DAVOS, Switzerland – Greta Thunberg isn’t easily intimidated. The 17-year-old Swedish activist wasted little time on Tuesday to push back against U.S. President Donald Trump’s description of climate campaigners as “the perennial prophets of doom” who predict the “apocalypse.” Though Trump didn’t mention her directly in his speech at the World Economic Forum in the Swiss Alpine resort of Davos, it was clear he had his sights on Thunberg, who shot to fame a year ago by staging a regular strike at her school and sparked a global environmental movement.