Human-caused climate change is driven by greenhouse gas emissions—and it is already affecting health and safety in measurable ways. (1).
As greenhouse gas concentrations rise, the climate shifts in ways that increase heat, intensify extreme weather, and raise air-quality risks. Those impacts are not experienced equally: communities with fewer resources often have less protection from heat, smoke, and other climate-related hazards. That’s why climate solutions and air-quality protections are often linked—and why equity matters when planning who gets protection, cooling, clean indoor air, and early warning systems first.
Greenhouse gases aren’t the same pollutants tracked in the daily Air Quality Index (AQI). But they set the conditions that influence air quality—like hotter temperatures, longer ozone seasons, and more wildfire smoke—so they affect what we breathe in everyday life.
What are greenhouse gases?
Greenhouse gases trap heat in the Earth’s atmosphere, contributing to the “greenhouse effect” that warms the planet and drives climate change. They do this by absorbing some of the heat energy the Earth gives off, making the atmosphere act a bit more like a thicker blanket.
EPA reports greenhouse gas emissions in carbon dioxide–equivalent (CO2e). CO2e puts different gases on the same scale so their impact can be compared (2).
To calculate CO2e, EPA multiplies the amount of a gas by its global warming potential (GWP). GWP shows how much heat 1 metric ton of a gas traps over 100 years compared with 1 metric ton of carbon dioxide (CO2).
For example, methane has a 100-year global warming potential of about 28–30 under IPCC AR5. That means 1 metric ton of methane traps roughly 28–30 times more heat over 100 years than 1 metric ton of carbon dioxide (CO2). So if a facility emits 10 metric tons of methane, that equals about 280–300 metric tons of CO2e.
The U.S. Inventory uses metric units and 100-year GWPs from the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5)
Not all greenhouse gases behave the same. Three factors determine how much they warm the planet:
- How much of it is in the air (its concentration)
- How long it lasts in the atmosphere
- How strongly it traps heat per ton—captured by GWP
Scientists measure greenhouse gases in parts per million (ppm), parts per billion (ppb), and parts per trillion (ppt). These units show how much of a gas is present in the air.
One ppm is about like one drop of water mixed into about 13 gallons of liquid—roughly the size of a compact car’s fuel tank.
Some greenhouse gases last only a few years. Others stay in the atmosphere for thousands of years. Because they persist and mix throughout the atmosphere, their concentrations are broadly similar around the world, no matter where they were emitted.
Examples of greenhouse gases include (3)(4);
- Carbon dioxide (CO2)
- Methane (CH4)
- Nitrous oxide (N2O)
- Industrial gases and fluorinated gases (or F-gases)
A quick “who’s who” (and where they come from):
- Carbon dioxide (CO2) enters the atmosphere through burning fossil fuels (coal, natural gas, and oil), but also from solid waste, trees and other biological materials, and chemical reactions such as cement production. CO2 is also removed (“sequestered”) when plants absorb it as part of the biological carbon cycle.
- Methane (CH4) is emitted during the production and transport of coal, natural gas, and oil. It also comes from livestock and other agricultural practices, land use, seeps into the atmosphere from abandoned and orphaned oil and gas wells, and the decay of organic waste in municipal solid waste landfills.
- Nitrous oxide (N2O) is emitted during agricultural, land-use, and industrial activities; combustion of fossil fuels and solid waste; and during wastewater treatment.
- Fluorinated gases include hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF6), and nitrogen trifluoride (NF3). They’re synthetic gases used in a range of household, commercial, and industrial applications—often as substitutes for ozone-depleting substances. They’re typically emitted in smaller quantities than CO2, but many have very high GWPs—often thousands to tens of thousands—so small leaks can pack an outsized warming punch.
Note: Other substances in the atmosphere—like water vapor, ground-level ozone, and tiny particles/aerosols—can also influence climate, even though greenhouse gas inventories focus on the main heat-trapping gases above.
Where do greenhouse gases come from?
Greenhouse gases are emitted from both natural sources and human activity. However, human activity since the industrial period (in the mid-19th century) has dramatically increased the amount of these gases in our atmosphere, significantly contributing to global warming and subsequent climate change.
Human activity since the industrial period has greatly increased the amount of these gases in our atmosphere, significantly contributing to global warming and subsequent climate change.
Natural sources of greenhouse gases include (5)(6):
- Decomposition of organic matter
- Methane emissions from wetlands
- Respiration
- Soil processes
Human activities generate greenhouse gas emissions. Sectors that significantly contribute to emissions include (7):
- Burning fossil fuels (coal, oil, and natural gas)
- Transportation
- Electricity generation
- Industrial processes
- Commercial and residential heating
- Agriculture (including livestock and crop production)
- Land use changes (e.g., deforestation)
- Nitrogen-based fertilizers
Who emits the most greenhouse gases?
Global greenhouse gas emissions are unevenly distributed, but they’re a shared atmosphere problem—because long-lived greenhouse gases mix globally over time.
According to the 2025 Emissions Database for Global Atmospheric Research (EDGAR), the world’s largest emitters in 2024 were China, the United States, India, the EU27, Russia, and Indonesia. Together, these emitters accounted for 61.8% of global greenhouse gas emissions in 2024, alongside 51.4% of global population and 62.5% of global GDP.
Globally, EDGAR estimates total greenhouse gas emissions (excluding land use, land-use change, and forestry) reached 53.2 Gt CO₂-equivalent in 2024, an increase of 1.3% compared with 2023 (8).
This matters for air quality because the same sectors driving greenhouse gas emissions—especially fossil fuel use in power, industry, transport, and buildings—can also contribute to the air pollutants that show up locally as PM2.5 and ozone.
The effects of greenhouse gases
While individual greenhouse gases can directly harm human health—such as carbon dioxide in poorly ventilated spaces—their greatest threat comes from their collective role in driving climate change.
By intensifying the greenhouse effect, these gases are destabilizing Earth’s climate, with far-reaching consequences for both the planet and human societies.
How greenhouse gases can worsen air quality
Here’s the simplest way to think about it: greenhouse gases don’t usually “spike” in a neighborhood the way smoke can, but they can increase the frequency and intensity of air-quality stressors.
- Hotter days can mean more ozone: Ground-level ozone forms more readily in heat and sunlight, increasing the chance of unhealthy ozone days.
- Higher wildfire smoke risk in some regions: Warmer, drier conditions can increase wildfire risk, and wildfire smoke can drive major PM2.5 spikes.
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More “stagnation” events: Some weather patterns linked to a warming climate can trap pollutants closer to the ground, keeping dirty air around longer.
By warming the planet, greenhouse gases contribute to a cascade of environmental changes that affect people, ecosystems, and economies. Rising global temperatures mean that ice caps and glaciers are melting, causing sea levels to rise and potentially displace huge numbers of people.
Other consequences of climate change include (9):
- Increased likelihood and severity of forest fires
- Extended droughts and heat waves
- Increased flooding and storm intensity
- Water scarcity and declining water quality
- Loss of biodiversity and collapse of ecosystems
- Disruption of global food systems and agricultural productivity
Climate change is also predicted to increase concentrations and lingering durations of ground-level ozone pollution, further degrading air quality and public health.
Some climate impacts become much harder to reverse once certain thresholds are crossed. In 2025, one widely covered assessment warned that warm-water coral reefs face especially high risk as warming continues (10).
Climate change and environmental justice
When air quality worsens because of climate change-related heat, smoke, or ozone, the people most affected are often those with the least access to healthcare, clean indoor spaces, and resources to adapt.
Climate change fueled by greenhouse gases is not just an environmental issue—it is a profound matter of justice.
The communities least responsible for greenhouse gas emissions, including low-income populations, Indigenous groups, and people in the Global South, are often the most vulnerable to its impacts (11)(12)(13). These groups face heightened risks from extreme weather, food and water insecurity, displacement, and health crises, despite contributing the least to the problem.
Systemic inequities, such as limited access to resources, inadequate infrastructure, and historical marginalization, amplify these disparities. Addressing greenhouse gases and climate change requires centering equity: true progress demands both reducing greenhouse gas emissions and rectifying the injustices that make climate change a multiplier of inequality.
The takeaway
Greenhouse gases warm the planet—and that warming can worsen air quality by increasing ozone risk and intensifying wildfire smoke in many regions.
The most effective path forward combines emissions reductions (especially cutting sources of CO₂ and methane) with practical air-quality protection for individuals and communities: monitor local conditions, ventilate when outdoor air is clean, and use filtration during smoke or high-ozone periods.
