Politics can be very like romance. The transition from bold declarations to detailed policy prescriptions is often analogous to the journey from “I love you” to deciding on the location of the marital home and the most suitable schools for the offspring. As George Osborne is learning, it can be tough.

Challenged last autumn to be clear on his fundamental approach to taxation, the Shadow Chancellor cheered the Tory conference by telling it: “I want lower taxes.” Challenged since then to be more specific, and at the same time stick to the modernisers’ aggressive environmental agenda, he has mooted a “personal flight allowance” of perhaps one return short-haul flight per person per year at market rates, after which consumers would be taxed progressively for additional flights. The money raised would allow for welcome tax cuts elsewhere.

Here was a practical example of “pay as you burn” rather than “pay as you earn”, Mr Osborne ventured. Anticipating an angry response from the airlines, he also boasted that the proposal showed a willingness “to make the tough choices” that he believes are needed to cut carbon emissions. Mercifully, it shows no such thing. The Tories are still only offering choices, not making them, and in this case their next challenge is to show the flexibility to rethink half-thought ideas.

Philosophically, a personal flight allowance as outlined in the Greener Skies paper that Mr Osborne will present today runs counter to the drive for a smaller, less intrusive State that should underpin every new policy emerging from the Conservatives’ wideranging review. It would also be costly, complex to implement and easy to abuse. The same goes for the idea of adding VAT to fares: would, for instance, a flight that involves some business but a large amount of personal pleasure be taxed? Would professional net-workers be able to write off VAT on every flight as a business expense? This proposal would be open to ambiguity and almost as much cheating.

Why, in view of such practical flaws, have these reforms been floated? The political context provides an answer. Last week Tony Blair helped Angela Merkel, the German Chancellor, to hammer out an optimistic but still significant climate change deal at the EU summit in Brussels. Tomorrow, his Government will introduce a Climate Change Bill in Parliament. Having staked so much on outflanking Labour on the environment, the Tories cannot afford to cede the initiative. Part of the purpose of the green air miles proposal is to regain that initiative.

The Bill is not expected to include radical new plans for taxing international flights. There is a reason for this: unilateral tax schemes on international businesses put those who implement them at a potentially ruinous disadvantage, and as air travel grows, so does the importance to the UK’s economy of its role as a global aviation hub.

In fairness to the Tories, personal flight allowances are but one of several options they are considering to make the price of air travel, which currently incurs no fuel duty, better reflect its environmental impact. If the consensus on Man’s role in climate change is accurate — and the consensus is still open to question — that impact will only grow. But it is best mitigated by tough overall emissions limits, and fluid and transparent carbon trading mechanisms that are the best way of harnessing technology and markets to green ends. They will eventually put a fair price on air travel. Government meddling will not.