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Greenhouse gases
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Greenhouse gases and global warming

Our planet is surrounded by a gaseous atmosphere, which comprises primarily oxygen and nitrogen. Both are important in sustaining life on earth. However what controls the global temperature is a group of gases called greenhouse gases which congregate in the upper atmosphere. These gases of which carbon dioxide is the most important allow sunlight to penetrate the earth’s atmosphere, which is then absorbed by the earth. When some of these rays are reflected back (at longer wavelengths) (Figure 1), these gases have the ability to trap some of the Earth’s radiation which would otherwise be transmitted to outer space. While one reason for the existence of life on earth is the presence of some greenhouse gases which raises the average earth temperature from -18 C to +12 C, the present concern is about the rate of increase in this average temperature and what impacts this can induce. Without these gases, the earth would be frozen (like Mars) so some concentration of these gases is essential for life on earth.

Ray diagram of solar energy

Figure 1 Ray diagram of solar energy flows showing effect of greenhouse gases

Carbon dioxide is the most abundant greenhouse gas and its production is primarily linked to our use of fossil fuel and results as a by-product of the combustion of oil, gas or coal which is used to provide energy for activities like transport, heating our homes or generating electricity.

Methane has a much stronger warming impact than carbon dioxide though a much shorter half life. It is the main constituent of natural gas, one of our main fossil fuels, which is widely used for space and water heating and increasingly for generating electricity. When burnt, methane combines with oxygen to form carbon dioxide and water vapour, which is also a greenhouse gas.

Nitrous oxides also have a much stronger warming impact than carbon dioxide and are released naturally by bacteria, digestion of ruminant animals like cows and in the combustion of fossil fuels.

Fluorinated gases have the highest warming impact and are a family of synthetic chemicals primarily used as refrigerants for air conditioners and in refridgerators. These chemicals are being phased out by international agreement and are being replaced by naturally occurring compounds with low global impact warming and low ozone depletion potential. These compounds include hydrocarbons like butane, ammonia and carbon dioxide. So paradoxically carbon dioxide has beneficial as well as adverse environmental properties. The other two greenhouse gases present naturally are water vapour (as noted above) and ozone.

Water vapour is created by evaporation from the seas and sources on land like lakes and rivers. Global warming will accelerate the rate of evaporation which will impact the earth’s water cycle and could increase global warming further.

Ozone is an unstable form (isotope) of oxygen and exists naturally in the upper atmosphere. Apart from its ability as a greenhouse gas to absorb the earth’s radiation, more importantly it protects living species by absorbing the ultraviolet portion of sunlight falling on the earth’s surface. As ozone is broken down by fluorinated gases, gaps have appeared in the ozone layer and so the global intention is to phase out all fluorinated gases as set out the Montreal convention (1987) on ozone depletion. Ozone can be created by the reaction between sunlight and ground level pollution so the there can be an excess of ozone if the level of pollution is high. Whilst ozone itself is a greenhouse gas, it can react with other greenhouse gases to shorten their lifespan.



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