Why is global warming called the greenhouse effect? Greenhouses are not common in Africa, so don't be surprised if you have never seen one! They are used mainly in the cooler northern hemisphere to grow vegetables and flowers.

A greenhouse is made entirely of glass. When sunlight (shortwave radiation) strikes the glass, most of it passes through and warms up the plants, soil and air inside the greenhouse. As these objects warm up they give off heat, but these heat waves have a much longer wavelength than the incoming rays from the sun. This longwave radiation cannot easily pass through glass, it is re- radiated into the greenhouse, causing everything in it to heat up.

THE NATURAL GREENHOUSE EFFECT

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!

HOW HAVE PEOPLE ALTERED THE GREENHOUSE EFFECT?

Human population growth and related industrial expansion, have led to greater air pollution and a change in the composition of the earth's atmosphere. Some pollutants enhance the natural greenhouse effect, resulting in increased global atmospheric temperatures.

WHAT ARE THE GREENHOUSE GASES?

* 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.

* 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.

SIGNS THAT GLOBAL WARMING HAS BEGUN

* The average global temperature is about 0,5 degrees centigrade warmer than it was 100 years ago.

* 1990 is the warmest year on record, 1988 the second warmest, and 1980, 1981, 1983 and 1986 were all warmer than any other years in the last century. Preliminary analyses of 1991 temperature records indicate that this year was also above average.

* Snow and ice-cover have decreased this century, deep ocean temperatures have increased, and cloud cover over North America has also increased over this period. The latter indicates increased atmospheric water vapour.

* Over the last century, global sea levels have risen by between 100 mm and 200 mm.

FURTHER EFFECTS OF GLOBAL WARMING

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.

DID YOU KNOW?

* Many nations are reluctant to commit themselves to the costly changes necessary to reduce greenhouse gas emissions in view of the uncertainties surrounding the precise effects of global warming.

* Atmospheric CO2 concentration will be double the pre- industrial concentration in about 60 years time.

* Sea levels are likely to rise 60 mm each decade over the next century.

* Two-thirds of South Africa's population rely mainly on coal as an energy source. Providing electricity to these people will result in a nett reduction in CO2 emissions as a consequence of improved overall energy efficiency.

WHAT YOU CAN DO

Reduction of greenhouse gas emissions requires greater energy efficiency. Industrialists and governments have a key role to play here. But what can the individual do?

- Reduce electricity consumption.
- Use lift clubs, public transport, bicycles or your feet for transport.
- Reduce, reuse, recycle, and save energy - the manufacture of all products requires energy.

FURTHER READING

THE GREENHOUSE EFFECT - A PRACTICAL GUIDE TO THE WORLD'S CHANGING CLIMATE. S. Boyle and J. Ardill. Hodder and Stoughton, London, 1989.

ESKOM. DATABANK FACTSHEETS: PRACTICAL HINTS FOR SAVING ELECTRICITY (NO.1) and ESKOM, VILLAIN AND BENEFACTOR: FACTS AND MYTHS ON POLLUTION (NO.8).

PROJECT OUTREACH: THE CHANGING ATMOSPHERE. Issues no. 56 & 57. Share-Net, address below.

THE GREENHOUSE EFFECT: A TEACHERS MANUAL. M. Roos. Share-Net, address below. Greenhouse buster pack. Available from branches of the Wildlife Society, address below.

USEFUL ADDRESSES:

The Wildlife Society of Southern Africa. National office, PO Box 44344, Linden, 2104. Tel: (011) 486 3294/5 or 0938. Branches nationwide.

Eskom: Communications Department. PO Box 1091, Johannesburg, 2000. Tel. 011-8002499