Environmental activist Greta Thunberg has set sail from the UK, bound for UN climate summits in New York and Chile.
Fans gathered to wave her off on her voyage across the Atlantic.
The 16-year-old made a speech ahead of departing from Plymouth on the Malizia II, a racing yacht with underwater turbines.
Greta said she was dedicated "to do everything I can" to tackle climate change which was a "very big problem".
The teenager, who refuses to travel by air because of its environmental impact, said of climate sceptics: "There's always going to be people who don't understand or accept the united science, and I will just ignore them, as I'm only acting and communicating on the science."
She added she thought people's mindsets were changing "even if it's not enough, and not fast enough, that's something, it's not for nothing".
Greta was asked if she could make US President Donald Trump listen, and she answered with a simple "no", prompting laughter from the crowd.
"I'm not that special. I can't convince everyone," she said.
"I'm just going to do what I want to do and what will have most impact."
The captain of the Malizia II, Boris Herrmann, appeared alongside her.
He thanked Greta for her courage and called the issue of climate change a "race against time".
Greta said her two-week trip would pose challenges including seasickness, but said many people in the world were suffering a lot more than that.
"I was test sailing two days ago and we went out for several hours," she said.
"I didn't feel bad or anxious, I felt seasick for about one or two minutes, then it stopped.
"I will just have to see, get on the boat and and see what happens, and that is also very exciting."
The 18m (60ft) yacht they are using was built to compete in the 2016-17 round-the-world Vendée Globe race.
She told the BBC that travelling by sea shows "the climate change crisis is a real thing".
The Swedish teenager's "school strikes" have inspired a global climate change protest movement driven by young people.
'My daughter wants to be her'
Laura Jackson and daughters Cicely and Tilia, who took part in the school strikes, are planning to strike again on 20 September.Ms Jackson said: "It's very exciting for them to be part of this.
"They often feel that they're too small to make an impact and the strikes make them feel really empowered.
"My daughter Cicely wants to be a Greta."
Kerrie Roche-Walker, from Plymouth, said she believed in what Greta was doing.
"I feel it's so important at this time for all of us globally to support the causes that she stands for," she said.
Her grandson Eric, eight, added: "I'm excited to see Greta.
"She's doing her best for climate change. Waters are rising and people keep littering.
"The waters could rise too much. She's basically saving lives."
'Greta inspires me'
Emily Stevenson, 21, who made her own environmental statement when she appeared at her graduation wearing a dress made out of crisp packets, got to ask Greta a question.
She said the experience was "unreal".
"I don't have any words to describe how it feels," she said.
"Greta inspires me."