The greenhouse effect refers to heat trapping action of the atmosphere caused by atmospheric gases and results in the general increase in earth temperature. When the sun’s thermal radiation is able to pass through the atmosphere due to its short wavelength. However, upon reaching the earth surface, the radiation is reflected back into the atmosphere with a long wavelength and this time, the atmosphere acts as a ‘heat trap’ which absorb the re-radiated heat. This happens because some gases (namely water vapour, carbon dioxide and methane) absorb energy strongly at certain wavelengths in the infrared radiation.
The presence of water vapour, carbon dioxide (CO2) and other gases in the air has served the planet well. The ‘trapping’ of heat by the atmosphere has kept the Earth warm. Without the greenhouse effect, the earth’s surface would be about 25°C cooler than it is today. However, the emission of extra carbon dioxide from the burning fuels (petrol, coal, etc.) may be adding to this effect and causing what is called ‘global warming’
The ‘greenhouse gases’ that produce this warming include water vapour, carbon dioxide, methane, the oxides of nitrogen and CFCs (chlorofluorocarbons). These gases absorb heat from the Sun that is reflected from the Earth’s surface. They prevent this heat from being re-radiated back into space. So the Earth’s surface becomes warmer.
The levels of carbon dioxide released into the atmosphere are very high (about 5000 million tonnes per year). Large-scale deforestation, particularly of the tropical rainforests, to provide land for agriculture, also increases the level of carbon dioxide in the air because it removes trees j which absorb the gas for photosynthesis.
The effects of global warming are complex. The polar ice-caps would begin to melt as the temperature rose. The increase in sea level would threaten low-lying lands. Extensive climate change I is possible but is difficult to predict. In some regions the effects may be good; but others may suffer long droughts. There could be major shifts in desert and fertile regions. Tropical storms could | increase in their intensity.
There is evidence, from dying corals to an increase in warm-water fish in the North Sea, to show that global warming is taking place. Whether this is due to increased carbon dioxide levels remains to be proved. Certainly the concentration of CO2 in the Earth’s atmosphere has increased over the last century (in 1850 it was 0.027%, and in 1993 it was 0.036%).
Ozone layer depletion
A protective layer of ozone in the stratosphere prevents harmful ultra-violet radiation reaching the Earth’s surface. CFCs and other halogen compounds are thought to cause the damage. International agreement has been reached on restricting the use of these compounds (the Montreal Protocol). If the situation were allowed to grow worse, exposure to higher levels of ultraviolet radiation could cause more cases of skin cancer in humans, and cause damage to crops.
There are various types of pollution that occur over smaller, local areas. These usually occur in large cities, where there is heavy use of motor cars. These forms of pollution are:
- photochemical smog,
- high levels of carbon monoxide, and
- high levels of lead compounds.
Photochemical smog is formed in regions where there is a large amount of traffic. It was first identified in Los Angeles, and forms only under certain conditions. Severe photochemical smog often occurs in cities located in valleys, or surrounded by hills or mountains. On sunny, windless days, oxides of nitrogen are trapped by the hills in air that is close to the ground. A complicated series of reactions takes place and ground-level ozone is one of the products. In these circumstances ozone is harmful, particularly for asthma sufferers.
Carbon monoxide is the most common air pollutant in the industrialised world. It is produced when hydrocarbon fuels are incompletely burnt. Carbon monoxide is poisonous at levels of only 0.1%. Oxygen is very important to living things.
In our bodies it is carried round the body attached to haemoglobin in our red blood cells. Carbon monoxide is attracted to haemoglobin over 200 times more strongly than oxygen. In the blood, carbon monoxide prevents haemoglobin from carrying oxygen. This leads to dizziness and headaches. Prolonged exposure to carbon monoxide causes death.
Lead compounds are the most toxic heavy-metal pollutants in the air. Lead compounds form part of the ‘urban dust’ that forms in a city atmosphere.
People living next to busy roads can have high levels of lead in their blood. This has been linked with nervous disorders and with learning disabilities in children. The increased moves to use unleaded petrol are reducing this health hazard.