"We sail for home!" Greta Thunberg announced on Twitter on Wednesday.
The 16-year-old climate activist is sailing to Madrid from Virginia in the US to attend the COP25 climate conference in Spain, which starts in December.
Thunberg is on a 48ft catamaran called La Vagabonde, with an Australian couple who are travelling the world with their newborn child by boat and posting about their experience on YouTube.
Thunberg has been touring the US and meeting with climate activist groups across North America since August. She arrived in New York on another sailboat which had given her a carbon-neutral lift from Europe.
She has been to several US states including New York, California, North Dakota, Iowa, Virginia, each Friday joining the Youth for Climate Friday marches in the city she was in, while also appearing on national TV, giving speeches at the U.N., and meeting with celebrities and public figures involved in the climate movement.
She refuses to fly to protest the high level of CO2 produced by commercial flights, which greatly contributes to climate change.
Thunberg had originally planned to stay in America much longer, slowly making her way to South America, where Chile was expected to host the COP25 meeting.
But after a wave of protests erupted in the Chilean capital, Santiago, the country announced that it could no longer host the talks, and Spain offered Madrid as an alternative venue.
This left Thunberg in need of a boat ride back to Europe.
"It turns out I've traveled half around the world, the wrong way", she tweeted, adding: "If anyone could help me find transport I would be so grateful."