The greenhouse effect is what causes our planet to warm in the presence of increased concentrations of ‘greenhouse’ gases in the atmosphere.
Each day the Earth receives energy from the sun in the form of ultraviolet rays (light), and it releases some of this energy back into space in the form of infrared rays (heat).
However, some of this outbound energy gets absorbed by water vapour, carbon dioxide, methane, nitrous oxide and other ‘greenhouse’ gases, which can then re-emit the heat energy.
These gases act like a blanket that surrounds the Earth and keeps it warmer than it would otherwise be — just as the glass panes of a greenhouse allow the sun’s energy to enter but prevent heat from escaping.
Without this natural process — known as the greenhouse effect — our planet would on average about 30 degrees Celsius cooler.
Human activities have, however, artificially raised the concentration of greenhouse gases in the atmosphere and scientists conclude that this is why the planet has warmed in recent history.
Measurements from Antarctic ice cores show that in pre-industrial times the concentration of carbon dioxide in the atmosphere was about 280 parts per million (ppm) by volume. Since then it has risen rapidly, and reached 387ppm by 2009.
Major sources of these emissions include transport, power generation and industrial activities that depend on fossil fuels, which release carbon dioxide and other greenhouse gases when they burn.
Deforestation is another major source, accounting for about one-fifth of all greenhouse-gas emissions from human activities. When forests are cleared or burned this releases carbon dioxide that had been locked away in the living plant material.
Agriculture is also a big source, and not only if forests are cleared to grow crops. Most fertilisers and pesticides are produced using fossil fuels. Large scale intensive agriculture produces high emissions as it depends heavily on these inputs and well as on fuel for mechanised farming and transport.
Farms are also a major source of a powerful greenhouse gas called methane, which is produced by bacteria that live in waterlogged rice fields and in the digestive tract of cows, which emit the gas when they burp.
Although these methane emissions are small compared to those of carbon dioxide from other sources, they are still important because methane has a far stronger warming effect. A single molecule of methane traps about 20 times more heat than one of carbon dioxide.
The greenhouse effect is not a new discovery. It was discovered by Joseph Fourier in 1824, experimented on by John Tyndall in 1858, and first reported quantitatively by Svante Arrhenius in 1896.
Since then scientists have provided growing evidence that not only are greenhouse gases increasing in concentration, but also that this threatens dangerous climate change (see Intergovernmental Panel on Climate Change).
Efforts to reduce the atmospheric concentration of greenhouse gases are termed ‘mitigation’.
Experts agree that the fastest, cheapest way to do this is to improve energy efficiency by, for instance, improved insulation and reduced use of electricity.
Other ways to mitigate climate change include actions that remove greenhouse gases from the atmosphere such as by planting trees or using algae to absorb carbon dioxide as the grow.
Other mitigation actions stop such gases from entering the atmosphere in the first place.
Mitigation is a major focus of the UN Framework Convention on Climate Change and its subsidiary treaty, the Kyoto Protocol.
Greenhouse-gas emissions vary greatly both within and between countries – but media reports do not always make these variations clear.
While China now emits more than any other nation, its emissions per person are still much lower than those in most other countries. For a comparison, Wikipedia has tables that rank countries according to their total (here) and per capita (here) emissions.
The Carbon Disclosure Project is a good source of information on what companies are doing to measure, report and reduce their emissions. It has information from over 2000 companies in 60 countries.
To find out what countries are doing to reduce emissions, journalists can check their national communications to the UN Framework Convention on Climate Change, which also has a register of greenhouse gases produced by industrialized nations.
Journalists who report on local weather conditions such as temperatures should also check what is happening around the world. For instance, January 2010 was extremely cold in the United Kingdom and eastern United States, which led some people to think the greenhouse effect was not real. But globally, January 2010 was the hottest on record.
It is also important to remember that the El Niño / La Niña cycle of warmth and cooling in the Pacific Ocean affects temperatures worldwide too.