Greenhouse gas emissions 101 – I

Before we begin, if you want to understand the general big picture about what climate is, Earth’s climate history, and/or about climate change, you can read this very comprehensive post.

A greenhouse is a structure, made of glass or plastic, which captures heat inside it so that it’s insides are warmer and drier than the atmosphere outdoors. Greenhouses are situated outdoors so they have a regular supply of sunlight. We’ve all experienced closed indoor spaces with glass façades that heat up due to receiving sunlight, and require specific cooling solutions that encourage air flow, or artificial cooling through air conditioners, such as sitting inside a car or a room with all its windows closed on warm days. These hot-car experiences are also due to the greenhouse effect.

This effect happens because sunlight, which is primarily composed of (a tiny amount of) ultraviolet (UV) light, visible light, and near-infrared (NIR) radiation, easily passes through greenhouse covers (glass or plastic) into the inside of greenhouse, where the objects, plants, and soil absorb the heat, and become warmer. These warmed up objects now radiate heat in the form of long-wavelength thermal infrared (IR) radiation, which, unlike the incoming shortwave radiation (UV, visible light, NIR) is absorbed into the greenhouse envelope (a building’s envelope is the skin of the building- all the outside walls). Since the building envelop has now absorbed the heat, the structure and its insides warm up and stay warm. In short: this effect allows heat energy inside, but doesn’t allow all of it to escape.¹²

Similarly, greenhouse gases are gas molecules in Earth’s atmosphere that absorb heat emanating from the planet’s surface- that is, they act sort of like the transparent skin of a greenhouse which absorbs heat so that the plants inside can be warm in cold weather.¹²

Here’s how it works: Solar energy travels through the atmosphere and warms Earth’s surface. As the planet radiates this heat back toward space, it does so primarily as long-wavelength infrared radiation, which is the same form of heat that gets trapped in a physical greenhouse. Greenhouse gases in the atmosphere absorb this infrared radiation. Instead of letting it escape to space, they re-radiate it in all directions, with much of it directed back downward toward Earth’s surface. This creates a second source of heating (the first being our Sun), amplifying the warming effect and keeping our planet warmer than it would otherwise be.¹²

A point to note is that in an actual greenhouse building, the warm air inside cannot mix with the cooler air outside it. Similarly, because there is nothing to mix with, the air inside the planet cannot be diluted with cooler air.

The greenhouse effect has directly caused life as we know it now to exist on this planet (other forms of life could still exist without it, who knows), as without this natural greenhouse effect, Earth would be a frozen, inhospitable world. Temperatures would average around -18°C instead of the habitable 15°C we currently enjoy.¹² But we’re now enjoying too much of a good thing, and the planet is now heating up more than is good for the life that evolved to live in it when the average temperature was the aforementioned the habitable 15°C: it’s not that no life will survive, it’s just that much of it won’t, leading to general ecosystem collapse, and life will be very uncomfortable for the humans who do make it to the hotter planet.^345678910

What does parts per million/ billion/ trillion mean?¹¹
ppm/ ppb/ ppt are notations scientists who study climate use to understand how much of the greenhouse gases in question is present in the atmosphere. Different greenhouse gases are measured in different units depending on their concentration levels. Carbon dioxide, which is relatively abundant in the atmosphere, is measured in parts per million. Methane, which exists in much lower concentrations, is measured in parts per billion. The most potent synthetic gases, such as the fluorinated gases like SF₆ and NF₃, are measured in parts per trillion, because even seemingly insignificant amounts have significant warming effects.

Besides, saying “the atmosphere contains 0.000194 of a percent of methane” is far less convenient than saying “the atmosphere contains 1,942 ppb of methane”.

Thus, if a scientist is measuring how many molecules of CO2 are present in our vast atmosphere, and if the atmospheric concentration of CO2 is measured to be 400 ppm, this means that out of every 1 million air molecules, 400 are CO2 molecules, and the remaining 999,600 molecules are other gases. The same principle applies to measuring ppb and ppt. The conversion between these units is the same as for regular numbers:

• 1 ppm = 1,000 ppb
• 1 ppm = 1,000,000 ppt
• 1 ppb = 1,000 ppt

Here’s how Global Warming Potential is measured¹²¹³
GWP measures how much heat a greenhouse gas traps in the atmosphere typically calculated over a 100-year time horizon, in comparison to the amount of heat trapped in the atmosphere by CO2. It’s calculated by the Intergovernmental Panel on Climate Change (IPCC) based on the intensity of infrared absorption by each gas and how long emissions remain in the atmosphere. The unit of measurement is called Carbon Dioxide Equivalent (CO₂e).

Carbon Dioxide Equivalents (CO₂e) provide a standardised way to express the impact of different greenhouse gases using a single, comparable metric. CO₂e is calculated by multiplying the quantity of a greenhouse gas emitted by its Global Warming Potential. The formula is:​

CO₂e = Mass of GHG emitted × GWP of the gas

For example, if you emit one million metric tons of methane (with a GWP of 30) and one million metric tons of nitrous oxide (GWP of 273), this is equivalent to 30 million and 273 million metric tons of CO₂, respectively.¹⁴​

This standardisation is crucial for several reasons because it allows comparison across GHGs and amounts of emissions, so no matter the gas that has been emitted or the amount of it emitted, it is easy to understand for everyone the effect it will have on the planet. It will also help compare emissions reduction opportunities across different sectors and gases, and help compile comprehensive national and corporate GHG inventories that include all greenhouse gases. Essentially, it provides a common language for understanding greenhouse gas emissions.

Radiative Forcing Vs. GWP¹⁵¹⁶
Radiative forcing (RF) is a measure of how much a substance or factor disrupts the balance of energy entering and leaving Earth’s atmosphere. It is expressed in watts per square meter (W/m²), representing the amount of energy imbalance imposed on the climate system: it quantifies how much extra energy is being trapped in the atmosphere by a given agent (greenhouse gas, aerosol, or solar change). Therefore,

• Positive radiative forcing = warming effect (energy trapped)
• Negative radiative forcing = cooling effect (energy lost to space)

In comparison, GWP is a simplified index that converts radiative forcing into a single comparable number by expressing it relative to CO₂.

GWP = Total radiative forcing from 1 kg of substance over time horizon / Total radiative forcing from 1 kg of CO₂

This formula is asking if 1 kilo of a substance is released into the atmosphere, how many kilograms of CO₂ would produce the same total warming effect.

Radiative forcing tells you the immediate, direct physics of climate impact. It’s precise but complex because each substance has a different RF value. GWP is a policy-friendly simplification that lets users compare “apples to apples”, so that if 1 million tons of methane (GWP 30) are emitted, vs. 1 million tons of N₂O (GWP 273), it is instantly known that the N₂O causes ~9× more warming.

Let’s take a look at the main GHGs
You can read more about pollution (natural and anthropogenic here).

Carbon Dioxide (CO₂)¹⁷ is the most abundant and significant human-caused greenhouse gas, accounting for approximately three-quarters of all anthropogenic GHG emissions. Before the Industrial Revolution, atmospheric CO₂ concentration was about 280 parts per million (ppm). By May 2023, it had reached a record 424 ppm, which is a level not seen in approximately three million years.​ Aside from it’s abundance in the atmosphere, CO₂ is also a particularly concerning GHG because of its atmospheric persistence. While about 50% of emitted CO₂ is absorbed by land and ocean sinks within roughly 30 years, about 80% of the excess persists in the atmosphere for centuries to millennia, with some fractions remaining for tens of thousands of years. This means that the CO₂ we emit today will continue warming the planet for generations.​

Methane (CH₄)¹⁷ is the second most important greenhouse gas after carbon dioxide. Although it exists in much smaller quantities than CO₂, methane is extraordinarily potent: one ton of methane traps as much heat as 30 tons of carbon dioxide.​¹⁴

Methane is emitted from both natural and human sources. Natural sources include wetlands, tundra, and oceans, accounting for about 36% of total methane emissions. Human activities produce the remaining 64%, with the largest contributions coming from agriculture, particularly livestock farming through enteric fermentation (this is a digestive processes in ruminant animals where microbes in their gut ferment food, producing methane as a byproduct) and rice cultivation. Other significant sources include landfills, biomass burning, and fugitive emissions from oil and gas production (unintentional, uncontrolled leaks of gases and vapors that escape the control equipment, sometimes due to poorly maintained infrastructure).¹³

The good news about methane is its relatively short atmospheric lifetime of approximately 12 years. This means that reducing methane emissions can have a more immediate impact on slowing global warming compared to CO₂, even though its effects are less persistent over the long term.​

Nitrous Oxide (N₂O), also known as laughing gas, is a long-lived and potent greenhouse gas with a Global Warming Potential 273 times higher than CO₂. It has an average atmospheric lifetime of 109-132 years.​¹⁴

Nitrous oxide emissions come from both natural and anthropogenic sources. Major natural sources include soils under natural vegetation, tundra, and the oceans. Human sources, which account for over one-third of total emissions, primarily stem from agricultural practices—especially the use of synthetic and organic fertilisers, soil cultivation, and livestock manure management.¹³¹⁴¹⁷ Additional sources include biomass or fossil fuel combustion, industrial processes, and wastewater treatment.​¹³¹⁴¹⁷

Fluorinated Gases¹⁸ represent a family of synthetic, powerful greenhouse gases including hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF₆), and nitrogen trifluoride (NF₃). These gases are emitted from various household, commercial, and industrial applications, particularly as refrigerants and in electrical transmission equipment.​

While fluorinated gases are present in much smaller quantities than CO₂, methane, or nitrous oxide, they are extraordinarily potent. Some have Global Warming Potentials thousands of times higher than CO₂. For example, SF₆ has a GWP of 24,300, and some HFCs have GWPs exceeding 10,000. Additionally, many fluorinated gases have extremely long atmospheric lifetimes, ranging from 16 years to over 500 years for certain CFCs, meaning they persist in the atmosphere for decades or even centuries.​¹⁴

Water Vapor (H₂O) is technically the strongest and most abundant greenhouse gas. However, its concentration is largely controlled by atmospheric temperature rather than direct human emissions. As air becomes warmer, it can hold more moisture, creating a feedback loop: warming from other greenhouse gases increases water vapor, which in turn amplifies warming. This makes water vapor a climate feedback mechanism rather than a primary driver of climate change.¹²¹⁹

Greenhouse Gas	Atmospheric Concentration1718	Global Warming Potential (100-yr)14	Warming Contribution17	Primary Sources & Their Contributions20
Carbon Dioxide (CO₂)	Pre-industrial: 280 ppm | Current: 423.9 ppm (↑152%)	1 (baseline)	~74.5% of total GHG emissions; 42% of radiative forcing increase since 1990	Fossil fuel combustion: 74.5% of total – Electricity/heat: 29% – Transportation: 15% – Industry: 24% – Deforestation: 6.5-12%
Methane (CH₄) – non-fossil	Pre-industrial: 730 ppb | Current: 1,942 ppb (↑166%)	27.0	~17.9% of total GHG emissions; 16% of warming from long-lived GHGs	Agriculture: 42% (livestock 27%, rice 9%) – Fossil fuel extraction: 23% – Landfills/waste: 16% – Natural wetlands: 36%
Methane (CH₄) – fossil*	—	29.8	—	Fossil fuel fugitive emissions from oil & gas systems and coal mining
Nitrous Oxide (N₂O)	Pre-industrial: 270 ppb | Current: 338 ppb (↑25%)	273	~4.8% of total GHG emissions; third most important long-lived GHG	Agriculture: 74-75% (synthetic fertilisers 30-50% of agricultural emissions) – Industrial processes – Biomass burning
Water Vapor (H₂O)	Pre-industrial: 0-4% (variable) | Current: 0-4% (variable), increasing 1-2%/decade	Not directly comparable (feedback amplifier)	41-67% of total greenhouse effect (but as feedback, not primary driver)	Natural evaporation from oceans/land – Acts as feedback amplifier (increases 7% per 1°C warming) – Not directly emitted by humans
Tropospheric Ozone (O₃)	Pre-industrial: 20-25 ppb | Current: 20-100 ppb (varies by location)	Varies regionally	Third most important GHG after CO₂ and CH₄; significant regional warming	Not directly emitted – Forms from: NOx + VOCs + sunlight – Sources: Transportation, industry, biomass burning
HFC-134a	Pre-industrial: 0 ppt | Current: 96.9 ppt	1,530	Part of 2.8% F-gases contribution	Refrigeration and air conditioning: largest use – Aerosol propellants – Foam blowing – Summer emissions 2-3× winter
HFC-23	Pre-industrial: 0 ppt | Current: Low but significant	14,600	Highest CO₂-eq among HFCs despite low concentration	Byproduct of HCFC-22 production – Industrial processes
HCFC-22	Pre-industrial: 0 ppt | Current: Declining post-ban	1,960	Part of declining HCFC contribution	Refrigeration/Air Conditioning: primary source (97% of HCFC use) – Being phased out under Montreal Protocol
HFC-152a	Pre-industrial: 0 ppt | Current: 9.93 ppt	164	Part of 2.8% F-gases contribution	Aerosol propellants – Foam blowing – Refrigeration
Sulfur Hexafluoride (SF₆)	Pre-industrial: Near 0 ppt | Current: 6.7 ppt	24,300	Part of 2.8% F-gases contribution; Highest CO₂-eq among all FGHGs	Electrical equipment: switchgear, transformers – Magnesium production – Semiconductor manufacturing
Perfluoromethane (CF₄)	Pre-industrial: 34.7 ppt | Current: 76 ppt	7,380	Part of 2.8% F-gases contribution	Aluminum production – Semiconductor manufacturing – Small natural sources: ~10 tonnes/year
Perfluoroethane (C₂F₆)	Pre-industrial: Near 0 ppt | Current: 2.9 ppt	12,400	Part of 2.8% F-gases contribution	Semiconductor manufacturing: 1,800 tonnes/year – Aluminum smelting
Nitrogen Trifluoride (NF₃)	Pre-industrial: 0 ppt | Current: Growing	17,400	Part of 2.8% F-gases contribution	Semiconductor/electronics manufacturing – Flat panel displays
CFC-12	Pre-industrial: 0 ppt | Current: Declining (banned)	12,500	Declining contribution; negative forcing from ozone depletion	Previously: refrigeration (primary), aerosols – Now banned; emissions from existing equipment
CFC-11	Pre-industrial: 0 ppt | Current: Declining (banned)	6,230	Declining contribution; negative forcing from ozone depletion	Previously: refrigeration, foam, aerosols – Now banned; emissions from existing equipment/foams
Black Carbon (BC/Soot)2122	Pre-industrial: Low natural levels | Current: No direct measurement in ppm/ppb	450–900 (100-yr GWP)*	Second or third most important climate forcer after CO₂ in some regions	Diesel engines – Coal power plants – Biomass burning: wood, agricultural waste (67% of human emissions) – Residential cooking/heating – Wildfires – Ranking: Fossil fuel > biofuel > biomass burning
CFCs (Total)	Pre-industrial: 0 ppt | Current: Declining overall	6,230–12,500	Negative forcing due to ozone depletion (cooling effect)	Banned under Montreal Protocol – Residual emissions from existing equipment/foams
HFCs (Total)	Pre-industrial: 0 ppt | Current: 89 ppt total	164–14,600	~2.8% combined with PFCs and SF₆; grown 310% since 1990	Refrigeration/AC sector: largest source (replacing CFCs/HCFCs) – Increased 310% since 1990
PFCs (Total)	Pre-industrial: 34.7 ppt | Current: 82 ppt total	7,380–12,400	~2.8% combined with HFCs and SF₆	Industrial processes – Aluminum production – Semiconductor manufacturing
HCFCs (Total)	Pre-industrial: 0 ppt | Current: Declining	90–1,960	Declining; negative forcing from ozone depletion offset by GHG warming	Transitional CFC replacement being phased out – HCFC-22 and HCFC-141b represent 97% of HCFC use

Key:

• ppm = parts per million; ppb = parts per billion; ppt = parts per trillion
• GWP (Global Warming Potential) is measured relative to CO₂ over a 100-year timeframe (IPCC AR6, August 2024)¹⁴
• F-gases (fluorinated gases) collectively contribute 2.8% of total greenhouse gas emissions but have grown 310% since 1990
• Water vapor is technically the most abundant greenhouse gas but acts primarily as a feedback mechanism rather than a forcing agent
• Black carbon is not measured in atmospheric concentration like other GHGs because it’s a particulate (soot) rather than a gas, and has a very short atmospheric lifetime (days to weeks). The GWP range reflects uncertainty in mixing state and location; IPCC AR6 provides radiative forcing (+0.44 W/m²) rather than a formal GWP.
• *Methane split: IPCC AR6 differentiates between fossil and non-fossil methane due to different atmospheric fates. Use CH₄ non-fossil (27.0) for biogenic sources and combustion; use CH₄ fossil (29.8) for fugitive emissions from oil & gas and coal mining where the carbon is of fossil origin.¹⁴²³ This is because fossil methane (GWP 29.8) adds carbon that was locked underground for millions of years to the active carbon cycle, representing a net addition of CO₂ when oxidised, whereas biogenic methane (GWP 27.0) comes from carbon that was recently in the atmosphere (absorbed by plants, eaten by livestock, etc.), so its oxidation just adds back the same carbon that was already in the atmosphere until recently and there is no net addition in the long term.²⁴

Sources of GHG emissions

1. The Energy Sector is the largest contributor to greenhouse gas emissions, producing approximately 34% of total net anthropogenic GHG emissions in 2019.²⁵ Within this sector, electricity and heat generation are the single largest emitters, accounting for over 25% of global emissions, with coal-fired power stations alone responsible for about 20% of global greenhouse gas emissions.²⁶ In 2022, 60% of electricity in many countries still came from burning fossil fuels, primarily coal and natural gas.²⁷ And of course, energy underpins every other sector, whether through fuel for agricultural tractors, for building space conditioning, or any other mechanical activity.
2. ​Industrial activities come next at 24% of global emissions. These emissions are usually from one of two sources: energy consumption for manufacturing processes, and direct emissions from chemical reactions necessary to produce goods from raw materials.²⁵²⁸ Within industry, cement production and metal production, especially steel, are particularly emission-intensive.²⁸ Since 1990, industrial processes have grown by a massive 225%, the fastest growth rate of any emissions source, driven by rapid industrialisation in developing countries.²⁰
3. Agriculture, Forestry, and Land Use contributed approximately 22% of global emissions in 2019.²⁵ This is an interesting sector because it’s a major source of non-CO₂ greenhouse gases.²⁹ Agriculture is the largest contributor to methane emissions globally, primarily from livestock farming and rice cultivation, which occurs in flooded fields where anaerobic conditions produce methane.²⁹ The sector also produces significant nitrous oxide emissions, primarily from the application of synthetic and organic fertilisers to soils.²⁹ Additionally, deforestation and land-use changes release stored carbon when forests are cleared for agriculture or development.²⁹​
4. Transportation accounts for approximately 15% of global emissions in 2019.²⁵ The vast majority of transportation emissions come from road vehicles (cars, trucks, buses, motorcycles, etc.) which rely overwhelmingly on petroleum-based fuels.³⁰ Aviation and maritime shipping also contribute significantly, with international aviation and shipping representing growing sources of emissions as global trade and travel expand.³⁰ Since 1990, transportation emissions have grown by 66%, making it one of the fastest-growing sources of greenhouse gases.²⁰³⁰ The sector’s heavy dependence on fossil fuels and the long replacement cycles for vehicles make it particularly challenging to decarbonise quickly.³⁰
5. And finally, Buildings, whether Commercial or Residential, directly contribute approximately 6% of global emissions through fossil fuels burned for heating and cooling, as well as refrigerants used in air conditioning systems.²⁵ However, when indirect emissions from electricity use are included, buildings account for a much larger share, which is about 28% in the United States, because buildings consume approximately 75% of electricity generated, primarily for heating, ventilation, air conditioning, lighting, and appliances.³¹³²

Sources

1. IRENA – Power to Heat and Cooling: Status
2. What is the greenhouse effect?
3. The Greenhouse Effect
4. 1.5 Degrees C Target Explained
5. IPCC AR6 Working Group II – Chapter 2
6. Science Magazine – Climate Study
7. What does the latest IPCC report mean for wildlife?
8. Nature – Climate Research Article
9. Is Earth becoming too hot for humans? Climate change facts & risks
10. Too Hot to Handle: How Climate Change May Make Some Places Too Hot to Live
11. Taylor & Francis Online – Climate Research
12. EPA – Global Greenhouse Gas Overview
13. UNFCCC – Global Warming Potentials
14. EPA – Understanding Global Warming Potentials
15. GHG Protocol – IPCC Global Warming Potential Values
16. EPA – Climate Change Indicators: Climate Forcing
17. IPCC – TAR Chapter 6: Radiative Forcing of Climate Change
18. IPCC AR6 Synthesis Report – Longer Report
19. IPCC AR6 Updated GWP Values for HFCs and HFOs
20. OpenLearn – Climate Change and Renewable Energy
21. World Resources Institute – 4 Charts Explain Greenhouse Gas Emissions by Sector
22. Climate and Clean Air Coalition – Black Carbon
23. Visualizing Energy – Global Black Carbon Emissions 1750-2022
24. IPCC AR6 WGIII – Annex II: Definitions, Units and Conventions
25. Carbon Brief – Q&A: What the ‘controversial’ GWP* methane metric means for farming emissions
26. IPCC AR6 Working Group III – Chapter 2: Emissions Trends and Drivers
27. World Nuclear Association – Carbon Dioxide Emissions From Electricity
28. Visual Capitalist – Coal Still Dominates Global Electricity Generation
29. UNECE – Pathways to Carbon-Neutrality in Energy-Intensive Steel
30. IPCC AR6 Working Group III – Chapter 7: Agriculture, Forestry, and Other Land Uses
31. UNFCCC – Greenhouse Gas Data Booklet
32. EIA – U.S. Electricity Generation by Energy Source