CO2 and the Greenhouse Effect

injx · Posted: Fri Nov 10, 2006 10:51 am

CO2 and the Greenhouse Effect

Climate change is a topic that seems to be discussed quite regularly in the news these days, with rising levels of CO2, carbon dioxide, taking the blame. But how can this innocent molecule, naturally present in our atmosphere, cause such drastic effects to the planet? Let's find out.

A molecule of carbon dioxide consists of three simple atoms: a central carbon atom and two oxygen atoms, each on opposite sides of the centre. In its solid phase it is known as 'dry ice', as at atmospheric pressure it cannot enter a liquid phase – it passes directly from solid to gas (or vice versa) when heated or cooled.

In the molecule, the oxygen atoms are bound to the carbon atom in an equilibrium position. If one of the oxygen atoms is displaced by a small amount, the forces involved will cause it to move back towards its equilibrium position. This is similar to a pendulum – when you displace a pendulum, it always accelerates back towards the centre and eventually comes to rest at its equilibrium.

This behaviour can be modelled as if the oxygen atoms were attached by springs to the carbon atom, and this also makes the situation easier to visualise.

This means that the molecule can vibrate in three different ways, what we call modes of vibration. The first mode is symmetric and involves the oxygen atoms either both moving away from the centre or both towards the centre. The second is asymmetric and involves the oxygen atoms moving either both to the left, or both to the right, while the carbon moves in the opposite direction. The third is transverse – meaning the oxygen atoms both move up or down, while the carbon moves oppositely – which is called bending of the molecule.

Symmetric Mode

Asymmetric Mode

Bending Mode

More importantly, each of these modes has a natural frequency of oscillation attached to it, which can be directly derived from the masses and forces involved in the molecule. Two of these modes are important: the asymmetric mode has a natural frequency corresponding to a wavelength of 4260 nm, and the bending mode 14990 nm, meaning both modes can absorb infrared energy. The symmetric mode can not absorb any electromagnetic radiation, for reasons which I won't go into.

Although the CO2 molecule may vibrate arbitrarily, any such vibration is simply a special combination of these three basic modes.

Here on Earth, we receive energy from the Sun. This energy comes in the form of electromagnetic radiation, some of which is visible to the eye (that is commonly called 'light'). Although this radiation is spread over many different wavelengths, its peak is in the visible region, at the wavelength of the colour yellow – this is why the sun appears yellow to us. However there is comparatively little in the infrared region.

Most of this energy is absorbed by the surface of the Earth, rather than the atmosphere. This energy raises the temperature of the surface, and the Earth then radiates energy back out as infrared heat.

This is where the CO2 comes in. Instead of the infrared energy leaving the Earth, it is absorbed by the CO2 molecules in the atmosphere and excites the two vibrational modes. This energy is then trapped within the Earth and its atmosphere, making it difficult for the Earth to cool and thus raising its average temperature.

Although there are many more causes of global warming, including other greenhouse gases such as water vapour, increasing carbon dioxide levels is of direct consequence of human industrialisation and thus is one that we, as humans, have the most control over.

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