What are greenhouse gases?

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 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 greenhouse gas effect

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.

Greenhouse gas effect step 1


2. Energy from the sun

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

Greenhouse gas effect step 2


3. Earth loses some heat

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

Greenhouse gas effect step 3


4. Some energy is lost to space

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

Greenhouse gas effect step 4


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.

Greenhouse gas effect step 5


Why is the greenhouse gas effect important?

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

However, long-term measurements of 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 is known as the Keeling curve.

The graph below, 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.

Keeling Curve graph from www.co2.earth

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.

This 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.