Climate change describes the gradual increase of the air temperature in the earth’s lower atmosphere. Why is climate change called the greenhouse effect? Greenhouses are not common in Botswana, 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, and thus causes everything in the greenhouse 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 keep the Earth an average of 33°C warmers 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°C below freezing.

  How have People made the Atmosphere Warmer?

Human population growth and related industrial expansion, leads to greater air pollution and a change in the composition of the Earth’s atmosphere. Some pollutants enhance global warming, resulting in increased global atmospheric temperatures.

  What Gases are Responsible for Climate Change?

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. 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) doubled in concentration, mainly as a result of agricultural activities, between 1750 and 1990.

Nitrous oxide (N2O) also a product of burning fossil fuels, 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 Climate Change has Begun

The average global temperature is about 0.5°C warmer than it was 100 years ago.

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 Climate Change

If current pollution trends continue, some scientists estimates that the Earth could probably be about 1°C warmer by 2025 and 3°C 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 15000 years ago, was only 5oc 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 120m lower than at present. In those days, if you had wanted to go for a swim at Cape Agulhas you would first have 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.