What is Climate Change?

Energy coming from the sun as visible radiation (or sunlight) is re-emitted back from earth to space.  The greenhouse effect is a natural phenomenon whereby some of this energy remains trapped, absorbed by naturally occurring gases in the atmosphere, thus maintaining the temperature of the earth’s surface at a temperature some 33°C warmer than it would otherwise be and enabling life as we know it to exist.  As a result of human activities, the atmospheric concentrations of some of these ‘greenhouse gases’ (GHGs)—including carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O)—have increased, predominantly since the start of the Industrial Revolution in the 1750s. Since that time, changes in the global climate have also occurred, and a statistical link has now been established between human activity and observed climate change phenomena.

The term ‘greenhouse gases’ as defined by the Kyoto Protocol includes

• carbon dioxide (CO2)

• methane (CH4)

• nitrous oxide (N2O)

• hydrofluorocarbons (HFCs)

• perfluorocarbons (PFCs)

• sulphur hexafluoride (SF6)

The Kyoto Protocol requires a percentage reduction in the total ‘basket’ of these six greenhouse gases. The UK’s domestic targets are specifically to reduce emissions of CO2.

Global Warming Potential (GWP)

This is a system of multipliers devised to enable warming effects of different gases to be compared.  The cumulative warming effect, over a specified time period, of an emission of a unit of CO2 is assigned the value of 1. Effects of emissions of a unit of non-CO2 greenhouse gases are estimated as multiples. For example, over the next 100 years, a gram of methane (CH4) in the atmosphere is estimated as having 21 times the warming effect as a gram of carbon dioxide; thus methane's 100-year GWP is  21. Estimates of GWP vary depending on the time-scale considered (eg, 20, 50-, or 100-year GWP), because the effects of some GHGs are more persistent than others.  The GWPs of CO2, CH4, and N2O are presently estimated to be 1, 23 and 296, respectively.  Some industrially produced gases such as sulphur hexafluoride (SF6), perfluorocarbons (PFCs), and hydrofluorocarbons (HFCs) have extremely high GWPs. Emissions of these gases have a much greater effect on global warming than an equal emission (by weight) of the naturally occurring gases. Most of these gases have GWPs 1,300 – 23,900 times that of CO2. PFCs and HFCs are ‘families’ of gases, so have a range of GWPs attributed to them.

The information shown in this panel has been extracted from the UK Joint Committee on the Draft Climate Change Bill (July 2007).