Greenhouse Gas Emissions

Introduction

Increasing atmospheric concentrations of greenhouse gases (GHGs) originating from anthropogenic activities are leading to enhanced warming of the atmosphere and global climate change. The major greenhouse gases are carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) all of which have both natural and anthropogenic sources. In contrast, the three industrial gases: hydrofluorocarbons (HFC), perfluorocarbons (PFC) and sulphur hexafluoride (SF₆), are potent greenhouse gases but do not occur in nature, and hence only originate from anthropogenic 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 target for the UK is to achieve a reduction of the global warming potential of the six greenhouse gases of 12.5% by 2008-2012 (based on 1990 emissions estimates). Consequently the UK is required to compile annual emission inventories of these greenhouse gases and report the emissions to international bodies, to demonstrate compliance with this legislation. The UK Greenhouse Gas Inventory is submitted to UNFCCC in the Common Reporting Format (CRF). The Common Reporting Format gives net carbon emissions, that is emissions minus removals. The data in this report is presented here in a UN/ECE reporting format (see Annex 1) and quotes land use change and forestry emissions and removals separately. More detailed information may be found in the annual report on UK GHG emissions produced by the NAEI (Salway et al, 2002).

The 2000 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 (Chapters 4, 5 and 5 respectively).

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 2000

Direct GHG	Emissions (kt) in 2000	GWP (100 years)	Global Warming Equivalence (equivalent kt of CO2)
CO2	147,532	1	540,951
CH4	2,427	21	50,960
N2O	141	310	43,813
HFCs	3.4	560-12100^2	9,316
PFCs	0.097	6000-7400^2	668
SF6	64.4	23900	1,540

1 Calculated on IPCC basis and includes land-use change emissions. CO₂ includes emissions only and does not include land use change and forestry removals

2 Range of GWP for species used

During the period 1990-2000 there has been a decrease in UK emissions of CO₂, CH₄, N₂O, HFC and PFC but considerable increases in SF₆ emissions. The overall effect has been a decrease in global warming potential from UK emissions. Figure 2.1 shows greenhouse gas emissions (comprising CO₂ , CH₄ , N₂O , HFCs, PFCs and SF₆) as CO₂ equivalent for 1990 to 2000. Reliable emission estimates of HFCs, PFCs, SF₆ and N₂O (from adipic acid production) are not available prior to 1990.

Figure 2.1 Total Global Warming Potential Associated with UK Emissions of GHGs

CO₂ is the major contributor to greenhouse gas emissions in the UK 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, 2002) 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 anthropogenic 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 (N₂O), 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). 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, 2000). 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 due to their role in the depletion of ozone in the stratosphere. A more detailed description of the usage, emissions and methodology for calculating emissions of these gases is given by MCG (2000).

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 other 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 inventory for 1999 are included in Appendices 1 and 2, respectively which can be accessed from the methodology section of the NAEI website (http://www.naei.org.uk)

CO₂ emission estimates

Total CO₂ emissions

Carbon dioxide emissions in 2000 have reduced by 22% since 1970, as shown in Figure 2.2. However, this decline has not been steady, and peaks were observed in 1973 and 1979 which were due to the state of the economy, high oil prices and severe winters in these years. Emissions fell again during the early eighties reflecting the recession during this period and the coal miners strike of 1984. Since the mid-1980s the emissions profile has been much smoother showing an overall reduction in emissions. There are small increases in several sectors. The elevated emission from the domestic sector in 1996 is considered to be due to the colder than average winter (indicated by lower than average mean air temperatures).

The major emissions of carbon dioxide arise from the combustion of fossil fuels in power generation, and the transport, domestic and industrial sectors (Figure 2.2, Table 2.2). The level of emissions depends on the fuel mix and the fuel consumption data. Details of UK fuel consumption are given annually in the Department of Trade and Industry's Digest of United Kingdom Energy Statistics (DTI, 2000, 2001). The fuel consumption data used to calculate the pollutant emission totals in the NAEI are given in Table 2.3; fuels which are used as feedstocks are omitted (principally natural gas used for the production of ammonia, methanol and acetic acid and some use of LPG and OPG in petrochemical plants).
Table 2.2 UK Emissions of CO₂ as Carbon by UN/ECE¹ Source Category and Fuel (Mt)

	1970	1980	1990	1991	1992	1993	1994	1995	1996	1997	1998	1999	2000	2000%
BY UN/ECE CATEGORY^2
Comb. in Energy Prod.
Public Power	57.0	58.3	54.1	53.3	50.4	45.5	44.4	44.1	43.5	39.6	40.6	38.8	42.0	28%
Petroleum Refining Plants	5.6	5.5	4.8	5.0	5.1	5.4	5.2	5.4	5.6	5.5	5.6	5.1	4.5	3%
Other Comb. & Trans.	8.2	2.9	3.8	3.8	3.8	3.8	4.4	4.8	5.3	5.4	5.8	5.9	5.9	4%
Comb. in Comm/Inst/Res
Residential Plant	26.5	23.2	21.5	23.9	23.1	24.2	22.9	21.7	24.7	22.7	23.2	23.1	23.4	16%
Comm/Pub/Agri Comb.	12.3	9.9	8.5	9.3	9.0	8.8	8.7	8.8	9.3	8.6	8.5	8.4	8.4	6%
Combustion in Industry
Iron & Steel Comb.	15.8	5.2	6.2	6.2	6.1	6.1	6.5	6.5	6.6	6.7	6.3	6.2	5.8	4%
Other Ind. Comb.	32.5	24.9	18.3	18.4	18.3	17.9	18.0	17.4	17.5	17.6	17.2	17.1	16.7	11%
Production Processes	4.8	3.9	4.0	3.3	3.2	3.2	3.5	3.5	3.8	3.6	3.5	3.7	3.7	3%
Extr./Distrib. of Fossil Fuels	0.1	0.1	0.4	0.2	0.2	0.1	0.2	0.2	0.2	0.2	0.2	0.24	0.2	0%
Road Transport	16.3	21.2	29.7	29.5	29.9	30.3	30.4	30.1	31.3	31.7	31.5	31.7	31.5	21%
Other Trans/Mach
Off-Road Sources	3.1	3.0	3.2	3.0	2.9	2.9	2.8	2.7	2.8	2.6	2.4	2.3	2.2	1%
Other^3	2.3	2.0	1.7	1.8	1.8	1.8	1.7	1.7	1.7	1.7	1.7	1.6	1.6	1%
Waste	0.3	3.4	2.3	2.1	2.0	2.0	2.6	2.0	2.0	1.6	1.6	1.4	1.4	1%
Agricul & Land Use Change	0.5	0.4	0.4	0.5	0.5	0.3	0.3	0.4	0.4	0.4	0.3	0.2	0.2	0%
By FUEL TYPE
Solid	90.3	70.6	61.4	61.0	57.4	49.4	46.0	42.7	39.6	34.4	34.3	30.5	32.0	22%
Petroleum	67.7	54.5	52.9	52.8	53.0	52.9	51.7	50.2	51.4	49.4	48.1	47.0	45.8	31%
Gas	22.7	32.2	38.8	41.0	40.7	45.0	47.8	50.7	57.8	58.7	60.7	63.5	64.9	44%
Non-Fuel	4.7	7.0	6.2	5.4	5.2	5.0	6.0	5.5	5.7	5.5	5.4	5.0	4.8	3%
TOTAL	185.3	164.2	159.2	160.2	156.3	152.3	151.5	149.2	154.6	147.9	148.4	146.0	147.5	100%

1 UK emissions reported in IPCC format (Salway, 2001) differ slightly due to the different source categories used.

2 See Annex 1 for definition of UN/ECE Categories

3 Railways, civil aircraft, shipping, naval vessels and military aircraft.

Figure 2.2 Time Series of CO₂ Emissions

Figure 2.3, Spatially Disaggregated UK Emissions of CO₂

Electricity supply industry

The electricity supply industry is the major consumer of fossil fuels, and hence the major source of carbon dioxide emissions in the UK (Tables 2.2 and 2.3). There have been significant changes in the generating mix between 1980 and 2000, as shown in Figure 2.4. The level of CO₂ emissions is determined by both the fuel mix and the generating technology used. During the 1970s the electricity supply industry was dominated by coal and fuel oil fired thermal power stations, and coal and oil consumption increased to meet the rising demand for electricity. The use of coal for power generation peaked in 1980 at 89.6 Mt and has subsequently declined. The fall has not been steady, showing minima in 1982 and 1984 due to recession in the early 1980s and the miners strike of 1984. During the late 1980s and early 1990s, the closure of inefficient plants led to an overall increase in the thermal efficiency of the conventional thermal power plants, and the contribution of nuclear power generation increased with the greater utilisation of existing nuclear plants and the commissioning of Sizewell B in 1995. The use of oil generation peaked in 1972 and apart from increased consumption during the miners strike of 1984 has been in decline ever since. Two oil-fired stations were converted to burn Orimulsion^® (an emulsion of bitumen and water) although this practice has been discontinued, largely on environmental grounds. More recently, the privatisation of the power industry has resulted in a move away from coal and oil generation towards combined cycle gas turbines (CCGT). The use of gas in power generation has increased by a huge amount. Further increases may be expected as and when more CCGT stations come on line.

Table 2.3 UK Fuel Consumption

Fuel	Consumer		1970	1980	1990	1995	1996	1997	1998	1999	2000
Coal	Major Power Prod.	Mtonnes	77	90	83	58	53	45	47	40	45
Coal	Industry	Mtonnes	22	9	8	6	5	5	4	4	3
Coal	Domestic	Mtonnes	20	9	4	3	3	3	2	3	2
Coal	Others	Mtonnes	38	16	14	10	10	10	10	9	10
Other Solid Fuels	All Consumers	Mtonnes	26	10	11	10	10	10	9	9	9
Motor Spirit	Road Transport	Mtonnes	14	19	24	22	22	22	22	22	21
Gas Oil	Road Transport	Mtonnes	5	6	11	13	14	15	15	16	16
Gas Oil	Industry	Mtonnes	5	5	3	3	3	3	3	3	3
Gas Oil	Others	Mtonnes	8	8	6	6	6	6	6	5	5
Fuel Oil	Major Power Prod.	Mtonnes	12	6	6	2	2	1	1	1	1
Fuel Oil	Refineries	Mtonnes	4	4	2	2	2	2	2	2	1
Fuel Oil	Industry	Mtonnes	21	10	4	3	3	2	2	1	1
Fuel Oil	Others	Mtonnes	10	4	2	3	3	3	2	2	1
Orimulsion	Major Power Prod.	Mtonnes	0	0	0	1	1	0	0	0	0
Burning Oil	Domestic	Mtonnes	2	2	2	2	3	3	3	2	2
Burning Oil	Others	Mtonnes	4	0	0	1	1	1	1	1	1
Aviation Turbine Fuel	Air Transport	Mtonnes	3	5	7	8	8	8	9	10	11
Other Pet. Products	All Consumers	Mtonnes	1	1	1	2	2	2	2	2	2
Petroleum Gases	Refineries	Mtherms	1011	987	1303	1382	1460	1378	1318	1278	1308
Petroleum Gases	Others	Mtherms	700	717	680	858	858	760	742	728	687
Natural Gas	Major Power Prod.	Mtherms	60	55	3	4540	6029	7633	8063	9622	9683
Natural Gas	Industry	Mtherms	710	6616	7016	7967	8783	9152	9698	10111	10531
Natural Gas	Domestic	Mtherms	627	8420	10250	11124	12824	11790	12144	12218	12622
Natural Gas	Others	Mtherms	2206	2188	3021	3945	4375	4172	4382	4502	4801
Other Gases	All Consumers	Mtherms	7009	1126	1371	1268	1339	1394	1397	1409	1430

Figure 2.4 Generating Mix (1980-2000) Million Tonnes Oil Equivalent (Mtoe).

The effect of these changes in the power sector are clearly reflected in the carbon dioxide emissions. Since 1970 electricity generation has increased by nearly 50% but emissions have decreased by around 26%. Despite a slight increase in coal consumption between 1999 and 2000, emissions have fallen by 22% between 1990 and 2000. This is due specifically to:

· The greater efficiency of the CCGT stations compared with conventional stations - around 47% as opposed to 36%.

· The calorific value of natural gas per unit mass carbon being higher than that of coal and oil (the inventory takes account of unrefined gas or sour gas used by some plant).

· and to a lesser extent, the proportion of nuclear generated electricity increasing from 21% to 25%.

The overall effect of the fuel and technology changes are also clearly illustrated in Figure 2.5 which shows that the average CO₂ emission (from power generation) per kWh electricity generated decreases from 245 tonnes/GWh in 1970 to 123 tonnes/GWh in 2000. This trend is likely to continue into the future through the use of more advanced technology and abatement equipment. However, the extent to which nuclear power is used in the future is expected to have a large impact on air emissions. A decreased use of nuclear power for electricity production is expected to give rise to increased consumption of fossil fuels, and the increased use of coal between 1999 and 2000 is evident.

Figure 2.5 Average CO₂ Emission per unit of Electricity Generated (ktonne/GWh)

Domestic

The domestic use of coal (including anthracite) shows an overall decline between 1970 and 2000, falling by 91%; domestic use of smokeless solid fuels (including coke) has also fallen significantly. This reflects a trend away from solid fuels towards alternatives such as electricity and gas in the domestic sector. Over the same period the domestic use of natural gas has increased by a factor of over 20.

Industrial

The 2000 industrial emissions show a decrease of 53% since 1970. The curve exhibits peaks in 1973, 1979, and 1988 which are due in part to the cold winters in these years but mainly to the various levels of economic activity. According to DTI (2001), industrial energy consumption in 2000 has declined since 1970 by 43% reflecting the decline in a number of the energy intensive industries conducted in the UK and improvements in energy efficiency. Over this period, there has also been a marked move away from coal and oil consumption to gas and to a lesser extent electricity.

Transport

Total emissions from the transport sector have steadily increased since 1970. Of these, road transport emissions have risen by 94% and currently account for 89% of the total transport/mobile machinery emissions in 2000. This also equates to 24% of the total UK carbon dioxide emissions. Emissions fell a little during 1974-75 reflecting the increase in motor fuel prices after the oil crisis. The steady increase in fuel use by most forms of transport reflects the increased demand for transport in the UK between 1970 and 2000. The increased use of private motor vehicles has resulted in a 50% increase in the consumption of petrol from 1970 to 2000. However, petrol consumption has declined by 12% since 1990, which is a result of the increase in popularity of diesel cars, and the increased fuel efficiency of petrol driven cars. Increased use of goods vehicles is reflected in the increase in DERV consumption by a factor of more than 3 since 1970.

Agriculture/forests/land use change

The effect of changing land-use can result in either net emission or net absorption of CO₂ , particularly on a global scale. For example, forest clearing for agricultural use could be a net source of CO₂. Recently the Intergovernmental Panel on Climate Change agreed new guidelines for preparing national inventories (IPCC, 1997). Land use change and forestry estimates are included in the UK Greenhouse Gas Inventory (Salway et al, 2002) for the years 1990-2000. The estimates have yet to be included in the NAEI which covers the longer period 1970-2000 (the entry under "Agricul. & Land use change" in Table 2.2 corresponds to the CO₂ emissions arising from the application of lime to soils). For comparative purposes the land use change estimates of carbon emissions and removals are summarised in Table 2.4 (Milne, 2000). The new guidelines for Land Use Change and Forestry include emissions from liming of soils under ‘D CO₂ Emissions and Removals from Soils’. These are already included in the NAEI under Agricultural Soils.

Table 2.4 Emissions & Removals of CO₂¹ (as CO₂) from Land Use Change & Forestry (2000)

Sources	Emissions (kt)	Removals (kt)
A. Changes in Forest and Other Woody Biomass Stocks ^1	-	10,553
B. Forest and Grassland conversion	-	-
C. Abandonment of Managed Lands	-	-
D. CO2 Emissions and Removals from Soil ^2	11,441	-
E. Other ^3	3,567	1,100
Total	15,007	11,653

¹ Removals include removals to forest and soil litter, and to forest products.

² Emissions include removals to soil due to set aside of arable land

³ Emissions include emissions from soil due to upland drainage, lowland drainage and peat extraction. Removals are increases in crop biomass.

CH₄ emission Estimates

Total CH₄ emissions

Since 1970, the total methane emission in the UK has declined by 35%, although this has not been a steady decrease with emissions actually increasing throughout the seventies, reaching a peak in 1983 (Figure 2.6). 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 2.6 Time series of CH₄ Emissions

Landfill

Landfills are estimated to account for 27% of the UK’s methane emissions in 2000. 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 the mid 1980’s 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.

Agriculture

The 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 and have declined extremely slowly since 1974. The methane emission is dependent 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.

Table 2.5 UK Emissions of Methane by UN/ECE¹ Source Category (kt)

	1970	1980	1990	1991	1992	1993	1994	1995	1996	1997	1998	1999	2000	2000%
BY UN/ECE CATEGORY^2
Comb. in Energy Prod.
Public Power	2	2	6	6	8	9	10	12	14	16	19	24	28	1%
Petroleum Refining Plants	1	1	1	1	1	1	1	1	1	1	1	1	1	0%
Other Comb. & Trans.	1	1	1	1	2	2	2	3	3	4	3	8	7	0%
Comb. in Comm/Inst/Res
Residential Plant	326	139	65	68	62	61	47	35	37	34	36	39	29	1%
Comm/Pub/Agri Comb.	2	2	4	5	4	4	4	4	5	4	4	4	4	0%
Combustion in Industry	25	12	13	13	12	12	13	13	13	14	14	13	12	1%
Production Processes	9	8	8	8	9	7	9	7	8	7	5	4	4	0%
Extr./Distrib. of Fossil Fuels
Coal Mines	1540	1269	818	838	803	724	456	504	473	445	375	310	264	11%
Gas Leakage	88	354	398	387	382	377	372	368	362	349	346	346	333	14%
Offshore Oil&Gas	13	43	93	84	87	80	77	73	67	67	62	44	40	2%
Gasoline Distribution								0	0	0	0	0	0	0%
Road Transport	18	23	30	29	28	26	25	23	22	21	19	17	16	1%
Other Trans/Mach ^3	3	2	2	2	2	2	2	2	2	2	2	2	2	0%
Waste
Landfill	764	889	1117	1080	1031	984	945	912	872	826	774	716	660	27%
Non Landfill Waste	32	66	55	51	54	54	62	65	64	59	59	56	58	2%
Agricul. & Land Use Change
Animal Wastes	106	113	111	110	110	111	112	110	111	111	112	110	105	4%
Enteric Fermentation	781	870	908	895	900	900	907	897	905	893	894	894	864	36%
Non Livestock Agricult.	14	21	13	11	8	0	0	0	0	0	0	0	0	0%
By FUEL TYPE
Solid	331	140	67	69	63	62	48	35	37	34	36	38	28	1%
Petroleum	26	29	34	33	32	31	29	28	27	25	23	21	19	1%
Gas	18	11	21	23	23	25	27	30	34	37	39	49	51	2%
Non-Fuel	3350	3635	3523	3464	3384	3237	2940	2935	2862	2756	2626	2480	2328	96%
TOTAL	3724	3815	3645	3589	3503	3354	3044	3028	2960	2852	2724	2589	2427	100%

1 UK emissions reported in IPCC format (Salway, 2002) differ slightly due to the different source categories used.

2 See Annex 1 for definition of UN/ECE Categories

3 Railways, civil aircraft, shipping, naval vessels, military aircraft and off-road sources.

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 41% of total UK emissions but by 2000 this had reduced to just 11%. 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.

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 14% 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 (and successor companies) together with data on their gas main replacement programme. Emissions have declined as old mains are replaced by new ones.

Offshore oil and gas

Methane emissions from offshore activities only account for 2% of total UK emissions but have declined in recent years (despite the increase in the number of installations). Emissions are estimated on the basis of data provided by the operating companies to the UK Offshore Operators Association. 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.

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 stopped dumping sewage at sea in 1998.

N₂O Emission ESTimates

The major source of nitrous oxide emissions in the UK are from agricultural activities. Lower levels of emissions arise from industrial processes, combustion processes in the power generation sector and from road transport (Table 2.6 and Figure 2.7).

Table 2.6 UK Emissions of Nitrous Oxide (N₂O) by UN/ECE¹ Category(kt)

	1970	1980	1990	1991	1992	1993	1994	1995	1996	1997	1998	1999	2000	2000%
BY UN/ECE CATEGORY^2
Comb. in Energy Prod.
Public Power			6.2	6.1	5.8	5.4	5.5	5.6	5.8	5.8	6.2	6.3	6.7	5%
Petroleum Refining Plants			0.4	0.4	0.4	0.4	0.4	0.4	0.4	0.4	0.4	0.4	0.3	0%
Other Comb. & Trans.			0.8	0.8	0.8	0.9	1.0	1.0	1.1	1.2	1.3	1.3	1.4	1%
Comb. in Comm/Inst/Res			1.2	1.3	1.2	1.2	1.1	0.9	0.9	0.8	0.8	0.8	0.7	0%
Combustion in Industry			2.1	2.1	2.3	2.1	2.1	2.1	1.9	1.8	1.6	1.6	1.4	1%
Production Processes			94.5	88.5	71.6	61.1	71.5	61.3	65.5	67.0	59.5	18.2	20.0	14%
Extract/Distrib Fossil Fuels			0.1	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0%
Road Transport			3.3	3.4	3.8	4.7	5.9	7.1	8.4	9.7	10.8	11.9	12.7	9%
Other Trans/Mach^3			3.7	3.8	3.8	3.7	3.6	3.5	3.6	3.4	3.4	3.2	3.1	2%
Waste			3.7	3.7	3.7	3.7	3.7	3.7	3.8	3.8	3.8	3.8	4.0	3%
Land Use Change
Animal Wastes			4.9	4.9	4.8	4.8	4.9	4.8	4.9	4.9	4.9	4.9	4.6	3%
Non Livestock Agricult.			98.2	97.7	92.4	90.7	93.1	93.6	94.2	97.1	94.3	92.4	86.4	61%
By FUEL TYPE
Solid			8.6	8.7	8.3	7.1	6.7	6.2	5.7	4.9	4.8	4.3	4.3	3%
Petroleum			7.8	8.0	8.5	9.3	10.4	11.5	12.9	13.9	14.9	15.8	16.5	12%
Gas			1.2	1.2	1.3	1.9	2.5	2.9	3.6	4.3	4.6	5.2	5.4	4%
Non-Fuel			201.3	194.8	172.6	160.3	173.3	163.5	168.5	172.9	162.6	119.4	115.1	81%
TOTAL			218.9	212.7	190.6	178.7	192.8	184.1	190.7	196.0	186.9	144.7	141.3	100%

1 UK emissions reported in IPCC format (Salway, 2001) differ slightly due to the different source categories used.

2 See Annex 1 for definition of UN/ECE Categories

3 Including railways, shipping, naval vessels, military aircraft and off-road sources

Figure 2.7 Time Series of N₂O Emissions

Agriculture

The calculation of emissions from agricultural soils and animal wastes have been considerably extended in recent years. In particular, the publication of the Revised 1996 IPCC Guidelines (IPCC, 1997) has enabled a greater number of sources to be considered. Further details of the methodology are given in Appendices 1 and 2.

The emissions from agricultural soils currently account for around 61% of total UK emissions. The most significant sources are fertiliser application and emissions indirectly from leaching.

Production processes

The production processes sector is comprised of emissions from adipic acid manufacture (a feedstock for nylon) and nitric acid manufacture. This sector accounts for 14% of the total nitrous oxide emissions in 2000. The extent of the emission depends on the production of these acids, hence the time series reflects production levels. However, the UK manufacturer of adipic acid commissioned an abatement unit in 1998 which has significantly reduced the UK total emission.

Power generation

The contribution from public power generation has been relatively constant between 1990 and 2000 in spite of the trend away from coal towards natural gas combustion.

Road transport

Emissions from the road transport sector have increased significantly since 1992. This is as a direct result of the introduction of three way catalytic converters which produce significantly larger emissions of nitrous oxide than cars not equipped with abatement technology. Between 1990 and 2000 the proportion of the car kilometres with converters has increased from <1% to 60% (Figure 2.8) and emissions of N₂O have greatly increased. The contribution of road transport to the total N₂O emission is small, but is expected to increase. Conversely emissions from the majority of the currently significant sectors are expected to decrease with time.

Figure 2.8 The growth in number of cars with catalytic converters

*Includes cars with failed catalysts

HFCs, PFCs and SF₆ Emission Estimates

Hydrofluorocarbons

The UK emissions of HFCs are shown in Table 2.7. The emissions are reported in terms of their global warming potential (GWP). The HFC emissions comprise many species each with its own GWP, hence it is more helpful to express emissions in terms of GWP as Carbon equivalent. It is not currently possible to give emission estimates for individual HFCs because some of these are considered commercially sensitive data within the industries involved.

HFCs had limited usage primarily as refrigerants blended with CFCs. However, CFCs and HCFCs are being phased out under the Montreal protocol, and hence HFCs are now being used increasingly as:

· substitutes for CFCs and HCFCs in domestic, commercial and industrial refrigeration and air conditioning

· substitutes for CFCs in plastic foam blowing

· substitutes for CFCs for some medical aerosols

· substitutes for CFCs for industrial and specialist aerosols

· fire fighting fluids

Salway (2002) reports the emissions, estimation methodology and ongoing improvements in some detail. The UK reports both actual and potential emissions of HFCs, although here only the actual emissions are presented.

Refrigeration is the largest source and contributed 36% of the total in 2000. Here emissions arise due to leakage from refrigeration and air conditioning equipment during its life time and from losses during manufacture and the recovery of the refrigerants on decommissioning.

There has been a large decrease in emissions from “Halocarbon production” from 1998 to 1999. This is a result of an abatement system fitted to a plant producing HCFCs.

In the case of closed foams where the fluid is retained within the foam there will be some leakage from the foam during its lifetime and on disposal but with open foams all losses occur during manufacture. Since 1990, the use of HFCs in aerosols has greatly increased, and this source sector now accounts for 13% of the total emission.

The total 2000 HFC UK emission has decreased by 24% compared with the 1990 emission and is characterised by the increasing use of HFCs offset by the large reduction in emissions from halocarbon production between 1998 and 2000.

Table 2.7 UK Emissions of HFCs (ktonnes C equivalent)

	1990	1991	1992	1993	1994	1995	1996	1997	1998	1999	2000	2000%
Halocarbon production	3102	3229	3357	3485	3614	3807	3898	4266	4509	1179	1177	46%
Foams & Firefighting	0	0	0	0	0	1	5	9	11	15	37	1%
Refrigeration	0	2	6	26	121	228	333	482	628	775	915	36%
Aerosols^1	0	3	4	8	32	111	207	274	356	376	412	16%
Total	3102	3234	3367	3519	3767	4147	4443	5031	5504	2346	2541	100%

¹ Includes metered dose inhalers.

Perfluorocarbons

Table 2.8 shows the UK emissions of PFCs reported as Ktonnes C equivalent. It is not currently possible to give emission estimates for individual PFCs because some of these are considered commercially sensitive data within the industries involved. PFCs had limited usage prior to the phase out of CFCs in the electronics and electrical industry. PFCs are now used in:

· etching processes in the semiconductor industry

· chemical vapour deposition in the electronics industry

· soldering processes

· leak testing of electrical components

· cooling electrical components , for example in supercomputers and radar systems.

Other uses include

· refrigerant blended with HFC

· firefighting in specialist applications

· cushioning in the soles of training shoes

Other minor uses of CFCs, which now use PFCs, were in cosmetics and as a tracer gas.

The largest source of PFCs for 2000, representing over half of the total, is from the electronics sector, where emissions arise from the manufacture of semiconductors. PFCs are also formed as a by-product of aluminium smelting. Emission from this sector account for 30% of the UK total in 2000. The emissions are caused by the anode effect which occurs when alumina concentrations become too low in the smelter. This can cause very high electrical current and decomposition of the salt - fluorine bath. The fluorine released reacts with the carbon anode, creating PFC compounds CF₄ and C₂F₆ . Total emissions have declined by 71% since 1990 reflecting steps taken by the industry to reduce emissions.

Table 2.8 UK Emissions of PFCs (Ktonnes C equivalent)

	1990	1991	1992	1993	1994	1995	1996	1997	1998	1999	2000	2000%
Fire-fighting	0	0	0	0	0	0	0	1	1	1	1	1%
Aluminium Production	554	425	203	129	111	102	81	65	61	57	55	30%
Refrigeration	0	0	0	0	1	2	4	5	7	8	9	5%
Electronics & Training Shoes	68	64	58	92	155	194	162	110	109	119	117	64%
Total	622	488	262	221	267	298	247	180	178	185	182	100%

Sulphur Hexafluoride

SF₆ is used in the following applications:

· insulation medium in high voltage applications such as switchgear and circuit breakers

· cover gas in magnesium foundries to protect the molten magnesium from re-oxidising when it is cast

· degasser in aluminium casting applications, though its use in the UK is rather limited

· insulating gas in double glazing applications, replacing vacuum as an insulation technique

· plasma etching of polysilicon and nitrite surfaces

· atmospheric tracer for scientific studies

· cushioning in soles of training shoes

Table 2.9 shows the UK emissions of SF₆. The largest source is from magnesium manufacture, where SF₆ is used as a cover gas. It is not possible to recover the SF₆, and hence the total consumption of SF₆ is released to atmosphere- emissions from this sector account for 39% of the UK total in 2000. Emissions in 2000 are higher than those in 1990 because SF₆ has been increasingly used in place of SO₂.

The other main sources are from the manufacture of trainers, and electrical/electronic equipment. The use of SF₆ as a cushioning in trainers will be phased out in the near future. Emissions from the electrical insulation arise during the manufacture and filling of electrical switchgear and from leakage and maintenance during the equipment’s lifetime. This application has only been in use for the last 20 to 30 years and little of the equipment has been decommissioned. It is expected that users will take great care over future fluid recovery so that emissions will be minimised. SF₆ emissions have increased by 113% since 1990.

Table 2.9 UK Emissions of Sulphur Hexafluoride (tonnes)

	1990	1991	1992	1993	1994	1995	1996	1997	1998	1999	2000	2000%
SF6 Cover Gas	20.0	20.0	20.0	20.0	25.0	25.0	30.0	30.0	25.0	25.0	25.0	39%
Electrical Insulation	5.3	5.6	6.0	6.3	6.4	8.4	8.9	8.2	22.0	22.7	23.5	36%
Electronics & Trainers	0.0	1.8	3.9	5.9	8.0	10.0	10.8	11.7	12.5	13.3	14.2	22%
Electrical equipment	5.0	5.0	5.0	5.0	5.0	4.0	3.4	3.0	2.6	2.2	1.8	3%
Total	30.3	32.5	34.8	37.2	44.4	47.4	53.2	52.8	62.1	63.2	64.4	100%

ACCURACY of emission estimates OF GREENHOUSE GASES

Quantitative estimates of the uncertainties in the greenhouse gas emissions were calculated using direct simulation, a technique similar to Monte Carlo Simulation. This corresponds to the IPCC Tier 2 approach. This work is described in detail by Eggleston et al (1998) though the estimates reported here have been revised to reflect changes in the 2000 Inventory. (Salway et al, 1991)

Table 2.10 Uncertainty of the Emission Inventories

Pollutant	Estimated Uncertainty %
Carbon Dioxide	± 2
Methane	± 21
Nitrous Oxide ^a	-75% to +264%
HFCs	± 25
PFCs	± 19
SF6	± 13

a - Skewed uncertainty distribution.

It should be noted that these uncertainties primarily arise from emission factor uncertainties. Activity data is considered to be more reliable and better characterised. As a result it can be assumed that the trends identified from time series plots are considerably more reliable than an absolute emission total.