Greenhouse gases are gases that trap heat in the atmosphere. These gases raise the temperature of the earth through the "greenhouse effect". Without the greenhouse effect, the Earth would not be warm enough for humans to live. But if the greenhouse effect becomes too strong because more greenhouse gases are being emitted, it could cause problems for humans, plants, and animals.
Greenhouse gases are produced through both natural processes and human activities. Water vapor is an example of a greenhouse gas that occurs naturally. The principal greenhouse gases that enter the atmosphere because of human activities are: carbon dioxide, methane, nitrous oxide, and fluorinated gases.
In the US, our energy-related activities account for over three-quarters of our human-generated greenhouse gas emissions, mostly in the form of carbon dioxide emissions from burning fossil fuels. More than half the energy-related emissions come from large sources such as power plants and factories, while about a third comes from transportation. Industrial processes (such as the production of cement, steel, and aluminum), agriculture, other land use, and waste management are also important sources of greenhouse gas emissions in the United States.