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Global warming

The diagram below shows variations in global temperature over the past 100 years. Note that the y-axis shows the difference in degrees Celsius from the 1900 mean (average) temperature.

Graph showing global mean surface temperatures, 1900-2000

Global mean surface temperature 1900-2000

You may be asked to describe what a line graph like this shows. Broadly, this graph shows that the overall trend is a rise in global surface temperature from 1900 to 2000.

In more detail it shows:

  • there was a drop in mean (average) temperature of 0.1 degree in the first 10 years of the 20th century
  • the temperature then increased, reaching 0.3 degrees above the 1900 mean by 1940
  • the average temperature then dropped back to 0.2 degrees above the 1900 figure by 1950 and fluctuated around that figure until 1975
  • in the past 25 years of the century the temperature rose rapidly to 0.7 degrees higher than 1900 by the year 2000

Remember that the gradient of this line graph shows how quickly or slowly the temperature is changing - the steeper the slope, the faster the change.

The temperature varied due to a combination of physical and human factors.

Physical causes

Physical causes of global warming and cooling include:

  • variations in solar energy - sunspot activity raises global temperature
  • volcanic eruptions - large quantities of volcanic dust in the atmosphere [atmosphere: The Earth's atmosphere is the envelope of gasses that surround the earth. The gasses are held there by the Earth's gravity. The important gasses in the atmosphere are nitrogen, oxygen and carbon dioxide. shield the earth from incoming insolation [insolation: solar radiation received in the Earth's atmosphere or at its surface., lowering global temperature. For example, the eruption of Mount Pinatubo in 1991 caused a dip in global temperatures in the early 1990s
  • Milankovitch cycles or variations in the tilt and/or orbit of the earth around the sun
  • changing oceanic circulation such as the periodic warming (El Nino) and cooling (La Nina) of areas of the tropical Pacific Ocean

These physical causes of global temperature change have always existed and have been responsible for alternate heating and cooling cycles of the earth's temperature.

Human causes

The human causes of global warming have been in the news a lot in recent years - you can probably think of a few examples. Human factors are the result of growing population and economic developments.

They include:

  • the burning of fossil fuels for transport, industry and power, producing carbon dioxide
  • world-wide deforestation, sometimes involving rainforest burning, which also produces carbon dioxide
  • car exhausts and nitrogen fertilisers, producing nitrous oxide
  • CFCs found in fridges, air conditioning and aerosols and as a biproduct of the production of polystyrene packaging, like pizza and burger boxes
  • methane, produced from rice fields, landfill sites and from both ends of cattle

These different greenhouse gases [greenhouse gases: The main greenhouse gases in the atmosphere are carbon dioxide, methane and nitrous oxide. They are naturally occurring in atmosphere, but are believed to have increased through burning more oil, petrol, and coal, while forests have been removed. - carbon dioxide, methane, nitrous oxide - have caused an enhanced greenhouse effect [greenhouse effect: The build up of greenhouse gases, such as Carbon Dioxide [CO2], in the atmosphere, is believed retain more heat in the atmosphere than would usually be the case. Naturally more of the radiation (heat) from the sun would be reflected back into space., trapping some outgoing infra-red radiation and keeping the earth warmer than it might otherwise be.

Collectively, these physical and human causes have produced the pattern seen in the graph above.


Britain from Above

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See how the technology of satellite radar has enabled vast weather systems to be detected, mapped, monitored and forecasted.


How the movement of sea water affects climate

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The Arctic

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Evidence of global warming


James Lovelock

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Environmentalist James Lovelock on global warming

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