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What is global warming? How does it occur?
Global Warming is the increase of Earth's average surface temperature due to effect of greenhouse gases, such as carbon dioxide emissions from burning fossil fuels or from deforestation, which trap heat that would otherwise escape from Earth. This is a type of greenhouse effect.Carbon dioxide and other air pollution that is collecting in the atmosphere like a thickening blanket, trapping the sun's heat and causing the planet to warm up.
Human beings have increased the CO2 concentration in the atmosphere by about thirty percent, which is an extremely significant increase, even on inter-glacial timescales.  It is believed that human beings are responsible for this because the increase is almost perfectly correlated with increases in fossil fuel combustion, and also due other evidence, such as changes in the ratios of different carbon isotopes in atmospheric CO2 that are consistent with "anthropogenic" (human caused) emissions. The simple fact is, that under "business as usual" conditions, we'll soon reach carbon dioxide concentrations that haven't been seen on Earth in the last 50 million years.

Combustion of Fossil Fuels, for electricity generation, transportation, and heating, and also the manufacture of cement, all result in the total worldwide emission of about 22 billion tons of carbon dioxide to the atmosphere each year. About a third of this comes from electricity generation, and another third from transportation, and a third from all other sources.

This enormous input of CO2 is causing the atmospheric levels of CO2 to rise dramatically. The following graph shows the CO2 levels over the past 160 thousand  years (the upper curve, with units indicated on the right hand side of the graph). The current level, and projected increase over the next hundred years if we do not curb emissions, are also shown (the part of the curve which goes way up high, to the right of the current level, is the projected CO2 rise). The projected increase in CO2 is very startling and disturbing.
The term greenhouse effect is used to describe the warming effect that certain gases have on the temperature of the earth's atmosphere under normal conditions.

Sunlight (shortwave radiation) passes easily through the earth's atmosphere. Once it strikes and warms the earth's surface, longwave radiation is given off and goes back into the atmosphere. While some of this longwave radiation or heat escapes into space, most of it is absorbed or held by carbon dioxide and other gases that exist in small quantities in the atmosphere. Thus these gases form a `blanket' that keeps the earth an average of 33 degrees centigrade warmer than it would be if this greenhouse effect did not occur. Without these gases the whole planet would be an icy wasteland with an average temperature of 16 degrees centigrade below freezing!
Water vapour is the main greenhouse gas. Human activities are not known to have had a significant influence on the atmospheric concentration of water vapour.

* Carbon dioxide (CO2) is the pollutant most responsible for increased global warming. It is released into the atmosphere mainly through burning of fossil fuels (e.g. coal, petrol, diesel). In addition, widespread destruction of natural vegetation, particularly forests, has contributed to increased atmospheric CO2 levels (see Enviro Facts "Deforestation"). This has occurred for two reasons. First, plants take up CO2 through the process of photosynthesis. The destruction of vegetation, as occurs in deforestation, reduces the amount of CO2 that is removed from the atmosphere. Second, when forests are cleared, and burnt or left to rot, CO2 is released.

* Methane (CH4) has doubled in concentration, mainly as a result of agricultural activities, between 1750 and 1990.

* Nitrous oxide (N2O), also a product of burning fossil fuel, has increased by 8% over the same period.
If current pollution trends continue, scientists conclude that the earth could probably be about 1 degree centigrade warmer by 2025 and 3 degrees centigrade warmer by 2100. This rapid temperature rise could have several effects:

* These changes in global temperature, although apparently small, could cause very large changes in climate. For example, the last Ice Age, which ended approximately 15 000 years ago, was only 5 degrees centigrade colder than current temperatures, but the resulting climate changes were massive: most of North America was covered in a layer of ice about 1,5 km thick and sea levels in the Cape were about 120 m lower than at present. In those days, if you had wanted to go for a swim at Cape Agulhas you would first have had to walk about 150 km to reach the sea!

* A rapid extinction of species.

* Rising sea levels - water expands as it warms and glaciers melt, adding water to the oceans, thus we can expect widespread flooding of coastal areas as sea levels rise.

* Greater frequency and scale of extreme weather conditions, e.g. drought and flood.

* Changes in the distribution of disease-bearing organisms so that people, domestic animals, and crops might be exposed to diseases previously absent from an area.

* Chlorofluorocarbons (CFCs), in addition to damaging the ozone layer, are potent greenhouse gases. Their concentrations in the atmosphere are increasing by about 4% every year.


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