Where are greenhouse gases coming from?

In the United States, greenhouse gas (GHG) emissions come primarily from the burning of fossil fuels in energy use.

Carbon Dioxide

Energy-related carbon dioxide emissions (CO₂), resulting from the combustion of petroleum, coal, and natural gas, represented about 81% of total U.S. human-caused (anthropogenic) GHG emissions in in 2009. About 1% of green house gases are CO₂ from other human activities like cement manufacturing.

Methane and Other Gases
Another greenhouse gas, methane (CH₄), comes from landfills, coal mines, oil and natural gas operations, and agriculture; it represented about 11% of total U.S. GHG emissions in 2009. Nitrous oxide (N₂O) emissions, at about 3% of total GHG emissions, came from the use of nitrogen fertilizers, burning fossil fuels, and certain industrial and waste management processes. Several human-made gases, hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6), that are released as byproducts of industrial processes and through leakage, represented about 3% of total emissions.

The Energy Connection
Fossil fuels are made up of hydrogen and carbon. When fossil fuels are burned, the carbon combines with oxygen to create carbon dioxide. The amount of carbon dioxide produced depends on the carbon content of the fuel; for example, for each unit of energy produced, natural gas emits about half and petroleum fuels about three-quarters of the carbon dioxide produced by coal.

Most of Our Carbon Dioxide Emissions Come from Coal and Petroleum Use

Coal Is the Dominant Emissions Source Related to Electricity Generation

Source:
http://www.eia.gov/energyexplained/index.cfm?page=environment_where_ghg_come_from