Greenhouse gases are gases that trap heat in the Earth’s atmosphere. Some examples of greenhouse gasses include carbon dioxide, methane and nitrous oxide. According to the United States Environmental Protection Agency, carbon dioxide enters the atmosphere through burning fossil fuels (coal, natural gas, and oil), solid waste, trees and other biological materials, and also as a result of certain chemical reactions (e.g., manufacture of cement). Carbon dioxide is removed from the atmosphere (or “sequestered”) when it is absorbed by plants as part of the biological carbon cycle. A GHG Inventory estimates the quantity of GHG emissions associated with five specific areas of community energy-use sources and activities taking place during a chosen analysis year. What a GHG Inventory does not measure is the emissions of other industrial emissions like heavy metals.