2. Greenhouse Gas Emissions

Increasing atmospheric concentrations of greenhouse gases originating from man's activities are leading to enhanced warming of the atmosphere and global climate change. The major greenhouse gases are carbon dioxide, methane and nitrous oxide all of which have both natural and anthropogenic sources. In contrast, the three industrial gases: hydrofluorocarbons, perfluorocarbons and sulphur hexafluoride, are potent greenhouse gases but are only emitted from man-made sources. These six greenhouse gases comprise the 'basket of emissions' against which reduction targets were agreed at the Third Conference of the Parties of the United Nations Framework Convention on Climate Change in Kyoto, Japan in December 1997. The 1996 emissions for each of these six greenhouse gases are summarised in Table 2.1, and their inventories are discussed in the following sections. Inventories for the three indirect greenhouse gases (carbon monoxide, nitrogen oxides and non-methane volatile organic compounds) are included in subsequent chapters.

The total global warming potential of UK greenhouse gas emissions has been calculated using their global warming potentials (GWPs), which measures their effectiveness in global warming relative to CO₂), agreed by IPCC for a 100 year time horizon (IPCC, 1996).
Table 2.1 GWP for UK Emissions of Greenhouse Gases in 1996

Direct GHG	Emissions (kt) in 1996	GWP (100 years)	Global Warming Equivalence - emissions GWP (equivalent kt of CO2)
CO2	593,4221	1	593,422
CH4	3,712	21	77,948
N2O	189	310	58,690
HFCs	2.15	560-121002	16110
PFCs	0.079	6000-74002	549
SF6	0.035	23900	844
Calculated on IPCC basis and includes land-use change emissions Range of GWP for species used

During the period 1990-1996 there has been a decrease in UK emissions of CO₂, CH₄ and N₂O but small increases in the three new greenhouse gas emissions HFCs, PFCs and SF₆. The overall effect has been a decrease in global warming potential from UK emissions. Figure 2.1 shows emissions as CO₂ equivalent and includes CO₂ , CH₄ and N₂O estimates from 1970 to 1996 and HFCs, PFCs and SF₆ from 1990 to 1996 only. N₂O emissions from adipic acid are not available prior to 1990.

Figure 2.1 Total Global Warming Potential Associated with UK Emissions of Greenhouse Gases

Carbon dioxide is the major contributor to the UK's greenhouse gas emissions and arises predominately from the combustion of fossil fuels. Non-fossil fuel sources are more difficult to assess due to the importance of CO₂ in respiratory processes and its role within the global carbon cycle. Following internationally agreed conventions, the NAEI excludes emissions of CO₂ originating from recently photosynthesised carbon as these will be part of the carbon cycle. Hence emissions from biomass combustion, the organic component of waste incineration, landfill and sewage treatment are not included in the total. Emissions of other pollutants from biomass combustion are included in the appropriate inventories. The NAEI also currently excludes CO₂ emissions from the effect of changing land use although these emissions are estimated in the UK Greenhouse Gas Inventory (Salway, 1998) and are discussed below.

Methane, like carbon dioxide, is naturally occurring and is part of the global carbon cycle. However, the magnitudes of sinks and sources of methane are not well known. Methane in the atmosphere is eventually oxidised to CO₂ and the most recent IPCC estimate of its lifetime in the atmosphere is 12±3 years (IPCC, 1996). Methane has a much greater warming effect on the climate than carbon dioxide (Table 2.1). The major sources of methane are waste disposal, agriculture, coal mining and leakage from the gas distribution system. Due to the nature of these sources the estimation of methane emissions is very uncertain although the methodologies are continuously being improved. Early estimates of methane emissions by sector were based on the findings of the Watt Committee on Energy (Williams, 1994), however many have now been revised to take into account new information and to ensure consistency with the methodologies recommended by the IPCC 1996 Revised Guidelines (IPCC, 1997).

The third direct greenhouse gas, nitrous oxide, is emitted from natural and anthropogenic sources (agriculture, biomass burning, coal combustion and some industrial processes). As N₂O has a GWP of 310, it is a powerful greenhouse gas. However, emissions from the UK are low, so the overall contribution to global warming is relatively small (although still significant). Increases in the emissions from agricultural soils and the treatment of animal wastes, compared to those published in the 1995 NAEI, have resulted from the use of the Revised 1996 IPCC Guidelines (IPCC, 1997). A full set of detailed emission factors for man-made sources, e.g. combustion, are not yet available however emissions are estimated using the default values given in the guidelines and more detailed data on coal combustion based on UK literature

The three industrial greenhouse gases included in the 'basket of emissions' agreed at Kyoto, namely hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulphur hexafluoride (SF₆), have very high GWPs but the quantities emitted to the atmosphere are far smaller than the emissions of CO₂ Hence the contribution of these gases to global warming was equivalent to just 3% of the total contribution from CO₂ emissions in 1995 (MCG, 1996). These are gases with particular industrial applications; HFCs and PFCs are substitutes for chlorofluorocarbons (CFC) and hydrochlorofluorocarbons (HCFC) which are being phased out under the Montreal Protocol because they deplete ozone levels in the stratosphere. A more detailed description of the usage, emissions and emissions methodology of these gases is given by MCG (1996).

The following sections present the inventories for each of the six greenhouse gases. To date, most international attention has focused on carbon dioxide and methane. Hence estimates of these pollutants were included in the UK inventory a number of years before the latter four pollutants. This is reflected in the longer time series of data available for carbon dioxide and methane. The overall accuracy of the inventories is also discussed. Full details of the methodology used to compile the inventory and the changes which have been made since the publication of the 1995 inventory are included in Appendices 1 and 2, respectively.