With growing awareness of elevated carbon dioxide levels and climate change, attention is turning to individual behavior as a source of global carbon emissions. According to Shui and Dowlatabadi (2005), consumers in aggregate were directly responsible for28% of US energy consumption and 41% of US CO2 emissions in the year 1997. In response to this focus on individuals, numerous websites have been created to help calculate an individual's “carbon footprint,” or an estimate of the carbon dioxide emissions that an individual is directly responsible for over a given period of time. These calculators typically divide the individual's profile into household activities and transportaion, and based on differing formulations of user inputthey produce a quantified amount of carbon dioxide or carbon dioxide equivalents emitted, generally in units of mass of CO2 per year. These calculators are provided by government agencies, non-governmental organizations, and private companies. Some of these carbon calculator providers also promote methods for mitigating carbon dioxide emissions through offsets or investments in renewable energy technology. Even when calculators are not coupled with mitigation measures, by providing estimates of individual contributions they play a fundamental role in promoting carbon emission reductions through individual behavior change.