Stockholm — Greta Thunberg, the Swedish teenager whose social media-savvy brand of eco-activism has inspired tens of thousands of students in Europe to skip classes and protest for faster action against climate change, said Monday that she plans to take her message to America the old-fashioned way: by boat. The 16-year-old tweeted that she’ll sail across the Atlantic aboard a high-tech racing yacht, leaving Britain next month to attend U.N. climate summits in New York in September and Santiago, Chile, in December.