The distinction between absolute emissions and emissions intensity is fairly simple in concept, but may be confusing in practice. “Absolute emissions” is the common measure of emissions that is used by protocols and measuring standards. While the actual calculations may be challenging for a number of reasons (e.g., difficulty in defining boundaries, quantifying activities that result in emissions, or determining proper emissions factors), the concept is fairly simple. Emissions are quantified for some entity, for example a company, university, city, or country, and reported -- usually in terms of tons of carbon dioxide. For example, if Ace Manufacturing Company reports that it produced one million tons of CO2 in 2003, it is reporting its absolute emissions.

Whereas absolute emissions quantifies the amount of carbon dioxide and other greenhouse gases produced, emissions intensity reports the amount of emissions per some unit of economic output. This can be done for a company relative to the company’s total profits, or in terms of units of a good produced. For a country, emissions intensity might be calculated as tons of carbon dioxide relative to that country’s gross national product.

Using emissions intensity
As a one-time snapshot of emissions from one entity, the difference between emissions intensity and absolute emissions doesn’t make much sense. There are two instances, however, in which emissions intensity may be illustrative. The first is when comparing emissions over time. If in 1990 Ace Manufacturing produced five hundred thousand tons of CO2, and in 2003 it produced one million tons, its absolute emissions doubled. However, if in that time Ace’s output doubled, then its emissions intensity stayed the same, even while its emissions doubled. If during this time Ace’s output quadrupled, then its emissions intensity was cut in half, showing that the company has become twice as efficient, even though its emissions have increased.

Comparing emissions intensity may also make sense when comparing the emissions of two entities of different size. A country whose economy account for one-sixteenth of the world economy may argue, for example, that its emissions levels are appropriate as long as they account for only one-sixteenth of the world’s CO2 emissions. That country’s plan for addressing emissions may then call for a specific reduction in emissions intensity, rather than a reduction in absolute emissions.

Many governments and other bodies (e.g., environmental organizations, protocols) believe that the proper measure of emissions, at least at the macro level, is in absolute terms. They point out that the build-up of carbon dioxide in the atmosphere, and its associated impacts, is based on absolute emissions. In fact, most international legislative and regulatory efforts, including the Kyoto Protocol, address absolute emissions, not emissions intensity.