Greenhouse gas emissions by the United States

The United States was one of the largest emitters of greenhouse gases in the world as of the early 2020s, emitting roughly 5,222 million metric tons of carbon dioxide and equivalents in 2020. At the same time, the country has been the largest historical emitter of greenhouse gases, releasing more into the atmosphere through human activity than any other country in history. Some of the most common greenhouse gases—gases that trap heat inside Earth’s atmosphere—are carbon dioxide, methane, and nitrous oxide. Human activity, such as burning fossil fuels and cutting down trees, has greatly increased the concentration of greenhouse gases in the atmosphere. Scientists believe that higher greenhouse-gas concentrations are responsible for causing climate change, which is the long-term shift of Earth’s temperature and climate patterns.

Since the United States has the highest historical greenhouse-gas output in the early 2020s, many people believed that it should take on a central role in the global fight to reduce the negative effects of climate change, which had already caused economic instability and social unrest in some parts of the world by the early 2020s.

Overview

Types and Sources of Greenhouse Gases

When released into the atmosphere, greenhouse gases cause Earth to warm. Some greenhouse gases occur naturally, such as carbon dioxide, methane, and nitrous oxide. However, others are not found in nature and have been created by humans. For example, people make sulfur hexafluoride to use in electric systems and other applications. Sulfur hexafluoride is the world’s most potent greenhouse gas, which means that it can trap the most radiation in the atmosphere. However, Earth’s atmosphere contains a much lower concentration of this gas than other greenhouse gases, such as carbon dioxide. Therefore, other greenhouse gases have a more significant impact on climate change than sulfur hexafluoride.

Human activity is the only way that human-made greenhouse gases such as sulfur hexafluoride get into the atmosphere. However, naturally occurring greenhouse gases are in the atmosphere because of human activity and biochemical processes in nature, such as photosynthesis, respiration, and decomposition. These processes help the gases cycle between the environment and the atmosphere. For example, trees and other plants sequester, or trap, atmospheric carbon dioxide to use during photosynthesis. However, after a tree dies and begins to decompose, it emits some of its carbon back into the atmosphere, so cutting down trees and clearing land contributes to climate change. Other human activities also affect greenhouse-gas concentrations in the atmosphere. For example, people produce carbon dioxide by burning fossil fuels.

Fortunately, people can also take actions that reduce greenhouse gases in the atmosphere. For example, they can plant trees, which sequester carbon dioxide.

Human Greenhouse-Gas Emissions

The concentrations of atmospheric greenhouse gases such as carbon dioxide, methane, and nitrous oxide have increased dramatically since the 1700s. Atmospheric carbon dioxide increased from roughly 280 parts per million (ppm) in the mid-1700s to roughly 415 ppm in 2021. Although the concentration of carbon dioxide can be affected by natural processes and human actions, scientists believe that human activity is responsible for the dramatic increase of the gas since the 1700s; they have found a strong correlation between the rise in atmospheric carbon dioxide levels and the rise in human carbon dioxide emissions. Also, from the 1700s to 2021, atmospheric methane concentrations nearly tripled, reaching roughly 1,900 parts per billion (ppb) in 2021 (Tollefson, 2022).

Human greenhouse-gas emissions were extremely limited until the mid-1700s. At that time, the Industrial Revolution reshaped societies throughout the world by steering them away from agrarian economies and toward economies driven by mechanical manufacturing and industry. During the Industrial Revolution, people began burning coal and other fossil fuels, which emitted carbon dioxide and other greenhouse gases into the atmosphere. Scientists believe that the emitted gases built up over decades, causing climate change.

Although the concentration of carbon dioxide and other greenhouse gases increased significantly from the 1700s to the 2000s, only a few countries are responsible for most of the emissions that fueled the increase. At the beginning of the Industrial Revolution, the United Kingdom (UK) was the largest emitter of carbon dioxide. In 1850, the UK produced six times as much carbon dioxide as the next-largest producer, the United States. In roughly 1911, the United States overtook the UK in carbon dioxide production. The United States remained the top producer of carbon dioxide emissions until 2006 when China surpassed it.

Greenhouse-Gas Emissions in the United States

In 2020, the United States emitted 5,222 million metric tons of carbon dioxide (after accounting for sequestrations; “Inventory of U.S.,” 2022). The 2020 emissions were roughly 21 percent lower than the country’s emissions in 2005 and about 7 percent lower than in 1990. This decline occurred for numerous reasons including improved technology, economic changes, and population changes.

The United States produces more carbon dioxide than any other greenhouse gas. As of the early 2020s, carbon dioxide made up approximately 79 percent of the United States’ total greenhouse-gas emissions. In 2020, the burning of fossil fuels was responsible for about 94 percent of the carbon-dioxide emissions in the United States. The United States also emitted carbon dioxide through cement production and other industrial processes. Carbon dioxide was also the greenhouse gas with the largest decrease in production between 2005 and 2020 in the United States. This reduction was caused in part by the country’s pivot from coal to natural gas as a fuel for electricity production, as burning natural gas produces less carbon dioxide than burning coal. Land use in the United States also affects its net carbon dioxide production, with long-term forests sequestering more carbon than other types of land.

The next most-abundant greenhouse gas produced in the United States is methane, which made up roughly 11 percent of the country’s greenhouse-gas production in 2020. The agricultural industry was one of the largest contributors to methane emissions in 2020. Livestock, including cattle and sheep, produce methane as part of their digestion processes. The energy industry is also responsible for a significant portion of the United States’ methane emissions. Methane is a component of natural gas and escapes into the atmosphere when natural gas is produced, transported, and used (“Overview of greenhouse gases,” 2022).

Nitrous oxide made up roughly 7 percent of US greenhouse-gas emissions in 2020. The country produced nitrous oxide through agriculture, industrial processes, energy production, and other industries. As of 2020, roughly 74 percent of the United States’ nitrous oxide production came from its agricultural sector. The sector with the next-largest emissions was the wastewater treatment industry, but it accounted for only 6 percent of the country’s total nitrous oxide emissions. Nitrogen is cycled out of the atmosphere during the nitrogen cycle, mostly by bacteria that breaks down the substance in soil and the ocean. The United States reduced its nitrous oxide production by about 5 percent from 1990 to 2020.

About 3 percent of the greenhouse gases in the United States produced in 2020 were fluorinated gases. Between 1990 and 2020, the United States increased its production of fluorinated gases by about 90 percent. This occurred when the US government limited ozone depleting substances (ODSs), such as chlorofluorocarbons (CFCs). People substituted CFCs and other ODSs with fluorinated gases.

Sources of Greenhouse-Gas Emission in the United States

Various industries and economic sectors contribute to the United States’ greenhouse-gas emissions, and the greenhouse gases they produce are not the same. The economic sectors that produced the most greenhouse gases as of 2020s were transportation, electricity generation, industry, agriculture, and homes and businesses.

Transportation was the sector with the highest contribution of greenhouse gases in the United States in 2020, contributing roughly 27 percent of all greenhouse gases produce by the country that year. This was largely because of the industry’s reliance on petroleum-based fuels. The transportation sector moves people and goods by car, truck, train, ship, plane, and other vehicles. Carbon dioxide was the most produced greenhouse gas by the transportation industry, which increased its carbon dioxide output by 7 percent from 1990 to 2020 because of increased demand for travel and use of larger vehicles. The US government increased fuel efficiency requirements at that time, and this change helped to cut down on the overall increase of carbon dioxide emissions from this sector. The transportation industry emitted relatively small amounts of methane and nitrous oxide in 2020 (“Inventory of U.S.,” 2022).

The electrical generation sector contributed about 25 percent of the United States’ greenhouse gas emissions in 2020. The total emissions from this sector decreased by about 21 percent from 1990 to 2020 in part because of an increase in renewable energy sources. In 1990 nearly all power in the United States was generated through the burning of fossil fuels. Furthermore, using natural gas in place of coal, which produces more carbon dioxide than natural gas, helped reduce carbon-dioxide emissions.

The industrial sector produced about 24 percent of the total of the United States’ greenhouse gases in 2020.The industrial sector emissions in the United States reduced by about 14 percent between 1990 and 2020, mainly because of changes in fuel types, especially the replacement of coal by natural gas. This reduction also occurred because of changes in the US economy, such as the manufacturing sector, which began making materials and products that required less energy to produce.

The agricultural sector contributed about 11 percent of the United States’ greenhouse gases in 2020. Contributing to this sector’s greenhouse-gas emission were manure management, livestock production, and soil management (e.g., through chemical fertilizer). The agricultural sector was the largest producer of nitrous oxide in 2020.

Businesses and homes, or the commercial and residential sectors, produced roughly 7 and 6 percent, respectively, of the total greenhouse gases created in the United States in 2020. Unlike many other sectors, the commercial and residential sectors increased overall greenhouse-gas production from 1990 to 2020. Annual fluctuations are common in these sectors, as changing weather conditions affect the demand for heating fuel.

Further Insights

When greenhouse-gas emissions from human activities increase, they build up in the atmosphere, where they contribute to a process called the greenhouse effect. This process is fueled by energy from the sun. Earth receives the sun’s energy through visible light, ultraviolet radiation, and infrared radiation. Most of the sun’s energy is in the form of infrared radiation, which fuels Earth’s climate system and warms the planet. Greenhouse gases act like a blanket around Earth, keeping the heat energy trapped in the atmosphere. Therefore, an increase in atmospheric greenhouse gases will most likely lead to an increase in Earth’s temperature. Furthermore, greenhouse gases can make it easier for more infrared radiation to reach Earth’s surface. Since the energy fuels Earth’s climate systems, an increase in energy in the atmosphere will cause the climate to change.

Earth’s climate system consists of many interrelated components. When one component changes, it often affects other components in the system. For example, an increase in temperature can reduce the amount of snow cover on land. White snow reflects more radiation than darker land, so when the snow melts, Earth’s surface absorbs more heat and becomes even hotter.

Climate change has occurred in the past, with changes in atmospheric greenhouse gases making Earth colder or warmer. Climate change can greatly alter life on Earth by affecting weather patterns (including extreme temperatures and storms) and sea levels. Many organizations, governments, and individuals want to limit the amount of greenhouse gases that human activities are allowed to release into the atmosphere.

Viewpoints

Scientists discovered that greenhouse gases were a threat to the environment in the 1960s. At that time, some scientists began predicting that a warming planet could lead to climate changes. By the 1990s, most scientists, governments, and organizations throughout the world asserted that reducing greenhouse-gas emissions was necessary to prevent the most catastrophic effects of climate change. In 1992, countries around the world, including the United States, joined the United Nations Framework Convention on Climate Change, which called for a reduction of greenhouse-gas emissions. In the late 1990s, most countries signed the Kyoto Protocol, which also called for countries to reduce greenhouse-gas emissions. The United States was one of the only developed nations that refused to adopt the protocol. However, in 2015, the country adopted the Paris Agreement (PA), which was another international treaty that set standards for countries to follow to reduce greenhouse gases and mitigate the harm caused by climate change. However, two years later in 2017, newly elected US President Donald Trump withdrew the United States from the treaty. Although President Joe Biden reentered the PA in 2021, the country had already faced criticism for its exit.

Although the United States was no longer the largest carbon-dioxide emitter by the 2010s, it was still the largest historical carbon-dioxide emitter. As of 2021, the United States had produced roughly 20 percent of the total carbon-dioxide emissions ever created by human activity. The next-largest historical emitter at that time was China, which had produced roughly 11 percent of all carbon-dioxide emissions until 2021.

The United States’ historic contributions to greenhouse-gas emissions and its reluctance to join the international community in reducing them generated heavy criticism from climate activists, climate scientists, and leaders of other governments. The United States faced especially tough criticism because it had released historic levels of greenhouse gases to build its own economy, while many other countries would never be able to use fossil fuels to rapidly expand their economies.

Those who support climate justice initiatives often assert that countries like the United States whose economies greatly benefited from record-breaking greenhouse-gas emissions should lead the efforts to reduce them globally. Many people also believe that the United States and countries with similar economic histories should stop using fossil fuels first, allowing countries with smaller economies to use the fuels for a longer time. Despite the United States’ lack of action regarding greenhouse-gas emission reduction, a 2020 poll from Pew Research Center found that most Americans (65 percent) believed that the US government should do more to address climate change (Tyson, A., & Kennedy, 2020).

About the Author

Elizabeth Mohn earned a bachelor’s degree in communications in 2006. She has developed social sciences content for more than a decade.

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