2.3 CH₄ emission Estimates

Since 1970, the total methane emission in the UK has declined by around 16%, although this has not been a steady decrease with emissions actually increasing throughout the seventies, reaching a peak in 1980 (Figure 2.5). There was a temporary fall in emissions in 1984 as a result of the miners strike reducing emissions from coal mines. The major sources of methane are landfill sites, livestock in the agricultural sector, leakage during the transmission and distribution of natural gas and coal mines (Table 2.5). The patterns of emissions from each of these sectors are discussed in the following sections.

Figure 2.5 Timeseries of CH₄ Emissions

1. UK emissions reported in IPCC format (Salway, 1998) differ slightly due to the different source categories used.
   
2. Railways, civil aircraft, shipping, naval vessels, military aircraft and off-road sources.

Landfills are estimated to account for around 46% of the UK's methane emissions. However, the estimation is relatively complex as it needs to take account of the wide range of different types of landfill sites in the UK and the variation in methane emissions during the lifetime of a landfill site. Methane emissions are derived from estimates of the amount of putrescible waste disposed of to landfill. Based on a model of the kinetics of anaerobic digestion the rate of methane production is estimated. Corrections are then applied for methane recovery, utilisation, flaring and oxidation by capping soil on the different landfill sites.

The methane emission from landfill shows a gradual increase to a peak in 1986 followed by a decline because of the implementation of methane recovery systems. This trend is likely to continue since all new landfill sites after 1994 must collect and utilise (or flare) the methane emissions. Similarly, since 1994 the combustion of landfill gas has been required at all existing sites in the UK which have significant remaining capacity and where significant gas production is likely. The uncertainties associated with the estimation of methane from landfills are large and it is likely that these estimates will be further refined in the future as more information becomes available.

2.3.3 Agriculture

The second largest source of methane emissions is the agricultural sector where the emissions arise from enteric fermentation in the guts of ruminant animals and from animal wastes. Emissions increased in the early seventies but have declined slowly since 1974. The emission depends on the numbers and types of farm animals with dairy cattle being the most significant source. The recent decline results from the reduction in dairy cattle numbers in line with the 1992 CAP reforms and gradual increases in animal productivity. It is expected that there will be further reductions in animal numbers leading to a continued reduction in the emission of methane.

2.3.4 Coal mining

Methane emissions from coal mining have reduced significantly during the past 20 years reflecting the reduction in UK coal production. In 1970 the emission accounted for 35% of total UK emissions but by 1996 this had reduced to just 9%. The strong correlation between coal production and methane emission is clearly illustrated by the large fall in emissions during the 1984 miners strike. The reduction in mining emissions is the most important contributor to the overall fall in methane emissions.

2.3.5 Leakage from the gas distribution system

Methane leakage from the gas distribution system increased substantially between 1970 and 1990 reflecting the growth in gas sales. It currently accounts for 10% of UK methane emissions. Emissions are estimated based on the throughput of gas and hence are rather uncertain. However, since 1990, emission estimates are based on actual leakage measurements made by British Gas together with data on their gas main replacement programme. Emissions have declined as old mains are replaced by new ones.

2.3.6 Offshore oil and gas

Methane emissions from offshore activities only account for 2% of total UK emissions but they are increasing as the number of installations increases. Emissions are estimated on the basis of the number and type of platforms and terminals. Methane emissions from cold venting, natural gas use, well testing, fugitive emissions and flaring from offshore platforms are now all estimated separately. Of these venting is the main source of methane. Other sources are fuel oil and gas combustion and emissions from tanker loading and unloading.

2.3.7 Sewage disposal

Methane emissions from sewage disposal are fairly uncertain but are likely to be quite small. They depend on the mode of disposal: sea dumping; land spreading or incineration. The proportion disposed of in landfills is allocated to the landfill estimate. Emissions are likely to rise as a result of the EC Urban Waste Water Treatment Directive but the rate of increase will depend on the disposal routes adopted. The UK will stop dumping sewage at sea by 1998.