3.3 Greenhouse gases
- What are greenhouse gases?
Many of the gases that occur in the Earth's atmosphere are greenhouse gases. They include carbon dioxide, methane and water vapour. Greenhouse gases occur both naturally and due to human activities.
A greenhouse gas is one which traps heat radiated from the surface of the earth. The term comes from the fact that these gases act like a greenhouse to limit the loss of heat from a system*. The diagram below shows the greenhouse effect, the way in which these gases help warm the Earth.
The steps are:
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1. The Earth is surrounded by its atmosphere, containing greenhouse gases like water vapour and carbon dioxide
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2. The Earth's surface is warmed by energy from the sun (in the form of short wave radiation)
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3. The Earth radiates some of this heat, as long wave radiation
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4. Much of this heat radiates out into space, cooling the planet
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5. Some heat is trapped by greenhouse gases, which re-radiate the heat in all directons, reducing the amount of cooling. |
Without a layer of greenhouse gases the earth would experience huge extremes of temperature and would be a hostile place to live.
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Long-term measurements of atmospheric carbon dioxide concentration have been made at the Mauna Loa observatory in Hawaii. These data show that carbon dioxide concentration is increasing in the earth's atmosphere. This graph is known as the Keeling curve. The graph on the right, showing the steady rise in CO2 concentration is from CO2 Now. (by the way ppm is 'parts per million', a unit of concentration). This steady rise in CO2 is worrying many people.
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* In fact the term 'greenhouse effect' is a little misleading because greenhouses warm up primarily through the prevention of convective cooling by the glass (airflow is reduced), whereas greenhouse gases trap radiated heat from the earth's surface and re-radiate it.