In the brilliant farce Le Dîner de Cons, a group of bored, rich businessmen have set each other an ongoing challenge: to entice the biggest idiot they can find to come to their weekly dinners. A publisher called Pierre Brochant finds a man he is sure will be the week's prize idiot, and invites him to his apartment before setting off for the dinner. But Brochant's triumph backfires horribly, as the man he brings into his home proceeds unwittingly to destroy his life.

Could Fraser Nelson have accepted a similar challenge? He's a publisher of a kind - editor of the Spectator magazine in fact – and, like Brochant, he has just made the biggest blunder of his career. He has invited a serial promoter of nonsense into his magazine and, to show off his remarkable find, has put him on the front cover, where his claims are promoted as straight fact.

Nelson's "find" is a man some of us found years ago and have seen as a source of wild entertainment ever since. He's called Nils-Axel Mörner, and among his claims to fame are that he possesses paranormal abilities to find water and metal using a dowsing rod, and that he has discovered "the Hong Kong of the [ancient] Greeks" in Sweden.

The celebrated debunker of cobblers James Randi challenged Mörner to demonstrate his expertise with a dowsing rod, but he "consistently refused to be tested". He did however, allow his paranormal abilities to be examined on Swedish television, using a test that Mörner himself devised: dowsing for a packet of sugar concealed under one of 10 cups. Needless to say, he failed, blaming, as such people so often do, "interference" and "influences".

In 2007, Mörner and his collaborator, a homeopath and amateur archaeologist called Bob Lind, were reprimanded by the Scania County archaeologist in Sweden for damaging an Iron Age cemetery during their quest to demonstrate the "Bronze Age calendar alignments", which would somehow help to show that this local graveyard was in fact an ancient Hellenic trading centre.

Reviewing such claims, the archaeologist and chair of the Swedish Skeptics Society, Martin Rundkvist, comments that if Nils-Axel Mörner is associated with a project, it's "a solid guarantee for high-grade woo."

Now Mörner turns up on the front cover of the Spectator, under the headline "The Sea Level Scam: the rise and rise of a global scare story". His wild assertions are published in the magazine without qualification or challenge. Far from it: they are proclaimed in the headline as "The truth about sea levels". Yet they are as far from the truth as his claims about dowsing and archaeology.

Mörner maintains that places such as the Maldives, Bangladesh and Tuvalu "need not fear rising sea levels." There is, he says, "no ongoing sea-level rise" and no link between sea levels and climate change. He makes the false claim that the rate of sea-level rise accepted by most climate scientists "has been based on just one tide gauge in Hong Kong" (does he have a thing about Hong Kong?).

These claims have already been comprehensively debunked. To sustain them, Mörner relies on misinterpretations of scientific data so grave that even an arts graduate such as Fraser Nelson should have been able to spot them.

In his Spectator article, Mörner makes much of his research trips to the Maldives. These culminated in a 2004 paper published in the journal Global and Planetary Change. In it, Mörner uses an apparently random series of observations – including the discovery of a skeletal "reef woman" buried in a 800-year-old coral reef – to postulate that sea level rise in the Maldives is a figment of scientists' imagination. How this paper got published is a mystery that only the journal's editors can explain.

It was comprehensively debunked within a year in the same journal by Philip Woodworth, an oceanographer based in the UK, who wrote acidly that 'reef woman' "is hardly definitive as a sea level marker" and that Mörner's convoluted arguments – which also relied on anecdotal accounts by fishermen sailing over shallow rocks – were "hard to understand" and ultimately "implausible". A follow-up critical comment by the Australian oceanographer Paul Kench and colleagues notes that Mörner's paper "contains a number of unqualified and unreferenced assertions" which fail to stand up to scrutiny, does not follow carbon-dating conventions, and that "standard information is missing".

Mörner, a geologist by training, was not the only author of this paper: the others were Michael Tooley, an expert in English historical gardens, and Goran Possnert, a nuclear physics engineer currently working on a Swedish project called the Human Regenerative Map: not exactly the qualifications you would expect for people working in the highly specialised and complex disciplines of oceanography and sea level change.

In reality, three tide gauges exist in the Maldives, whose outputs are all available online and can be inspected by the public via the UK-based scientific collaborative research effort the Permanent Service for Mean Sea Level. One is in the south at Gan, which shows a clear and consistent rise since before 1990. A second is located at Hanimaadhoo in the north of the country, and shows no consistent trend from 1992. The third is at the capital Male', and shows a strong upward trend from the early 1990s until 2009.

Here is the irony – in his attempt at scientific cherry-picking, Mörner chose the wrong tree. Had he travelled to the western side of the Indian Ocean – to Zanzibar or Mauritius – he would have found incontestable evidence of sea-levels falling, dutifully recorded by their tide gauges. Does this invalidate global sea-level rise? Of course not – because sea levels fluctuate all the time due to winds and currents, even over several years in every location, a truly global picture can only be obtained from hundreds of tide gauges operating over multi-decadal periods.

For shorter time-periods, estimates of sea level change depend on satellite data. Mörner however chooses not to believe the published satellite record, probably because it shows a clear upward trend across the global oceans of 3.3mm a year. This conscious rejection of the established satellite data comes about, the Spectator reveals, because of something Mörner claims to have overheard several years ago at a scientific conference in Moscow which he interprets as evidence of a conspiracy.

Mörner also claims in the Spectator article to speak on behalf of the INQUA (the International Union for Quaternary Research) commission on Sea Level Changes and Coastal Evolution, whose members he says are "the world's true experts on sea level" – as opposed to the IPCC, which he asserts has "hijacked and distorted" the data. Mörner was indeed president of this commission until 2003. However, as documented by the Carbon Brief, INQUA now clearly dissociates itself from Mörner's views. Current president of the INQUA commission on Coastal and Marine Processes, Professor Roland Gehrels of the University of Plymouth, says his view do not represent 99% of its members, and the organisation has previously stated that it is "distressed" that Mörner continues to falsely "represent himself in his former capacity."

In recent years, before being discovered by Mr Nelson, Mörner had largely been reduced to self-publishing pamphlets on the web and penning overblown diatribes in loony-tunes publications which bear as much relationship to scientific literature as the Spectator does. One of these – titled There Is No Alarming Sea Level Rise! – was published in an online publication called 21st Century Science and Technology. It might sound impressive, but this magazine is in fact a vehicle for the views of Lyndon Larouche. Larouche is the US demagogue who in 1989 received a 15-year sentence for conspiracy, mail fraud and tax code violations. He has claimed that the British royal family is running an international drugs syndicate, that Henry Kissinger is a communist agent, that the British government is controlled by Jewish bankers and that Barack Obama is a puppet of the renewed British Empire, which is supposedly trying to start a third world war over Syria (see end for references). He sees science and empiricism as yet another conspiracy, and uses 21st Century Science and Technology to wage war against them.

Mörner's conspiratorial claims also appear in a pamphlet with a foreword by our old friend Lord Monckton, called Sea level is not rising. In this paper his thesis, whose grounds appear to shift with every article he writes, rests partly on yet another putative scientific conspiracy: "In the Maldives, a group of Australian environmental scientists uprooted a 50-year-old tree by the shoreline, aiming to conceal the fact that its location indicated that sea level had not been rising."

But it is the pamphlet's Figure 10 which tells you all you need to know about their methods. It must win the prize for the most comically distorted illustration ever produced in the annals of climate change denial. That is not for want of competition. As you can see, it rotates the graph of satellite-observed sea levels until the line appears flat, whereupon the illustration declares that there is "no trend"! Need we add that the pamphlet was published by a "thinktank" run by a Ukip candidate?

This is not the first time that the Spectator has championed groundless claims about climate science. In 2009, before Fraser Nelson became editor, it published a cover story extolling the claims of Ian Plimer, who, like Nils-Axel Mörner, is a retired geologist. His work had also been widely ridiculed by scientists for its hilarious schoolboy errors and its fudging and manipulation of the data. Plimer inflicted further damage on the magazine after he frantically tried to evade the hard questions arising from this article during a television debate.

Now Fraser Nelson has tried to do the same thing. The Guardian asked him on Twitter whether any of the editors or senior staff at the Spectator has a science degree. He repeatedly tried to duck this question, as well as the other question we asked: who checked the article and what were their qualifications? At one point, instead of answering these simple queries, he blustered: "But pls do your thought crime piece, ur always on great form when hunting heretics!"

The Spectator has a long and inglorious record of scientific balls-ups of this magnitude. As Ben Goldacre has shown, Fraser Nelson has also championed a wildly misleading film about Aids and promoted scare stories about vaccinations.

Nelson's defence when challenged on any of this (when you can get an answer out of him at all), is the standard rightwing canard that those who criticise him or his contributors are enemies of free speech, witch-hunters or thought police, opposed to debate. (We should, as the tweeter Paul Crowley suggested, institute a new version of Godwin's law: a rightwinger, when his claims are challenged, will soon denounce his opponents as thought police. Let's call it Crowley's Law.)

But exposing the false claims people publish is, of course, part of the debate. Perhaps Nelson would rather we silenced ourselves and didn't challenge him in the name of, er, free speech. Perhaps – as his claim that debunking the wild inaccuracies he has published is the equivalent of hunting heretics suggests – he regards criticism as illegitimate. In either case, his employers should now be asking themselves some serious questions about their editor's judgment. They might gently suggest to him that he could be better off overseeing a magazine like Conspiracy Digest.

• Mark Lynas is a climate writer, Visting Research Associate at the Oxford University Centre for the Environment and also climate adviser to the President of the Maldives.

References for Larouche's history and beliefs:

Terry Kirby, 21st July 2004. The Cult and the Candidate. The Independent

Chip Bertlet, 20th December 1990.

Roger Boyes, 7th November 2003. Blame the Jews. The Times

David Bamford, 30th July 1987, Turkish Officials Carpeted. The Guardian

Michael White, 3rd May 1986. Will the Democrats wear this Whig? The Guardian

Francis Wheen, 21st August 1996. Branded: Lord Rees-Mogg, international terrorist. The Guardian

Extract from Chip Berlet and Matthew N. Lyons, 2000. Right-Wing Populism in America: Too Close for Comfort

Guilford Press, New York