Methane

Methane (CH4) is, after carbon dioxide, the most important trace gas species responsible for radiative forcing (sometime known as the "Greenhouse effect"), contributing a global forcing of 0.47 W m-2 compared to 1.56 W m-2 from carbon dioxide. Atmospheric concentrations have risen from a pre-industrial concentration of around 700 ppbv to the present concentration of around 1750 ppbv (IPCC, 1996) and are currently increasing at a rate of around 10 ppbv year-1, although this is a significant improvement over the growth rate during the 1970's of around 20 ppbv year-1.

Globally, the annual (inventory) emission of CH4 is 535 Tg year-1, of which the largest natural source is wetlands (predominantly high latitude) at 115 Tg. The sources are summarised in Table 1.

Table 1: Global Sources

Emissions (Tg CH4 year-1)

Wetlands (N)

115

Termites (N)

20

Oceans (N)

10

Other (N)

15

Fossil fuel derived (A)

100

Enteric fermentation (A)

85

Rice paddies (A)

60

Biomass burning (A)

40

Landfills (A)

40

Animal waste (A)

25

Domestic sewage (A)

25

TOTAL

535

The relative contributions to the UK methane emission are rather atypical of the global values. Total UK emissions for 1997 were 3.7 Tg (0.7% of global emissions) declared to IPCC and 3.8 Tg in total and the breakdown of emissions from each sector is given in Table 2 (derived from Salway, 1998 and Williams, 1994). Almost half UK emissions come from landfill, the other significant sources being from enteric fermentation and fossil fuel derived sources. Within this latter category, the majority of the emission comes from fugitive release of methane during storage and transportation of fossil fuels.

Table 2: UK Sources

Emissions (Tg CH4 year-1)

Wetlands (N)

0.1

Fossil fuel derived (A)

0.90

Enteric fermentation (A)

0.94

Landfills (A)

1.72

Animal waste (A)

0.12

Domestic sewage (A)

0.03

TOTAL

3.81

Disaggregated methane emissions for Great Britain have recently been calculated (Fowler et al, 2000) based on detailed information on landfill inventories, livestock distribution (Sutton et al, 1996), mining activities and population distribution. Disaggregated figures for Northern Ireland are not available. This disaggregation produces a significantly lower emission factor than that of Salway (1998) and is summarised in Table 3.

Table 3: Sources

England & Wales

Scotland

Great Britain

Landfill

741

426

1167

Livestock

842

205

1047

Coal mining

236

19

255

Gas distribution

399

41

440

Wetland

-�

�120

120

Total

2218

811

3029

This indicates that Scotland produces 27% of the UK methane emission with the dominant sources being landfills and agriculture, although the UK natural wetlands (of which the majority are in Scotland) comprise 15% of the Scottish emission. The bulk of the non-wetland emission in concentrated in the Glasgow-Edinburgh-Dundee corridor and the Aberdeen urban area (see Figure 1 below).

In terms of the effects of methane on habitats and species, there is no direct, chemical effect on animals or plants. There are, however, secondary effects arising from climate change which will bring about, for example, changes in air temperature, hydrology and sea level. Until the exact nature of these climatic changes is known, the effect on individuals or populations is difficult to assess. Further secondary effects on atmospheric chemistry will arise as the main sink for methane in the atmosphere is by reaction with the hydroxyl radical.

non-wetland emission of CH4

Figure 1: The bulk of the non-wetland emission of CH4 is in concentrated in the Glasgow-Edinburgh-Dundee corridor and the Aberdeen urban area

References: 

Fowler, D.; Hargreaves, K.; Skiba, U.; Bower, K. 2000 Direct Measurements of the UK Source Strength of Radiatively Active Gases Final Report to DETR
Sutton, M.A.; Dragosits, U.; Fowler, D.; Place, C.J. 1996 A preliminary map of the distribution of methane emissions from livestock sources in Great Britain Draft Report
Williams, A. 1994 Methane Emissions Report of a working group appointed by the Watt Committee on Energy