Dominic Lawson: So many people seeking to be the new messiahs - but what's the point of it all?

We have a new beatitude. Blessed is the tax raiser, for he shall save the earth. I am not just referring to Gordon Brown's pre-Budget report. On the same day that the Chancellor sought our salvation through such alleged good works as a doubling of air passenger duty and ending the three-year freeze in fuel duty, various of this country's religious leaders published a joint letter declaring their own green conversion.

Dr Rowan Williams, Cardinal Cormac Murphy O'Connor, and Dr Jonathan Sacks, together with the heads of Britain's Sikhs and Hindus declared that "Research has shown that the future of our planet is at risk from environmental destruction. We are consuming today our children's tomorrow. There is no greater challenge for the present generation than securing the future of the planet ... While there are no easy answers, the religious voice provides a moral framework to address the issues of environmental destruction and sustainability."

That last bit gives their game away. What these gentlemen are seeking is to regain their moral market share. In recent years the environmentalist movement has brilliantly appropriated both the messianic claim - "We can save mankind" - and the notion of Armageddon from their traditional purveyors.

British Christianity's official leaders, to judge from their new missive to the masses, no longer regard the greatest challenge to be preparation for the End of the World -the outmoded view of Jesus Christ--but to prevent that outcome altogether. Faced with deadly competition from the sun-worshippers otherwise known as the modern environmental movement, the men of God have exchanged spiritual Salvationism for what they suppose is a science-based form of earthly salvation.

Doctrinally, this does provide a few problems for them. The Bible is not exactly an environmentalist's handbook. Such guidance as the God of Jews and Christians gave is to be found in Genesis 1.28: "Then God blessed them [Adam and Eve] and God said to them: "Be fruitful and multiply and fill the earth and subdue it; have dominion over the fish of the sea, over the birds of the air and over every living thing that moves on the earth." It seems to me that the descendants of Adam and Eve have done a pretty good job of carrying out those instructions.

To be fair to them, our religious leaders were not attempting to augment Gordon Brown's "Green Budget" but to endorse Prince Charles' announcement on Wednesday that he was going to reduce the royal impact on the environment by abjuring his customary private jets and helicopters in favour of scheduled flights and trains, and taking delivery of a fleet of Jaguar cars adapted to run on biodiesel.

The Prince of Wales declared that others must consider making similarly painful sacrifices: "Few accountants and business decision-makers ask: how much of our critical natural resource is left? How many miles of polar ice cap has our business helped melt last year? By how many inches have we raised sea levels? ... These are not comfortable questions, but, by God, they need to be asked." There, you see: God's right across this debate.

I wonder what my accountant would say if he were to address himself to these questions in the way the heir to the throne demands. "How much of our critical natural resource is left? Haven't a clue - and nor has anyone else down at the Coach and Horses. How many miles of polar ice cap have my clients helped melt last year? Sorry, but if you can't supply me with a proper form to send them, this is pointless. How many inches have they added to global sea levels? Sorry, same problem. Oh, you want me just to guess? Let's say a billionth of a millimetre. Does that mean they have to pay more tax?"

But as the late Frankie Howerd used to say, we shouldn't mock. Since we never know who has written a speech given by any member of the Royal Family, we wouldn't know who we are really ridiculing.

A similar indulgence cannot be granted to the Chancellor of the Exchequer. To read the pre-Budget statement in full is to witness the English language suffering almost unendurable torment. On page 157, in the chapter entitled "Protecting the Environment", we are told that "Fiscal measures can tackle negative externalities by internalising environmental costs into prices and encourage the behavioural changes needed to move to a more sustainable economy."

This, I think, is the intellectual justification for the decision to double air passenger duty to £10 for short-haul economy flights, with similar increases for other flights, up to a maximum of £80 for first-class long-haul flights. According to the pre-Budget report, "this will reduce carbon emissions by 300,000 metric tonnes by 2010-2011."

It's very clever of the Treasury to know many flights will no longer be taken over the next few years as a result of the extra £10 on most people's flying costs. Or it would be if they did know. Over the past few years, oil prices have tripled. Over the same period there has been a sharp increase in air travel. Who would have forecast that response?

In a sense, Gordon Brown's critics, such as The Friends of the Earth, are right when they say that the proposed changes in air passengers duty will do very little to change people's behaviour; but the scale of duty increase which might have such an effect is not going to be attempted by any politician subject to democratic election.

Gordon Brown is, I suspect, still scarred by the popular uprising in 2000 otherwise known as the fuel-tax protest. It is tempting for some to dismiss that episode as a stunt by a well-organised group of road hauliers; but the New Labour government remembers only too well that during that fortnight the apparently dormant Conservative Party under William Hague suddenly took a substantial lead in the opinion polls. So impressed were the Liberal Democrats that at their party conference that year they tore up their most treasured environmental policy; Charles Kennedy called for a freeze on fuel taxes, with the immortal phrase " Politicians have got to listen as well as lead."

Of course Mr Brown is not really doubling air passenger duty to save the earth: British flights are responsible for no more than a few thousandths of 1 per cent of global carbon emissions. Even if all climate change was caused by man-made carbon emissions - which it absolutely is not--this is a matter of the utmost insignificance. Or at least, it is a matter of the utmost insignificance except for a family of extremely limited means, for whom a very small increase in price might have an impact.

This is what you would expect: most "Green taxes" will have a disproportionate impact on the poor. Still, at least Dr Rowan Williams and Cardinal Cormac Murphy O'Connor will be able to reassure them that theirs is still the kingdom of heaven.

d.lawson@independent.co.uk