A greenhouse gas (GHG) is a gas that traps heat in the atmosphere. Greenhouse gases are so named because they function similarly to the glass in a greenhouse. They pose no hindrance to sunlight, which passes straight through and generates heat when it reaches the ground. However, when that heat attempts to rise and dissipate, it cannot penetrate the gases/glass and instead builds up, raising the ambient temperature. This phenomenon is known as the greenhouse effect.
Greenhouse gases are actually essential to keeping the Earth warm; without them, the global temperature would average about 0°F. However, human activity—particularly certain agricultural practices and the burning of fossil fuels—has created an overabundance of greenhouse gases in the atmosphere. This is widely understood to be the primary cause of unnatural global warming. The world’s countries produce and release varying amounts of greenhouse gases into the atmosphere. The overall emissions level in a country can be explained by the size of its population, its GDP, its energy sector, and more.
* Full data in table after text
According to the International Energy Agency, global carbon dioxide emissions from fuel combustion reached a new record of 36.3 billion tons (36.3 Gt) in 2021. Overall, greenhouse gas emissions fell 9% from 2019 to 2020, largely as a result of COVID-19-related lockdowns, which limited the use of motor vehicles (and in turn greatly reduced the emission of GHGs in vehicle exhaust). However, early data indicate that GHG emissions not only rose in 2021, but reached the highest global level yet recorded. Both coal and renewable power (wind, hydro, solar, etc.) rose to their highest recorded levels of consumption in 2021.
|Gas||% of emissions in U.S. 2020 (EPA)|
|Carbon Dioxide (CO₂)||78.8%|
|Nitrous Oxide (N₂O)||7.1%|
The primary greenhouse gases generated by humans are carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and the "fluorinated gases" such as hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and nitrogen trifluoride. Water vapor and ozone (O₃) could also be classified as greenhouse gases, but both offer substantial benefits that far outweigh their influence upon global warming.
According to the Environmental Protection Agency (EPA), the economic sectors that produce the largest amounts of greenhouse gas emissions are electricity and heat production (25%), agriculture, forestry, and other land use (24%), industry (21%), and transportation (14%).
|Energy Sector||% of GHGs contributed|
|Transportation (gasoline, diesel, etc)||27%|
|Electricity generation (coal, natural gas)||25%|
|Industry (manufacture of metals, cement, chemicals)||24%|
|Agriculture (livestock, non-organic farming & processing)||11%|
|Commercial (heating, cooking)||7%|
|Residential (heating, cooking)||6%|
Carbon dioxide (CO₂) is the most abundant greenhouse gas, and by a massive margin. CO₂ makes up approximately 4/5 of all human-generated greenhouse gas emissions. It emitted primarily through the burning of fossil fuels (coal, natural gas, oil) either in motor vehicles as fuel or in power plants to generate electricity. Additional sources include solid waste, biological materials, and certain chemical reactions.
Since 1970, global carbon dioxide emissions have increased by 90%, with emissions from fossil fuel combustion and industrial processes contributing to about 78% of the total greenhouse gas emissions increase.
|Energy Sector||% of CO₂ contributed|
|Transportation (gasoline, diesel, etc)||33%|
|Electricity generation (coal, natural gas)||31%|
|Industry (manufacture of metals, cement, chemicals)||16%|
|Residential and Commercial (heating, cooking)||12%|
Although it is emitted in much smaller volumes than carbon dioxide, methane has a much greater impact in the atmosphere and is roughly 25 times as damaging as an equal volume of CO₂ over 100 years. Methane is released into the atmosphere by both natural sources (35-50%) such as wetlands and by human activities (50-65%).
Human-generated sources of methane include natural gas production, coal mining, wastewater treatment, and landfills. Arguably the best-known source of atmospheric methane is the rearing of livestock. The size of the global meat industry is such that the animals' collective flatulence and biological waste releases a considerable amount of methane into the atmosphere.
|Energy Sector||% of CO₂ contributed|
|Natural Gas and Petroleum Production||32%|
|Enteric Fermentation (animal flatulence)||27%|
|Landfills (decay of organic matter)||17%|
With an atmospheric impact 300 times that of carbon dioxide, nitrous oxide is a major greenhouse gas. It is primarily released through agricultural soil management practices (74%) such as the application of fertilizer. Chemical production, the combustion of fossil fuels, and the treatment of wastewater also contribute to N₂O emissions. It is also part of the natural nitrogen cycle.
The fluorinated gases, including hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and nitrogen trifluoride, are almost entirely man-made gases generated by various chemical and industrial processes. 93% of the fluorinated gases released into the atmosphere come from their use in various industrial roles including that of refrigerants, aerosol propellants, solvents, fire retardants, foam blowing agents, gaseous insulators. Although they appear in only trace amounts in the atmosphere, fluorinated gases are tremendously impactful, with an effect roughly 10,000 to 22,800 times that of an equivalent volume of carbon dioxide.
China has the highest greenhouse gas emissions of any country in the world, and released 9,877 megatons (9,877 million tons) of GHGs in 2019. The country’s economic growth has primarily been powered by coal, which produces up to twice the amount of carbon dioxide as other fossil fuels. China’s industrial sector is the primary coal consumer. Manufacturing, agriculture, mining, and construction make up about 62.5% of China’s energy use and 49% of its coal use. What's more, China's coal use rose in 2021, as a 10% spike in demand for electricity (fueled by the post-pandemic economic recovery) coincided with a spike in natural gas prices, leading to an increased reliance on coal-powered electrical plants.
The United States is the second-largest contributor of CO₂ emissions, responsible for 4,745 megatons of GHGs in 2019. U.S. net emissions decreased by 12% between 2005 and 2017, with the electric power sector emissions falling 27% as a result of increased use of renewable energy, shifting from coal to natural gas, and a leveling of electricity demand. The transportation sector was the largest contributor to emissions in 2020, responsible for 27% of emissions, followed by electricity (25%), and industry (24%).
India, like China, has a large population—the second-largest in the world at 1.4 billion people—and is the third-largest contributor of carbon dioxide emissions, emitting 2,310 megatons of CO₂ in 2019. Cattle, coal power plants, and rice paddies are the country’s major sources of emissions, which continue to rise rapidly. The country has pledged a 33-35% reduction in its emissions by 2030 compared to 2005 levels.
Russia is the fourth-largest contributor of CO₂ emissions, emitting 1,640 megatons of carbon dioxide in 2019. Russia’s per capita emissions (10.8 tons in 2020) are among the highest in the world, much higher than that of the U.K. (4.6), France (3.8), or Ukraine (3.7)—though still lower than the United States (13.0). The majority of greenhouse gas emissions in Russia come from the energy industry (78.9%), nearly half of which comes from the production of electricity and heat for the general population.
Japan is the fifth-largest contributor of greenhouse gases and the fifth and final nation that contributes more than a thousand (1,056) megatons per year. Japan is the only G7 country still building new coal-fired power plants. Japan has some unambitious climate change goals and is facing both criticism and pressure from the international community.
Germany is responsible for 644 million tons of carbon dioxide emitted in 2019, a 6.3% reduction in greenhouse gases compared to 2018. Since 1990, Germany has reduced its emissions by 35.7%, primarily by shutting down coal-fired power plants, expanding wind and solar energy, and successfully reforming European emissions trading. Germany’s goal is to cut emissions by 55% by 2030 and to run on 80% renewable energy sources by 2050.
After emitting a record 605.9 megatons of CO₂ in 2018, South Korea lowered its emissions to 586 million tons in 2019. While emissions from electricity, coal, and steel production have increased, South Korea has committed to greening its energy platform by decommissioning old and inefficient coal and nuclear power plants and transitioning to more efficient systems, including a blossoming hydrogen industry.
Iran is the eighth-largest emitter of greenhouse gases in the world, emitting 583 million tons of CO₂ in 2019. Between 1990 and 2018, Iran’s CO₂ emissions rose by roughly 5% annually. The burning of natural gas and oil are the two leading contributors to Iran’s carbon emissions. Iran is rich in resources with enormous oil and gas reserves; however, it still has considerable potential to produce renewable energy, such as solar power. Iran has pledged to reduce carbon emissions by 4% by 2030, but increased sanctions and a lack of trade have negatively impacted the country’s economy, hindering the use of resources for climate initiatives.
Canada emitted 571 million tons of carbon dioxide in 2019. Canada is warming up twice as quickly as the rest of the world, despite the country's many hydroelectric dams and nuclear power plants, which do not require fossil fuels to produce electricity. Oil and gas production is Canada’s largest emitting sector, accounting for about 45% of emissions, followed by transportation, which accounts for about 28% of emissions. Since 1990, Canada’s gross domestic product (GDP) has more than tripled and its population has grown by 6 million people, yet overall total greenhouse gas emissions have risen less than 30% and per-capita emissions have declined.
Saudi Arabia is the tenth-largest emitter of greenhouse gases, emitting 495 million tons of CO₂ in 2019. Saudi Arabia’s economy is highly oil-dependent, and Saudi Aramco, the official Saudi Arabian Oil Company, has contributed the most to global carbon dioxide emissions since the 1960s. However, the 2020 COVID-19 pandemic lockdowns triggered a massive drop in global demand for oil, and made clear that Saudi Arabia needs to diversify away from oil for both environmental and economic reasons. Like most Middle Eastern countries, Saudi Arabia has massive potential to generate solar power.
China has the greatest abundance of greenhouse gas emissions on Earth.