Such a modern dilemma. You are invited to a climate change conference overseas. How do you travel? If you are going to Google’s Davos-on-Sea in Sicily, as Prince Harry and Leonardo DiCaprio did last week, you take international jets and helicopters, hoping your good intentions will offset your CO2 emissions. If you are Greta Thunberg, you find a prince in a shining carbon-fibre boat to take you.

Next month Thunberg is heading to the UN Climate Action Summit in New York courtesy of Pierre Casiraghi, nephew of Prince Albert of Monaco. A stranger seafaring pair is hard to imagine, but the 16-year-old climate activist and Casiraghi, a dashing 31-year-old with a sailor’s beard and a former model for a wife, will spend two weeks together in a 60ft yacht, crossing the Atlantic. Her reason is flygskam, Swedish for “flight shame”. His? Well, it may be Jacques Cousteau.

Fancy a Ride?

Thunberg, whose protests outside the Swedish parliament last year sparked a global climate strike movement, appealed for ideas on how to get to New York without damaging the planet, and was met with an offer from the blue-blooded sailor. He sails the Malizia II, a monohull racing boat that was recently upgraded to be carbon neutral. “We are probably one of the few boats in the world that is totally zero fossil fuel,” he said when we spoke last week.

Greta Thunberg in Plymouth, England, on August 13, 2019.

His team had been having a debrief after the Rolex Giraglia, a race in the Mediterranean, when someone raised Thunberg’s plea. “I thought, ‘We can do that,’ ” Casiraghi said. He urged one of his team to make a call and a few days later Thunberg’s team rang back. “They had a lot of questions and they had been exploring different kinds of boats to get across … We explained this is a racing boat, that there is little comfort on the boat, but she seems fine with that.”

They will set out from a port in the south of England this month. Thunberg won’t be tacking or jibing. She is travelling with her father, Svante, and a cameraman. “The guests will be able to do what they want — rest or be up, take some books and enjoy the ride,” Casiraghi says, playing the perfect host and perhaps overselling the luxury of the experience. The boat has only four bunks. Casiraghi and his long-term sailing partner Boris Herrmann will man the boat and switch in and out of the fourth bunk, sleeping for just an hour or two.

The food is in sachets, the sort that mountaineers or astronauts eat — just add hot water — and there will be the occasional pot of rice cooked over a camping stove. The toilet is a bucket, owing to lack of space. They did ask Thunberg’s team if they wanted a special one fitted. The answer seems to have been: “No, we’ll manage.”

The Malizia II has hydrofoils — wings that lift the boat above the water — and gets up to 34 knots (40mph) at full tilt. On the ocean it can be “very bumpy”, Casiraghi says. “At speed it will jump above the waves. You’ve got to get used to moving around because it is a bit like being on a rollercoaster.” With the precious cargo of Thunberg on board, they are not intending to go full pelt.

The Sailor’s Stripes

A speed-loving scion of a dynasty in a tax haven doesn’t sound like the obvious candidate for an ocean eco-warrior, but he’s not the first in the family.

“As a kid, I’d like to go on small sailing boats,” he says. “It is very common in Monaco and the south of France.” He didn’t get to know his father, Stefano Casiraghi, a financier and powerboat racing champion and the second husband of Princess Caroline of Monaco, the daughter of Prince Rainier and Grace Kelly. Casiraghi Sr died when his powerboat flipped during a race off the coast of Monaco. Pierre was three at the time.

The Grimaldi family, who have ruled Monaco since the 14th century, have taken a different approach to water. “My ancestor was the first admiral of the French king against the English in the Hundred Years’ War,” Casiraghi mentions casually.

The Malizia II leans in. Let’s hope Thunberg has a strong stomach …

Since the end of the 19th century they have also been pioneers in ecology. His great-great-great-grandfather Prince Albert I founded the Oceanographic Museum of Monaco in 1906. It is still a research centre, specialising in reproducing corals in warmer waters, and also saves injured turtles. “I grew up with my house right in front of the museum,” Casiraghi says. “Albert I was the first to put a functioning lab on a boat. He was a visionary, foreseeing problems we are having now in the seas in the 1920s.”

Then came the deep-sea conservationist in the red woolly hat. “Jacques Cousteau was the director from 1957 to 1988,” Casiraghi says. “All his missions were launched from Monaco, funded partly by Monaco. Few people know this. He was very close to Prince Rainier, my grandfather. They went diving together in the late Fifties — that was really the beginning of diving.”

“[Jacques Cousteau] was very close to Prince Rainier, my grandfather.”

How does his family feel about his adventures at sea? Casiraghi is a regular in Hello! magazine, snapped at parties with Beatrice Borromeo, an Italian aristocrat. They married four years ago — European royalty and the singer Lana Del Rey attended the wedding — and have two sons.

He is cautious when speaking about personal matters. “Everyone supports … that’s a big word …” He tails off, then picks up again. “I’m a reasonable person, I would think. I’m not here to take unreasonable risks.” Casiraghi says that he takes “a measured risk”, the odds made better in modern sailing by good meteorological data. “You can trip on the boat and break your arm, but you can do that at home.”

He started sailing competitively ten years ago and in 2013 joined the crew of a boat, the Maserati, sailing from Cape Town to Rio de Janeiro. On board were Giovanni Soldini, a fêted Italian sailor, and Herrmann. They achieved a record crossing of 10 days and 11 hours, which still holds.

Trouble Ahead?

This will be the first time he has done this transatlantic route, and conditions aren’t optimal. “It is not the best moment to get across in August,” Casiraghi says. Because it is hurricane season in the South Atlantic they will take a more northerly course to New York, a slower journey with less favourable winds. Even so, those hurricane weather systems can move north. “We’ve been seeing it in the last five years more and more,” he says, then wonders whether climate change might be the reason.

The Malizia II doesn’t use any fuel, even for batteries. There are two solar panels, but the real power comes from the hydro-generators — turbines fixed to either side of the boat that turn with the forward motion. They have also been working with BMW on battery storage. “Like Formula One, at a high level of sport you test new technologies that slowly trickle down to the mass market.” In what must be a homage to Albert I, the Malizia II also carries scientific equipment on board to measure CO2, sending data to the Max Planck Institute in Germany for analysis.

As an ocean-goer, Casiraghi has noticed environmental changes. He recalls going diving in Indonesia and in Australia, and seeing coral reefs dying. “And I’ve seen a significant reduction of fish in the Mediterranean, which I’ve been swimming in since I was a child.”

The Malizia II doesn’t use any fuel, even for batteries.

Has Casiraghi changed how he lives as a result? “I’m more sensitive. I try to do as little plastic as possible. We separate trash,” he says, but also notes that “change for society is slow”. He cites Norway and Sweden as great recyclers, but this has been a project built up over decades.

How to effect change, then? “Governments are here for the greater good,” he says. “They have to legislate to help us get through this issue.” This point might have been better made to Prince Albert rather than me. As the head of state in Monaco, he has the power to initiate legislation. Albert does also run a foundation that funds environmental projects and education.

Does Casiraghi feel any flygskam? “Flying … er, I take the plane,” he says, reluctantly. “I think it is a small part of emissions. Industry is a big part, marine transport is huge. I think probably a tenth of the cargo ships produce more than all the cars in the world.” That’s true for sulphur oxide, but that’s because of the specific fuel that cargo ships use. On CO2, cargo and aviation both come in at about 3 per cent, depending on the method of reporting.

After our conversation I discover that Casiraghi is also a shareholder in Monacair, founded by his father, which furnishes those in need in the principality with helicopters — not a small market in one of the world’s great tax havens.

I joke that during two weeks on a small boat with Thunberg his conversion may be even more substantial. “I’m not here to advocate change or how to do things,” Casiraghi says, insouciantly. “I’m, you know, really just transporting Greta. And it was just a way to help her get across. We had a zero-emission boat. It’s very simple.”