Answer: Sources of Greenhouse Gases (GHGs)
Greenhouse Gases (GHGs) are atmospheric gases that absorb and re-emit infrared radiation, trapping heat in the Earth's atmosphere and causing the greenhouse effect. The main GHGs are Carbon Dioxide (CO2), Methane (CH4), Nitrous Oxide (N2O), and Fluorinated gases.
1. Overview of GHG Contributions
| Greenhouse Gas |
Formula |
Share of Emissions |
GWP (100-year) |
Atmospheric Lifetime |
| Carbon Dioxide |
CO2 |
76% |
1 |
100-1000+ years |
| Methane |
CH4 |
16% |
28-36 |
12 years |
| Nitrous Oxide |
N2O |
6% |
265-298 |
121 years |
| Fluorinated Gases |
HFCs, PFCs, SF6 |
2% |
1,000-23,500 |
Varies (years to millennia) |
2. Carbon Dioxide (CO2) - The Primary GHG
Carbon Dioxide Sources GWP = 1 (Reference)
A. Fossil Fuel Combustion (~75% of CO2)
- Power Generation: Coal, oil, and natural gas burning in thermal power plants - largest single source (~40% of energy-related CO2)
- Transportation: Petrol/diesel vehicles, aviation, shipping (~20% of emissions)
- Industrial Energy: Heating and processes in manufacturing
- Residential/Commercial: Heating, cooking, water heating
B. Industrial Processes (~5-6%)
- Cement Production: Calcination of limestone (CaCO3 → CaO + CO2) releases ~0.9 tonnes CO2 per tonne cement
- Steel Manufacturing: Blast furnace reactions using coke
- Chemical Industry: Ammonia, petrochemical production
C. Land Use Change (~10%)
- Deforestation: Clearing forests releases stored carbon
- Forest Fires: Natural and human-caused fires
- Peatland Drainage: Exposes organic carbon to decomposition
- Agricultural Expansion: Converting forests to cropland
3. Methane (CH4) - High Warming Potential
Methane Sources GWP = 28-36
A. Agricultural Sources (~40% of CH4)
- Enteric Fermentation: Digestive process in ruminant animals (cattle, sheep, goats) produces methane - single cow emits ~100 kg CH4/year
- Rice Paddies: Anaerobic conditions in flooded rice fields - major source in Asia
- Manure Management: Animal waste decomposition
B. Energy Sector (~35% of CH4)
- Natural Gas Leaks: Extraction, processing, transmission, distribution
- Coal Mining: Methane released from coal seams (Coal Mine Methane - CMM)
- Oil Production: Associated gas venting and flaring
C. Waste Management (~20% of CH4)
- Landfills: Anaerobic decomposition of organic waste produces landfill gas (50-60% CH4)
- Wastewater Treatment: Sewage treatment plants
- Open Dumps: Uncontrolled waste decomposition
D. Natural Sources
- Wetlands: Largest natural source of methane
- Termites: Digestive processes
- Permafrost Thawing: Releases trapped methane (climate feedback)
4. Nitrous Oxide (N2O) - Agricultural Connection
Nitrous Oxide Sources GWP = 265-298
A. Agricultural Sources (~60% of N2O)
- Synthetic Fertilizers: Nitrogen fertilizers (urea, ammonium nitrate) undergo nitrification/denitrification
- Animal Manure: Nitrogen in manure converts to N2O
- Crop Residue: Decomposition of nitrogen-rich plant matter
- Soil Management: Tillage practices affect soil nitrogen cycles
B. Industrial Sources (~20% of N2O)
- Nitric Acid Production: Used in fertilizer manufacturing
- Adipic Acid Production: Used in nylon manufacturing
- Caprolactam Production: Synthetic fiber precursor
C. Combustion Sources
- Fossil Fuels: Vehicle engines, power plants
- Biomass Burning: Forest fires, agricultural burning
5. Fluorinated Gases (F-gases) - Synthetic and Potent
Fluorinated Gas Sources GWP = 1,000-23,500
| F-Gas Type |
Sources |
GWP |
| HFCs (Hydrofluorocarbons) |
Refrigeration, air conditioning, aerosols, foam blowing |
12-14,800 |
| PFCs (Perfluorocarbons) |
Aluminum smelting, semiconductor manufacturing |
7,390-12,200 |
| SF6 (Sulfur Hexafluoride) |
Electrical switchgear, magnesium production |
23,500 |
| NF3 (Nitrogen Trifluoride) |
Electronics/semiconductor manufacturing |
17,200 |
6. Other GHGs
- Water Vapor (H2O): Most abundant GHG but acts as feedback, not driver. Concentration depends on temperature.
- Tropospheric Ozone (O3): Not directly emitted; formed from NOx and VOCs in sunlight. Contributes to smog and warming.
- CFCs/HCFCs: Ozone-depleting substances also potent GHGs (being phased out under Montreal Protocol)
Key Insight: While CO2 dominates in quantity, short-lived climate pollutants like CH4 and high-GWP F-gases offer significant near-term mitigation opportunities due to their higher warming potential per molecule.
7. Sectoral Breakdown of Global Emissions
| Sector |
Share of GHG Emissions |
Primary Gases |
| Energy (Electricity & Heat) |
25% |
CO2 |
| Agriculture, Forestry, Land Use |
24% |
CO2, CH4, N2O |
| Industry |
21% |
CO2, F-gases |
| Transportation |
14% |
CO2, N2O |
| Buildings |
6% |
CO2 |
| Other Energy |
10% |
CO2, CH4 |
Conclusion
Greenhouse gas sources span all sectors of the economy and human activity. While CO2 from fossil fuels is the dominant contributor, addressing methane from agriculture and waste, nitrous oxide from fertilizers, and high-GWP fluorinated gases is equally important for comprehensive climate mitigation. Understanding sources enables targeted policies - transitioning energy systems, reforming agricultural practices, improving waste management, and phasing down F-gases are all essential components of emission reduction strategies.
Sources: Module 1 Notes | IPCC AR6 | EPA | Global Carbon Project | WRI Climate Watch