Greenhouse gas emissions: sources, persistence, and warming potential

Where the main gases come from

Greenhouse gases trap heat in the atmosphere. To understand how they differ, it is not enough to list their sources: it also matters how much of each gas accumulates, how long it remains in the air, and how much it contributes to warming per unit of mass. The United States Environmental Protection Agency (EPA) describes the sources and criteria for comparing the main gases in its overview of greenhouse gases.

Carbon dioxide (CO₂) enters the atmosphere when fossil fuels—coal, natural gas, and oil—are burned, when solid waste is incinerated, through the burning of trees and other biological materials, and as a result of certain chemical reactions, including cement production. It can also be removed from the air: plants absorb it as part of the biological carbon cycle. Its atmospheric balance therefore reflects both emissions and absorption processes.

Methane (CH₄) is emitted during the production and transport of coal, natural gas, and oil. The EPA also identifies sources associated with livestock and other agricultural practices, land use, and the decomposition of organic waste in municipal landfills. These emission pathways differ from those characteristic of CO₂, although both gases contribute to the greenhouse effect.

Nitrous oxide (N₂O) is associated with agricultural activities, land-use changes and practices, and industrial processes. It is also emitted during the combustion of fossil fuels and solid waste, and during wastewater treatment. Fluorinated gases—including hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and nitrogen trifluoride—are synthetic compounds originating from various domestic, commercial, and industrial applications and processes.

The EPA indicates that fluorinated gases are generally emitted in smaller quantities than other greenhouse gases, but can be potent: for the same mass, they trap substantially more heat than CO₂. Some, particularly hydrofluorocarbons, are used as substitutes for substances that deplete stratospheric ozone. This describes a use of certain compounds; it does not mean that all fluorinated gases have the same applications.

Three factors for comparing their effects

The EPA organizes the comparison of climate impacts around three factors. The first is atmospheric abundance, meaning the amount of a gas present in the air. The agency explains that greater greenhouse gas emissions lead to higher atmospheric concentrations. Depending on the gas and scale, these are expressed in parts per million, parts per billion, or parts per trillion.

The second factor is persistence time. The gases considered can remain in the atmosphere from a few years to thousands of years. The EPA notes that they all persist long enough to mix thoroughly: in general terms, the amount measured is similar in different parts of the world, regardless of where it was emitted. This helps explain why atmospheric concentrations do not necessarily correspond to the immediate location of a source.

The third factor is effectiveness at retaining energy. Some gases absorb more energy per unit of emitted mass than others. The global warming potential (GWP) is used to compare them: it measures how much energy is absorbed by the emission of one ton of a gas over a given period, usually a 100-year horizon, relative to the emission of one ton of CO₂. A higher GWP indicates greater absorption per ton over that horizon; on its own, it is not a measure of total emissions or of the gas’s abundance.

The table summarizes the sources and qualifications described by the EPA; it does not rank gases by their total contribution to climate change. That kind of comparison would also require emissions and concentration data for a defined scope and period, in addition to their properties.

What it means to express emissions as CO₂ equivalent

Emissions of different gases can be expressed as carbon dioxide equivalent (CO₂e). According to the EPA, conversion consists of multiplying a gas’s emissions by its GWP. This accounts for the fact that, per unit of mass, some gases cause more warming than CO₂ over the selected horizon. The result allows quantities to be compared using a common unit, but its interpretation depends on the GWP applied and the time period considered.

The EPA states that the GWP values shown on its gas pages reflect those used in the U.S. Inventory of Greenhouse Gas Emissions and Sinks, which are based on the IPCC Fifth Assessment Report (AR5). The agency specifies that all its emissions estimates come from that inventory, which covers 1990–2022. Therefore, figures or equivalencies from that framework should not be presented as universal estimates or confused with a comparison based on updated GWP values.

Atmospheric measurement is another aspect, related to but distinct from comparing impacts. NOAA explains that when measuring CO₂, it reports the mole fraction in dry air: the proportion of CO₂ molecules relative to the total number of air molecules after water vapor has been removed. NOAA notes that this mole fraction does not change when air expands as it warms or rises into an area of lower pressure, or when water evaporates or condenses into droplets. NOAA’s explanation of how atmospheric CO₂ is measured provides methodological context; it does not constitute independent corroboration of the emission sources or GWPs described by the EPA.

What these differences allow us to conclude

The comparison distinguishes three questions that should not be conflated: where a gas is emitted, how long it can remain in the atmosphere, and how much heat a given mass absorbs over a specified horizon. CO₂ also has a removal process identified by the EPA—absorption by plants—while the article highlights the high potency of fluorinated gases per unit of mass, even though their emissions are generally lower than those of other gases.

GWP helps translate emissions into CO₂e, but that equivalence alone does not provide a complete description of the climate. Atmospheric abundance, persistence time, and absorption effectiveness are different dimensions. Likewise, NASA GISS documentation on definitions and methods for temperature analysis concerns how temperature records are constructed and compared; it does not provide direct evidence for quantifying the sources or properties of the gases described here.

Consequently, a rigorous explanation of the differences between gases must preserve the distinction between emissions, concentration, and warming potential. EPA information provides an institutional framework for describing sources and comparison criteria, while the NOAA and NASA references serve here only to situate distinct methodological issues. None of these contextual references replaces the specific evidence needed to assess total emissions or trends in a particular place and period. GWP comparisons always depend on the time horizon chosen.