According to the United States Environmental Protection Agency, carbon dioxide is the primary greenhouse gas emitted through human activities. Carbon dioxide is naturally present in the Earth’s atmosphere as part of its carbon cycle, but human activity is altering that cycle by adding more carbon dioxide to the atmosphere. In addition to adding more carbon dioxide to that natural cycle, human activity is influencing the ability of natural sinks, such as oceans, forests and plants, to remove carbon from the atmosphere.
The combustion of fossil fuels for energy and transportation is the primary carbon dioxide-emitting human activity. In fact, the EPA notes that electricity, transportation and industry are the three main sources of carbon dioxide emissions in the United States. Electricity accounted for roughly 37 percent of total carbon dioxide emissions and 31 percent of total greenhouse gas emissions in the United States in 2013. Transportation, which involves the combustion of gasoline and diesel fuel to transport people and goods, was the second largest source of carbon dioxide emissions in 2013, accounting for 31 percent.
In addition to automobiles, transportation-related sources of carbon dioxide emissions include airplanes, marine transportation and trains. Though much of the world has grown increasingly eco-conscious over the last decade or so, carbon dioxide emissions remain high, as such emissions in the United States increased by 7 percent between 1990 and 2013. However, the 2014 U.S. Greenhouse Gas Inventory Report, which tracked total annual U.S. emissions and removals dating back to 1990, indicated a 9 percent decline in greenhouse gas emissions between 2005 and 2013, suggesting that efforts to reduce emissions were beginning to bear fruit.