Since the mid-nineteenth century, we have been aware that greenhouse gases affect the Earth’s climate. In 1856, an American woman, Eunice Foote, wrote that changing amounts of carbon dioxide (CO2) in the atmosphere could change the climate. Therefore, this concept is nothing new. Yet, we continue to emit more greenhouse gases into the atmosphere every day.

What is a greenhouse gas?

Greenhouse gases contribute to the greenhouse effect. This means that in the atmosphere, they absorb heat emitted from the Earth’s surface and prevent it from escaping into space. Some of these gases, such as CO2, methane and nitrous oxide, are naturally found in low concentrations in the atmosphere. They help make life on Earth possible by trapping heat. However, humans have significantly increased the number of greenhouse gases in the atmosphere, leading to dangerous levels of global warming.

The longevity of greenhouse gases

Each of the greenhouse gases can remain in the atmosphere for varying lengths of time. Some may stay there for just a few years. Others can last for thousands of years. All of them exist in the atmosphere sufficiently long enough to mix evenly with the other gases there. As a result, the amount of each greenhouse gas in the atmosphere is roughly the same wherever measurements are taken around the world. Proximity to sources of emissions makes little difference.

What are the 7 types of greenhouse gases?

Greenhouse gases: Carbon dioxide (CO2)

The most prevalent greenhouse gas is carbon dioxide. Some of it comes from natural sources. These include “outgassing from the ocean, decomposing vegetation and other biomass, venting volcanoes, naturally occurring wildfires, and even belches from ruminant animals”. However, the atmospheric concentration of carbon dioxide has increased by 50 per cent since the Industrial Revolution. This is not due to natural sources.

As of 2021, there are 419 CO2 molecules for every one million molecules of air. This is the highest level of carbon dioxide in the air for the past four and a half million years. Humans are directly responsible for this enormous increase. 89 per cent of human-caused carbon dioxide emissions come from burning fossil fuels, like coal, oil and natural gas. The remaining 10.6 per cent is from land-use changes. This includes deforestation and fires.

Why is carbon dioxide so bad?

Carbon dioxide is the principal driving force behind climate change. In part, this is because it stays in the atmosphere for an extremely long time – between 300 and 1,000 years. Consequently, carbon dioxide that was released by burning coal in the 1850s can still cause global warming today. As we continue to use fossil fuels, more of this greenhouse gas is accumulating in the atmosphere.

Greenhouse gases: Methane (CH4) 

Methane is the second-most common type of greenhouse gas. It is also the second-most important driver of climate change after carbon dioxide. However, its global warming potential is 86 times that of CO2 across the first 20 years it is in the atmosphere. Luckily, CH4 does not stay in the atmosphere for long. After about a decade, it converts to carbon dioxide. Nevertheless, over a 100-year period, methane’s global warming potential is 28 times higher than carbon dioxide.

Cows in a cattle shed

Similar to carbon dioxide, methane comes from both natural and artificial sources. Humans are responsible for 60 per cent of all methane emissions. Much of these emanate from agriculture, particularly raising cattle and other ruminant livestock. Growing rice in flooded paddy fields also causes both methane and nitrous oxide emissions. 

Fossil fuels and methane emissions

Fossil fuels are another significant source of methane emissions, particularly natural gas. For example, Australia’s methane emissions are rising due to the expansion of its natural gas industry. The expanding use of natural gas has contributed to an atmospheric concentration of methane two and a half times higher in 2019 than in pre-industrial times. This is no surprise, given that natural gas is almost entirely methane. CH4 can form as much as 99 per cent of the gas used for cooking, heating houses and industrial processes.

The natural gas industry’s greenhouse gas emissions

The natural gas industry fuels global warming as methane leaks from its extraction, transportation and end-use. The Environmental Protection Agency in the USA estimates a leakage rate of 1.4 per cent. However, studies coordinated by the Environmental Defence Fund found a rate of 2.3 per cent. That is a 60 per cent increase. It represents at least 11.8 million tonnes of wasted gas, enough to supply 10 million homes for an entire year and is worth about USD $2 billion.

Greenhouse gases: Nitrous oxide (N2O)

Nitrous oxide exists in the atmosphere for 114 years on average. As this falls between carbon dioxide and methane, N2O has been described as a ‘middle-ground’ amongst the greenhouse gases. But, one kilogram of N2O has 300 times the global warming potential of a kilogram of carbon dioxide. Both its potency and relative longevity in the atmosphere make nitrous oxide a serious threat to our climate.

Sources of N2O

The majority of nitrous oxide emissions come from agriculture. In the decade up to 2016, agricultural practices caused almost 70 per cent of global N2O emissions. Almost 110 million tonnes of nitrogen are spread over crops each year as a synthetic fertiliser. Another 100 million tonnes of nitrogen are put onto farmland as manure from livestock. The nutrient helps plants to grow more abundantly. However, it also releases N2O.

Only about half of the nitrogen from fertilisers placed onto soil gets taken up by plants. The remaining half either washes away in groundwater or bonds with oxygen to become nitrous oxide or other gases. Largely as a result of this agricultural practice, human-caused nitrous oxide emissions have risen by 30 per cent over the past 40 years. 

Even making nitrogen fertiliser has a heavy environmental cost. Producing it uses one per cent of the world’s energy use and creates 1.4 per cent of carbon dioxide emissions. In 2018, atmospheric concentrations of N20 reached 331 parts per billion. That is 22 per cent higher than pre-industrial levels.

Greenhouse gases: Fluorinated gases

Fluorinated gases include perfluorocarbons (PCFs), sulphur hexafluoride (SF6) and hydrofluorocarbons (HFCs). HFCs are used for multiple different applications, including refrigerants in refrigeration and air-conditioning; as blowing agents for foam; and in fire extinguishers and aerosols. The electronics, cosmetic and pharmaceutical sectors most commonly use PFCs. Meanwhile, SF6 finds utility as an insulating gas and during the production of magnesium and aluminium. 

Contribution to global warming

Unlike the other greenhouse gases mentioned, fluorinated gases do not come from natural sources. They are all human-made. Fluorinated gases include a whole family of substances. In many cases, they are used as a substitute for ozone-depleting alternatives as they do not damage the ozone layer. But, they can have significant global warming potential. Some trap up to 23,000 times as much heat as an equivalent quantity of carbon dioxide.

Not only are fluorinated gases some of the worst greenhouse gases for their global warming potential, but they also have long atmospheric lifetimes. PFCs and SF6 can last for thousands of years and are only removed when sunlight in the upper atmosphere destroys them. Due to their extremely harmful properties, a global phasedown of HFCs was agreed in 2018 under the Kigali Amendment. 198 international parties have agreed to gradually reduce the production and use of these dangerous greenhouse gases. 

Which is the most dangerous GHG? 

A case could be made that any of these greenhouse gases are the most dangerous for our planet. For example, CO2 is the most dangerous because it is the most common and lasts a long time. Estimates are that CO2 is responsible for about three-quarters of total emissions when all greenhouse gases are converted to CO2 warming equivalents.

Graph showing average annual greenhouse gas emissions

Equally, methane could be described as the most dangerous greenhouse gas because it is the second-most prevalent and has a far greater global warming potential. While it only lasts in the atmosphere as methane for a decade or so, it subsequently converts to CO2. Consequently, it continues to trap heat for hundreds of years.

Likewise, both fluorinated gases and nitrous oxide have considerably worse global warming potentials than CO2. NO2 can last in the atmosphere for well over a century, and it traps heat at a rate 300 times that of CO2. Fluorinated gases can be even more damaging than this. For instance, HFC 23 contributes 14,800 times as much to warming the planet than equivalent amounts of CO2. It also lasts in the atmosphere for about 228 years. This makes it an extremely potent greenhouse gas.

What can we do to reduce greenhouse gas emissions?

Fossil fuel consumption

By far, the biggest source of greenhouse gas emissions is the ongoing combustion of fossil fuels. 93 per cent of CO2 emissions come from using fossil fuels, mainly for generating electricity and heat, transportation, manufacturing and consumption. Transitioning our energy generation away from coal, oil and natural gas, and replacing these dirty sources with renewable alternatives, is crucial for reducing our emissions.

Similarly, fossil fuels are also responsible for other emissions, namely methane. The entire journey of natural gas, from production to end-use, leaks methane. Australia’s natural gas industry causes about one-fifth of the country’s methane emissions. However, this figure is almost certainly an underestimation. 

Coal mining also produces vast amounts of methane. The Bowen Basin releases about 1.6 million tonnes of methane annually. Due to its potency, that is on par with 134 million tonnes of CO2 or the emissions of a mid-sized European country. Global warming will continue to increase until we stop using these harmful fossil fuels.

Agriculture and industries

Besides fossil fuel consumption, agricultural practices need to change to reduce global greenhouse gas emissions. Raising livestock creates 7.1 gigatonnes of CO2-equivalent emissions worldwide each year. That represents 14.5 per cent of all human-caused greenhouse gas emissions. Decreasing meat and dairy production would make a huge difference. Cutting these products out is the single-biggest way an individual can reduce their environmental impact.

Similarly, it is extremely important to stop producing fluorinated gases. Aluminium and semiconductor manufacturing are some of the key emitters of these potent gases, though they are found in a wide range of substances. The anticipated phasedown of HFCs will prevent 80 gigatonnes of CO2 equivalents until 2050. It is extremely important that all signatories to the Kigali Amendment keep their word and cut out these extremely potent greenhouse gases.