions in 2019.


What is the Global Warming Potential (GWP) of GHGs?

The ability of a gas to retain heat in the atmosphere over a given period is measured by its Global Warming Potential (GWP).

Carbon dioxide (CO₂), being considered the reference, is assigned a value of 1. In order to be able to compare the effects of different greenhouse gases, their GWPs are converted into CO₂ equivalents over a period of one hundred years, which is the reference period used by the IPCC. This conversion makes it possible to establish a hierarchy of greenhouse gases in terms of their impact on global warming. The Global Warming Potential is therefore the warming power of one mass of a GHG compared with the warming power of the same mass of C02.

The lifespan of different GHGs in the atmosphere is not the same. This fact must also be taken into account when considering the importance of a GHG in the fight against global warming and the strategy to adopt. CO2 has a long lifespan of over 100 years, whereas methane only has a lifespan of around 12 years. The CO₂ accumulation effect in the atmosphere due to its long lifetime makes it the main GHG despite its lower GWP.

Another possible measure is the Global Temperature Potential (GTP). Whereas the GWP is a measure of the heat absorbed over a given period due to the emissions of a gas, the GTP is a measure of the change in temperature at the end of that period (always in relation to CO2).

Global Warming Potential (GWP) values - IPCC 6th Assessment Report

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