Greenhouse gases

But greenhouse gases also pose a problem. Thanks to human activity since the Industrial Revolution, in particular the combustion of fossil fuels such as gasoline or coal, the abundance of GHGs has been steadily rising, trapping ever more heat in the atmosphere and causing global warming and climate change. 

Greenhouse gases also occur naturally, however, as volcanoes erupt or humans breathe. Others, meanwhile, are only produced due to human activities, and some of these – such as sulphur hexafluoride – are the most potent when it comes to trapping heat near the Earth’s surface.

A report from the World Meteorological Organization, which is an agency of the United Nations, shows that concentrations in the atmosphere of greenhouse gases such as carbon dioxide have never been higher, and mankind’s carbon footprint is set to get bigger. Other studies, meanwhile, show that the emissions of greenhouse gases in countries such as the United States are actually falling, thanks to greater efficiency and structural changes to the economy, including greater use of renewable energies. 

What are greenhouse gases? 

Greenhouse gases are gases such as carbon dioxide (CO₂) that are present in the Earth’s atmosphere, and that trap heat from the sun. Some of these gases are released into the atmosphere due to natural processes on Earth, such as the eruption of a volcano, when organic matter decomposes, or when humans and animals breathe. But others are pumped into the air due to human activities, such as the burning of fossil fuels and the use of fertilisers. What’s more, several greenhouse gases, known as fluorinated gases, rarely occur naturally on Earth at all, and are only present in the atmosphere because of humans. All greenhouse gases, however, stop heat from the sun from leaving the atmosphere, keeping the temperature on Earth at an average 14ºC (57ºF). If it weren’t for these gases – which literally act like the walls of a greenhouse, hence the name – temperatures would fall to as low as 18ºC (-0.4ºF), which would be too cold for life to survive on the planet.

How abundant are GHG in the atmosphere?

Since the advent of machinery, steam power and factory production during the Industrial Revolution in the 18th century, human activities have seen ever-larger quantities of greenhouse gases present in the Earth’s atmosphere. Some of the gases can remain in the Earth’s atmosphere for millenia, while others will disappear in a matter of just years. They become mixed up in the atmosphere due to wind patterns, meaning that their concentration is similar throughout the globe. Most have a long lifetime.

For some context, the most abundant non-greenhouse gases in the Earth’s atmosphere are nitrogen, which accounts for 78% of the total (excluding water vapour), followed by oxygen, which makes up 21% of the total. The most-common greenhouse gas, meanwhile, is carbon dioxide, which accounts for 0.042% of the total gases in the atmosphere. While that percentage may sound relatively low, the amount of carbon dioxide in the atmosphere has greatly increased due to human activities such as burning fossil fuels.

Deforestation has also played a part, given that fewer trees means less CO₂ is absorbed from the atmosphere for photosynthesis. What’s more, carbon stored in trees is released back into the atmosphere as CO₂ when they are felled. The end result is that more heat from the sun is trapped in the atmosphere, something that worsens global warming.

According to a report from the World Meteorological Organization, in 2023 the global averaged concentrations of carbon dioxide reached a full 51% above pre-industrial levels for the first time ever. What’s more, the global atmospheric concentrations of carbon dioxide, methane and nitrous oxide have never been higher in the last 800,000 years on Earth, according to figures from the United States Environmental Protection Agency (EPA).

The abundance of a greenhouse gas in the atmosphere can also be measured according to the number of greenhouse gas molecules per billion molecules of air, and is expressed in parts per billion (ppb). In the case of methane, for example, the EPA reports that the concentration in the atmosphere has more than doubled since before the Industrial Era, to greater than 1,900ppb in 2023. And in the case of nitrous oxide, the mean abundance is low, coming in at 336.9 ppb in 2023. But it is much more effective at trapping heat than carbon dioxide, and its abundance compared to pre-industrial times has increased by some 25%. 

The most abundant greenhouse gas in the atmosphere, however, is water vapour. This greenhouse gas – basically, water in a gas state – is nearly always present in the atmosphere, and accounts for 4% of total volume. The concentration of H₂O in the air is dependent on temperature as well as weather conditions, rather than being directly affected by human activities. 

What are the main greenhouse gases?

The main greenhouse gases in the Earth’s atmosphere are carbon dioxide, methane, nitrous oxide and water vapour. 

• Carbon dioxide (CO₂) is the main contributor to climate change, and is released from the burning of fossil fuels, as well as via natural processes such as animals and humans breathing, volcanic eruptions and plant respiration.
• Methane (CH₄) is released naturally into the environment as organic matter decomposes, but its presence in the atmosphere is also due to human activities such as cattle and rice farming, landfill waste sites and the production of oil and gas.
• Nitrous oxide (N₂O) is produced when fossil fuels or biomasses are burned, as well as when commercial and organic fertilisers are used, and when nitric acid is produced. 
• Water vapour is nearly always present in the atmosphere, and increases due to meteorological conditions such as high heat and humidity.

Carbon Dioxide Emissions

Global energy-related emissions of carbon dioxide increased by 1.1% in 2023, according to data from the International Energy Agency (IEA). An extra 410 million tonnes of the gas were pumped into the atmosphere compared to the year before, setting a new record high of 37.4 billion tonnes of global emissions during the year. CO₂ is the primary greenhouse gas that is emitted through human activities, although it does come from a wide range of natural sources too, such as volcanoes erupting, humans and animals breathing and plant respiration. Carbon dioxide is removed from the atmosphere as part of the so-called biological carbon cycle, which is when it is absorbed by plants.

Some carbon dioxide is absorbed quickly, but some will remain in the atmosphere for thousands of years. This is due to the slow process that sees carbon transferred to ocean sediments. It can also be removed from the atmosphere thanks to technologies known as Carbon Capture and Sequestration (CCS). This kind of process captures CO₂ from a coal power plant before it can reach the atmosphere, transporting it deep underground via a pipeline into an area such as an abandoned oil field, where it can be stored without seeping out.

  SEE INFOGRAPHIC: Total greenhouse gas emissions in 2023 [PDF]

The main sources of CO₂ emissions in the United States

Transportation is by some measure the biggest economic sector when it comes to the emission of carbon dioxide in the United States. According to the Energy Information Administration (EIA), the burning of gasoline, diesel and other fossil fuels to move both people and goods accounted for 39% of total CO₂ emissions in 2023.

The second-biggest source was electricity generation. Some 30% of CO₂ emissions in the US in 2023 came from the combustion of fossil fuels such as coal, oil and natural gas to power homes, businesses and industry. Not all of these fuels are the same, however, with coal emitting more CO₂ to produce the same amount of electricity compared to the use of natural gas or oil.

  SEE INFOGRAPHIC: US energy-related CO₂ emissions by sector, 1990-2023 [PDF]

Then comes industry, which accounted for 20% of US carbon dioxide emissions in 2023. Industrial processes often involve the burning of fossil fuels to generate electricity, but other processes in the same sector cause emissions of CO₂ from chemical reactions, such as producing cement, iron and steel, and chemicals.

Methane (CH₄)

Methane accounts for around 30% of the increase in global temperatures since the start of the Industrial Era, according to the IEA. Its lifetime in the atmosphere is much shorter than that of carbon dioxide, at around 12 years compared to several centuries, but it is much more efficient at trapping radiation compared to CO₂. This means that its impact on global warming is 28 times greater than that of CO₂ over a 100-year period. 
 

Main methane emission activities 

On a global scale, somewhere between 50 and 65% of total methane emissions come from human activities, including natural gas and petroleum systems, landfill, coal mining and agriculture.

Nitrous oxide (N₂O)

Nitrous oxide accounted for 6% of all greenhouse gas emissions from human activities in 2022, according to the EPA. The gas stays in the atmosphere for around 120 years before it is either destroyed by chemical reactions, or is removed by a sink. It is naturally present in the Earth’s atmosphere, and is not just in the air due to human activities. 
 

Main nitrous oxide emission activities

On a global scale, 40% of N₂O emissions come from human activities, such as land use, industry and waste treatment, among others. 

• Agricultural soil management accounted for 68% of nitrous oxide emissions in 2023, according to Statista. The gas is produced when certain fertilisers are used, as well as when organic matter from farming is burned. Forest and grassland fires also emit the gas. 
• Processes including industrial activities such as nitric acid production, the manufacture of semiconductors and the use of nitrogen-based compounds in plastics are collectively the second-biggest emitter, at 11% of the total. 
• The power industry is the third-biggest emitter of nitrous oxide, at 5% of the total according to Statista, with a varying amount of N₂O emitted depending on the type of fuel and combustion. 

Fluorinated gases 

Fluorinated gases, in general, are the most potent and longest lasting of greenhouse gases. What’s more, they are produced almost entirely from human activities, and have very few natural sources. The four main types of fluorinated gases are hydrofluorocarbons (HFCs), sulphur hexafluoride (SF₆), perfluorocarbons (PFCs) and nitrogen trifluoride (NF₃). Some of these GHGs can last for thousands of years, and in many cases are only removed from the Earth’s atmosphere when they are destroyed by sunlight in the upper levels. Due to their long lifetimes, they are well mixed in the atmosphere, and their levels are similar throughout the globe.
 

Main source of fluorinated gas emissions 

Fluorinated gases are mostly emitted into the atmosphere through their use as refrigerants, as well as industrial processes such as the manufacturing of aluminium and semiconductors. 

• Substitution of ozone-depleting substances is a major source of fluorinated GHG emissions, with hydrofluorocarbons used for a variety of purposes, including solvents, fire retardants, aerosol propellants and refrigerants. The latter accounts for most of the emissions, such as from air-conditioning units. These gases were developed to replace chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), which were found to be creating a hole in the ozone layer. 
• Industry is also a big emitter of fluorinated gases, with perfluorocarbons released as a byproduct of aluminium production. They are also used in the manufacture of semiconductors. 
• The transmission and distribution of electricity also emits fluorinated GHGs, with sulphur hexafluoride employed as an insulating gas in electrical equipment such as circuit breakers. SF₆ is the most-potent greenhouse gas that the Intergovernmental Panel on Climate Change has ever evaluated, with a Global Warming Potential (GWP) of 23,500, compared to the baseline of one for carbon dioxide. 

Main consequences of GHG 

The Earth is often referred to as the "Goldilocks" planet, a concept used in the scientific field to describe planets that are neither too hot nor too cold, but rather maintain a temperature that allows life to thrive. If it weren't for the greenhouse effect of gases that trap heat in the atmosphere, the planet would be too cold to sustain life. Greenhouse gases are, therefore, an essential part of the atmosphere.
 

  SEE INFOGRAPHIC: Which countries generate the most greenhouse gases [PDF]

There was also a sharp fall in emissions from 2019 to 2020, although that was mainly due to the impact of the Covid-19 pandemic on travel and economic activity. But the return to normality once the global health crisis was over saw an increase of 5.7% from 2020 to 2022, as CO₂ emissions rose due mainly to increased fossil fuel combustion.

From 1990 to 2022, meanwhile, emissions of carbon dioxide in the United States did actually fall, by 2%, as did methane (down 19%) and nitrous oxide (5%). According to the EPA this is thought to be due to increased energy efficiency, as well as structural changes to the economy. The emissions of fluorinated gases, however, increased by 58% over the same time period thanks to their use in commercial, industrial and household sectors. 

U.S. greenhouse gas emissions and trends 

According to a report released in November 2023 by the World Meteorological Organization, the abundance of GHG in the atmosphere hit a new record in 2022, and the rising trend looks set to continue, despite efforts by governments and international bodies to reduce emissions and improve sustainability. The report found that global averaged concentrations of CO2 were a full 50% above levels from the pre-industrial era in 2022, and continued to grow in 2023. Concentrations of methane and nitrous oxide increased.

However, in terms of emissions, there is evidence of a downward trend in the United States. According to the EPA, GHG emissions fell in 2022 by 3% compared to 1990. Compare that to a high of 15.2% above 1990 levels in 2007. 

  SEE INFOGRAPHIC: U.S. Greenhouse gas emissions by gas, 1990-2022 [PDF]

There was also a sharp fall in emissions from 2019 to 2020, although that was mainly due to the impact of the Covid-19 pandemic on travel and economic activity. But the return to normality once the global health crisis was over saw an increase of 5.7% from 2020 to 2022, as CO2 emissions rose due mainly to increased fossil fuel combustion.

From 1990 to 2022, meanwhile, emissions of carbon dioxide in the United States did actually fall, by 2%, as did methane (down 19%) and nitrous oxide (5%). According to the EPA this is thought to be due to increased energy efficiency, as well as structural changes to the economy. The emissions of fluorinated gases, however, increased by 58% over the same time period thanks to their use in commercial, industrial and household sectors. 

  SEE INFOGRAPHIC: How to reduce greenhouse gases [PDF]