How our carbon calculator works.
The average combined MPG for all US cars and light trucks on the road today is 19.8 MPG. (Source: 2005 Highway Statistics from the U.S. Department of Transportation, Federal Highway Division). A passenger vehicle in the US is driven an average of 12000 miles per year. A gallon of gasoline emits approximately 19.4 lbs of CO2. By calculating the number of gallons used each year, we can determine the vehicle's carbon footprint.
The US Department of Energy provides statistics on natural gas consumption per state, and the number of residential consumers per state so an average usage per consumer can be calculated. US DOE also provides data on the emissions based on types of electricity used in each state. All emissions are converted to equivalent carbon dioxide (CO2E) using a factor of 21:1 for methane and 310:1 for nitrous oxide. The natural gas conversion factor is 12.0593 lbs per CCF.
Those averages are used to calculate the typical home's emissions based on the state of residence chosen. According to the National Home Builders Association, the average square footage of a single family home is 2330. Entering a home size at least 30% larger will increase the home footprint by 20%. Entering a home size at least 30% smaller will decrease the home footprint by 20%. Choosing an apartment will decrease the home footprint by 50%, and choosing a townhome will decrease the home footprint by 25%.
The average american makes 1.7 round trip flights per year, according to a Gallup study. The Bureau of Transportation Statistics calculates approximately 850 billion revenue passenger miles on US air carriers in 2007. With a US population of approximately 300 million, that calculates to 2800 air miles per american, which for rounding sake has been equated to 2 short-hop round trips per year. Because longer flights pollute less per mile, the flights have been broken down into short hops, domestic flights and international. The carbon emissions for each respectively are 1.84, 1.28 and 1.13 lbs per mile. This is considering both the actual CO2 emissions from the fuel consumed, and the radiative forcing index, which takes into account the increase in impact of these greenhouse gasses as they are being released at a very high altitude. We have chosen a radiative forcing index of 2, which is recommended by the IPCC.
The CO2 emissions from the calculated distance are determined by multiplying the specified miles traveled by one of the CO2e figures below, which are the estimated pounds of CO2 emissions per passenger mile, based on flight distance.
flight length pounds co2 per mile RF pounds co2 per mile Short Hop (average 500 miles) 0.6386 1.2133 Cross Country (average 2000 miles) 0.4470 0.8493 Overseas (average 4000 miles) 0.3903 0.7416