On white-crested swells under leaden skies, the teenage climate activist Greta Thunberg has set sail from Plymouth on arguably her most daunting challenge yet.

A two-week crossing of the Atlantic during hurricane season in a zero-carbon yacht – harnessing wind, sun, and hydro power – is the first obstacle. But it is unlikely to be the toughest in an odyssey through the Americas over many months.

This will be both the ultimate gap year and a journey into the heart of climate darkness: first to the United States of president Donald Trump, who has promised to pull out of the Paris climate agreement, and then down to South America, possibly including Brazil where president Jair Bolsonaro is overseeing a surge of Amazon deforestation.

In between, the 16-year-old Swede will add her increasingly influential voice to appeals for deeper emissions cuts at two crucial global gatherings: the Climate Action Summit in New York on 23 September and the the UN climate conference in Santiago in early December.

The reception awaiting her on the other side is likely to be mixed, with the climate issue a polarising point in US politics.

In a taste of the hostility that is likely to come from supporters of the fossil fuel industry, Steve Milloy, a Fox News contributor and former member of the Trump transition team, described Thunberg on Twitter earlier this week as “the ignorant teenage climate puppet”.

The young founder of the school climate strike movement appeared unfazed in a quayside press conference before she boarded the vessel. “There’ll always be people who don’t understand or accept the science. I’ll ignore them,” she said. “Climate delayers want to shift the focus from the climate crisis to something else. I won’t worry about that. I’ll do what I need.”

Speaking to a throng of several dozen reporters from around the world, she said her primary goal was to raise awareness among the public about the climate emergency. “People (need to) come together and put pressure on people in power so they have to do something,” she said.

Asked if she will meet Donald Trump, the teenager said it would be a waste of time because the US president hasn’t been persuaded by the experts he has already spoken to. “I’m not that special. I can’t convince everyone,” she said.

The voyage is a demonstration of her declared values, which revolve around reducing emissions. A flight to New York would have been much quicker, but it would pump close to 1,000kg of carbon dioxide into the atmosphere. Conventional cruise ships often have an even bigger footprint.

Instead, Thunberg – along with her father, a cameraman and a two-man crew – are taking a zero carbon option.

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The Malizia II is an 18 metre (60ft) racing yacht that was built for round-the-world challenges and has just completed the Fastnet race, which runs every two years. While the wind propels it, the vessel generates the power for lighting and communication through solar panels and underwater turbines. The racing team have removed sponsorship logos from the hull and emblazoned Greta’s slogan “Unite Behind the Science” on the mainsail.

The yacht is designed for speed rather than luxury so conditions will range from basic to difficult. There is no toilet or shower on the boat, only blue plastic buckets. Inside the cabins the lights are dim so Thunberg will need a headlamp or torch to read and keep her journal. Internet access is also likely to be patchy, so her 883,000 Twitter followers may have to wait longer than usual for updates via satellite phone. Her diet will be freeze-dried vegan meals – she has given up meat, which is a major source of emissions.

The young Swede is braced for sea sickness. Although the waves were small as she left Mayflower Marina in Plymouth, she will be fortunate if they remain that way. August is part of the hurricane season in the Atlantic. Even in moderate swells, the vessel is noisy and bumpy.

The experienced German captain, Boris Hermann, said the 3,500 nautical mile journey would demonstrate that it is possible to cope without fossil fuels and get closer to nature. “I want to show that this can be positive and exciting,” he said. “And that solidarity with Greta is not limited to eco-activists.”

Facebook Twitter Pinterest Make America Greta Again placard seen as Climate change activist Greta Thunberg sets sail for New York in the 60ft Malizia II yacht. Photograph: Finnbarr Webster/Getty Images

Thunberg accepts that solar yachts are not for everyone but says she is taking the opportunity to demonstrate there are alternatives. “I don’t tell people what to do. People can do what they want.”

The journey marks a new stage in the remarkable rise of the young Swede, who was unknown outside of her family and school until she started a climate strike last August. She has been diagnosed with Aspergers and has at various times experienced depression, anxiety and selective mutism. Today, however, she has become the spokesperson for the global climate movement and its most recognisable face.

Her Fridays for Future campaign has now mobilised more than a million students across the globe. She has been invited to UN summits, feted at the World Economic Forum in Davos, nominated for the Nobel peace prize, collaborated on a song with the band the 1975, appeared on the cover of countless magazines and been credited with injecting new life into the climate movement.

Last month, the head of the trillion-dollar Organization of the Petroleum Exporting Countries (Opec) described the campaign by Greta and others as the greatest threat to the fossil fuel industry.

“Some things are actually changing, like the mindsets of people. It’s not fast enough, but it’s something,” she said.

Facebook Twitter Pinterest Thunberg gets a hug before she begins her voyage. Photograph: Kirsty Wigglesworth/PA

The scale of the movement will be tested on 20 September, when Thunberg and others have called for a global general strike for the climate.

By that time Thunberg should be in the US, where she plans to meet the UN secretary general, António Guterres, as well as US politicians. She has been promised a warm welcome from the congresswoman Alexandria Ocasio-Cortez, with whom she has already discussed strategies to raise climate ambitions and mobilise campaigners.

Supporters hope Thunberg’s 12 months off school will be a world-changing gap year. After the US, Thunberg will head south to the UN climate summit in Santiago. Asked if she would also visit Brazil, Thunberg was vague. “I’ll travel around the whole continent,” she said. “I’m taking it step by step … I’ll travel for I don’t know how long. I want it to be loose. Not a tight schedule so that it can change as time goes by.” That includes the return voyage. “I don’t know yet how I will get home.”

As she walked towards the yacht in her black waterproofs, there were cheers, applause and cries of “Safe journey.” By the time the vessel pulled into the harbour, the skies cleared, and better weather is forecast this evening and next week

On Tuesday, Thunberg will pass the first-year anniversary of her campaign in the middle of the Atlantic. “I will see how I will celebrate. I don’t know yet. I will think about it,” she said. “I think it will be quite an adventure.”

• This article was amended on 2 September 2019 to clarify references to the yacht being solar powered. The main power source is wind.