Piers Corbyn is an hour late. But when he finally shambles into the placid garden of a café where we meet – in Islington, inevitably – he doesn’t disappoint. “GREETINGS, CITIZEN!” he bellows into his white Samsung brick, as he arrives. “I’m with the New Statesperson at the moment…”

Eventually, he hangs up.

“People keep ringing me, asking for more forecasts,” is his excuse for his lateness. “So I fobbed them off and came here.”

He folds his bandy frame into one of the garden chairs at our wooden cable spool table. He wears a grey and brown striped jumper and ink-stained suit trousers. A look topped off with his flyaway sweep of grey curls.

“Nutty professor” and “brother of Jeremy” are the main media characterisations of Piers these days, ever since his younger brother – who is two years his junior – became Labour leader.

But Piers has always attracted a level of fascination from the press, as an ardent climate change denier. The 69-year-old longrange weather forecaster – who doesn’t believe in man-made global warming and insists Earth is cooling – has long been described as a “controversial meteorologist”.

Piers Corbyn makes a prediction. Photo: YouTube screengrab

He makes solar activity-based calculations (using the mysterious “Solar Lunar Action Technique” – but he doesn’t reveal his methods) to predict weather events up to a year in advance, and climate forecasts up to two decades ahead. His forecasting business is called WeatherAction. He has occasionally made correct predictions, such as the UK’s coldest December in 100 years in 2010. Climate change sceptics love him. Boris Johnson has called him “my old chum” and “the world’s foremost meteorological soothsayer” in his Telegraph column.

Reaching out to hold a branch hanging down from an ash tree that shelters the garden, Piers claims: “CO₂ now is at a very low level. And that’s because these things we see above us are so efficient. A carbon dioxide molecule comes here and gets gobbled up.

“These people who want to make genetically-modified microbes which absorb more CO₂ – well, that would be the end of life as we know it, because trees will die, everything would die, and we’d go with it,” he growls.

“This is Dr Strangelove insanity. They ought to be stopped, these people. In fact, they ought to be locked up. I would go as far as saying they should be locked up.” He pauses sheepishly, and wags his finger at my notebook. “I’d perhaps not say that…”

His fury at the scientific consensus that humans are destroying the planet is palpable. “If it’s true that carbon dioxide is a problem, then the powers-that-be ought to attack the main producers of carbon dioxide. Termites produce ten times more CO₂ than mankind, so we should have a war on termites. But we haven’t seen any of that,” he concludes. “There’s no war on termites.”

Piers Corbyn with a map of the UK. Photo: YouTube screengrab

Piers’ unorthodox views are often used by the press to embarrass Jeremy, but he finds the association equally frustrating. “They want to portray me as a mad man on climate change, and the brother of my brother,” he says. “They find anything bad happening, if Piers Corbyn’s within a mile, they mention that he’s my brother.”

A recent example is his criticism of Louise Ellman MP’s condemnation of Jeremy Corbyn for failing to stamp out antisemitism in the party. “ABSURD!” he tweeted. “JC+All #Corbyns are committed #AntiNazi. #Zionists cant cope with anyone supporting rights for #Palestine”.

Jeremy defended his brother, saying, “We actually fundamentally agree – we are a family that were brought up fighting racism from the day we were born.”

Piers today stands by his comments: “As an object of fact, my grandfather was called Benjamin, my father’s second name was Benjamin, Jeremy’s eldest son is called Benjamin. My mother’s name was Naomi,” he says.

“So there’s all this Jewishness around – our names, anyway. We’re probably distantly related to some – I dunno. I’ve been confused with Jews, people have said ‘you look Jewish’, and I’ve said, ‘no, I’m not to my knowledge’. I’ve employed Jewish people, including ones who wear the…” he gestures with his hand hovering above his crown. “Kippahs. Quite devout ones. I had Jewish friends in Imperial College . . . and I’ve written scientific papers with Jewish people. So there’s no issue of me being anti-Jewish at all.”

Is there an antisemitism problem in the Labour party? “Not that I’ve witnessed myself,” says Piers. “All I can say is, in my experience in Bermondsey and around the country, I’ve never seen it.”

Piers Corbyn as a young campaigner. Photo: Getty

Piers is pro-Brexit, and reveals the extent to which his brother – now campaigning (if lukewarmly) to Remain – agreed with his stance. “In [the] 1975 [referendum], my father was for the EU and my mother was against. I voted for it, and I think Jeremy voted against. But I’m not sure,” he explains. “Then he became against the EU, and was in the Tony Benn view of things – that it’s a capitalist club. And I agreed with him about that completely.

“And I agree very much with what [Labour MP] Graham Stringer said in a recent interview – he said he’d been with Jeremy through every lobby, against Maastricht, against this and that. And he said he thought Jeremy was doing a party management operation on this.”

But the biggest question of all: does Jeremy agree with his brother on climate change? “The way I see it on climate change is that he’s got to manage what the party is saying,” Piers replies. “He can’t just jump around on views, whether or not he agrees with me. I’m not saying whether he does or he doesn’t. I’m doing an NCND – no confirmation, no denial,” he chuckles.

Piers feels the left has been led astray by the green movement. And somehow manages to compare this to Nazi Germany. “After the Berlin Wall came down, various people on the left got lost, and my former associates – or friends, even – in the International Marxist Group and related bodies developed this idea of red-green politics, eco-revolution,” he muses. “Where they seem to equate large-state spending with socialism. Well, I don’t think that’s a good argument, because, of course, Hitler had large-state spending, but he wasn’t socialist.”

Prior to WeatherAction, Piers was a housing and squatters’ right activist in London, and a Labour councillor in Southwark from 1986-90. But he left the Labour party in 2002, after 26 years as a member, over the selling off of the Aylesbury Estate in Southwark. He is now in the process of rejoining, mainly due to brotherly loyalty.

“I need to get in there, support what they’re doing,” he smiles, his thin square-framed spectacles flashing in the afternoon sunlight. “The good things they’re doing on housing.”

So exercised is he about housing inequality – and what he sees as the “theft” of properties by the super-rich in London – that he condemns Labour’s London mayoral candidate for his “pro-business” intentions. “Obviously, I will support Sadiq Khan over the Tory whathisnameis,” he says. “But both of them are hand-in-glove in bed with developers. And Sadiq Khan has been receiving handouts from them, you know . . .

“Which business [will he support]?” he asks of Khan, who nominated his brother for Labour leader. “If it’s the property business, then HELP! Breadmaking, maybe? Coffee parlours? I dunno. I don’t know what it means, and I don’t think he knows what it means.”

Piers Corbyn campaigning in the Seventies. Photo: Getty

Piers’ accent is speckled with the lingering vowels and dropped t’s of a south Londoner, but he also echoes the faint crackle of West Country we occasionally hear in Jeremy. The brothers, with their two elder siblings, David and Andrew, spent their early childhood in a Wiltshire village, before moving to Shropshire when Piers was nine and Jeremy was seven. It was this Shropshire house, the idyllic Yew Tree Manor, which is often pictured in papers to illustrate the Corbyns’ affluence.

The boys did have a comfortable childhood, attending prep schools and then the local grammar, but Piers recalls helping his parents do up their Shropshire house, which he says was run-down when they bought it. He and Jeremy repointed the brickwork. The Corbyn boys did all manner of handiwork, and helped their father – who was an electrical engineer – with mechanical projects.

Piers began monitoring the weather at eight years old, when he dug out his own dew pond. He built an observation station, and made his own Stevenson screen to shelter his home-made weather instruments (which included an anemometer to measure wind speed, made out of copper sheets and pieces of an old bicycle wheel and curtain rod, and an oil drum dug into the ground to measure soil density).

“My father and mother encouraged us to do our own thing,” Piers says. “I remember Jeremy made a sundial. And my elder brother, Andrew, made a forge. He had an anvil and he made tools.”

Jeremy’s team may try to distance the Labour leader from his brother’s views, but their similarities come from the same place – a political family. Both have a single-minded commitment to their principles, even when the winds of change are buffeting against them.

“We were all political,” Piers explains. “My parents were. They met during [an event supporting the anti-fascists in] the Spanish Civil War. They came from this sort of old school where Communism and science, or socialism and science, went together for progress.

“Science was objective, and you could see it worked. And there was a scientific analysis of society, as by Marx – and it was scientific, therefore it was true. So, you know, support the Soviet Union – shut your eyes when you see bad things there, you know, stuff like that,” he admits. “They were never in the Communist party; they were only in the Labour party. But obviously the Labour left then was quite sympathetic to the Soviet Union . . . So that was their sort of background, and I suppose we were brought up believing those things.”

Piers Corbyn is appearing at HowTheLightGetsIn, the philosophy and music festival at Hay 26 May – 5 June. Book your tickets and accommodation for debates and parties here.