As you have read from the “Concepts” section, MO Theory can be used to calculate the bond order and thus the bond strength of a molecule. Generally speaking, a higher bond order would mean that a molecule possesses higher bond strength. Bond strength, on the other hand, is related to the frequency of bond vibration via Hooke’s Law. This is shown in the following equation (click to enlarge):

Credits: The McGraw-Hill Companies, Inc.

From the equation, we can see that the greater the bond strength, the greater the frequency of infrared radiation (IR) absorbed.

So what does all this have to do with global warming?

We know that the Sun emits radiation in the form of Ultraviolet (UV) and Infrared Radiation, in addition to radiation with wavelengths falling in the spectrum of visible light. Greenhouse gases such as CO2 and CH4 absorb heat from the Sun, therefore creating the greenhouse effect whereby the Earth is able to maintain a stable average temperature of ~16ºC. It is this effect which enables living things to survive on Earth.

In recent years, however, the percentage of greenhouse gases in our atmosphere has increased, arguably due to human activity. The Industrial Revolution has caused unprecedented levels of CO2 to be released in the atmosphere, and the concentration of methane, CH4, has also increased due to increased cattle-rearing. This has led to the “enhanced greenhouse effect”, which means that more infrared radiation is trapped in the Earth’s atmosphere than before, instead of being reflected out into space. Take a look at the picture below.

Credit: Will Elder, National Park Service

I am sure that you more-or-less have an idea of what this entails, so I won’t go into the details of that here.

Our purpose now, then, is to explain: How and why do greenhouse gases such as CO2 and CH4 trap heat?

For a simple explanation, take a look at the video below from 10:16-11:50.

If you’re too lazy to fast-forward the video till the time interval stated above, here’s the words version of the explanation:

Greenhouse gases absorb IR of a certain frequency because the bonds between the atoms in their molecules absorb IR of that frequency in order to vibrate and become excited. When IR from the Sun reaches the Earth, CO2 and CH4 are some examples of molecules which can absorb IR. They will absorb around 5-10% of IR, and the rest will be reflected out by the Earth into space.

When these molecules come back down to their lower energy states, they will release the energy in the form of photons again. When this energy is emitted, it will radiate out in all directions. Some of the energy will be radiated out into space, while the rest is radiated back to the Earth’s surface, therefore causing the Earth to become hotter.

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There! Now you know how MO Theory can be linked to Global Warming!

🙂