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Greta Thunberg is sailing across the Atlantic, again. It’s much sooner than she had planned, but not before she makes her mark in the United States.

Ms. Thunberg, the 16-year-old Swedish climate activist, set sail from Hampton, Va., on Wednesday morning. This time, she’s hitching a ride with an Australian couple that sails around the world in a 48-foot catamaran called La Vagabonde and chronicles their travels on YouTube.

La Vagabonde will take roughly three weeks to reach Spain, where Ms. Thunberg hopes to arrive in time for the next round of United Nations-sponsored climate talks.

“I decided to sail to highlight the fact that you can’t live sustainably in today’s society,” Ms. Thunberg said by phone from Hampton on Tuesday afternoon. “You have to go to the extreme.”