Table of Contents
Overview
Major greenhouse gases are the gases in the atmosphere that trap heat and contribute significantly to the greenhouse effect. They differ in how strongly they absorb heat, how long they stay in the atmosphere, and where they come from. Understanding their specific roles is essential for interpreting climate policies, emission targets, and mitigation strategies in later chapters.
How Greenhouse Gases Are Compared
Greenhouse gases are often compared using a metric called Global Warming Potential, or GWP. GWP measures how much heat a given mass of a gas will trap over a specific period, usually 100 years, compared to the same mass of carbon dioxide, CO₂. By definition, the 100 year GWP of CO₂ is 1. A gas with GWP of 28 traps 28 times more heat per kilogram than CO₂ over 100 years.
Key rule: The higher the GWP and the longer the atmospheric lifetime, the more powerful and persistent the warming effect of a greenhouse gas.
In this chapter, whenever you see a GWP value, you can mentally compare it to CO₂ with GWP = 1 over 100 years.
Carbon Dioxide (CO₂)
Carbon dioxide is the most important long lived anthropogenic greenhouse gas. It is not the strongest gas molecule for molecule, but it is emitted in very large quantities and stays in the climate system for a very long time.
Human related CO₂ emissions mainly come from the combustion of fossil fuels such as coal, oil, and natural gas, from industrial processes such as cement production, and from land use changes such as deforestation and degradation of soils. CO₂ molecules can be taken up by plants through photosynthesis and by oceans, but a fraction of emitted CO₂ remains in the atmosphere for hundreds to thousands of years.
CO₂ has a 100 year GWP of 1 by definition. Although its warming effect per kilogram is lower than that of several other gases, its enormous emission volume and long persistence make it the dominant driver of long term climate change.
Key statement: Carbon dioxide is the primary driver of human caused climate change, due to both its large emission volumes and its long atmospheric lifetime.
Methane (CH₄)
Methane is a potent greenhouse gas with a stronger warming effect per molecule than CO₂ but a much shorter atmospheric lifetime. Typical sources include the extraction and transport of fossil fuels, agriculture especially enteric fermentation in ruminant animals like cows and sheep and rice paddies, landfills and waste, and natural sources such as wetlands.
Methane stays in the atmosphere for about 10 to 12 years on average before it is broken down by chemical reactions. Its 100 year GWP is commonly estimated around 28 to 34, depending on the scientific source and assumptions. This means that over a century, one kilogram of CH₄ warms the planet roughly 28 to 34 times more than one kilogram of CO₂.
When a shorter time frame is used, for example 20 years, the GWP of methane is much higher, because most of its warming occurs soon after emission before it breaks down.
Important fact: Methane has a high GWP over short and medium time scales, so rapid methane reductions can yield relatively quick climate benefits.
Methane also contributes indirectly to ozone formation in the lower atmosphere, which has its own climate and health impacts, but that mechanism belongs in more specialized discussions.
Nitrous Oxide (N₂O)
Nitrous oxide is a long lived greenhouse gas with a very high GWP. It is emitted mainly from agricultural soils where nitrogen fertilizers and manure are applied, from some industrial processes, and from combustion sources such as engines and biomass burning. Microorganisms in soils and water bodies convert nitrogen compounds into N₂O, especially when nitrogen is abundant and conditions are favorable.
N₂O has an atmospheric lifetime of around 100 to 120 years. Its 100 year GWP is approximately 265 to 298. This means that, per kilogram, nitrous oxide is almost three hundred times more potent than carbon dioxide over a century.
N₂O also plays a role in stratospheric ozone depletion because it participates in chemical reactions that affect ozone, but this is addressed separately in more detailed atmospheric chemistry contexts.
Key statement: Nitrous oxide is both a powerful and long lived greenhouse gas, making agricultural and industrial N₂O emissions a serious climate concern even though their mass is smaller than CO₂ emissions.
Fluorinated Gases (F-gases)
Fluorinated gases are a group of synthetic greenhouse gases that do not occur naturally in significant amounts. They are designed for industrial and commercial uses such as refrigeration and air conditioning, electrical insulation, semiconductor manufacturing, and some specialized applications.
The three main classes are hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF₆). Each class includes many individual chemicals with different properties, but they share two common characteristics. They have very high GWPs and they can have long atmospheric lifetimes.
HFCs are used primarily as refrigerants and in foams. They were introduced to replace older substances that damaged the ozone layer, yet many HFCs themselves are powerful greenhouse gases. Their GWPs range roughly from the hundreds to the thousands. Their lifetimes are often measured in years to a few decades, shorter than some other fluorinated gases.
PFCs are often byproducts of aluminium production and semiconductor manufacturing. They are chemically very stable. Their atmospheric lifetimes can reach thousands of years, and some PFCs have GWPs in the thousands to over ten thousand.
Sulfur hexafluoride, SF₆, is used mainly as an insulating gas in high voltage electrical equipment. SF₆ has one of the highest known GWPs, commonly quoted above 20,000 on a 100 year basis, and an atmospheric lifetime estimated around 3,000 years.
Important rule: Even small leaks of fluorinated gases can have very large climate impacts because many F gases have GWPs hundreds to tens of thousands of times higher than CO₂.
Because they are man made and linked to specific applications, F gases are often targeted by technical standards, containment requirements, and substitution policies.
Water Vapor
Water vapor is the most abundant greenhouse gas in the atmosphere and is very effective at absorbing infrared radiation. However, it behaves differently from the major long lived greenhouse gases described above.
The concentration of water vapor in the atmosphere is controlled mainly by temperature and the availability of surface water, not directly by human emissions of water. When the planet warms due to increased CO₂ and other long lived gases, the atmosphere can hold more water vapor. This added water vapor then amplifies the original warming.
For climate change discussions, water vapor is therefore treated mainly as a feedback to warming rather than as a direct driver from human emissions. Human activities can influence humidity locally for example through irrigation or power plant cooling, but these effects are small compared to the global feedback driven by temperature.
Key statement: Water vapor strongly enhances warming but is considered a feedback, while long lived gases like CO₂ and CH₄ are the main direct human controlled drivers of climate change.
Ozone (O₃) And Other Short Lived Climate Forcers
Ozone is a greenhouse gas present in different layers of the atmosphere. In the stratosphere, ozone forms the ozone layer, which shields the Earth from harmful ultraviolet radiation. In the lower atmosphere, or troposphere, ozone is a pollutant and a short lived greenhouse gas.
Tropospheric ozone is not emitted directly in large amounts. Instead, it forms from reactions involving nitrogen oxides, carbon monoxide, methane, and volatile organic compounds in the presence of sunlight. These precursors come from sources such as vehicles, industrial activities, biomass burning, and natural emissions from vegetation.
Because ozone in the lower atmosphere has a relatively short lifetime often days to weeks it is classified as a short lived climate forcer rather than a long lived greenhouse gas. It does contribute to warming, especially in regions with high air pollution, and also causes health and ecosystem damage.
There are other short lived species that affect climate, such as some aerosols and black carbon. They influence the Earth’s energy balance but are not greenhouse gases in the strict sense, because they are particles rather than gases. Their detailed roles belong in more focused discussions of air pollution and radiative forcing.
Summary Of Major Greenhouse Gases
In practical climate policy and reporting, major greenhouse gases are usually grouped as CO₂, CH₄, N₂O, and the family of fluorinated gases. Water vapor and ozone are crucial for the overall greenhouse effect and climate response, but they are often treated separately, as feedbacks or short lived forcers.
To compare emissions of different gases, many inventories and targets express totals in terms of carbon dioxide equivalents. The basic idea is to multiply the emitted mass of each gas by its GWP and then sum.
Formula: If $m_i$ is the mass of gas $i$ and $GWP_i$ its global warming potential, then total emissions in CO₂ equivalent are
$$\text{CO₂e} = \sum_i m_i \times GWP_i.$$
This conversion allows different gases to be combined into a single number for accounting and target setting. However, it also hides differences in lifetimes and time scales. Later chapters on climate policy and mitigation will build on this understanding of the major greenhouse gases and their distinct characteristics.