Greenhouse gases trap heat in the atmosphere and make the planet warmer. Human activities are responsible for almost all of the increase in greenhouse gases in the atmosphere over the last 150 years. The primary sources of greenhouse gas emissions are:

• Burning fossil fuels: The largest source of greenhouse gas emissions from human activities in the United States is from burning fossil fuels for electricity, heat, and transportation. This includes burning coal, oil, and natural gas for energy use.

• Deforestation: Cutting down forests also adds enormous amounts of greenhouse gases to those naturally occurring in the atmosphere.

• Livestock farming: Livestock, especially ruminants such as cattle, produce methane as part of their normal digestive processes.

• Industrial processes: Greenhouse gas emissions from industry primarily come from burning fossil fuels for energy, as well as greenhouse gas emissions from certain chemical reactions necessary to produce goods from raw materials.

• Waste management: Greenhouse gas emissions from this sector come from direct emissions including fossil fuel combustion for heating and cooking needs, management of waste and wastewater, and leaks from refrigerants in homes and businesses, as well as indirect emissions that occur offsite but are associated with the use of electricity by homes and businesses.

The main greenhouse gases responsible for the greenhouse effect include carbon dioxide, methane, nitrous oxide, and water vapor. Many of these greenhouse gases occur naturally, but human activities are increasing the concentrations of some of them in the atmosphere. Carbon dioxide is the most significant gas that causes global warming via the greenhouse effect. Methane is a more powerful greenhouse gas than carbon dioxide, but has a shorter atmospheric lifetime. Nitrous oxide, like carbon dioxide, is a long-lived greenhouse gas that accumulates in the atmosphere over decades to centuries.