The carbon footprint is the estimation of carbon dioxide (CO2) released into the atmosphere by human activities, notably the burning of fossil fuels (see Greenhouse gases). Typically, this is calculated by evaluating the amount of time a person spends traveling in various vehicles; how much electricity, natural gas, fuel oil, etc. they use in their home; what they do, eat, wear, and buy; whether or not they practice environmentalism (e.g. recycling); and what country they live in. This calculation is then used to determine what someone has to do to "offset" their carbon footprint's impact on climate change. This almost always involves paying exorbitant amounts of money to companies that supposedly plant trees or build alternate energy producing facilities.
Liberal politicians have trumpeted the imposition of carbon caps and carbon-trading, in order to control the carbon footprints of companies and people. Although market-based carbon cap-and-trade schemes are being implemented to certain extents and with some measured success in New Zealand and in the European Union, and a similar, successful program for sulfur dioxide and nitrogen oxides was implemented in the U.S. under President George H.W. Bush as an amendment to the Clean Air Act, a "carbon currency" in the U.S. still has many logistical hurdles and has the potential for abuse.
The Intergovernmental Panel on Climate Change produced a report for the International Energy Agency (IEA) which states that the price for reducing human CO2 50% by the year 2050 will cost an estimated $45 trillion dollars or 1.1 per cent of average annual global GDP.
Detractors argue that CO2 is just a tiny component of the overall mechanisms that causes the climate to change. Human driven CO2 is even less a component. 
- New Zealand Ministry for the Environment. "Emissions Trading Fast Facts." September, 2008. http://www.mfe.govt.nz/publications/climate/emissions-factsheets/index.html
- New Zealand Ministry for the Environment. "Summary of the proposed changes to NZ-ETS. September 2009. http://www.mfe.govt.nz/publications/climate/emissions-trading-bulletin-11/index.html
- Parker, Larry. Climate Change: The European Union's Emissions Trading System. July 31, 2006. Congressional Research Service, Library of Congress. http://fpc.state.gov/documents/organization/70317.pdf
- U.S. Environmental Protection Agency. "Acid Rain Program 2007 Progress Report." http://www.epa.gov/airmarkt/progress/arp07.html
- MSNBC Devil is in details of carbon cap system
- , Businessgreen.com, IEA calls for $45 trillion energy "revolution", June 6, 2008
- , CFP, Medieval Environmentalists’ attack CO2, January21, 2008