The revelations that Exxon concealed its early findings that fossil fuels cause global warming have sparked a criminal investigation by New York’s attorney general and calls for a federal probe like the one against Big Tobacco. But some aren’t waiting for the justice system to act. During the recent U.N. climate summit in Paris, environmental activists held a “mock trial” charging Exxon with “climate crimes.” Hundreds from around the world—including participants in COP21—packed into a large warehouse-like cultural space to hear a stirring indictment of Exxon. A tribunal of judges heard testimony from witnesses that included scientists, energy experts and residents of frontline communities threatened by climate change. The witnesses were questioned by two leading environmentalists acting as chief prosecutors: Bill McKibben, co-founder of 350.org, and journalist Naomi Klein.