Like the lights of the fourth plane in the queue to land at Heathrow, we could see this coming from several miles off. Sure enough, David Cameron in a speech on Monday and George Osborne in the Budget said that the country needs more airport capacity in the south-east of England. As we report today, the Prime Minister and the Chancellor want to press ahead with a new scheme – or revive an old one – to turn RAF Northolt, just six miles away, into a "sixth terminal" for Heathrow.

This newspaper was never wholly persuaded by Mr Cameron's conversion to a green aviation policy, although we naturally welcomed the Conservatives' manifesto promise to cancel the building of a third runway at Heathrow. Equally, we were pleased with the coalition agreement, which confirmed the policy on Heathrow and also said: "We will refuse permission for additional runways at Gatwick and Stansted." After the next election, however, Gatwick in particular might be back in play.

We were right, therefore, to doubt how far the greening of the Transport Department really went. Justine Greening, the Secretary of State since October, is the MP for Putney, one of many west London Conservative constituencies suffering from aircraft noise, whose voters do not want more. It was clear, even before he became Prime Minister, that Mr Cameron's opposition to more runways owed more to London nimbyism and to gesture politics than to a policy to minimise climate change.

So it proved when, earlier this year, Mr Cameron showed a more open mind than before on "Boris Island" – the plan for an airport in the Thames estuary. This is the sort of scheme that gave futurism a bad name in the 1970s. The Prime Minister's support for the Mayor of London's project exposed his hypocrisy. The argument from climate change was abandoned; all that seemed to matter was that the new runways were in the middle of the sea where few voters lived.

Now it seems that, if Mr Cameron could be persuaded of the so-called business case for more airport capacity, the voting power of west London's residents may not be such an obstacle, this far from the next election.

And so, only two months later, Mr Cameron, who once confessed that "I'm not a deeply ideological person", seems to have been influenced by the Chancellor's impatience with environmentalism and by his desperation to foster economic growth. Never mind that the best way to promote growth in the short term would be to cut public spending less sharply. Never mind that last week's Budget was a disguised admission that the Government's economic policy had been found wanting. Hence the mistaken decisions to cut the 50p top rate of income tax and to pursue "infrastructure" projects such as the Heathrow U-turn, neither of which will affect growth for many years and both of which come at a significant cost in other dimensions of our national life.

Let us restate the green case against a "predict and provide" policy of ever-increasing air traffic. We accept that it is easier for a newspaper than it is for a practising politician to say that the growth of air traffic should be constrained. But aviation is the fastest-growing source of carbon dioxide emissions, mainly because there has been no international agreement to tax jet fuel. The most efficient way to minimise global warming would be to use the price mechanism to suppress demand for flying. Because that is difficult to achieve globally, limited airport capacity may be the most practical way of achieving the same ends. If we had a competitive market in landing slots at airports in the South- east, their price could act as a substitute carbon tax. That would help to allocate them efficiently. It could mean more expensive holidays, but this is a price worth paying in the long run.

The Independent on Sunday recognises that there are good arguments on the other side, which Mr Osborne has been pressing. We put ourselves at a competitive disadvantage if we unilaterally increase the cost of flying from British airports. But air travel may not be that important for new economic growth in the late 21st century. A greater prize is within our reach: that of a leadership role for Britain in low-carbon enterprise and low-carbon technology. Building a better 20th-century infrastructure is no role for a forward-looking government.

Mr Cameron once insisted that protecting the environment and promoting the economy were not in conflict. Whatever happened to his brave vision of green growth?