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Greenhouse effect

Just what is this effect and what does it have to do with greenhouses?

Greenhouse effect

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Melting icecaps, stranded polar bears and sinking pacific islands – blame it on the greenhouse effect causing global warming.

The greenhouse effect is also responsible for the natural temperature regulation of our planet. Without it, the Earth start to freeze and become uninhabitable.

Almost all our energy comes from the sun, and this is in the form of radiation at visible wavelengths (colours of the rainbow), which penetrates through the atmosphere. About 30% of radiation is reflected back to space but the remaining 70% is absorbed by the oceans, land and the atmosphere.

The 70% of radiation absorbed by the Earth is emitted back into the atmosphere in the form of infrared. Some of the heat is radiated back into space but about 40% is trapped in the lower atmosphere by greenhouse gases such as carbon dioxide, methane, and even water.

This process is called the greenhouse effect, and it's basically a giant version of what goes on in your backyard greenhouse.

So why all the fuss about the greenhouse effect?
Scientists are concerned with what they call the enhanced greenhouse effect, and how it leads to global warming, which is believed to be caused by human activity.

Since the industrial revolution, there has been a 30% increase in carbon dioxide in the atmosphere – far more than you would expect to occur naturally. It is now highest than it has been in 420,000 years and possibly higher than it has been in 20 million years.

The main greenhouse gases responsible for global warming are: water, carbon dioxide, methane, nitrous oxide, chlorofluorocarbons (CFCs) and ozone.

The amount of infrared radiation (global warming potential or 'GWP') that these gases absorb varies enormously. For example the gas nitrogen trifluoride, used in the manufacture of flat screen televisions, absorbs 17,000 times as much radiation as carbon dioxide.

Carbon dioxide
For ease of accounting the global warming potential of gases is converted into carbon dioxide equivalence (CO2e). All but CFCs and nitrogen trifluoride occur naturally. However, humans have also artificially increased the amount of carbon dioxide, methane, nitrous oxide greenhouse gases through their activities.

Since the industrial revolution, the amount of carbon dioxide has risen due to the burning of fossil fuels coal, oil and gas. At the same time, deforestation has reduced the amount of carbon being absorbed by plants.

Expansion of domestic grazing and rice growing, coupled with landfills, coal mining and oil and gas extraction has increased methane levels.

Chlorofluorocarbons used as refrigerators and aerosols are extremely potent greenhouse gas.

Nitrous oxide levels have increased due to the application of fertilisers, and burning of fossil fuels but are a relatively minor source of greenhouse gases.

The big picture
The result of pumping all these extra greenhouse gases into the atmosphere is a warming of the globe above what we would normally expect. According to the Intergovernmental Panel of Climate Change(IPCC), temperatures are expected to rise between 1.4 and 6.4 C by 2100.

Scientists' greatest concern is the point at which will we heat the Earth so much that it melts ice caps, which will reduce the surface area that heat can be reflected by, increasing emissions levels and creating a runaway greenhouse effect.

Some scientists think we have already reached this level.

The effect of warming is uncertain, but scientists predict sea level rises, storms and unpredictable weather patterns, which are likely to affect agriculture, survival of threatened species and result in flooding of inhabited areas.