Michael E. Mann has been a central figure in the battle for the environment since the “hockey stick” graph made him a target for climate change deniers 25 years ago. Now on Penn’s faculty and heading the Center for Science, Sustainability, and the Media, he’s fending off a new generation of “inactivists” comprised of climate change deflectors on the right and doomists on the left to get out the message that it’s still within our power to save the planet.