Climate change, and all that

There’s a lot of talk in the media and around us about climate change and the greenhouse effect, but very few people actually tell you what it is and how it works. It is hard for us to tackle a problem this serious without understanding it. If you want to know more, this is a small package of all the science that you need to understand the greenhouse effect. We hope you find it interesting.

What is the greenhouse effect?

In order to understand the greenhouse effect, you first have to know a little about radiation and gas molecules.

Electromagnetic radiation in general

The shortest wavelength, most energetic and therefore potentially destructive kind of radiation is gamma rays (normally a by-product of a nuclear reaction, as they require enormous energy to produce). Progressively weaker and longer in wavelength down the spectrum are: x-ray, ultra violet, blue, yellow, red, infra red (which is heat), microwaves, then finally radio waves.

These waves can be reflected, scattered, bent, focused or absorbed depending on what they encounter. The outcome depends on various factors, the most basic being the relationship between the size of the object they hit and the wavelength of the wave. Some ‘springy’ objects vibrate when hit, as a bell resonates when struck. If they are hit by a wave with a wavelength that is similar to their natural vibrating frequency (i.e. the frequency at which they vibrate when struck), then the wave can be absorbed and re-emitted at a lower frequency.

The sun’s radiation

As we all know, the sun emits electromagnetic radiation. Use this link to see a graph showing the varying intensity of the sun’s output over the whole electromagnetic spectrum, http://en.wikipedia.org/wiki/File:Solar_Spectrum.png Luckily for us, there is no gamma radiation emerging from the sun, and the UV output is modest. The peak comes in the visible light spectrum with plenty of blue light (which is more energetic than red). For organisms trying to harness the energy found in light, the trick is to use the highest energy (i.e. shortest) wavelength without it being so energetic it destroys you. Blue is just right. There is also plenty of longer wave radiation (i.e. heat), but this tails off in a parabola as the wavelength gets longer. The sun is exceptional. Other stars have very different profiles; some radiate mainly in UV, others mainly in red and infra red.

Earth’s atmosphere contains various gases with molecules of different shapes, sizes and natural ‘springy’ vibrating frequencies. The ‘springs’ in this case are the molecular bonds between the atoms. As you can see from the chart, some of the sun’s radiation is scattered by these gases, and some is absorbed by them. Only a little over half gets through to ground level on Earth. (Incidentally the preferential scattering of a bit of the blue light is what makes the sky blue: see http://en.wikipedia.org/wiki/File:Spectrum_of_blue_sky.png)

The mechanism for the greenhouse effect

You will notice from the graph that the atmosphere is relatively transparent to the visible light part of the spectrum (the graph peaks at this point; the red area represents the radiation that gets through). Most of it travels straight through the air until it hits the ground/sea etc., whereupon it is then partly absorbed and partly reflected. The absorbed light donates its energy to any object it hits, heating it up a bit. (Note: The lighter the object’s colour, the less it will absorb and the more it will reflect.) The object then re-radiates this energy as heat in the lower frequency infra-red spectrum. If there were no atmosphere, most of this heat would radiate back into space. Likewise, if the only gases were oxygen and nitrogen, which do not have the right sort of ‘springs’, then the heat would still radiate back into space.

As it happens, any gas that has three or more atoms per molecule has the necessary ‘springs’, and will absorb strongly at certain points on the infra-red spectrum unique to each gas – its absorption signature if you like. There is a long list of these gases: water vapour, CO2, methane, oxides of nitrogen etc. In general, the bigger the molecule, the more effectively it absorbs. For example, methane (CH4), which is made up of five atoms, absorbs up to 20 times as much infra-red radiation as CO2, which is made up of three.

As the molecules absorb this heat radiation, they get hotter and move around faster, bumping into other molecules more, transferring heat energy randomly and directly to them like bumper cars. Gradually this heats the whole atmosphere until a new balance is found. Both these effects, the absorbing and the bumping, increase with atmospheric pressure as well, so that in a dense atmosphere like that of Venus the surface temperatures become high enough to melt lead.

On earth we owe our existence to the greenhouse effect. Without it, earth would be a frozen snowball, with an average temperature of some -33C. It is a powerful effect, which has enabled life to thrive on our hospitably warm planet.

How can you alter the greenhouse?

These rules of physics apply throughout the universe. If you accumulate any gases made up of molecules containing three or more atoms in any planet’s atmosphere, or if you increase atmospheric pressure (by adding more gas, or increasing gravity), or both, then you will increase the greenhouse warming effect. Whether this will warm a planet depends on what else is going on at the same time – there are many other ways planets heat and cool.

Why now?      Our strategy      Our philosophy      Election 2010     Our policies