Factsheets

The Greenhouse Effect

The greenhouse effect is a natural phenomenon that insulates the Earth from the cold of space. As solar radiation reaches the Earth, it is absorbed and re-emitted as infrared energy. Greenhouse gases (GHGs) in the atmosphere prevent some of this heat from escaping into space, instead reflecting the energy back to further warm the surface.¹ The greenhouse effect was first described in the 1820s. By the late 1800s, scientists had identified specific GHGs and human activities, such as burning coal, that intensify this.² Anthropogenic (human-caused) GHG emissions are changing the Earth’s energy balance between incoming solar radiation and outgoing heat, amplifying the greenhouse effect and resulting in climate change.¹

Greenhouse Gases

• Of the primary GHGs, water vapor (H₂O), carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) can occur naturally. F-gases—hydrofluorocarbons (HFCs), chlorofluorocarbons (CFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF₆)—arise exclusively from industrial processes.¹
• Water vapor is the most abundant GHG in the atmosphere. Its concentration is regulated by temperature and other meteorological factors, rather than direct human activities.¹
• Global Warming Potentials (GWPs) measure the relative effectiveness of GHGs in trapping heat in the atmosphere over a given period. CO₂, the dominant anthropogenic GHG, serves as the reference with a GWP of 1. N₂O has a 100-year GWP of 273, meaning it traps 273 times more heat than the same amount of CO₂ over the same time period.¹
• GHG emissions are reported in terms of carbon dioxide equivalents (CO₂e), calculated by multiplying the mass of each gas by its GWP.⁵
• In addition to trapping heat, ozone-depleting GHGs such as CFCs damage O₃ in the stratosphere, reducing its ability to block ultraviolet radiation.¹
   

Atmospheric Concentrations

• Since 1750, atmospheric concentrations of CO₂, CH₄, and N₂O have risen by 151%, 265%, and 125%, respectively,³ to levels unprecedented in the past 800,000 years.¹
• Before the Industrial Revolution, CO₂ levels remained about 290 parts per million (ppm).¹ By February 2025, the global monthly average concentration had increased to 426 ppm.⁶

Sources of Greenhouse Gas Emissions

• Fossil fuel combustion is the primary anthropogenic source of CO₂, which is also released through land-use changes such as deforestation, land clearing, and soil degradation.²⁶
• CH₄ arises from agricultural activities, waste management, energy production and use, and biomass burning.²⁶
• Anthropogenic N₂O emissions mostly come from agricultural activities, like fertilizer use, as well as chemical production and fossil fuel combustion.²⁶
• Industrial processes, refrigeration, and consumer products lead to emissions of HFCs (substitutes for O₃-depleting CFCs),PFCs, and SF₆.^7,26

Emissions and Trends

Global

• In 2023, global anthropogenic GHG emissions totaled 52.9 Gt CO₂e, a 62% increase since 1990.⁸
• Average annual GHG emissions were 56 Gt CO₂e from 2010–2019—a record high, and 10 Gt CO₂e more than 2000–2009.⁴
• CO₂ from fossil fuel combustion accounts for 64% of global anthropogenic GHG emissions.⁴ In 2024, global energy-related CO₂ emissions reached a record high of 37.8 Gt—up 54% since 2000.^11,27 Global energy-related CO₂ emissions are projected to increase by 15% from 2022–2050.¹⁵
• F-gases are the fastest-growing GHGs. In 2023, global F-gas emissions increased by 4.2%, CH₄ by 1.3%, N₂O by 1.1%.¹⁰
• The U.S. has emitted the most cumulative CO₂, followed by the EU and China. India and the African Union have produced a minor share despite large populations.¹⁰
• Since 2005, China has surpassed the U.S. as the largest source of CO₂ emissions.¹⁴ In 2023, China’s per capita GHG emissions were twice the global average, while the U.S. was three times the global average.⁸
• Globally, the top 10% of income earners were responsible for 48% of emissions—two-thirds of whom live in developed countries. The bottom 50% contributed just 12%.¹² See the Environmental Justice Factsheet.
• CO₂ emissions in the Americas are expected to fall 0.2% annually; a 0.7% decline in the U.S. and 0.5–1.4% increases everywhere else. Total emissions in Africa are projected to increase 1.2% yearly, but will remain the lowest globally.¹⁵ 

United States

• The U.S. represents 4.2% of the world’s total population,¹⁶ and 11% of annual anthropogenic GHG emissions in 2023.⁸
• CO₂ accounts for 80% of U.S. GHG emissions. The largest source of CO₂ in the U.S. is fossil fuel combustion (74%), primarily from transportation and power generation.⁵
• In 2022, CO₂ emissions were 1.5% lower than in 1990 and 18% lower than in 2005. GHG emissions were 3% below 1990 levels; an average annual decline of 0.1%.⁵
• Since 2005, GHG and fossil fuel emissions fell by 15% and 18% respectively, while GDP grew 36%.⁵
• Livestock, natural gas, and landfills are the primary sources of CH₄.5 CH₄ emissions from the oil and gas industry were 13 Mt (2.3% of gross production)—60% higher than EPA estimates. The climate impact rivals CO₂ emissions from combustion.¹⁷
• HFC and PFC emissions increased since 1990, driven by the phase-out of O₃-depleting CFCs, making up 90% of F-gas emissions in 2022. SF₆ emissions declined by 80%, attributed to a rise in prices in the 1990s and EPA intervention.⁵

• Transportation has been the largest source of U.S. GHG emissions since 2017, accounting for 28% in 2022. Emissions were 8% below 2005 levels but 18% above 1990, driven by growing travel demand, urban sprawl, and low fuel prices.⁵
• Passenger cars and light-duty trucks emitted 370 and 660 Mt CO₂e, respectively, accounting for 57% of U.S. transportation emissions and 16% of total U.S. emissions.⁵
• The electric power sector accounts for 25% of total U.S. GHG emissions. This is the only sector showing major declines; down 16% since 1990 and 36% since 2005.⁵
• The residential sector accounts for 15% of U.S. emissions, driven by electricity use and consumption of natural gas and petroleum.⁵
• Emissions have risen since 1990 due to population growth, migration to warmer regions, and larger houses. Recent trends reflect a shift toward energy efficiency.⁵
• The land use and forestry sector offsets 14% of U.S. emissions, absorbing CO₂ through plant and tree growth.⁵

Solutions

• Stabilizing global temperatures and limiting climate change impacts require net-zero or net-negative emissions.²⁰
• Any rise in global warming harms human health, with greater mortality, economic, and security risks above 1.5 °C. Limiting warming to 1.5 °C is projected to reduce ecosystem loss and species extinction compared to 2 °C.²⁰
• GHG emissions can be reduced by transitioning from fossil fuels, improving energy efficiency, adopting sustainable agriculture and forest management, and conserving critical ecosystems.²²
• International cooperation to mitigate climate change is established through the 1992 UN Framework Convention on Climate Change (UNFCCC) and its legal instruments—the 1997 Kyoto Protocol and the 2015 Paris Agreement.⁹ See the Climate Change: Policy and Mitigation Factsheet.
• Nationally Determined Contributions (NDCs) are countries’ self-defined climate pledges under the Paris Agreement, outlining their commitments to limit global warming to 1.5 °C.23 As of September 2024, the 168 latest NDCs cover 95% of total global emissions in 2019.²⁴
• Global GHG emissions must fall 43% from 2019 levels by 2030 and reach net-zero by 2050 to limit warming to 1.5 °C.²¹
• Full implementation of latest NDCs cuts emissions by just 5.9%; this requires financial and technology transfer. Without transfers, emissions could rise 0.8%.²⁴
• Current NDCs would use 86% of the carbon budget—for a 50% chance of limiting warming to 1.5°C—by 2030, leaving only two years' worth of emissions after 2030, and requiring even steeper post-2030 reductions to stay on track for 1.5 °C or well below 2 °C warming.²⁴
• Under current assumptions, 21st century warming is projected to reach 2.1 °C – 2.8 °C.²⁴
• As of September 2024, 60% of the world's largest companies have set net-zero emissions targets, covering $31T or 67% of aggregate annual revenue.²⁵

Global CO₂ Emissions Budget²⁴

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