You might think your feet stopped growing in high school, but there's a footprint of a sootier variety that's likely to have grown a lot since then. Carbon footprints measure how much carbon dioxide (CO2) you produce in your daily life. Since CO2 is one of the biggest contributors to global warming, most do-it-yourself environmentalists want to produce less of it. Carbon footprints act as a benchmark -- kind of like a weigh-in before a diet -- so that people striving to reduce or even eliminate their carbon output know where they stand to start with.
Carbon calculators, programs hosted by a variety of Web sites, turn easy-to-supply numbers like average power use and gas mileage into a single figure: usually a monthly or annual total of CO2 output measured in tons. With such a clear-cut total, it's easy to see the environmental payoff of seemingly small changes like replacing an inefficient appliance or teleconferencing instead of flying to a business meeting. Learn what kind of tracks you're leaving in How Carbon Footprints Work.