Many of the gases that occur in the Earth's atmosphere are 'greenhouse gases' or GHGs. They include carbon dioxide, methane and water vapour. Greenhouse gases occur both naturally and due to human activities.

A greenhouse gas is one that traps heat radiated from the surface of the earth. The term comes from the fact that these gases act a little like a greenhouse to limit the loss of heat from a system. Actually the term 'greenhouse effect' is a little misleading. Greenhouses (glasshouses) warm up primarily because the glass reduces airflow so that convective cooling is reduced, whereas greenhouse gases trap radiated heat from the earth's surface and re-radiate it.

Here we explain the greenhouse effect, the way in which these gases help to warm the Earth.

1. Greenhouse gases in the atmosphere

The Earth is surrounded by its atmosphere, containing greenhouse gases like water vapour and carbon dioxide.

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2. Energy from the sun

The Earth's surface is warmed by energy from the sun (in the form of short wave radiation)

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3. Earth loses some heat

The Earth radiates some of this heat, as long wave radiation.

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4. Some energy is lost to space

Much of this heat radiates out into space, cooling the planet.

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5. Some heat is trapped

Some heat is trapped by greenhouse gases, which re-radiate the heat in all directons, reducing the amount of cooling.

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Without a layer of greenhouse gases the earth would experience huge extremes of temperature and would be a challenging place to live. We depend upon greenhouse gases for our survival.

However, long-term measurements of the atmospheric carbon dioxide (CO₂) concentration have been made at the Mauna Loa Observatory in Hawaii. These data show that the carbon dioxide concentration in the atmosphere is increasing. This graph of CO₂ concentrations is known as the Keeling curve.

This fascinating video from NASA shows the output from a high resolution computer model of carbon dioxide in the Earth's atmosphere. It clearly shows how CO₂ emissions change during the course of a single year, how emissions are carried around the planet and how plants absorb carbon dioxide through photosynthesis. You can see sources of the greenhouse gas such as power plants, industrial facilities, vehicles and forest fires.

The image below is from CO2-Earth and shows when the atmospheric CO₂ concentration rose above 350 ppm (in 1990) and then above 400 ppm in 2016. The concentration is measured in 'parts per million' (ppm).

This steady rise in CO₂ is worrying many people.

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Carbon dioxide is not the only greenhouse gas that is increasing in the earth's atmosphere. Concentrations of methane and nitrous oxide are also rising. You can read about the combined warming effect of the earth's greenhouse gases (an index called the Annual Greenhouse Gas Index or AGGI) on the National Oceanic and Atmospheric Administration's website.