Climate Change > The greenhouse gases

The greenhouse gases

One of the characteristics of the gases that contribute to climate warming is that they are transparent to short-wave radiation from the sun that reaches the Earth, but they are able to absorb some of the heat that is radiated from the surface of the Earth.

Of the current climate-influencing emissions, carbon dioxide accounts for around 70 per cent of the effect, followed by methane, around 20 per cent, then nitrous oxide and fluorinated gases, around 5 per cent each (ignoring the climatic influence of ozone).

Carbon dioxide (CO2)

Fossil fuels are the main source of carbon dioxide emissions. Emissions of energy-related carbon dioxide totalled 24 billion tonnes in the year 2000, or 6.5 billion tonnes when calculated as carbon. Changes in land use (mainly deforestation) over the last 20 years are estimated to have contributed roughly one quarter of the total emissions of carbon dioxide, or around 1.5 billion tonnes of carbon per year.

Emissions per capita in developed countries are on average six times as high as those in developing countries (emissions from deforestation excluded). The US alone produces around a quarter of all the carbon dioxide from fossil fuels worldwide.

Per capita emissions of carbon dioxide from the burning of fossil fuels, 2003 data. Source: Wikipedia.

Once released from fossil storage, carbon dioxide remains in the atmosphere for a very long time and can affect the climate long into the future.

Methane (CH4)

Methane is formed naturally by the bacterial decomposition of organic matter under oxygen-free conditions. As a result of various types of human activity, emissions of methane have roughly doubled. Rice cultivation, cattle breeding, emissions from coal mines and the leakage of fossil gas represent significant anthropogenic sources around the world, as do the treatment of wastewater and organic waste.

Nitrous oxide (N2O)

Our knowledge of the extent of emissions and the factors that control them is incomplete, but denitrification is the main source of nitrous oxide in the atmosphere. This process, which is carried out by micro-organisms, occurs naturally in the soil. However, the more nitrogen is made available to plants by adding it in the form of fertilizer or through the deposition of airborne nitrogen, the more nitrous oxide is formed. Another source of emissions is combustion.

Particles

Particles in the atmosphere also affect the radiation balance. Sulphate particles reflect incoming sunlight and hence reduce the amount of solar energy that reaches the Earth's surface. Particles of black carbon can both absorb heat and reflect incident light. Their net effect on the climate is therefore difficult to assess.

Last modified: 21 July 2007.