"Carbon footprint" has become a catchphrase in the last year or two. We find out just what a carbon footprint is, how it’s calculated, and how much it matters. Mark Z. Jacobson is Professor in the Department of Civil and Environmental Engineering at Stanford University and Catherine S. Norman is Assistant Professor in the Department of Geography and Environmental Engineering at Johns Hopkins University.

Weigh in: Do you have any idea what your carbon footprint is? Would knowing your carbon footprint make you change your daily habits?