Greenhouse Gases

Earth’s atmosphere is composed almost entirely of the gases nitrogen (78%) and oxygen (21%), but several gases that exist in trace amounts—concentrations of less than a fraction of a percent—have an outsize impact on Earth’s climate. These are greenhouse gases—gases that allow solar radiation to pass through the atmosphere and warm the Earth, but that absorb the heat that the Earth radiates back to space, trapping it like a comforter traps body heat to keep us warm on a cold night. Small changes in the atmospheric concentration of these gases (for example going from 0.02% to 0.2% of the atmosphere) can lead to big changes in Earth’s temperature and climate, making the difference between ice ages, when mastodons roamed the Earth, and the sweltering heat in which the dinosaurs lived.

How Impactful are Different Greenhouse Gases?

Carbon dioxide (CO2) is the most abundant greenhouse gas. Each year, hundreds to thousands of times more CO2 is emitted into the atmosphere by human activity than any other greenhouse gas. However, other gases like methane, nitrous oxide, and halogenated compounds (industrial-derived gases that contain fluorine, chlorine, or bromine) have an outsize greenhouse effect relative to their concentration because of two critical characteristics: their radiative efficiency and their atmospheric residence time.

Radiative efficiency is a greenhouse gas’s ability to absorb energy and radiate it back to Earth. Atmospheric lifetime is a measure of how long a gas stays in the atmosphere before natural processes remove it (e.g., through breakdown via chemical reactions, or cycling into the ocean or biosphere).

In order to compare the relative impact of different greenhouse gases, these characteristics are incorporated into a measurement called the Global Warming Potential (GWP). GWP is a measure of the radiative efficiency of each unit of gas (by mass) over a specified period of time, expressed relative to the radiative efficiency of carbon dioxide (CO2). GWP is often calculated over 100 years, though it can be done for any time period. Gases with GWPs greater than one will warm the Earth more than an equal amount of CO2 over the same period, and the higher the GWP, the greater the warming influence. A gas with a long lifetime, but relatively low radiative efficiency, might exert more warming influence than a gas that has a comparatively high radiative efficiency but leaves the atmosphere faster than the time window of interest. This is reflected in a higher GWP in the long-lived, low radiative efficiency gas.

The table below presents atmospheric lifetime and GWP values for the three most significant greenhouse gases and select examples of halogenated compounds (of which there are more than one hundred) from the Sixth IPCC Assessment Report (AR6) released in 2021. These values are periodically updated by the scientific community as new research refines estimates of radiative properties and atmospheric removal mechanisms (sinks) for each gas.

Some gases (like CO2) are made by both natural and manmade processes, while others (like hydrofluorocarbons) are only the result of human industrial activity. The table below shows the relative concentrations of these major greenhouse gases in 1750 (well before the industrial revolution) and in 2019, and their sources. Despite carbon dioxide’s comparatively low GWP among major greenhouse gases, the large human-caused increase in its atmospheric concentration has caused the majority (about 65 percent) of global warming. Methane is similar—despite having a GWP that is also much lower than several other greenhouse gases, it accounts for about 16 percent of warming since pre-industrial times.

The table also shows the annual emissions rate of each greenhouse gas in 2021, normalized to CO₂-equivalent. CO₂-equivalent (calculated by multiplying a gas’s GWP with its annual emissions by mass) provides a simple way to compare the relative climate impact of different gases and to understand the total emissions impact of human activities. Such measurements allow us to assess how much warming we might expect given current emissions trajectories, and the importance of interventions that help reduce non-CO₂ greenhouse gas emissions (such as reducing fertilizer use or stopping methane leaks) as we work to minimize and mitigate climate change.