Greenhouse gases (GHGs) play a pivotal role in shaping the Earth's climate. They are naturally occurring or human-made gases that trap heat in the Earth's atmosphere.
Essentially, they act as a "blanket" by allowing sunlight to pass through the atmosphere and reach the Earth's surface.
There are eight GHGs, four of which are natural and four of which are anthropogenic (through human-made causes).
What are the different GHGs? Let's take a look at each one in more detail.
Carbon dioxide (CO2)
Carbon dioxide (CO2) is a colourless and odourless gas composed of one carbon atom bonded to two oxygen atoms. It is a naturally occurring component of Earth's atmosphere, and it plays a crucial role in the planet's carbon cycle and the regulation of temperature.
CO2 is both a product of natural processes, such as respiration and volcanic activity. It's also a by-product of human activities, particularly the burning of fossil fuels like coal, oil and natural gas.
Methane (CH4) is a colourless and odourless gas composed of one carbon atom bonded to four hydrogen atoms. Methane occurs naturally through various biological and geological processes, including microbial activity in wetlands, the digestive processes of animals and the decomposition of organic matter.
Human activities, such as agriculture (especially livestock digestion and manure management), the extraction and distribution of fossil fuels, landfills and wastewater treatment, contribute to elevated methane levels in the atmosphere.
Methane is a powerful GHG, with a high ‘global warming potential’. While it has a shorter atmospheric lifetime compared to CO2, it is much more effective at trapping heat in the atmosphere.
In fact, pound for pound, methane's impact on global warming is over 25 times greater than that of CO2 over a 100-year period. This heightened potency makes methane emissions a significant concern for climate mitigation efforts.
Nitrous oxide (N2O)
Nitrous oxide (N2O) is a colourless and odourless gas composed of two nitrogen atoms bonded to one oxygen atom.
Nitrous oxide is produced through both natural processes, such as microbial activity in soils and oceans, and human activities like agricultural practices (fertiliser application and manure management), the combustion of fossil fuels and industrial processes.
While nitrous oxide is present in much smaller concentrations in the atmosphere compared to CO2 and methane, it is even more effective at trapping heat. Pound for pound, nitrous oxide's impact on global warming is over 300 times greater than that of CO2 over a 100-year period.
Water vapour (H2O)
Water vapour is the gaseous form of water (H2O) present in the Earth's atmosphere. It is created naturally when liquid water evaporates due to heat or when ice changes directly into vapour through a process called sublimation.
Water vapour is a critical component of the Earth's climate system and weather patterns.
Water vapour has the ability to absorb and emit heat energy, which makes it a GHG. Its concentration in the atmosphere is influenced by temperature – warmer air can hold more water vapour, while cooler air can hold less.
This relationship between temperature and water vapour content contributes to the Earth's weather patterns and climate variability.
Perfluorocarbons (PFCs) are a group of synthetic compounds composed of carbon and fluorine atoms. They are entirely human-made and do not occur naturally in the environment.
PFCs are known for their unique properties, including high thermal stability, chemical inertness and electrical insulation, which make them valuable in various industrial applications.
PFCs are potent GHGs with a high global warming potential, thousands of times greater than that of CO2 over a relatively short time frame..
They are highly stable and can persist in the atmosphere for a long time, sometimes lasting for centuries. This extended atmospheric lifespan allows them to accumulate and contribute to long-term climate impacts.
Due to their potent greenhouse effects and environmental persistence, many PFCs have been targeted for phase-out under international agreements like the Kyoto Protocol and the Montreal Protocol. Efforts to mitigate the impact of PFCs include seeking alternatives in industrial processes and consumer products.
Sulphur hexafluoride (SF6)
Sulphur hexafluoride (SF6) is a synthetic gas consisting of one sulphur atom and six fluorine atoms. It is a human-made compound that does not occur naturally in the environment.
SF6 is known for its remarkable insulating and arc-quenching properties, making it widely used in the electrical industry for various applications.
Similar to PFCs, SF6 is a potent GHG with a high global warming potential, thousands of times greater than that of CO2 over a relatively short time frame.
It also has a long atmospheric lifespan and can remain in the atmosphere for centuries, contributing to its cumulative effect on global warming.
As a result, there has been growing international concern about SF6 emissions. The electrical industry is actively seeking alternatives to SF6 with similar insulating properties, but have lower environmental impacts.
Nitrogen trifluoride (NF3)
Nitrogen trifluoride (NF3) is a synthetic chemical compound composed of one nitrogen atom and three fluorine atoms. It is a human-made substance that does not occur naturally in the environment.
NF3 is primarily used in industrial processes, particularly in the electronics and semiconductor manufacturing sectors.
NF3 has a relatively high global warming potential, but lower than some other GHGs like SF6. Similar to other fluorinated gases, NF3 has a long atmospheric lifespan and it can persist in the atmosphere for several centuries.
Concerns about NF3 emissions and its potential environmental impact have led to efforts to monitor and report emissions, but it is not as extensively monitored as other GHGs, which poses challenges in accurately quantifying its contribution to climate change.
Hydrofluorocarbons (HFCs) are a group of synthetic chemical compounds composed of hydrogen, fluorine and carbon atoms. They are created by replacing hydrogen atoms in hydrocarbons with fluorine atoms.
HFCs are commonly used as refrigerants in air conditioning, refrigeration, and heat pump systems. They are also used in aerosol propellants, foam-blowing agents and as solvents.
HFCs were developed as alternatives to ozone-depleting substances like chlorofluorocarbons (CFCs) and halons due to their lower impact on the ozone layer. This was because, unlike CFCs, HFCs do not contain chlorine or bromine, which are harmful to the ozone layer. As a result, they were initially considered a more environmentally friendly option.
However, although HFCs do not harm the ozone layer, many have a high global warming potential – often much higher than that of CO2, contributing significantly to climate change when emitted into the atmosphere.
Efforts have been focussed on transitioning to other, less harmful, alternatives. For example, the Kigali Amendment to the Montreal Protocol aims to phase down the use of high global warming potential HFCs to mitigate their impact on climate change.
To find out more about how GHGs are measured, check out our comprehensive guide on carbon accounting.