Greenhouse Gases

Amazingly, without greenhouse gases, Earth would be around 30 °C colder and life would be confined to the oceans! Contributing to the greenhouse effect, these gases trap heat within the atmosphere, warming the earth’s system in the process. In modern times, human activity has resulted in massive amounts of these greenhouse gases being released into the atmosphere, which has been one of the major factors contributing to global warming.

We'll learn about greenhouse gases, their sources, the greenhouse effect, and their impact on global warming, and we'll see some examples.

Greenhouse Gases Definition

Greenhouse gases are substances which absorb infrared radiation reflected from the Earth’s surface and release this radiation back towards the earth, warming the planet in the process. These gases act in the same way as the glass walls of a greenhouse.

The Names of Greenhouse Gases

Some examples of greenhouse gases are:

• Water vapour
• Carbon dioxide
• Methane
• Nitrous oxide
• Chlorofluorocarbons

The Greenhouse Effect

The greenhouse effect is a natural phenomenon which has allowed terrestrial life to thrive. Without it, the terrestrial environment would be too cold and life would be restricted to the sea. The natural greenhouse effect occurred because of ocean water evaporating and forming water vapour, which is a greenhouse gas. The Earth’s natural internal variability caused there to be enough water vapour in the atmosphere to create a greenhouse effect large enough that life on our planet could be sustained. The greenhouse effect maintains an average temperature of 15 °C on Earth.

This is how the greenhouse effect works (Fig. 1):

• The majority of the solar radiation from the sun passes through the atmosphere and is absorbed by the Earth’s surface, warming its temperatures in the process.

• Some radiation is absorbed by greenhouse gases and re-emitted, while other radiation is reflected by the atmosphere or the Earth’s surface.

• Some of the infrared radiation reflected by the Earth’s surface will be absorbed by greenhouse gases in the atmosphere and re-emitted in all directions, including back towards the Earth, resulting in further warming of the surface.

• Greenhouse gases will re-emit infrared radiation in all directions, meaning that some radiation will be sent back into space while some will warm the atmosphere.

Modern Global Warming

Since the Industrial Revolution of the 1800s plus developments in technology and agriculture, masses of unwanted greenhouse gases have been released into the atmosphere. The release of these gases has added to the greenhouse effect, resulting in the evaporation of more ocean water releasing water vapour into the atmosphere, which amplifies the greenhouse effect. This has led to global temperatures increasing to unprecedented levels. There are certain negative feedbacks which the Earth exhibits to counteract these changes, such as increased cloud formation, but these are not enough to halt global warming alone. Anthropogenic greenhouse gas emissions must be massively reduced or stopped completely to reduce the impacts of climate change on the planet.

Greenhouse Gases Diagram

The following diagram shows the greenhouse effect.

Fig. 1 - The Earth's greenhouse effect.

Sources of Greenhouse Gases

The sources of greenhouse gas emissions are diverse, but the vast majority stem from burning fossil fuels. Fossil fuels are non-renewable, carbon-rich deposits. When burned, they release energy - but also carbon dioxide and other greenhouse gases.

Fig. 2 shows the greenhouse gas emissions by main sectors in the UK for the year 2019. For example, it shows that transport, electricity and heat, and buildings, are the three most important sources of these emissions. The burning of fossil fuels is part of most of these sectors’ activities, as these are used for transport, production of some of the electricity and heat, construction, industry processes, and so on.

Fig. 2 - Greenhouse gas emissions by sector for the United Kingdom for the year 2019 (industry here includes mostly chemical and cement production). Source: Ritchie et al. 2020.

Examples of Greenhouse Gases

Here are some of the greenhouse gases you should be aware of:

Carbon Dioxide

Carbon dioxide is the most well known of the anthropogenic greenhouse gases, and for good reason too. It contributes to 60% of human-induced greenhouse gas emissions. Carbon dioxide occurs naturally in volcanoes, hot springs and glaciers. Before the exponential increase in carbon dioxide releasing processes (around the time of the industrial revolution in 1850), atmospheric carbon dioxide levels were kept in check by photosynthetic organisms and the dissolution of CO₂ into the oceans. Here are some of the ways that humans release carbon dioxide into the atmosphere:

• Fossil fuels: the combustion of fossil fuels (coal, gas and oil) for energy releases massive amounts of carbon dioxide into the atmosphere. Electricity, industry and transportation all burn fossil fuels for energy.

• Deforestation: deforestation not only releases carbon dioxide via the combustion of trees but also actively reduces the producer population of planet Earth, so less carbon dioxide is taken up.

• Agriculture: the heavy machinery and transportation vehicles involved in agriculture burn fossil fuels for energy.

Water Vapour

Water vapour is actually the largest contributor to global warming out of any of the greenhouse gases, it's just lesser talked about seeing as humans do not cause water vapour emissions. The original development of the greenhouse effect, and the subsequent inhabitable planet, was attributable to an increasing water vapour content in the atmosphere. With the unprecedented levels of warming we are currently experiencing, more and more ocean water is evaporating and forming atmospheric water vapour. This water vapour is adding considerably to the greenhouse effect by sitting in the lower atmosphere and re-releasing infrared radiation towards the Earth from here. Water vapour is responsible for the Earth’s negative feedback to warming in cloud formation, which reflects solar radiation back into space.

Methane

Fig. 3 - Example of a peat bog swamp. Peat bogs release huge amounts of GHGs.

Nitrous Oxide

Nitrous oxide is an extremely dangerous greenhouse gas and possesses a warming ability almost 300 times greater than carbon dioxide, whilst also remaining in the atmosphere for over 100 years. Nitrous oxide occurs naturally in the ground and is released as a byproduct in the combustion of fossil fuels, in industrial processes, and in agriculture. The majority of nitrous oxide release comes from agriculture, as nitrogen-containing fertilisers are used worldwide to supplement infertile soils with the nutrients required for plant growth. This is a major problem because it is very difficult to reduce agricultural processes without putting food security at risk at the same time.

Ozone

Ozone is classified as the third most environmentally impacting greenhouse gas, after carbon dioxide and methane. Ozone forms in the upper layers of the atmosphere when ultraviolet radiation will cause an oxygen molecule to break into two oxygen atoms, and then subsequently react with another oxygen molecule, forming ozone (O₃). Nitrogen oxides and volatile organic compounds are often involved in ozone formation. The combustion of fossil fuels, fertilisers, vehicles and industrial methods can all lead to the formation of ozone. Ozone is present in two parts of the atmosphere: the stratosphere and the troposphere. Occurring naturally in the stratosphere, here it forms the ozone layer and is harmless. It is in the troposphere where ozone acts as a greenhouse gas.

Fig. 4 - A simple chemical equation for the formation of ozone. UV = ultraviolet radiation.

Role of the Ozone Layer in the Atmosphere

The ozone layer is situated in the upper stratosphere, roughly 15 miles above Earth’s surface. This layer protects the Earth from harmful ultraviolet rays from the sun. These rays will split oxygen molecules in two, forming oxygen atoms, which then react with existing oxygen molecules to form ozone.

Ozone Layer Depletion

The depletion of the ozone layer is a problem which we face today, and is exacerbated due to ozone-depleting substances released from human activity. Ozone-depleting substances include chlorofluorocarbons (CFCs), which are industrial solvents, and methyl chloroform, which is used in a variety of industrial chemical processes.

Hopefully, this article has given you more information on what greenhouse gases are and what their roles are. Remember that greenhouse gases actually help sustain life and are naturally occurring, however, human activities add to them to the point where they become detrimental.