Ms. Thunberg has never done anything like this before. She said she was looking forward to being without the familiar luxuries, to “being so limited.” She acknowledged being a bit nervous. “Whether it’s seasickness or homesickness or just anxiety or I don’t know,” she said. “I don’t know how tough this journey will be.”

Also, she said, she will really miss her two dogs.

She has packed many books (she is currently reading “Quiet,” a book about introverts, like her); eight writing journals, some partly filled; and boxes of freeze-dried vegan meals. (Ms. Thunber g stopped eating meat a few years ago, because of the emissions associated with animal protein.)

There is a satellite phone on board, so she plans to send some pictures and text messages from her voyage to friends who will upload them on her social media accounts. Going to the toilet will mean going to the back of the boat with a bucket. Her drinking water will come from a tiny desalination machine that treats seawater.

“By doing this it also shows how impossible it is today to live sustainable,” she said. “That, in order to travel with zero emissions, that we have to sail like this across the Atlantic Ocean.”

The epic journey of the Malizia II is the latest stage in an epic journey that Ms. Thunberg has been on for the last few years. As a child, doctors told her she had Asperger’s syndrome. In early adolescence she battled severe depression, so much so that she stopped eating for a while and stopped growing.