Planet Earth should in theory be capable of supporting life, including human life, for thousands of years to come - indeed, some estimates say millions. It all depends on the human race. Not long ago the greatest danger seemed to come from nuclear war between the superpowers. Now the major threat is environmental damage, particularly the heating up of the atmosphere owing  to the discharge into it of so-called greenhouse gases such as methane and carbon dioxide. The prospect is not of an overnight catastrophe wiping out the human race in one sweep, but of a planet which can currently sustain a human population of six billion - and growing - finding within a generation or two that, due to global warming, the sustainable population is only a fraction of that - and falling. The effect would be something like that of the Irish potato famine, but on a global scale and lasting for centuries. Clearly millions would die in acute hardship and misery, and the poorest and most vulnerable would be the first to go.

However, there are still those who deny the reality of global warming. At the extreme end of that spectrum are those who allege that the scientific evidence has been cooked. More common is the acceptance of global warming as a natural phenomenon, caused by changes in the radiation from the sun. Both positions deny that there is any obligation on humanity to change its ways in order to head off an environmental catastrophe. A particular - and revealing - dislike is of any intervention that would interrupt the functioning of market forces (or the profitability of the oil industry).

Is one free to deny global warming if one so chooses? Is it a moral matter? To say so is not necessarily to want to go as far as the Bishop of London, Dr Richard Chartres, who faces the prospect of travelling to an international conference in Romania by train - a 37-hour journey with four changes. He famously said not long ago that air travel was a sin because of the greenhouse gases emitted in the course of it; hence the long ordeal by hard seats and railway sandwiches to which he is about to subject himself. Nor do we have to choose between the Bishop of London and Cardinal Biffi, former Archbishop of Bologna, who said recently that the Antichrist could well appear to the faithful as an environmental campaigner. Indeed, his fuller portrait - ecumenist, ecologist and pacifist - bore a closer resemblance to St Francis of Assisi than to the Scarlet Beast of the Apocalypse.

Whether the science stands up is a matter of scientific, not moral, judgement, provided it is made in good faith. But there comes a point where a purely scientific prediction can no longer be ignored by moralists. Invoking the precautionary principle, the Catholic Church would be well within its remit on faith and morals to say that the probability of global warming is now more than a hypothesis, and to gamble with the survivability of the planet in return for short-term material and economic gains would be gravely sinful. And that would be to say, in turn, that denial of global warming is no longer a sustainable position, but morally wrong.

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