The 16-year-old arrived in the United States in August after sailing across the Atlantic on a racing boat from Plymouth, England.
Since then, she has traveled around the country advocating for climate change awareness and action.
When Chile’s unrest forced the U.N. climate venue to be changed, Thunberg realized she needed to head back across the Atlantic sooner than expected, The New York Times reported.
“It turns out I’ve traveled half around the world, the wrong way:) Now I need to find a way to cross the Atlantic in November... If anyone could help me find transport I would be so grateful,” she posted on Twitter on Nov. 1.
She had originally planned to travel south to Chile by bus and boat.
Thunberg told The Times she decided to sail back to Europe “to highlight the fact that you can't live sustainably in today’s society. You have to go to the extreme.”
She will be sailing with Australian YouTubers Riley Whitelum and Elayna Carausu who live on a 48-foot catamaran and travel around the world with their baby.
The trip should take around three weeks.
Thunberg has said she won’t fly because of air travel’s negative impacts on the environment.