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Introductory guide to greenhouse gases

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In the 1820s, Joseph Fourier calculated that, based on the distance between the Sun and the Earth, the average temperature should be minus 18 degrees Celsius. This is far from the actual global temperature average that is estimated at 13.9 degrees Celsius. So, Fourier hypothesised about the Earth’s capacity to absorb and reemit heat radiation in the atmosphere. This was the foundation of the greenhouse effect. This theory was later tested with various gases. When the experiments were inconclusive on gases found in large concentrations, such as oxygen and nitrogen, they were tried on trace gases such as water vapour and carbon dioxide. Despite their low concentrations, these greenhouse gases had a notable capacity for absorbing and reemitting infrared radiation.

Greenhouse gas effect
Figure 1: The greenhouse effect

What are greenhouse gases?

Greenhouse gases (GHGs) are naturally occurring gases that trap heat radiation, preventing it from exiting the atmosphere. This constitutes the greenhouse effect. GHGs are essential to maintaining a habitable temperature range for humans on Earth.

The principle GHGs are carbon dioxide, methane and nitrous oxide. These are naturally occurring but heightened by human activity, in particular industrial processes. There is debate about whether water vapour can also be classified as a greenhouse gas, as its concentration and therefore effect depends on temperature and meteorological conditions.

The emission of anthropogenic GHGs, or gases released by human activity, means that we are increasing the atmosphere’s absorption of infrared radiations. This in turn means that the atmosphere’s temperature rises, as these warming rays are not exiting the atmosphere.

However, the climatological impact of GHGs is not equal. To measure this, the Global Warming Potential is used as a relative measure of the amount of heat that gets trapped in the atmosphere by a specific GHG. It compares the amount of heat trapped by a certain mass of the gas in question to the amount of heat trapped by the same mass of carbon dioxide (CO2). Most commonly, the GWP of a GHG is calculated withing a 100-year range.

What is a greenhouse gas
Figure 2: Concentrations and GWPs of the main greenhouse gases

Why the focus on carbon dioxide?

When discussing climate change, we most often refer to carbon dioxide and net zero carbon emissions. However, CO2 is a relatively small part of the atmosphere’s total average composition, and we can see that its GWP is the lowest of all GHGs. So why is CO2 so important?

CO2 is a GHG that has continuously been released into the atmosphere by human activities for thousands of years. More recently, however, the rate at which CO2 has been released has spiked. This is a result of developed countries’ increasing use of fossil fuels since the Industrial Revolution. During this time, the use of coal and gas, which are major sources of CO2 when burned, allowed for a rapid economic expansion. NASA has estimated that atmospheric CO2 concentrations increased by 47% from the 1850-1900 baseline. This means the concentration has risen from nearly 280 parts per million (ppm), to 412 ppm in 2019. From 2000 to 2020 alone, there was an 11% increase.

What makes CO2 different is its’ atmospheric lifetime, also known as longevity. The table below shows the longevity of all the recognised GHGs, except for CO2. CO2’s lifetime is thought to be 300 to 1000 years, depending on rates of uptake by different removal processes on land and sea. Approximately 50% of CO2 is reported to be absorbed by oceans in a matter of decades. 30% is believed to be absorbed over a few centuries. An additional 20% is thought to remain in the atmosphere for thousands of years after it has been emitted.

A note on the sites of GHGs

Most discussions of GHGs imply atmospheric concentrations. There are also oceanic concentrations of CO2, and these can cause coral bleaching and biodiversity loss, most notably. Dissolved oceanic GHGs are not as much of a concern for rising temperatures, however.

What are CO2e and carbon credits?

The GWP has unified the way we measure climate change. We are able to calculate the impact of a GHG in relation its’ CO2 equivalent (CO2e). This allows for comparison of the climatological impact of polluting activities, regardless of the GHG being emitted. It is often used in kilogrammes or tonnes.

Measuring emissions in tCO2e is a standard. You might have seen tCO2e come up in your One Two Zero workbook or calculator. At Plannet Zero, we help SMEs measure their carbon footprint, as well as planning for a reduction strategy, and compensating emissions. If you already work with us.  Compensation is done through the purchase of carbon credits. One carbon credit is equal to one tonne of CO2e. For more information on how to start your carbon journey, please contact a member of our team, or read about carbon neutrality and net zero here.


Edited by Tiffany Cheung

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