Jeffrey Sachs has palled around with do-gooding celebs like Bono and Angelina Jolie, but also advised international power brokers like U.N. Secretary-General Ban Ki-moon. An economist and professor of sustainable development at Columbia University, Sachs is on a mission to solve the world's big problems: climate change, poverty, security, disease, and ecosystem degradation. More pragmatist than eco-purist, Sachs advocates solutions ranging from solar power and ultra-efficient cars to advanced coal technologies, chemical fertilizers, and genetically modified seeds. Grist's Amanda Griscom Little caught up with Sachs to hear about his new book, "Common Wealth: Economics for a Crowded Planet," and his vision for a more sustainable world.