Teenage climate activist Greta Thunberg set sail for New York on Wednesday, heading for a UN summit on a zero-emissions yacht skippered by a member of Monaco's ruling family.
The 16-year-old Swede, whose school strikes have inspired children across the world to protest against global warming, refuses to fly because of the carbon emissions caused by planes.
But she has been offered a lift on the Malizia II racing yacht, along with her father Svante and a filmmaker to document the journey, that will allow her to attend the UN talks in September with a clear conscience.
The voyage will take about a fortnight, a daunting prospect for someone who has never sailed before this week, but Thunberg said she was willing to accept a bit of seasickness and a lack of comfort.
"I am one of the very few people in the world who actually can do this, and I think I should take that chance," Thunberg told reporters in the English port of Plymouth, wearing her black Malizia branded sailing kit.
A few minutes later she taxied out for the Malizia on an electric motorboat and stepped onto the yacht as a group of people on a passing sailboat applauded her.
The 60-foot (18-meter) yacht is skippered by Pierre Casiraghi, vice president of the Monaco Yacht Club and a member of the principality's ruling family, and German round-the-world sailor Boris Herrmann.
It can travel at speeds of around 35 knots (70 kilometers an hour) but will be heading into the wind for much of the time so will be slower, and the captain wants a smooth ride.
"The objective is to arrive safe and sound in New York," Herrmann told AFP ahead of their departure.
Ahead of the UN summit on September 23, Thunberg will take part in youth demonstrations, before heading to Canada, Mexico and then to Chile for another UN conference in December. It is not clear how she will be traveling.
"I don't know yet how I will get home," she said.
'Pressure on people in power'
Thunberg has become a figurehead for climate action with her stark warnings of catastrophe if the world does not act now to cut carbon emissions and curb global warming.
She has received criticism and abuse for her uncompromising attitude but shows little concern at how she might be received among climate change deniers in the United States.
"I will just ignore them because I'm only acting and communicating the science, and if they don't like that, what have I got to do with that?" she said.
"I'm not concerned about the reactions. What I'm concerned about is whether we will do something or not, whether the people in power will react and act with necessary force."
Thunberg said she had seen some cause for optimism in how many children have joined her school strike to demand action.
"The mindset of many people is changing, even if that is not enough and even if it’s not fast enough it’s at least something," she said.
But she says she has nothing to say to US President Donald Trump.
The yacht is made for racing, with foils, or wings, that lift it out of the water for a faster and smoother ride.
Inside it is sparse, fitted with high-tech navigation equipment, an onboard ocean laboratory to monitor CO2 levels in the water, and four bunks -- Herrmann and Casiraghi will share one, sleeping in turns.
The toilet is a blue plastic bucket, complete with a biodegradable bag that can be thrown overboard, and meals will be freeze-dried packets of vegan food mixed with water heated on a tiny gas stove.
But state-of-the-art solar panels adorn the yacht's deck and sides while there are two hydro-generators, which together provide all the electricity they need on board.
Thunberg, who has spent hours on trains across Europe to spread her message, was relaxed about the basic conditions.
"You can't really ask for that much if you get to sail across the Atlantic for free," she told AFP ahead of the journey.
She is taking lots of books and board games to help pass the time -- games that do not require pieces that might slide away -- as well as a toy rabbit someone gave her.