About climate change: Greenhouse gases
The most important greenhouse gases are carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons and sulphur hexafluoride. These are the gases that are covered by the Kyoto Protocol.
Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are also powerful greenhouse gases but they are being progressively phased out under the Montreal Protocol as they also damage the stratospheric ozone layer. They are part of a longer list of greenhouse gases covered by the Kyoto Protocol.
Measuring the Global Warming Potential
Each greenhouse gas has a different capacity to cause global warming, depending on its radiative properties, its molecular weight and its lifetime in the atmosphere. Its so-called global warming potential (GWP) encapsulates these. The GWP is defined as the warming influence over a set time period of a gas relative to that of carbon dioxide. A 100-year time horizon is used in the Kyoto Protocol. When the warming effect of current greenhouse gas emissions over the next 100 years is calculated, the graph shows that carbon dioxide will be responsible for about two thirds of the expected future warming.
How the relative climate effects of greenhouse gases are compared
1. To compare the relative climate effects of greenhouse gases, it is necessary to assess their contribution to changes in the net downward infra-red radiation flux at the tropopause (the top of the lower atmosphere) over a period of time. Ultimately the best way to do this is by comparing different emission scenarios in climate models, but a simple working method has been derived for use by Parties to the UNFCCC. This provides the relative contribution of a unit emission of each gas, relative to the effect of a unit emission of carbon dioxide integrated over a fixed time period. A 100-year time horizon has been chosen by the Convention in view of the relatively long time scale for addressing climate change.
2. The factor is known as a global warming potential (GWP). It means for example, that 1 tonne of HFC-134 emitted to the atmosphere has 1,000 times the warming potential over 100 years of 1 tonne of carbon dioxide.
3. To compute the carbon dioxide equivalent of the emission of any gas, we multiply its emission by the GWP. This is often expressed as the carbon equivalent so we then multiply by 12/44, the ratio of the atomic weights of carbon and carbon dioxide. Thus, for example, an emission of 1 tonne of HFC-134 is equivalent to 1 x 1000 x 12/44 = 273 tonnes of carbon.
Greenhouse gas emissions inventory
The UK's National Atmospheric Emissions Inventory - funded by Defra, The National Assembly for Wales, The Scottish Executive and The Department of Environment, Northern Ireland - compiles the UK's annual greenhouse gas inventory. It is submitted annually to the UN Framework Convention on Climate Change (UNFCC).
The emissions inventory contains information on greenhouse gas, emissions from fuel consumption industrial production, agriculture and land use change and forestry. It also provides a disaggregated inventory for England, Scotland, Wales and Northern Ireland.
- UK's National Atmospheric Emissions Inventory website
- UK Greenhouse Gas Inventory from 1990 to 2004 (UK's Greenhouse Gas Inventory website)
Aerosols
See aerosols
Page last modified: 12 March 2007
Page published: 01 December, 2005