Chapter 2: GHG Types and Sources

2.1 Carbon Dioxide (CO₂) - The Primary Driver

CO₂ Profile

Chemical Formula	CO₂
Molecular Weight	44.01 g/mol
Global Warming Potential (100-year)	1 (baseline)
Atmospheric Lifetime	300-1,000 years (complex)
Pre-industrial Concentration	~280 ppm
Current Concentration (mid-2020s)	~420 ppm
Annual Increase	~2.5 ppm/year
Contribution to Warming	About two-thirds of long-lived GHG radiative forcing

Major CO₂ Emission Sources

Source Category	Relative Magnitude	Key Activities
Fossil Fuel Combustion	The large majority of anthropogenic CO₂ emissions	Coal, oil, natural gas burning
Land Use Change	Second-largest source (highly variable from year to year)	Deforestation, conversion to agriculture
Cement Production	Small but hard-to-abate process emissions	Calcination of limestone (decarbonation)
Other Industrial Processes	Minor	Chemical processes, metal smelting

Annual quantitative values should be taken from the Global Carbon Project's annual Global Carbon Budget and from national inventories.

The Carbon Cycle

CO₂ doesn't simply accumulate - it participates in the global carbon cycle. According to the Global Carbon Project's carbon budget estimates, the approximate partitioning is:

Oceans absorb about one-quarter of anthropogenic CO₂ (causing acidification)
Land ecosystems absorb nearly one-third (forests, soils)
The remainder, close to half, stays in the atmosphere (the "airborne fraction")
This distribution varies year-to-year based on climate variability

Sector-Specific CO₂ Emissions

Sector	Primary Sources	Mitigation Strategies
Electricity & Heat	Coal, natural gas power plants	Renewables, nuclear, CCS
Transportation	Road vehicles, aviation, shipping	EVs, efficiency, alternative fuels
Industry	Manufacturing, construction	Electrification, efficiency, hydrogen
Buildings	Heating, cooling, cooking	Insulation, heat pumps, solar
Other	Agriculture energy, fugitive emissions	Various

Within the energy sector, electricity and heat production is the largest source of CO₂, followed by transportation and industry.

2.2 Methane (CH₄) - The Potent Short-Term Forcer

CH₄ Profile

Chemical Formula	CH₄
Molecular Weight	16.04 g/mol
Global Warming Potential (100-year)	28 (84 over 20 years)
Atmospheric Lifetime	~12 years
Pre-industrial Concentration	~722 ppb
Current Concentration (mid-2020s)	~1,900 ppb
Annual Increase	~10 ppb/year
Contribution to Warming	About one-sixth of long-lived GHG radiative forcing

Methane's Dual Nature

Methane presents both a challenge and an opportunity:

Challenge: 28x more potent than CO₂ over 100 years
Opportunity: Short atmospheric lifetime means rapid climate benefits from reduction
Reducing methane emissions is one of the fastest ways to slow near-term warming

Methane Sources

According to Global Methane Budget studies, about 60% of total methane emissions come from anthropogenic sources, with the remainder from natural sources such as wetlands. The main sources are:

Source Category	Type	Description
Agriculture (Enteric)	Anthropogenic (one of the largest single sources)	Cattle and livestock digestion
Agriculture (Rice)	Anthropogenic	Anaerobic decomposition in flooded fields
Oil & Gas Systems	Anthropogenic (fossil)	Leaks and venting during extraction, processing, distribution
Coal Mining	Anthropogenic (fossil)	Underground and surface mining
Waste (Landfills)	Anthropogenic	Anaerobic decomposition of organic waste
Waste (Wastewater)	Anthropogenic	Treatment and discharge
Biomass Burning	Anthropogenic and natural	Wildfires, agricultural burning
Wetlands	Natural (largest natural source)	Not anthropogenic, but climate-sensitive

Methane Super-Emitters

Satellite observations have revealed "super-emitter" events - individual sources emitting several tonnes of methane per hour:

Oil & gas leaks: Faulty equipment, venting, flaring
Coal mine vents: Uncontrolled releases during extraction
Landfill hotspots: Poor waste management
These super-emitters represent low-hanging fruit for mitigation

2.3 Nitrous Oxide (N₂O) - The Long-Lived Oxidizer

N₂O Profile

Chemical Formula	N₂O
Molecular Weight	44.01 g/mol
Global Warming Potential (100-year)	265
Atmospheric Lifetime	~120 years
Pre-industrial Concentration	~270 ppb
Current Concentration (mid-2020s)	~335 ppb
Annual Increase	~1 ppb/year
Contribution to Warming	About 6% of long-lived GHG radiative forcing

N₂O Sources

Global N₂O emissions divide into natural sources, such as natural soils and oceans, and anthropogenic sources; agriculture accounts for the largest share of anthropogenic emissions.

Source Category	Type	Description
Agricultural Soils	Anthropogenic (largest)	Synthetic fertilizers, manure, crop residues
Manure Management	Anthropogenic	Storage and treatment of animal waste
Industrial Processes	Anthropogenic	Adipic acid, nitric acid production
Fossil Fuel Combustion	Anthropogenic	Mobile and stationary sources
Biomass Burning	Anthropogenic and natural	Wildfires, agricultural fires
Wastewater	Anthropogenic	Treatment processes
Natural Soils and Oceans	Natural	Non-anthropogenic background emissions

The Nitrogen Paradox

Nitrogen fertilizers have enabled global food security, feeding billions. Yet their overuse creates environmental problems:

Anthropogenic N₂O emissions have grown substantially over recent decades, driven mainly by agriculture
N₂O is also currently the most heavily emitted ozone-depleting substance
A substantial share of applied nitrogen is not taken up by crops and is lost to the environment
Improving nitrogen use efficiency (right-rate application, nitrification inhibitors, etc.) can reduce emissions

2.4 Fluorinated Gases (F-gases) - Industrial Powerhouses

F-gases are mostly synthetic compounds with no natural sources (CF₄ has a very small natural background concentration). While present in tiny concentrations, their extremely high warming potentials make them significant. The GWPs and lifetimes in this chapter are IPCC AR5 (100-year) values, as used by the specification.

Hydrofluorocarbons (HFCs)

HFC Examples

Gas	Formula	GWP (100yr)	Lifetime	Common Uses
HFC-134a	CH₂FCF₃	1,300	13.4 years	Automotive AC, refrigeration
HFC-32	CH₂F₂	677	5.2 years	Air conditioning
HFC-125	CHF₂CF₃	3,170	28.2 years	Refrigeration blends
HFC-143a	CH₃CF₃	4,800	47.1 years	Commercial refrigeration
HFC-23	CHF₃	12,400	222 years	HCFC-22 production byproduct

The Kigali Amendment

In 2016, the Montreal Protocol was amended to phase down HFCs:

Developed countries: 85% reduction by 2036 (baseline 2011-2013)
Developing countries: 80% reduction by 2045 or 85% by 2047, depending on group
Assessed as able to avoid up to 0.4°C of warming by 2100
Transition to low-GWP alternatives (HFOs, natural refrigerants)

Perfluorocarbons (PFCs)

Gas	Formula	GWP (100yr)	Lifetime	Primary Source
CF₄	CF₄	6,630	50,000 years	Aluminum production
C₂F₆	C₂F₆	11,100	10,000 years	Aluminum, semiconductors
C₃F₈	C₃F₈	8,900	2,600 years	Semiconductor etching

Sulfur Hexafluoride (SF₆)

SF₆ - The Most Potent GHG

Formula	SF₆
GWP (100-year)	23,500
Lifetime	3,200 years
Primary Use	Electrical switchgear insulation
Atmospheric Concentration Trend	Rising steadily

SF₆: Essential but Problematic

SF₆ is widely used in high-voltage electrical equipment, but:

1 kg SF₆ = warming impact of 23.5 tonnes CO₂
Electric grid expansion is increasing SF₆ use
Leak management during operation, maintenance, and decommissioning determines emissions
Alternatives exist but are more expensive (SF₆-free switchgear)

Nitrogen Trifluoride (NF₃)

Formula	NF₃
GWP (100-year)	16,100
Lifetime	500 years
Primary Use	Semiconductor manufacturing, LCD panels
Note	Added to the Kyoto Protocol's covered gases for the second commitment period (2013-2020) by the 2012 Doha Amendment

2.5 Cross-Sectoral Emission Patterns

Understanding emissions requires looking beyond individual gases to see how sectors contribute across the GHG spectrum. Sectoral global shares follow the table in Chapter 1 (specification Phase 1 §4.2).

Sector	Global Share (Specification)	Dominant Gas	Secondary Gases
Energy	73%	CO₂	CH₄ (fugitive emissions), N₂O
Agriculture	12%	CH₄, N₂O	CO₂ (urea and lime application)
Industrial Processes	6%	CO₂	F-gases, N₂O
Waste	3%	CH₄	N₂O, CO₂ (incineration)
Land Use	6%	CO₂	CH₄, N₂O

2.6 Regional Emission Patterns

National and regional emissions are taken from national inventories submitted to the UNFCCC and from independent databases such as EDGAR. Total and per-capita emissions rank countries very differently, so both indicators should be considered together.

Region	Emission Profile	Primary Drivers
China	Largest total emitter in the world	Coal power, industry, transport
United States	High total emissions, high per-capita emissions	Transport, power, industry
EU-27	Declining since 1990	Power, transport, industry
India	Total emissions rising, low per-capita emissions	Coal power, agriculture
Russia	High per-capita emissions	Oil & gas, power, industry
Rest of World	Collectively a large share of global emissions	Various

2.7 Temporal Trends in Emissions

Historical Emission Growth

1990s-2010s: Global emissions grew steadily alongside industrialization in emerging economies
2010s: Growth slowed as renewable energy deployment expanded
2020: The COVID-19 pandemic caused a large temporary drop in fossil CO₂ emissions
2021 onward: Emissions rebounded with economic recovery; whether global emissions have peaked is a key question

Year-by-year growth rates should be taken from primary sources such as the Global Carbon Project's annual reports.

2.8 Emission Intensity Metrics

Monitoring emissions relative to economic activity reveals decoupling trends. Globally, all three indicators have been improving since 1990, but absolute emissions remain high.

Metric	Definition	What a Decline Means
Carbon Intensity (CO₂/GDP)	CO₂ emissions per unit of economic output	Economy growing faster than emissions
Energy Intensity (Energy/GDP)	Energy consumption per unit of economic output	Improving energy efficiency
Grid Emission Factor (gCO₂/kWh)	CO₂ emissions per unit of electricity generated	Grid decarbonization underway

2.9 Monitoring Priorities by Gas

WIA-ENE-051 Gas-Specific Monitoring Recommendations

CO₂:

High-precision background monitoring (±0.1 ppm)
Continuous emission monitoring systems (CEMS) for large point sources
Satellite column measurements for regional budgets

CH₄:

Super-emitter detection via satellite (TROPOMI, GHGSat)
Leak detection and repair (LDAR) programs for oil & gas
Isotopic analysis to distinguish sources (fossil vs. biogenic)

N₂O:

Agricultural soil flux chambers for direct measurement
Industrial stack monitoring for adipic/nitric acid plants
Regional atmospheric inversions

F-gases:

Equipment-level leak detection
Production and import/export tracking
Atmospheric monitoring at select remote stations

2.10 Future Source Trends

Emerging Emission Concerns

Permafrost thaw: Potential release of the vast carbon stored in frozen soils as CO₂ and CH₄
Arctic methane: Destabilization of subsea clathrates (gas hydrates)
Tropical wetlands: Enhanced CH₄ under warming
Wildfire emissions: Increasing frequency and intensity
Data centers: Growing energy demand and potential refrigerant leaks

Key Takeaways

CO₂ accounts for about two-thirds of long-lived GHG radiative forcing, primarily from fossil fuels
CH₄ is 28x more potent than CO₂ and offers rapid climate benefits if reduced
N₂O from agriculture is increasing due to fertilizer use
F-gases have extremely high GWPs but are being phased down under Kigali Amendment
Different sectors emit distinct gas portfolios requiring tailored monitoring
Regional emission patterns reflect development stage and energy systems
Emission intensity is decreasing, but absolute emissions remain too high
Climate feedbacks may create new natural emission sources

Review Questions

Why does CO₂ have such a wide range for atmospheric lifetime (300-1,000 years)?
Calculate the CO₂e of releasing 100 tonnes of methane using GWP-100.
Which sectors are the main anthropogenic sources of methane?
Explain why reducing methane offers faster climate benefits than reducing CO₂.
What is the primary source of N₂O emissions globally?
Why are F-gases particularly concerning despite their low atmospheric concentrations?
What is the Kigali Amendment, and what is its expected climate impact?
How does emission intensity differ from absolute emissions?
Name three "super-emitter" source types for methane.
What role does isotopic analysis play in methane source attribution?