Measuring the National Carbon Footprint

How do you total up the greenhouse-gas emissions for an entire country?

Smog near the port in Long Beach, California

The United Nations reported this week that Kyoto Protocol signatories have reduced greenhouse-gas emissions to 5 percent below 1990 levels, four years ahead of the treaty schedule. Is it really possible for a country to measure how much carbon dioxide, methane, nitrous oxide, perfluorocarbons, sulfur hexafluoride, and other greenhouse gases it’s putting out?

Not exactly. For a country to get a direct readout of its national emissions, it would have to put a greenhouse-gas meter on every tailpipe, landfill, and cow. Since that’s impractical, government officials track the inputs rather than the outputs. In other words, they count up the number of gallons of fossil fuels consumed, cattle raised, and pounds of waste produced. From there, it’s just a matter of doing the right calculations.

Carbon dioxide is the easiest gas to measure in this way. When a unit of fossil fuel is combusted, it emits a predictable amount of carbon dioxide. While different combustion devices may produce differing levels of certain pollutants, such as sulfur dioxide or particulate matter, the poundage of carbon dioxide emitted depends in large part on the composition of the fuel. For example, combusting one gallon of gasoline reliably produces about 19 pounds of carbon dioxide. One ton of coal can be counted on to give off approximately 5,700 pounds. A cubic foot of natural gas produces 0.12 pounds of carbon dioxide. (Sequestration technology could in theory lower these numbers, but so far has had little impact on national emissions.) That means you can tally the amount of each type of fuel consumed in a year—via surveys of energy importers, producers, and suppliers—then multiply the totals separately.

Computing methane emissions is more complicated, because the ratio of inputs to outputs can change from one situation to the next. In the United States, about 24 percent of human-caused methane emissions comes from the decomposition of organic matter in landfills, but the amount that comes out of any one landfill depends on what kind of garbage the landfill contains and at what temperature. A further 21 percent of our emissions are spewed out by livestock—but a cow will be more or less gassy depending on the details of its diet and the bacteria in its gut.

The Intergovernmental Panel on Climate Change recommends a method for calculating methane emissions from livestock. The mathematical model looks at a national livestock population’s average age, weight, diet, and other factors to generate an “emission factor,” representing the average methane emission per head of cattle per day for that population. The emission factor can then be multiplied by the number of cattle, which is calculated via government surveys of ranchers and dairy farmers. There is a similar model for methane emissions from landfills, incorporating the weight of municipal waste, the fraction of waste deposited in landfills, the age of the waste, and climatic conditions.

Nitrous oxide estimates are even less reliable. The gas is released when microbes in the soil metabolize nitrogen-rich fertilizer—but the details of this reaction are very difficult to model due to variations in the native microflora and oxygen availability. The best we can do is use temperature data, soil conditions, fertilizer type, and crop type to make an imprecise guess about the overall rate of emission.

The difficulties in measuring certain gases means there’s likely to be some degree of error in the calculations. That problem is mitigated by the relative ease with which we can estimate carbon dioxide emissions. While methane, nitrous oxide, and other gases are more potent climate change agents, carbon dioxide remains the largest contributor to the problem by a sizable margin.

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Explainer thanks Paul McArdle of the Energy Information Administration and Roxanne Smith of the Environmental Protection Agency.