Our planet receives massive amounts of energy, as radiation, from the sun. This radiation strikes our atmosphere, a thick ‘blanket’ surrounding our planet; some of the radiation passes through this blanket and heats the surface of the planet; the rest is reflected back and dissipates in the frozen vacuum of space.
This blanket is a finely tuned instrument. It is invisible and yet all around us. It allows enough radiation through to heat the planet, and not so much that the planet boils. At the same time, it is able to ‘trap’ enough heat to keep the planet warm at night so that the planet’s surface does not freeze when the sun is not shining directly on it.
Our atmosphere primarily consists of nitrogen (78%) and oxygen (21%). These two gases sustain life – being present in all living organisms in the case of nitrogen, and being used to ‘breathe’ in the case of oxygen. Together nitrogen and oxygen account for 99% of the content of the atmosphere. However, they are not able to trap radiated heat from the surface of the planet.
It is the gases that make up the remaining 1% of our atmosphere that are responsible for trapping heat – they are the reason why the average surface temperature of the earth is 15 degrees centigrade and not minus 18 degrees centigrade.
The name for these gases is ‘greenhouse gases’ because they behave like a greenhouse and trap heat. The primary greenhouse gases are water vapour, carbon dioxide, methane, nitrous oxide and ozone. There are other greenhouse gases to consider, such as sulfur hexafluoride, hydrofluorocarbons, and perfluorocarbons.
Typically, water vapour accounts for about 1% of the atmosphere (consisting of two parts hydrogen and one part oxygen and thus counting towards the overall oxygen content in the atmosphere). Carbon dioxide accounts for about 0.039%, (or 390 parts per million) and is the most prolific greenhouse gas in the atmosphere.
Greenhouse gases are critical to the maintenance of the average temperature of the earth. They are essential to sustaining the equilibrium of the planet and are thus a good thing. By and large, greenhouse gases occur in the atmosphere as a result of natural processes occurring on the planet, such as natural forest fires, decaying plants and animals, and so on. At the same time there are natural ‘sinks’, such as oceans and forests, which ‘capture’ greenhouse gases so that the balance between emissions and absorption is maintained.
An increase in concentrations of greenhouse gases in the atmosphere can occur if the rate at which greenhouse gases are being emitted into the atmosphere exceeds the ability of the earth to absorb them. A good example of this is the effect of cutting down trees – if they are cut down they can no longer absorb carbon dioxide; if they are subsequently burned as fuel, carbon dioxide is released from the wood into the atmosphere.
It is generally accepted that increased concentrations of greenhouse gases in the atmosphere leads to higher average temperatures as more heat from the earth’s surface is trapped. This is known as global warming.
The Question: How important is our atmosphere for maintaining global average temperatures and thus our existence?