Greenhouse effect and Global Warming

Greenhouse gases such as water vapour, methane and carbon dioxide stop heat escaping from the Earth into space. An increased greenhouse effect can lead to global warming and climate change.

The greenhouse effect

Some gases in the Earth’s atmosphere stop heat radiating into space from the Earth. This is called the greenhouse effect and the gases involved are called greenhouse gases. They include:

• methane
• water vapour
• carbon dioxide.

The diagram shows how the greenhouse effect works.

Electromagnetic radiation at most wavelengths from the Sun passes through the Earth’s atmosphere.

1. The Earth absorbs electromagnetic radiation with short wavelengths and so warms up. Heat is radiated from the Earth as longer wavelength infrared radiation.
2. Some of this infrared radiation is absorbed by greenhouse gases in the atmosphere.
3. The atmosphere warms up.

   Global warming

   Some sources of greenhouse gases are natural and some are man-made. The table shows some of these sources.
   

   Some sources of greenhouse gas

   Greenhouse gas	Natural source	Man-made source
   Methane	Decomposing plant material	Rice paddy fields, cattle, coal mines
   Water vapour	Evaporation from oceans, lakes and rivers	Burning hydrocarbon fuels
   Carbon dioxide	Respiration by plants and animals, forest fires, volcanoes	Making cement, burning fossil fuels

   The amount of man-made water vapour is insignificant compared to the amount of water vapour from natural sources. However, emissions of methane and carbon dioxide are contributing to increased global warming.
   

   Global warming

   Human activities are causing the release of large amounts of carbon dioxide. These activities include:
   

   • deforestation - cutting down trees for fuel, farms, buildings and roads
   • increased use of energy (and so an increased use of fossil fuels).

   As the percentage of carbon dioxide in the atmosphere has increased, so in general has the Earth’s mean temperature.