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A natural function of the Earth's atmosphere is to keep in some of the heat that is lost from the Earth. This is known as the greenhouse effect.

• The atmosphere allows the heat from the Sun (short-wave radiation) to pass through to heat the Earth's surface.
• The Earth's surface then gives off heat (long-wave radiation).
• This heat is trapped by greenhouse gases (eg methane, carbon dioxide and nitrous oxide), which radiate the heat back towards Earth.
• This process heats up the Earth.

Human factors increasing global warming

Some human activities increase the greenhouse gases in the atmosphere:

• Burning fossil fuels, eg coal, gas and oil - these release carbon dioxide into the atmosphere.
• Deforestation - trees absorb carbon dioxide during photosynthesis. If they are cut down, there will be higher amounts of carbon dioxide in the atmosphere.
• Dumping waste in landfill - when the waste decomposes it produces methane.
• Agriculture - agricultural practices lead to the release of nitrogen oxides into the atmosphere.

Natural factors increasing global warming

There are also natural factors which contribute to increased global warming:

• Orbital changes - the Earth has natural warming and cooling periods caused by Milankovitch cycles or variations in the tilt and/or orbit of the Earth around the Sun (Wobble, roll and stretch theory).
• Volcanic activity - during a volcanic eruption carbon dioxide is released into the atmosphere.
• Solar output - there can be fluctuations in the amount of radiation from the sun. If there is high amount emitted there will be an increase in Earth's temperatures.