Official figures put UK emissions in 2005 at 655 million tonnes of carbon dioxide equivalent (MtCO2e), but difficulties in accurately measuring nitrous oxide emissions from agriculture in particular mean they could have been as low as 620MtCO2e - or as high as 762MtCO2e.
The NAO says that the Environment Department (DEFRA) could do more to improve the accuracy of figures by using legislation that requires firms like steel and cement manufacturers to provide accurate information on their emissions.
The NAO also highlights differences between the headline indicator favoured by the government, which follows UN reporting guidelines for the Kyoto Protocol, and a separate set of emissions statistics prepared for the UK environmental accounts by the Office of National Statistics.
While the NAO acknowledges that the UK’s estimates for Kyoto compliance follow best practice, it notes that the environmental accounts are "more comprehensive" because they include emissions from the UK’s share of international aviation and shipping. As a result they put UK emissions some 78MtCO2 higher than the Kyoto figures.
The difference is even more striking where long-term trends are concerned. The Kyoto figures suggest that UK emissions have fallen by more than 16% since 1990, but the NAO says the environmental accounts approach "demonstrates that there have been no reductions in UK carbon dioxide emissions."
Though the environmental accounts are more comprehensive, "they still underestimate the full impact of greenhouse gas emissions", says the NAO because they do not include "indirect" greenhouse gases like carbon monoxide. Nor do they take account of aviation’s increased climate impacts from releasing CO2 at higher altitudes.
Neither set of statistics consider the emissions embodied in services and products that are imported into the UK. The NAO recognises that such consumption-based reporting "might prove useful in highlighting policy issues", but it says that calculating the emissions would be "dauntingly complex", and would be too uncertain to "provide a robust, internationally agreed basis for reporting emissions".