This is an excerpt from EERE Network News, a weekly electronic newsletter.

May 15, 2002

EPA Examines Greenhouse Gas Emissions by Sector, Strength

The Environmental Protection Agency (EPA) released its final inventory of greenhouse gas emissions and sinks for 1990 through 2000 in April. The final report verifies the conclusions of the draft report, released in February, which found a 2.5 percent increase in emissions in 2000. For more details, see the February 27th edition of EREN Network News.

While the final report is no surprise, of greater interest is two re-analyses of the report by the EPA. The first, "Emissions by Economic Sector," divides the emissions into more intuitive groupings. This report shows electricity generation as the major source of U.S. greenhouse gas emissions, contributing 34 percent of the total in 2000. Transportation comes in second at 27 percent, and industry ranks third at 19 percent. Commercial buildings and agriculture contribute just 5 percent and 8 percent, respectively. And contributions from U.S. households - not counting their electricity use, but including the effects of the waste they generate - add up to about 8 percent of the total.

Rearranging the data to include electricity generation in each of the sectors (based on their electricity consumption) shuffles the order, placing industry just ahead of transportation, followed by residences, commercial buildings, and agriculture. Download the report: (PDF 102 KB). Download Acrobat Reader.

All of these analyses depend on counting the emissions of various greenhouse gases, such as methane, and applying a conversion factor that accounts for their strength as a greenhouse gas relative to carbon dioxide. The results are reported as simply "carbon dioxide equivalents." But here's a problem: scientists are tweaking and adjusting those conversion factors, which are known as Global Warming Potential (GWP) values. The most recent assessment report from the Intergovernmental Panel on Climate Change, for instance, changed the GWP for methane from 21 to 23. In other words, methane is now considered 23 times more powerful than carbon dioxide as a greenhouse gas.

The EPA's "Greenhouse Gases and Global Warming Potential Values" summarizes at these changes and asks an important question: do the changes revise our view of U.S. greenhouse gas emissions? The answer, thankfully, is no. Download the report: (PDF 82 KB). Download Acrobat Reader.