The Greenhouse Effect!
This is the natural process by which the Earth heats up and cools down over time. There is four stages to this process which are shown on the diagram below:
The diagram can be clicked on in order to skip to the paragraph on a particular part of normal global warming. The natural process of global warming occurs in four stages, we are currently in one now.
Firstly though I will explain the processes involved with the greenhouse gases. Greenhouse gases (CO2 is the example used in the diagram, although Ccl2f2 are often on exams) are found in the Troposphere of the Earths atmosphere. There is a table taken from Salters Advanced Chemistry Chemical Storylines book that shows most of the gases and their contributions to the effect, below. Greenhouse gases work very simply, the sun radiates electromagnetic radiation in a variety of wavelengths towards the Earth. The O-zone layer (which is also under threat from our CFC's) absorbs some of the high energy radiation and emits it to the Earths surface at a lower wavelength (often visible light). This hits the Earths surface and is absorbed again, and then emittedat a lower wavelength. This time the radiation is infra-red (heat). This infra-red radiation is intercepted by the greenhouses gases and emitted in all directions some of which returns to the surface for a second time. The more greenhouse gases there is in our atmosphere, the more ir radation will be returned to the Earths surface and so the hotter the surface will be. However it is important to know that this is a natural processes that keeps the Earth warming and leads to the occasional ice age. Click here to skip to the enhanced greenhouse effect!
| Gas | Tropospheric abundance (by volume)/% | Greenhouse factor |
| N2 | 78 | negligible |
| O2 | 21 | negligible |
| Ar | 1 | negligible |
| H2O(g) | 1* | 0.1 |
| CO2 | 3.5 x 10-2 | 1 |
| CH4 | 1.7 x 10-4 | 30 |
| N2O | 3.0 x 10-5 | 160 |
| CCl2F2 | 4.8 x 10-8 | 25 000 |
| CCl3F | 2.8 x 10-8 | 21 000 |
* = Averaged figure
Now I'll explain a little more about the four stages of global warming.
At this stage volcaneos are low in number and the levels of CO2 and other greenhouse gases are droping. This is due to the combined effect of the falling emmisions coupled with the increased levels of precipitation caused by the melting of the ice caps in hot house earth. The CO2 gas is absorbed in to the water! This causes the Earth to cool again and the polar ice caps are able to reform. However much life was killed during the hot house earth phase and so the levels of CO2 continue to drop, this leads to the next phase.
Now the Earth begins to lose most of its greenhouse gases and the planet freezes. This is often refered to as an ice age. However the water on the Earth (sea) or in the sky (clouds) all freeze on the surface as ice or snow. The water can no longer absorb gases such as carbon dioxide and so these gases are released back into the atmosphere. The Earth can slowly begin to warm and we move on to the next phase.
Now the Earth is warming up and the ice and snow recede of form ice caps again and life thrives where the ice melts (see succession). This means that the greenhouse gases are returned and the Earth warms to around the optimum level. Unfortunatly the animals continue to develop and so the greenhouse gas levels slowly rise again, leading to the next phase.
Well as the greenhouse gas levels continue to rise the Earth slow heats up. This causes increased amounts of desertification around the tropics and kills many plants and animals. The increased heat also causes many more forest fires and increased volcanic activity which raise the greenhouse gas levels further. This will cause the ice caps to melt and kill even more life this time in the higher latitudes. This as you should be able to see leads to the heat spiraling and will eventually create the conditions described in order to form the Snowball Earth Prologue phase again, and the cycle restarts.
The Enhanced Greenhouse Effect!
This is the bit you have all be waiting for and surprisingly it is so simple that it only takes one sentance to describe. The enhanced greenhouse effect is when humans speed up the natural greenhouse cycle by causing a rapid increase in greenhouse gases. And this finally leads us to the page I promised about the 1700's which shows how we have increased the levels of greenhouse gases in the atmosphere and why it is a modern issue.
Please Click Here!