By 2008, the level of CO2; in the tub was 385 parts per million (ppm) and rising by 2 or 3 ppm each year. To stop it at 450 ppm, Sterman says, a level many scientists consider dangerously high, the world would have to cut emissions by around 80 percent by 2050. When diplomats convene in Copenhagen this month to negotiate a global climate treaty, Sterman will be there to help, with software that shows immediately, based on the latest climate-model forecasts, how a proposed emissions cut will affect the level in the tub—and thus the temperature of the planet. His students are generally much better at bathtub dynamics by the end of his course, which gives him hope. “People can learn this,” he says. —Robert Kunzig