The Greenhouse effect - Global Warming

UNIT 2 - Module 4 (Resources) - AS OCR Chemistry


  • Created by: Kat
  • Created on: 05-04-12 17:03

What is the Greenhouse effect?

The Greenhouse effect is not always a bad thing. In fact, it stops us from being frozen solid completely! The Earth stays at a temperature we can survive in, because of the electromagnetic radiation that it recieves from the sun. This radiation enters through the Earth's atmosphere (relatively easily) and comes towards the surface of the Earth.

Some of the solar energy is absorbed on the surface of the Earth, but some of it is reflected back again - in the same direction of which it entered. (Out of the atmosphere).

Most of the infared radiation emitted by the Earth's surface itself makes it back into space; but there is some that doesn't quite get there. Some gases in the atmosphere absorb the radiation and emit energy - some of which comes back towards us, at the Earth's surface. This traps a lot of heat in the lower atmosphere of the Earth, as this is a continual process.

DEFINITION: GREENHOUSE EFFECT - The absorbtion and emission of infared radiation via atmospheric gases warms the lower atmosphere, and the Earth's surface. 

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Greenhouse Gases - Natural existences

Greenhouse gases can be naturally occuring. Water vapour - in fact - is an example of the most common (and naturally occurring) greenhouse gas; as is CO2 (Carbon Dioxide).

Water vapour can be produced by evapouration of large water sources, while Carbon Dioxide can be produced by...:

  • Volcanos and their eruptions
  • Respiration (Gas exchange / Breathing) of animals
  • Burning and decay of organic matter - Like plants (Which can happen naturally when natural fires are caused in droughts - or unnaturally, in human burning.)
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Greenhouse Gases - Not so natural

Greenhouse gases like Methane (the third largest contributing greenhouse gas) can also have an effect, and is far more the result of human interference. Methane can be...:

  • Emmitted during the production of coal, natural gas & Oil
  • A product of rotting materials in landfill sites
  • Released from certain anmials, expecially cows, as a product of digestion (Seen to have been worsened by humans, as intensive breeding of cows is common)

There are various other Greenhouse gases; but their concentration in the atmosphere is little compared to Water Vapour, Carbon Dioxide and Methane.

DEFINITION: GREENHOUSE GASES - Gases that, in some way, contribute to the Greenhouse effect.

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Wait - gases can absorb radiation?

Yes, yes they can! Carbon Dioxide, for example, is a linear molecule (O=C=O), can absorb infrared radiation, which causes it to vibrate. The vibrations of the molecule eventually emit energy in the form of it's own radiation. This can, in turn, be absorbed by another greenhouse gas molecule, or at the Earth's surface. (Ultimately causing heat!)

Water vapour and Methane do this similarly;

-In H2O (Water), the O-H Bonds absorb the radiation

-In CH4 (Methane), the C-H Bonds absorb the radiation

The amount a gas contributes to the Greenhouse effect is not only dependent on how much of it is present in the atmosphere (the concentration), but is also dependent on it's ability to absorb the infared raditation; in turn, it's ability to vibrate and the amount of radiation it, itself, produces.

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Potential global warming threats?

Because the greenhouse effect of a gas is so variable, a method has been created in order to scale the gases according to the possibility that they will be harmful. The ability of a gas to cause Global Warming is measured by giving each gas a 'Global Warming Potential' (GWP).

As well as being linked to the gas' ability to vibrate, GWP is also relative to the lifetime of the gas.

Some Chlorofluorocarbons (CFCS) are TWENTY FIVE THOUSAND times more efficient at absorbing infared radiation - but as this was spotted by chemists, and legislation introduced to stop the use of them, their effect is decreasing. 

Without human intereference, however, the abundance of molecules like CFCS in the atmosphere are delightfully low! :D

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