Methane is the second most important greenhouse gas after carbon dioxide, and its atmospheric concentration is rising at a rate of 1% per year. Emission inventories of the gas are still uncertain, as the major changes between last year's and this year's estimates in the Department of the Environment's annual statistical digest suggest (see table ).
The largest increase is in the data for offshore oil and gas production. Estimates of fugitive emissions from oil and gas platforms have been included for the first time. These account for about one-third of the total figure. The estimate assumes a 1% loss of gas production. The industry disputes the figure, but has yet to devise a more accurate method of estimation to disprove it.
Another factor behind the increase was the inclusion of estimates of gas venting from all oil and gas platforms for the first time. The Department of Energy (DEn) has previously supplied incomplete data, but estimates have now been made for all platforms in the UK sector of the North Sea.
The landfill data are based on studies of the UK's exploitable landfill gas resource by the DEn's Energy Technology Support Unit. The revised figures are based on a more comprehensive survey of gassing landfills, covering 453 large sites each containing over 200,000m3 of waste.
The amounts of gas known to be utilised or flared have been subtracted from the estimated landfill methane generation figure, and a further percentage has been deducted to allow for oxidation of methane in the soil. Critics say the result is too low because it does not allow for methane generation by many small landfills which do not produce exploitable quantities of the gas, or by closed landfills.
Meanwhile, the Watt Committee on Energy held a seminar in London in February at which scientists discussed methods of assessing emissions more accurately. A recurring theme of the meeting was the inadequate information made available to academics by UK companies.
British Gas was criticised for failing to provide details of how it arrived at data on gas leakage which were incorporated in the DoE's digest. The company has produced few figures on the lengths and ages of pipework in its distribution system, or other data needed to estimate the proportion of gas lost to the atmosphere. Its attitude has helped to foster suspicions that actual leakage rates may be several times more than 1% of throughput, the figure put forward by the company.
The debate began in 1989 when an American researcher claimed that leakage rates in the USA were 2.9-5.9% (ENDS Report 175, pp 3-5). In 1990, a study by Earth Resources Research for Greenpeace concluded that leakage from the UK system was somewhere between 1.9 and 10.8% of throughput, with 5.3% as the medium case.
British Gas has just announced plans to spend over £2 million on a survey to improve its leakage figures. The study will measure leakages from isolated lengths of pipe, chosen to represent different materials and conditions in the distribution network.
The need for an accurate emissions inventory is becoming more pressing because action to curb methane releases will be required by policy developments at EC and global level within the next few years.