Greenhouse gases, or GHGs, are compound gases that trap heat or longwave radiation in the atmosphere. Their presence in the atmosphere makes the Earth’s surface warmer. Sunlight or shortwave radiation easily passes through these gases and the atmosphere. This radiation is absorbed by the surface of the earth and released as heat or longwave radiation. The molecular structure of GHGs allows them to absorb the heat released or trap them in the atmosphere and re-emit them back to the earth. This heat-trapping phenomenon is known as the greenhouse effect. The accumulation of GHGs since the industrial revolution has accelerated this greenhouse effect, causing global warming and climate change.

The principal GHGs, also known as heat-trapping gases, are carbon dioxide, methane, nitrous oxide, and the fluorinated gases. Carbon dioxide composes 64.3% of GHGs. It enters the atmosphere through the burning of fossil fuels, solid waste, trees and wood products, and certain chemical reactions. It is removed from the atmosphere naturally by plants as part of the biological carbon cycle. Methane is released to the atmosphere by the production and transport of coal, natural gas and oil, livestock and agricultural practices, and the decay of organic waste in municipal solid waste landfills. Nitrous oxide (N2O) is emitted during agricultural and industrial activities and during the combustion of fossil fuels and solid waste. Fluorinated gases are synthetic gases from industrial processes. They include hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and nitrogen trifluoride. They are sometimes known as stratospheric ozone-depleting substances or high global warming potential (GWP) gases because they can be 1,000 times more powerful than carbon dioxide and can be in the atmosphere for thousands of years.

GHGs can also be classified into two different types: forcing GHGs and feedback GHGs. Forcing GHGs are the four above-mentioned gases: carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and fluorinated gases. They take many years to leave the atmosphere and don’t react to changes in temperature or air pressure, so they are not easily removed. Feedback GHGs are water vapor. They last in the atmosphere for only a few days and are highly active components of the climate system, meaning they respond rapidly to changes in conditions. They act as feedback to GHGs by circulating the greenhouse effect or by accentuating the warming effect of forcing GHGs.

Greenhouse gases can be evaluated in three ways:

1. How much of the gas is in the atmosphere?
   This refers to the concentration or abundance of a greenhouse gas. Some GHGs are produced in larger amounts than others. In popular discussions about climate change and GHGs, carbon dioxide tends to receive greater focus because it is the most abundant GHG in the atmosphere, accounting for roughly 75% of greenhouse gases.
2. How long does the gas remain in the atmosphere?
   Greenhouse gases vary in lifespan and remain in the atmosphere for very different amounts of time. For example, carbon dioxide, methane, and nitrous oxides have a relatively short lifespan in the atmosphere, ranging from twelve to 200 years. Hydrofluorocarbons and perfluorocarbons, on the other hand, can persist in the atmosphere for hundreds to thousands of years.
3. How much impact does the gas have on global temperatures?
   This refers to a greenhouse gas’ global warming potential (GWP) or impact on global temperatures. Some gases are more effective in absorbing energy or trapping heat than others. For instance, one pound of methane traps 21 times as much heat as one pound of carbon dioxide.

Human activities are the main source of GHGs. The burning of fossil fuel, deforestation, intensive livestock farming, the use of synthetic fertilizers, and industrial processes all contribute. Since the Industrial Revolution of the 18^th century, there has been a sharp and dangerous increase in GHGs in the atmosphere. This can be attributed in large part to industrial processes, electricity, heating of homes, and driving.

According to the Environmental Protection Agency (EPA), GHG emissions in the United States can be traced from different sectors.

Electricity production generated 25%, of U.S. GHG emissions in 2020. Approximately 60% of U.S. electricity comes from burning fossil fuels, mostly coal and natural gas.

Transportation accounted for 27% of 2020 U.S. GHG emissions. This is primarily from burning fossil fuel for cars, trucks, ships, trains, and planes. Over 90% of fuel burned is petroleum-based, including gasoline and diesel. The largest sources of transportation-related GHG emissions include passenger cars and light-duty trucks, including sport utility vehicles, pickup trucks, and minivans.

Industry accounted for 24% of 2020 U.S. GHG emissions. There are two classes of industrial emissions: direct and indirect emissions. Direct emissions come from burning fossil fuels for power or heat, from chemical reactions, and from leaks in industrial processes. Indirect emissions originate from burning fossil fuels at power plants to make electricity used for industrial facilities or to power machinery.

Agriculture accounted for 11% of 2020 U.S. GHG emissions. Emissions in this sector come from agricultural activities such as the cultivation of crops and livestock. Enteric fermentation, a part of the digestive process for many livestock animals, represents almost 40% of the emissions from agriculture. Manure from livestock also contributes to methane and nitrous oxide emissions. Manure management accounts for about 13% of total GHG emissions in this sector in the U.S.

The commercial and residential sector accounts for 13% of U.S. GHG emissions in 2020. These include all homes and commercial businesses (excluding industrial and agricultural activities). Emissions in this sector come from fossil fuel combustion for heating and cooking needs, management of waste and wastewater, and leaks from refrigerants in homes and businesses. Emissions from natural gas consumption used for cooking and heating represent about 79% of emissions in this sector. Organic waste sent to landfills emits methane. Wastewater treatment emits methane and nitrous oxide. Refrigeration leaks emit fluorinated gases.

The Earth’s temperature depends on the balance between energy entering and leaving the atmosphere. Imbalance can be caused by changes in the greenhouse effect, which in turn causes climate change.

Prior to the Industrial Revolution, climate varied naturally. However, climate changes since cannot be explained by natural causes. Rising temperatures, changing snow and rainfall patterns and more extreme climate events have been linked to the unnatural levels of carbon dioxide and other GHGs in the atmosphere.