Where does science end and politics begin? For over a decade we have seen an increasingly bitter debate between environmentalists and sceptics about the extent to which the globe is warming, who is responsible and what, if anything, we ought to do. Presented with two sets of “experts,” the public is left confused, as opinion polls show.

The real truth — that all major scientific questions about global warming have long been settled in a way that largely supports the environmentalist position — remains obscured by political trench warfare and media debate. This is why Al Gore’s documentary, An Inconvenient Truth — despite its largely accurate portrayal of climate science — was this week criticised as “one-sided” by an English High Court judge in a case about a British government plan to show the film in U.K. schools. He said the film contained nine scientific errors — but still ruled that pupils could see it.

That is not to say that Mr. Gore got everything 100 per cent right. It is true that the apocalyptic scenario of Gulf Stream shutdown (leaving Europe shivering in a new ice age) is now out of favour with oceanographers, and Mr. Gore was wrong to imply that the very close relationship between carbon dioxide and temperatures during ice age cycles proves cause and effect.

He should also have been clearer about the timescales involved with any collapse of the Greenland or West Antarctic ice sheets. Yes, if the Greenland sheet melted sea levels would rise by seven metres. But no one thinks that the entire ice sheet could melt in this century. This is an area of real scientific uncertainty: glaciologists are still struggling to understand ice-sheet dynamics.

Lake Chad and Kilimanjaro are even trickier. Yes, Lake Chad has lost 90 per cent of its water — but dams and overgrazing will have aggravated any fall in precipitation. Kilimanjaro’s glaciers, because they mostly sublimate (turn from ice directly into water vapour) rather than melt, are unusual in comparison with most mountain glaciers. But so what? Rising temperatures are clearly behind the glacial retreat observed in every major mountain range on the planet.

Moreover, the judge was wrong on coral bleaching — which is unambiguously related to rising sea temperatures — and in downplaying the sea level impacts experienced by atoll states. As it happens, Mr. Gore’s statement on this subject was based on a photo I took in Tuvalu in 2002, shown in the film, at a time when increasingly severe flooding during high tides was already a reality, driving negotiations with New Zealand about evacuating the entire population.

All these points, however, are trivial in the context of the film’s main argument, which is unambiguously correct in its portrayal of mainstream scientific understanding of climate change. The judge, to his credit, stated this clearly. But the case serves to illustrate how science and politics collide on climate change: so long as the political debate demands absolute scientific certainty as a prelude to serious action, a tiny seed of doubt on any issue — a single lake or mountain among 10,000 — can be used by the denial lobby to cast doubt on the entire global warming thesis, and so undermine public understanding.

Hence the need to move the debate from science and towards precaution. It is now very likely that global warming this century will present major challenges to the survival of human civilisation — and to our children’s and grandchildren’s lives. If we listen to the deniers, we are taking a very dangerous gamble — a bit like playing Russian roulette with five bullets and only one empty chamber. That’s not a game I want to play with my kids. — ©Guardian Newspapers Limited, 2007

(Mark Lynas is the author of Six Degrees: Our Future on a Hotter Planet [Fourth Estate].)