There’s no police tape across Michael Mann’s office doorway this morning. “Always a good start,” he says, juggling a cup of coffee as he slides his key into the lock.

Mann, a paleoclimatologist, wears a sport coat over a turtleneck. As he takes a seat at his desk, a narrow sunbeam angles through the window, spotlighting a jumble of books, journals and correspondence. Behind him, a framed picture of his six-year-old daughter rests near a certificate for the Nobel Peace Prize he shared in 2007. Propped into a corner is a hockey stick, a post-lecture gift from Middlebury College, which Mann jokingly says he keeps “for self-defense.”

Mann directs Penn State University’s Earth System Science Center. Several months ago, he arrived at his office with an armload of mail. Sitting at his desk, he tore open a hand-addressed envelope and began to pull out a letter. He watched as a small mass of white powder cascaded out of the folds and onto his fingers. Mann jerked backward, letting the letter drop and holding his breath as a tiny plume of particles wafted up, sparkling in the sunlight. He rose quickly and left the office, pulling the door shut behind him. “I went down to the restroom and washed my hands,” he says. “Then I called the police.”