On Dec. 3, La Vagabonde docked beneath a flight path to Portugal’s largest airport. Thunberg and her father stood on the deck, waving to the hundreds of people that had gathered on a cold, sunny day to welcome them back to Europe. Above their heads, planes droned, reminders of how easily Thunberg could have crossed the ocean by air, and of the cost of that convenience: the roughly 124,000 flights that take off every day spill millions of tons of greenhouse gases into the atmosphere. “I’m not traveling like this because I want everyone to do so,” Thunberg told reporters after she walked, a little wobbly at first, onto dry land for the first time in weeks. “I’m doing this to send a message that it is impossible to live sustainably today, and that needs to change.”