The Greenhouse Effect
The temperature of the Earth is determined by the balance of energy coming in from the sun in the form of visible radiation (sunlight) and energy being lost from the surface of the Earth to space. Energy coming from the sun passes through the atmosphere and warms the Earth - but the emitted infra-red radiation coming from the Earth's surface is partly absorbed by gases in the atmosphere and some of it is re-emitted downwards, further warming the surface of the Earth and the lower levels of the atmospshere. This effect has been called the 'greenhouse effect' because of a similar effect caused by glass in a greenhouse: it lets sunlight into the greenhouse but in turn traps a portion of infra-red radiation (heat) inside the greenhouse.

The change in balance between radiation coming into the atmosphere and radiation going out is known as 'radiative forcing'. A positive radiative forcing tends on average to warm the surface of the earth; negative forcing tends on average to cool the surface.

The diagram below provides an illustration of the 'greenhouse effect'.

Find out more about the Greenhouse Effect from UNFCCC Climate Change Information Kit - The Greenhouse Effect.

Greenhouse Gases
As mentioned above, the greenhouse effect is caused by increases in greenhouse gas concentrations in the atmosphere. The most important greenhouse gases in the atmosphere (in terms of this effect) are: carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perflurocarbons and sulphur hexafluoride.

Carbon dioxide: this is the most important of the greenhouse gases released by human activities. It is the main contributor to climate change because of the quantities released, especially through the burning of fossil fuels. When fossil fuels are burned, the carbon content is oxidised and released as carbon dioxide. Every tonne of carbon burned produces 3.7 tonnes of CO2. The global consumption of fossil fuels is estimated to release 22 billion tonnes of CO2 into the atmosphere every year - and the amounts are still climbing.
The table below shows the emissions of CO2 by selected countries (including South Africa) on a 'per capita' basis and in total

Methane: this is produced when vegetation is burned, digested or rotted without the presence of oxygen. Large amounts of methane are released by rubbish dumps, rice paddies and grazing cattle. Methane is significant because it has 21 times the heat trapping effect of carbon dioxide.

Nitrous Oxide: This occurs naturally in the environment but human activities are increasing the quantities. Nitrous oxide is released when chemical fertilizer is used in agriculture.

The table below (taken from www.grida.no) lists some of the main greenhouse gases and provides the following information for each: their concentrations in pre-industrial times and in 1994; atmospheric lifetimes; anthropogenic sources; and Global Warming Potential (GWP).
(GWP is an index defined as the cumulative radiative forcing between the present and some chosen time horizon caused by a unit mass of gas emitted now, expressed relative to a reference gas such as CO2, as is used here. GWP is an attempt to provide a simple measure of the relative radiative effects of different greenhouse gases.)

Greenhouse Gases - facts and figures

Much of the work on climate change currently being done by scientists involves modeling the atmospheric concentrations of these gases now and in the future under various scenarios, to determine what the cumulative impact might be on radiative forcing and global warming. This research has resulted in the following points.

• The atmospheric concentrations of the main greenhouse gases, carbon dioxide (CO2), methane (CH4) and Nitrous Oxide (N2O) have grown significantly since pre-industrial times – by about 30%, 145% and 15% respectively (values for 1992). These trends can be attributed largely to human activities such as fossil fuel use, land use change and agriculture
  
• Many greenhouse gases remain in the atmosphere for a long time (for Co2 and N2O- many decades to centuries) – hence they affect radiative forcing on long timescales.
  
• The direct radiative forcing of long-lived greenhouse gases is due primarily to increases in the concentrations of CO2 and N2O
  
• CFCs and HCFCs also have a net positive radiative force – however this is smaller than it could be because these gases have also caused stratospheric ozone depletion, which gives rise to a negative radiative forcing.
  
• Growth in the concentrations of CFCs (but not HCFCs) has slowed to about zero. The concentrations of both CFCs and HFCFs, and their consequent ozone depletion, are expected to decrease substantially by 2050 through the implementation of the Montreal Protocol and its Adjustments and Amendments.
  
• At present, some long-lived greenhouse gases (particularly HFCs (a CFC substitute), PFCs and SF6) contribute little to radiative forcing but their projected growth could contribute several percent to radiative forcing during the 21st century.
  
• If CO2 emissions are maintained at near current (1994) levels, they would lead to a nearly constant rate of increase in atmospheric concentrations for at least two centuries, reaching about 500 ppmv (parts per million by volume) (twice the pre-industrial level of 280ppmv) by the end of the 21st century.
  
• Carbon cycle models indicate that stabilization of atmospheric CO2 concentrations at, for example, 450 ppmv – could be achieved only if global anthropogenic CO2 emissions drop to 1990 levels 40 years from now and stay at below 1990 levels subsequently. If a level of 650ppmv was to be achieved, this reduction would have to happen by 140 years from now. Stabilization at a much higher level of 1000ppmv would happen only if emissions drop to 1990 levels 240 years from now.
  
• Any eventual stabilized concentration is governed more by the accumulated anthropogenic CO2 emissions from now until the time of stabilization than by the way those emissions change over the period. This means that for a given stabilized concentration value, higher emissions in early decades require lower emissions later on.
  
• Stabilization of CH4 and N2O concentrations at today’s levels would involve reductions in anthropogenic emissions of 8% and more than 50 % respectively.
  
  More information on Greenhouse Gases can be obtained from the UNFCCC Climate Change Information Kit by following the links below:
  Greenhouse Gases and Aerosols
  Future Changes in Greenhouse Gas Levels
  How human activities produce greenhouse gases
  Data on greenhouse gas emissions and sources
  
  Acknowledgement The diagrams featured on this page are from Grid Arendal/ UNEP as featured on their website www.grida.no