AS POLITICAL revolts go, it was a modest one. On Tuesday delegates at the Liberal Democrat conference in Glasgow voted down an amendment to the party's environmental policy which would have exempted Gatwick from its ban on airport expansion. Next year, after the General Election, Sir Howard Davies's review chooses between two possible expansion schemes at Heathrow and one at Gatwick. The Lib-Dem leadership wanted to go into the election with a pro-business flag that they could wave. Ordinary delegates argued, however, that they would rather canvass for a party that was clearly different from the others, bar the Greens. The policy banning airport expansion stays.

This will have an impact on the airports' future, of course, only if the Lib-Dems find themselves in another coalition. The Conservatives have pledged to support whichever airport scheme the Davies review comes up with. Labour has been opposed to a third runway at Heathrow, but is now signalling a softer approach, as indicated by the Shadow Chancellor, Ed Balls, at last month's party conference. Both the main parties, and many within the Liberal Democrat leadership, wish to be seen as "pro-business" which, interpreted, means pro-growth.

The airport industry has a huge PR budget, so it is hard to form a clear view of either the business case or the environmental impact of an expanded airport on the outskirts of London. The purpose of expansion is to create a hub airport - which both airports already are, to a degree - in which passengers connect from one flight to another. The Heathrow website praises the world's largest hub, the Hartsfield-Jackson International Airport in Atlanta, which, it says, "has a direct economic impact of more than about $32.5 billion for the metro Atlanta area economy", a difficult figure to verify.

The trouble with growth, unless is it accompanied by environmental regulation and technical innovation, is that carbon emissions grow proportionately. The car industry, for example, contributes substantially to the UK's carbon emissions, but that figure has remained relatively stable since 1990, despite an increase in vehicles and mileage, thanks to engines that consume less petrol and cleaner fuel. International airlines have not been subjected to such intense lobbying; besides which, the technology is more difficult. Air travel has been estimated to cause as much as 15 per cent of the UK's greenhouse-gas emissions: an alarming figure, given that most residents fly very rarely.

The argument is that a failure to expand would do nothing to save the planet: other cities would simply enlarge their airports and take the business that could have been ours. We have heard similar arguments for other questionable industries, not least the arms trade. In essence, this is to argue that wrongdoing is justified because others would do it. And it is wrongdoing. Britain will find it hard to honour its environmental promises unless there is a technological breakthrough, or the public and the industry show restraint.