Two days into her transatlantic voyage aboard a hyper-efficient sailing yacht, 16-year-old climate activist Greta Thunberg is already weathering rough seas — and a deluge of online attacks.

The Swedish teenager set sail Wednesday on her nearly 3,000-mile trip from Plymouth, England, to New York, where she plans to attend the United Nations Climate Action Summit in September.

Thunberg is making the journey aboard the 60-foot, carbon-neutral Malizia II sailboat to avoid flying. Air travel emits as much carbon dioxide annually as some of the world’s major economies, according to Tristan Smith, a lecturer at University College London’s Energy Institute. Cruise ships are also significant emitters.

A sailboat journey is an unusual method of cross-continental transportation in the 21st century, and Thunberg’s trip has drawn attention from across the world. Fans are cheering Thunberg on and tracking her progress. Critics, meanwhile, have been ruthlessly derisive.

Arron Banks, a British businessman who has bankrolled the Brexit campaign, mused on Twitter about a calamity befalling the teenage voyager.

“Freak yachting accidents do happen in August …” he wrote.

Banks’s comments drew a swift backlash from British politicians and celebrities. Green Party M ember of Parliament Caroline Lucas, whose encouraging words to Thunberg prompted Banks’s tweet, said the Brexit-backer’s message “makes me sick to the stomach."

“I have made a formal complaint to Twitter,” she wrote.

British actress Amanda Abbington also rallied behind Thunberg.

“If you are a grown up, fully-fledged adult and you are mocking this young girl for trying to save the planet then I genuinely feel sorry for you,” she wrote on Twitter. “I also think you are incredibly cruel, vicious and ignorant.”

Banks defended himself by saying that his tweet “was a joke,” adding “you lefties have no sense of humour.”

“Obviously I don’t hope she encounters a freak yachting accident!” he wrote, adding that Thunberg was “being used,” though he did not specify by whom.

Thunberg is no stranger to criticism. The teenager has emerged as one of the world’s most prominent voices on climate action after her school strikes in Stockholm inspired youth around the world to demand that global leaders address climate change. Thunberg was nominated for the Nobel Peace Prize in March.

While celebrated by many, her fame has also exposed her to vigorous opposition.

Conservative French politicians mocked Thunberg when she spoke to French lawmakers last month as a “prophetess in shorts.”

Steve Milloy, a former member of President Trump’s transition team, described her last week as “a teenage puppet.”

Thunberg has appeared undeterred, however, telling her critics last month “we become the bad guys who have to tell people these uncomfortable things because no one else wants to, or dares to.”

She has also vowed to ignore climate change skeptics.

“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,” she told reporters at a news conference ahead of her trip.

Thunberg hasn’t responded to the latest wave of criticism Wednesday. Internet access aboard the Malizia II is limited, although she was able to share a progress update Thursday on Twitter.

Thunberg is making the journey with her father, Svante, and a small team: Boris Herrmann, the 38-year-old German captain of the Malizia II; Pierre Casiraghi, who heads the yacht’s racing team; and documentary filmmaker Nathan Grossman.

The trip came together after Herrmann’s team offered to sail Thunberg to New York.

“We will make sure she will reach New York in the safest way possible,” he said. “I feel humbled that Greta accepted our offer as the lowest-carbon option to cross the Atlantic — despite the lack of comfort for her.”

Zero-emissions travel isn’t easy. The Malizia II is an elite racing boat but lacks many amenities.

It was built in 2015 out of composite material meant to reduce weight and withstand extreme weather. The vessel is powered by solar panels and an underwater turbine. It doesn’t have a toilet and much light, so Thunberg will have only a bucket as a bathroom and a headlamp by which to read and keep her journals (she’s reportedly packed eight).

She’ll eat freeze-dried meals, the New York Times reported, and drink seawater treated in a desalination machine. She plans to share sporadic communiques with the outside world by sending messages to her friends via satellite phone to post on social media. There are only two beds on board, which Thunberg and her father will sleep in. The rest of the team plans to sleep on beanbags.

The Malizia II’s mainsail is emblazoned with Thunberg’s slogan: “Unite Behind the Science.”

Before she set off, Thunberg told reporters she was looking forward to the trip, even though she expected to feel seasick.

“If it’s really hard I just have to think it’s only for two weeks, then I can go back to as usual,” she said.

So far, the sailing hasn’t been smooth. The first night “was bouncy and rough,” Herrmann tweeted Thursday morning.

Though it could remain choppy, the voyage has already begun to fulfill one of Thunberg’s goals: raising awareness about climate change.

Still, sailboats remain extremely unlikely to become a viable form of international travel anytime soon, said Smith, the University College London lecturer.

“The cost would be astronomical” for most people, he said. “Essentially very few people have the luxury or the ability to take two weeks over a voyage.”

But, he added, Thunberg’s trip may prompt many to reconsider how frequently and by what means they travel. Many Europeans have already begun to make the switch to rail for overland trips and cut back on flights across the ocean. And in the business world, some shipping companies are starting to harness wind power to make shipping more efficient, Smith said.