But the most dramatic turn away from environmentalism has come with this week's cabinet reshuffle. First there was the promotion to environment secretary of Owen Patterson: a Conservative who opposes windfarms, loves airports and supports fracking for shale gas (although his website does list "trees" as an interest). More pressingly, there is the issue of the expansion of Heathrow. Justine Greening, who was just last month vehemently defending the government's official opposition to a third runway, has been moved from the transport brief; her replacement, Patrick McLoughlin, will start talks on a cross-party commission reopening the subject. To cap it all, the prime minister has refused to make it clear that he will oppose a third runway beyond this parliament. We may thus assume that the way is being cleared for the aeronautical equivalent of the U-turn manoeuvre.

However appalling this may be, it should not be a surprise. Even the most lackadaisical student of this government can tell you that the coalition agreement states baldly: "We will cancel the third runway at Heathrow. We will refuse permission for additional runways at Gatwick and Stansted." What they omit to mention is just where those policy commitments are made: not in the sections on transport or economic policy but under climate change. Like a lot of this government's green commitments, this one was never synthesised with its economic ideology: it was, from the outset, just so much garnish. Nick Clegg might well believe that "going green has never made so much sense"; but George Osborne is certainly closer to Tory convictions when he argues against "saving the planet by putting our country out of business".

This does not disguise the speciousness of the economic arguments for a third runway at Heathrow. Even if the cabinet gave the green light tomorrow, the project would not create a significant number of new jobs for years to come. That goes double for Boris Johnson's dream of a new airport on an island in the Thames estuary. However hard the right may argue for more airports as an immediate economic boost, it is no such thing. The argument that Britain will be crippled by somehow stunting Heathrow is also disingenuous: as if the perpetual expansion of what is already the world's largest airport will somehow magically add points to GDP growth.

The case for Heathrow has always been less about the arguments than the lobby behind it: big business and the aviation industry. The concessions BAA and others typically offer are not worth much. A few years ago, they claimed that a snazzy Terminal 5 would put an end to all talk of new runways. We can take just as seriously the current promises to limit the planes taking off from any new runway.

But the policy arguments are likely to be rehearsed all over again in the next few months and years. The significance of this week lies in the politics. Mr Cameron has signalled that even if it means losing an able transport secretary in Ms Greening; even if it means Tory environmentalist Zac Goldsmith forcing a by-election, he appears minded to support a policy he previously fought hard.

Lib Dems such as climate secretary Ed Davey, party president Tim Farron and deputy leader Simon Hughes must surely be dismayed by this U-turn. They owe it to themselves and to all of us to give voice to that dismay.