Where are the gases coming from? A greenhouse gas emissions inventory report

An EPA report illustrates changes in greenhouse gas emissions over the 21 years between 1990 and 2010.

In April 2012 the U.S. Environmental Protection Agency (EPA) released the “Greenhouse Gas Emissions Inventory” report that quantifies the sources of greenhouse gas emissions in the United States and compares the quantity of greenhouse gases (GHG) emissions, by source, between 1990 and 2010. This report will be useful as businesses, commodity groups and individuals examine their contribution to the total GHG emissions.

The main greenhouse gases discussed in the report include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydroflurocarbons (HFCs), perfluorocarbons (PFCs) and sulfur hexafluoride (SF6). Of the GHG emissions discussed in the report, HFCs, PFCs and SF6, combined, contribute less than 2 percent of the global warming potential of the GHGs emitted in 2010. While these GHG emissions are important, because of the relative small amount emitted in the U.S., they will not be further discussed in this article.

Each greenhouse gas has a different capacity to absorb heat based on its chemistry and how long it lasts in the atmosphere. The Global Warming Potential is an index that represents the global warming impact of a particular greenhouse gas relative to carbon dioxide. The unit carbon dioxide-equivalents (CO2e) is used and helps when making inventories of or comparing greenhouse gases. For example, methane has 24 times and nitrous oxide has 298 times the global warming potential, or greenhouse effect’, of CO2. For more information on greenhouse gases, read the Michigan State University Extension Bulletin E3148 Greenhouse Gas Basics.

According to the EPA report, in 2010 the total U.S. GHG emissions was 6,821.8 teragrams[1] (Tg) CO2e, approximately 18 percent of the global GHG emissions and a 10.5 percent increase in total U.S. GHG emissions since 1990.  For reference, one Tg equals a 1,000,000 metric tons or 1,102,311 U.S. tons. Emissions of CO2 were the largest contributor to total GHG emissions, accounting for 5706.4 Tg (84 percent) of total U.S. GHG emissions. Fossil fuel combustion was the single largest contributor to U.S. GHG emissions, accounting for 94 percent of the CO2 and 79 percent of the total CO2e emitted in 2010.

Though there has been an increasing awareness of GHG emission during the 21 years covered in the report, emissions of CO2 continued to increase. Between 1990 and 2010, CO2 emissions increased 605.9 Tg, a 12 percent increase. While other sources of GHGs emissions showed either small increases or reductions, there was a significant increase in CO2 emissions from fossil fuel combustion. Electricity generation (+ 437.6 Tg CO2) and transportation fuels (+ 259.6 Tg CO2) accounted for the increase in CO2 emissions. Other areas of fossil fuel utilization, industrial (- 68.6 Tg CO2), residential (+ 1.9 Tg CO2) and commercial users (+ 5.2 Tg CO2) showed either much smaller increases or reductions in CO2 emissions.

As one would expect, economic recessions reduce CO2 emissions. According to the EPA report, U.S. carbon dioxide emissions peaked in 2007 at 6118.6 Tg CO2 then fell to 5500.5 Tg in 2009. Less manufacturing, construction and travel all contributed to reduced use of fossil fuels and lower CO2 emissions. The increased economic activity in 2010 was accompanied by a small increase in emissions, 205.9 Tg CO2, above the 2009 rate.

In 2010, agriculture’s net GHG emissions was 426.7 Tg CO2e, a 14 percent increase over the 367.7 Tg CO2e attributed to all agricultural activities in 1990. For this article agriculture’s total contribution to CO2e emissions was calculated by totaling the agricultural land use changes (-1.7 Tg CO2e) discussed in the “Land Use, Land Use Change and Forest” section of the EPA report with those emissions quantified in the agriculture section, including both crop and livestock production (+428.4 Tg CO2e). When agriculture’s sequestration of GHG’s attributed to land use changes is added to all other emissions attributed to the sector, agriculture accounted for 6.3 percent of the total 2010 CO2e emissions. To break this down further, of the total GHG emissions in the United States, agriculture was responsible for 74 percent of the nitrous oxide and 30 percent of the methane released in 2010.

If reducing GHG emissions becomes one of society’s goals, all sectors will need to participate in reducing their total emissions – that is – a portfolio approach is needed. Based on the amount of GHG emissions associated with fossil fuel combustion, therein lies the greatest opportunity for reduction, and it includes the transportation of food and feed and other agricultural products. If other sectors are asked to reduce their emissions, agriculture could benefit via payments to reduce GHG emissions or increase carbon storage in soils.

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