Greenhouse gases generated by British aviation could be far higher than the government's published figures, according to new data obtained from the Department for Transport (DfT). The DfT's published figures - which are also the ones used by airlines, and by Gordon Brown when he announced a £5 hike in air-passenger duty in his 6 December pre-Budget report - state that British aviation generates roughly 34 million tonnes of carbon dioxide a year.
To environmentalists, that figure has long sounded suspiciously small - and now they know why. The department has admitted that it works it out simply by counting the aircraft that take off from Britain. Those that land here are excluded.
That might appear reasonable if half the passengers on those planes were foreigners - but they are not. In fact, according to the Office for National Statistics, 70 per cent of the passengers landing and taking off from the UK are Britons, most of them taking cheap tourist flights booked and paid for here. Which means that the DfT should be adding up the emissions from all the passenger aircraft flying in to or out of Britain and attributing 70 per cent of them to the UK. That would increase the amount of aviation emissions by 14 million tonnes, bringing the total to roughly 48 million.
This is on top of the 570 million tonnes of carbon dioxide that Britain already admits to creating by other means. At present, under international treaties, aviation is excluded from all national emissions figures - a policy that makes environmentalists angry.
"It's becoming increasingly clear that British aviation is having a far greater impact on climate than generally realised and that the figures we are being given lack transparency," says Tim Yeo, the Conservative MP who chairs the Commons environmental audit committee. "Aviation simply cannot be allowed to keep on expanding. We need much higher taxes, or some other way of stopping it growing."
Yet aviation is booming as never before. Last year alone, approximately 200 million passengers passed through Britain's airports. The forecast is for that figure to rise to roughly 470 million by 2030. Under the DfT's official system for calculating emissions - the one based purely on outgoing flights - CO2 emissions would rise from 34 million to about 80 million tonnes in the same period. If, however, the proportion of domestic flyers stays the same, the real contribution would be approximately 112 million tonnes of CO2 by 2030.
By contrast, Britain has pledged to cut its emissions by 60 per cent by 2050. In other words, by then, the whole country should be emitting no more than 229 million tonnes of CO2.
The environmental audit committee has already warned that aviation could derail that policy. It pointed out in its most recent aviation report that the industry's growth is "unsustainable and unacceptable", and said: "Were such growth to occur it could totally destroy the government's commitment to a 60 per cent cut in carbon-dioxide emissions by 2050."
Brown's £5 tax hike looks unlikely to halt that growth. Studies by the Civil Aviation Authority suggest that air passengers would have to be presented with an increase of at least £30 before they would consider not travelling.
Most damaging activities
The revelations come as the industry faces increasing criticism for its determination to keep expanding regardless of the environmental impacts - and for producing whitewash reports to support that expansion.
Early in December, a study by Oxford Economic Forecasting claimed that UK airport congestion had cost the country £1.7bn in 2005. By contrast, it suggested, expansion of the nation's airports would boost the economy by £13bn a year. The report was keen on building another runway at Heathrow, suggesting this could add £7bn to the economy by 2030. What was made less clear was that the research had been funded by the industry, including BAA, the airport operator.
Hard on its heels came another report, this time from Sustainable Aviation, another industry-funded group that wants aviation brought into the European Emissions Trading Scheme (ETS). Under the ETS, fares would rise by between £3 and £27 according to the length of journey - which does not hit the £30 figure that the CAA says would put people off flying.
But why are the lobbyists working so hard just now? One answer is that civil servants are drafting a progress report on the 2003 aviation white paper. Another is that, on 20 December, the EU environment commissioner, Stavros Dimas, is to unveil the laws that will include airlines in the ETS from 2011.
By next spring, however, the aviation industry could find itself with a far nastier set of figures than the DfT's muddled statistics. That is when the Intergovernmental Panel on Climate Change (IPCC) will release its latest report on the pace and scale of global warming and its causes. It will contain a section on aviation, spelling out how flying is emerging as one of the most damaging of human activities.
Besides CO2, which is the gas commonly associated with global warming, aircraft emit a cocktail of other substances (including methane, oxides of nitrogen and water vapour) that multiplies their overall impact, in a phenomenon known as "radiative forcing".
The science of what happens to these chemicals six miles up in the air has been slow to emerge, but in 1999 the IPCC published its first major report on the subject, based on computer models and predictions. It concluded that, over the period 1992 to 2050, the overall radiative forcing by aircraft will be two to four times larger than the effect of aircraft CO2 alone.
That report was ignored by Britain in its aviation white paper. However, thanks to Osama Bin Laden, it is now supported by direct evidence. The 9/11 attacks grounded all commercial aircraft in the United States for three days - giving scientists the chance to observe what happened to an atmosphere that wasn't bombarded by aircraft emissions.
A report since published in Nature found that, in the absence of aircraft vapour trails and related cloud formation, the temperature difference between day and night increased by nearly 2°C. It was the first empirical evidence that aviation is exerting powerful effects on global climate.
"The impact of aviation on climate change is much greater than anyone thought," says Olivier Boucher, head of the climate chemistry ecosystem team at the Met Office's Hadley Centre in Exeter, which specialises in climate-change pre dictions. "We think aviation is warming the climate by a factor of about 2.7 more than you would expect from carbon-dioxide emissions alone. Measuring just carbon dioxide does not give a true picture."
Combining the corrected DfT figures with the effect of radiative forcing indicates that UK passengers are generating the equivalent of 96 million tonnes of CO2. By 2030, this could rise to a staggering 224 million tonnes. A high price for cheap and plentiful flights. The question is: are consumers willing to pay it?
Jonathan Leake is environment editor of the Sunday Times