A handful of gases are responsible for nearly all of the greenhouse effect: water vapor, carbon dioxide, methane, nitrous oxide, ozone, and a group of synthetic fluorinated gases. Water vapor accounts for roughly half of the natural greenhouse effect, while carbon dioxide is the dominant driver of the warming increase since the Industrial Revolution. Five gases alone, led by CO2 and methane, account for about 96% of the extra heat trapped by long-lived greenhouse gases.
How Greenhouse Gases Trap Heat
Sunlight passes through the atmosphere and warms the Earth’s surface. The surface then radiates that energy back upward, but at a longer wavelength in the infrared spectrum. Greenhouse gas molecules absorb these infrared photons instead of letting them escape to space. The bonds between atoms in these molecules bend and stretch when they absorb the light, effectively capturing the energy and re-emitting it in all directions, including back toward the ground. This is what keeps the planet warm enough to support life, and it’s also what drives additional warming when concentrations of these gases rise.
Nitrogen and oxygen, which make up 99% of the atmosphere, don’t absorb infrared radiation because of their simple two-atom structure. Greenhouse gases have three or more atoms per molecule, giving them the more complex bond structures needed to interact with infrared light.
Water Vapor: The Most Abundant Greenhouse Gas
Water vapor is responsible for about half of the greenhouse effect. It’s the most important greenhouse gas by sheer concentration. But unlike CO2 or methane, humans don’t directly control how much water vapor is in the atmosphere. Its concentration is governed by temperature: warmer air holds more moisture, and that extra moisture traps more heat, which raises temperatures further. This positive feedback loop is considered the most important amplifying mechanism in the climate system. Water vapor stays in the atmosphere for only about 8 to 10 days before cycling out as precipitation, so it responds to warming rather than initiating it.
Carbon Dioxide: The Biggest Driver of Warming
Carbon dioxide is the single largest contributor to the increase in heat-trapping since preindustrial times. It accounts for 66% of the total radiative forcing from long-lived greenhouse gases, adding about 2.33 watts per square meter of extra energy to the climate system. Since 1990, CO2 has been responsible for 81% of the increase in heat trapped by these gases.
The global average concentration hit a record 422.8 parts per million in 2024, up from about 280 ppm before the Industrial Revolution. The primary source is fossil fuel combustion. Annual emissions from burning coal, oil, and natural gas have climbed from roughly 11 billion tons of CO2 per year in the 1960s to an estimated 37.4 billion tons in 2024. Deforestation and land-use changes add to the total by releasing stored carbon and reducing the number of trees pulling CO2 from the air.
CO2 is particularly consequential because of its persistence. While it cycles through the atmosphere on timescales of years to decades, a significant fraction of each emission lingers for centuries, meaning its warming effect accumulates over time.
Methane: Potent but Short-Lived
Methane is the second-largest contributor to radiative forcing, responsible for about 16% of the total from long-lived greenhouse gases. Ton for ton, it traps roughly 28 times as much heat as CO2 over a 100-year period. The tradeoff is its lifespan: methane persists in the atmosphere for only about a decade before breaking down, while CO2 can last for centuries.
The major sources of methane are agriculture (particularly livestock digestion and rice paddies), fossil fuel extraction and distribution (natural gas leaks from wells and pipelines), landfills, and wetlands. Because methane is so potent in the short term, cutting methane emissions offers one of the fastest ways to slow the rate of warming.
Nitrous Oxide: The Agricultural Gas
Nitrous oxide is a less well-known greenhouse gas, but it has been the second-largest contributor to the increase in radiative forcing since 1990, surpassing methane on that measure. Agricultural land is responsible for 60% of total human-caused nitrous oxide emissions, and those emissions have risen 30% over the past four decades. The primary culprit is nitrogen fertilizer applied to cropland, especially for rice, wheat, and maize. Industrial processes and the burning of fossil fuels and biomass contribute the remainder.
What makes nitrous oxide especially significant is its longevity. It persists in the atmosphere for over a century, and over 100 years it traps roughly 265 times as much heat per ton as CO2.
Tropospheric Ozone
Ozone in the lower atmosphere (the troposphere) is the third most important human-influenced greenhouse gas after CO2 and methane. Unlike the protective ozone layer high in the stratosphere, tropospheric ozone forms at ground level when pollutants from vehicles, power plants, and industrial facilities react in sunlight. It absorbs infrared radiation escaping from the Earth’s surface. Concentrations are highest over industrialized regions in the northern hemisphere and over areas with heavy biomass burning in the southern hemisphere.
Tropospheric ozone is not emitted directly, so it doesn’t appear in standard emissions inventories the way CO2 or methane do. Its levels depend on controlling the precursor pollutants that create it.
Synthetic Fluorinated Gases
A family of human-made gases, including hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), nitrogen trifluoride, and sulfur hexafluoride, round out the list. These gases exist in tiny concentrations compared to CO2, but molecule for molecule they are extraordinarily powerful. Sulfur hexafluoride, used in electrical equipment, has a global warming potential nearly 24,000 times that of CO2 over 100 years and can persist in the atmosphere for 3,200 years. Some perfluorocarbons used in semiconductor manufacturing last up to 50,000 years.
Older synthetic gases like CFC-11 and CFC-12, originally used as refrigerants and in aerosol sprays, are also significant greenhouse gases. Together with CO2, methane, and nitrous oxide, these two CFCs account for about 96% of the radiative forcing from long-lived greenhouse gases. Their production has been phased out under the Montreal Protocol because they also destroy the ozone layer, but they decay so slowly that they still contribute measurable warming today.
Relative Contributions at a Glance
- Carbon dioxide: 66% of radiative forcing from long-lived greenhouse gases
- Methane: 16%
- Nitrous oxide and synthetic gases: the remaining 18%, with nitrous oxide as the largest share in that group
- Water vapor: about half of the total natural greenhouse effect, but acts as a feedback rather than a direct driver of change
- Tropospheric ozone: third most important human-influenced greenhouse gas, not included in the long-lived gas index because it forms indirectly
NOAA’s Annual Greenhouse Gas Index, which tracks cumulative warming from long-lived gases, reached 1.54 in 2024. That means the extra heat these gases add to the atmosphere has increased by 54% since 1990. The overwhelming share of that increase traces back to rising CO2 concentrations, with nitrous oxide and methane playing supporting but meaningful roles.