What is One Million Metric Ton of Carbon Dioxide-Equivalent?
By Joe Abraham | The University of Arizona | June 22, 2009
The amount of greenhouse gases emitted into the atmosphere from U.S. landfills in 2007 was equivalent to more than 132 million metric tons of carbon dioxide, or MMtCO2-e, according to U.S. Environmental Protection Agency estimates.1 To put this into perspective, about one metric ton of carbon dioxide (CO2) is produced to the meet the average monthly energy demand of the typical American household.
Comparing different sources of greenhouse gases isn’t as difficult as you may think.
Credit: ©Oleg Prikhodko, istockphoto.com
Greenhouse gas emissions are typically expressed in metric tons, an international unit of measurement equivalent to approximately 2,200 pounds. Using a single unit of measurement makes it easier to communicate how, for example, various sources of greenhouse gases—diesel trains, factories, and coal-fired power plants—compare as contributors to a warmer climate.
Individuals measuring the carbon footprint of their activities—driving a car, the energy used in their home, or flying for a vacation—are more likely to use online calculators that estimate greenhouse gas emissions from those activities in metric tons or pounds. The emission reduction options discussed on this Web site, however, are estimated in millions of metric tons (MMt) because they are scaled up to an entire state or economic sector and reflect emissions reduced over a period of time (in the case of southwestern state climate action plans, 2007 through 2020).
The following examples from the EPA help put metric tons of carbon dioxide (CO2) into a more manageable number for individuals:
- One metric ton of CO2 is released to the atmosphere for every 103 gallons of gasoline used. Using a car that gets 25 miles to the gallon, that’s just a bit more than 2,500 miles—about two months of driving for many Americans.
- About 12 metric tons of CO2 are released to the atmosphere each year as a result of the energy consumed by the average American home for heating, cooling, cooking, electricity use, and other energy needs.
- Using up 40 barbeque propane canisters is the equivalent of releasing a ton of carbon dioxide into the atmosphere.
What are CO2 “equivalents?”
CO2 equivalents (CO2-e) offer a universal standard measurement that allows for the comparison of different greenhouse gases based on their ability to trap heat in the atmosphere. There are many types of greenhouse gases, and some gases are more effective at warming the atmosphere than others because they trap heat more effectively and longer.
|Carbon dioxide||CO2||280 ppmv||385 ppmv||50-200||
Fossil fuel combustion
Land use conversion
|Methane||CH4||700 ppbv||1866 ppbv||12-17||
|Nitrous oxide||N2O||275 ppbv||323 ppbv||120-150||
|Hydrochloroflouro-carbons (HCFCs)||HCFC-22||0||210 pptv||13||Liquid coolants||125|
|Perfluorocarbon||CF4||0||110 pptv||50,000||Production of aluminium||6,500|
|Sulphur hexa-flouride||SF6||0||6.84 pptv||1,000||Production of magnesium||23,900|
ppmv = parts per million by volume; ppbv = parts per billion by volume;
pptv = parts per trillion by volume
|Credit: IPCC radiative forcing report; Climate change 1995; Carbon Dioxide Information Analysis Center; http://cdiac.ornl.gov/pns/current_ghg.html|
Scientists have estimated measures of both factors for many different greenhouse gases that together determine the “global warming potential” (GWP; Table 1) of each gas. A GWP can then be used as a multiplier to compare emissions of different greenhouse gases based on their ability to contribute to the greenhouse effect. The GWP of a greenhouse gas is relative to the warming potential of CO2, which is set at a value of 1. For example, the GWP value of methane is 21, which means that a metric ton of methane is approximately 21 times as effective at warming the atmosphere as is a metric ton of CO2.
Thus, in terms of CO2-equivalents, a metric ton of methane is the same as 21 metric tons of CO2. Similarly, a metric ton of nitrous oxide, which is approximately 310 times as effective at warming the atmosphere as a metric ton of CO2, is the same as 310 metric tons of CO2.
It is important to note there is significant variation in the amount of time each greenhouse gas will remain in the atmosphere (see Table 1), but the GWP of greenhouse gases is based on the ability to trap heat over a period of 100 years. While this helps compare different greenhouse gases using CO2-equivalents, it also introduces some potential error, especially for gases like perflourocarbons that are expected to remain in the atmosphere for tens of thousands of years. Thus, the GWP for gases that will remain in the atmosphere for more than 100 years likely underestimates the contribution of those gases to a warmer atmosphere beyond the next century.
U.S. Environmental Protection Agency greenhouse gas equivalencies calculator
| http://www.epa.gov/cleanenergy/energy-resources/calculator.html |
Detailed technical discussion of greenhouse gases and their radiative forcing by the Intergovernmental Panel on Climate Change working group I (2007)
| http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter2.pdf |
- U.S. Environmental Protection Agency. 2009. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 – 2007. U.S. EPA report EPA 430-R-09-004. Washington, D.C., 441
- U.S. Environmental Protection Agency. Greenhouse Gas Equivalencies Calculator. Site last visited June 18, 2009.