Why Do Gases Act As Greenhouse Gases?

One of my favourite things about science is learning about something which you can then apply to real life situations. Why is this table hard but this plastic bag soft? How can certain gases be toxic while others are completely harmless? What can I do to trick sniffer dogs on my way through security? Unfortunately, from my experience, learning science in an academic environment is taught from a position which is quite removed from real life situations. 

For example, in one of my classes I was taught about gases being infrared (IR) active – meaning they can absorb IR radiation – without any kind of context as to the consequences of this property (outside of how it can be used to identify molecules using lab equipment anyway). It wasn’t until over a year later when it actually fell into place as I was contemplating the climate crisis: being infrared active is the property which makes gases act as greenhouse gases. 

Carbon dioxide is the most well known greenhouse gas

As carbon dioxide is by far the most popular greenhouse gas – and as we all know, popularity is the only thing which really matters – I’m going to use this as the example to try and explain how this works. 

The structure of carbon dioxide – which can also be written as CO2 – is one carbon atom and two oxygen atoms. It is a linear molecule i.e shaped like a pole, or a straight line. Both the oxygens are attached to the carbon but they are not attached to each other, so it goes oxygen-carbon-oxygen all in a line. In a CO2 molecule, the bonds between the atoms means that the atoms are sharing electrons. So the carbon is sharing electrons with both oxygens, as they are bound to one another. However, as the oxygens are not, they do not share electrons.  

A drawing of the structure of carbon dioxide. The red hemi-spheres are oxygen and the black section is carbon. Source: Wikimedia Commons.

One of the properties of an oxygen atom is that it attracts electrons more than a carbon atom does. It’s also worth mentioning here that electrons have a negative charge. The result of this is that both the oxygens have a slightly negative charge, whereas the carbon has a slightly positive charge. This happens because the oxygens attract the bonding electrons towards them and away from the carbon.

A dipole moment is when two sides of a molecule have an opposing charge i.e one end is positive and one end is negative. Therefore, as both ends of the carbon dioxide have a negative charge there is no overall dipole moment. Both ends are negative because we have oxygen-carbon-oxygen and both of the oxygens have a slight negative charge.

IR active molecules absorb heat

So what I described to you above is the sort of the theoretical structure of a carbon dioxide molecule. In reality, molecules are able to move around. This means that the bonds can stretch, or they can bend, and this is called a vibration. 

A molecule is said to be IR active if there is a change in the dipole moment during a vibration. So, if the CO2 molecule develops a dipole upon stretching or bending, it will absorb IR radiation. In the case of CO2 there are 3 vibrations which develop a dipole, meaning that it can therefore absorb IR radiation.

An infrared picture taken in Stamford. Source: Vin Crosbie.

Absorbing IR radition traps heat in the lower atmosphere

Now, the fun part, as I mentioned in my opening line. As the sun shines on our planet, the solar radiation is in part reflected by parts of the Earth, such as clouds and snowy regions. The remaining solar radiation is absorbed by the land, oceans and atmosphere. This causes these things to warm up, and when they do, they begin to emit this heat into the air as IR radiation. When this heads towards space, it is absorbed by the IR active gases such as carbon dioxide which exist in our atmosphere. 

These IR active gases continue to absorb and reradiate the IR radiation, essentially trapping the heat energy in the lower atmosphere. As an analogy to close things off, you can almost think of the reradiation process like a group of friends throwing a ball around to prevent it from landing on the floor, where the floor would be the heat dissipating into space: certain gases are better at it, as are certain groups of friends. 

Liked what you read? Spread the word and share it on social media:

Jack McGovan is a recent graduate in chemistry with a specialisation in ‘Energy and Sustainable Chemistry’ from the University of Groningen, the Netherlands. Following a job as a student journalist covering the energy transition, he has moved to Berlin where he is following his passion for working towards creating a fairer and more sustainable world. Seeing a gap in the way in which the world of science was communicated, he founded Delta-S. By writing source based content, he hopes to communicate his findings to a wider audience.