Greenhouse Gases (GHGs)

Introduction

Certain gases present in the atmosphere have a special physical property—they can absorb and re-emit outgoing infrared radiation from the Earth’s surface.

These gases include:

• Carbon Dioxide (CO₂)
• Methane (CH₄)
• Nitrous Oxide (N₂O)
• Water Vapour
• Chlorofluorocarbons (CFCs)

Because they trap heat and prevent it from escaping back into space, they are called greenhouse gases (GHGs), and the warming they cause is known as the greenhouse effect.

👉 Think of them as a thermal blanket around the Earth—essential in moderation, dangerous in excess.

Evidence of Warming: How Much Has Earth Already Warmed?

Scientific measurements show that:

• Since 1880, the average global surface temperature has increased by about 1°C relative to the mid-20th century baseline (1951–1980).
• Even before that, between 1750 and 1880, there was an additional ~0.15°C warming.

This rise may look small numerically, but climatically, 1°C is enormous.

⚠️ If GHG emissions continue unchecked, scientists warn that global temperatures may rise by up to 5°C by the end of this century, a level associated with catastrophic ecological disruption.

Consequences of Rising Temperatures

Scientists expect this warming to trigger a chain reaction:

A. Climatic Instability

• Increase in extreme and irregular climatic events
• Higher frequency and intensity of phenomena like El Niño

B. Cryosphere Melting

• Accelerated melting of:
  • Polar ice caps
  • Mountain glaciers
  • Himalayan snow caps

C. Sea Level Rise

• Long-term melting leads to sea level rise
• This threatens:
  • Coastal cities
  • Low-lying islands
  • Delta regions

D. Loss of Critical Ecosystems

• Submergence and degradation of:
  • Coral reefs
  • Mangroves
  • Swamps and marshes

These ecosystems are vital because they provide maximum ecological services—coastal protection, carbon sequestration, biodiversity support, and fisheries.

Global Warming Potential (GWP)

Not all greenhouse gases are equally dangerous.

What is Global Warming Potential (GWP)?

GWP measures how much heat one unit of a gas can trap in the atmosphere relative to CO₂.
→ CO₂ is taken as the baseline (GWP = 1).
→ Other gases are compared to it.

GWP100

• GWP calculated over a 100-year time horizon
• Widely used in climate negotiations and IPCC reports

Sources of Major Greenhouse Gases

Greenhouse Gas	Major Sources
CO₂	Burning of fossil fuels, deforestation
CFCs	Refrigeration, insulation foams, aerosols, solvents
Methane (CH₄)	Paddy cultivation, livestock excreta, termites, wetlands, landfills, fossil fuel burning
Nitrous Oxide (N₂O)	Fertilisers, burning of fossil fuels, crop residue
Carbon Monoxide (CO)	Iron ore smelting, fossil fuel burning, e-waste burning

GWP and Lifetime of Key Greenhouse Gases

This table explains why some gases are far more dangerous than CO₂, even if emitted in smaller quantities.

Gas	GWP100	Atmospheric Lifetime
CO₂	1	50–200 years
Methane (CH₄)	28	12 years
Nitrous Oxide (N₂O)	265	120 years
HFCs	140–11,700	1–270 years
PFCs	6,500–9,200	800–50,000 years
SF₆	23,900	~3,200 years

👉 Key insight:
Some gases may stay for thousands of years, locking future generations into warming even if emissions stop today.

GWP* (Star): A New Way to Measure Climate Impact

To address limitations of GWP100, scientists introduced GWP* in 2018 (COP24, UNFCCC).

How is GWP* different?

Feature	GWP100	GWP*
Timeframe	Fixed 100 years	Any specified period
Method	Direct comparison with CO₂	Adjusts GWP using gas lifetime
Accuracy	Better for long-lived gases	Better for short-lived gases
Complexity	Simple	More complex
Usage	Widely used	Not yet widely adopted

Concerns with GWP*

Despite its scientific sophistication, GWP* has problems:

A. Omission of Non-CO₂ Effects

• Ignores:
  • Aerosol effects of black carbon
  • Impact of tropospheric ozone on methane lifetime

B. Tool for Greenwashing

• Depending on the chosen baseline year, the same emissions can be portrayed as:
  • Causing warming
  • Causing no warming
  • Or even causing cooling

Related Concepts

• Greenwashing: Misleading claims about environmental benefits
• Greenhushing: Deliberate omission of environmental information to avoid scrutiny

Both are increasingly relevant in climate governance and corporate responsibility.

Super Pollutants: Small Quantity, Massive Impact

Super pollutants are short-lived climate pollutants that:
➡️Remain in the atmosphere for less time than CO₂
➡️But have very high warming potential

They include:

• Methane
• Black carbon
• Hydrofluorocarbons (HFCs)
• Tropospheric ozone

👉 Controlling super pollutants offers a fast and effective pathway to slow near-term warming and improve air quality simultaneously.

Greenhouse Gas Emissions: CO₂ Dominates, but Super Pollutants Matter

• Carbon Dioxide (CO₂) is the largest contributor, accounting for ~74% of global greenhouse gas emissions.
  • Nearly 92% of CO₂ emissions arise from fossil fuel combustion (coal, oil, gas).
• Other Greenhouse Gases, though smaller in volume, have disproportionately high warming impact:
  • Methane (CH₄) and Nitrous Oxide (N₂O) are often called “super pollutants” due to their much higher global warming potential (GWP) in the short term compared to CO₂.
• Major Sources of Super Pollutants:
  • Agriculture (livestock, rice paddies, fertilisers)
  • Waste management (landfills, wastewater)
  • Gas flaring and leakages in energy systems
• Fluorinated Gases (HFCs, PFCs, SF₆, NF₃):
  • Originate mainly from industrial processes and refrigeration
  • Contribute a small share of total emissions but have extremely high GWP
• UPSC Takeaway:
  👉 While CO₂ reduction remains central, targeting super pollutants offers quick and cost-effective climate gains, making them a high-impact mitigation opportunity often overlooked in climate strategies.

Carbon Dioxide (CO₂)

Carbon dioxide is meteorologically one of the most important gases in Earth’s atmosphere.

Why is CO₂ so important?

• It is transparent to incoming solar radiation (short-wave radiation).
• But it is opaque to outgoing terrestrial radiation (long-wave infrared radiation).

This dual property allows CO₂ to:

• Absorb heat emitted by the Earth’s surface
• Re-radiate part of this heat back towards the surface

As a result, CO₂ plays a central role in the greenhouse effect and Earth’s heat energy budget.

Distribution and Concentration of CO₂

• CO₂ is denser than air, so its concentration is higher near the Earth’s surface, where warming directly affects humans and ecosystems.
• In 2024, global atmospheric CO₂ crossed 422 ppm.

This steady rise is one of the strongest scientific indicators of anthropogenic climate change.

Aviation and CO₂ Emissions

• The aviation sector contributes about 2.5% of total human-induced CO₂ emissions.
• When non-CO₂ effects (such as water vapour, contrails, nitrogen oxides) are included, aviation’s contribution rises to nearly 5% of historical global warming.

👉 This distinction is important for policy debates and carbon accounting, especially in international climate negotiations.

How Much Carbon Exists on Earth?

According to the US National Academy of Sciences, the total carbon stock on Earth is estimated at:

🔹 1.85 billion gigatons (Gt) of carbon

Now let me correct a common misconception.

👉 Volcanoes are NOT the main source of atmospheric CO₂ today.
Human emissions from fossil fuels and deforestation are 40–100 times greater than all volcanic emissions combined.

Distribution of Earth’s Carbon

A. Below the Surface

• 1.845 billion Gt of carbon lies below Earth’s surface
• About 315 million Gt is stored in continental and oceanic lithospheres

B. Above the Surface (43,500 Gt)

Reservoir	Carbon Stock
Deep ocean	37,000 Gt (85.1%)
Marine sediments	3,000 Gt (6.9%)
Terrestrial biosphere	2,000 Gt (4.6%)
Surface ocean	900 Gt (2%)
Atmosphere	590 Gt (1.4%)

👉 Key insight:
Even though the atmosphere holds only 1.4% of Earth’s carbon, small changes here have huge climatic consequences.

Ozone (O₃)

Ozone is also a greenhouse gas, but its impact depends on where it is located.

A. Stratospheric Ozone (Good Ozone)

• Concentrated in the stratosphere
• Absorbs harmful ultraviolet (UV) radiation
• Protects life on Earth

B. Tropospheric Ozone (Bad Ozone)

• Formed near the surface when:
  • Carbon Monoxide (CO)
  • Nitrogen Dioxide (NO₂)
  • Volatile Organic Compounds (VOCs)
    react in the presence of sunlight

Tropospheric ozone:

• Is a pollutant
• Acts as a greenhouse gas
• Damages crops, human health, and ecosystems

Water Vapour

Water vapour is highly variable in the atmosphere.

Concentration

• Ranges from:
  • 0.02% in cold, dry regions
  • To 4% in humid tropical climates

Vertical and Horizontal Distribution

• 90% of atmospheric moisture lies within 6 km of Earth’s surface
• Concentration:
  • Decreases with altitude
  • Decreases from equator to poles

Role in the Greenhouse Effect

Like CO₂, water vapour:

• Absorbs long-wave terrestrial radiation (night-time heat)
• Also absorbs part of short-wave solar radiation (visible and UV)

👉 Water vapour acts mainly as a feedback mechanism—warming increases evaporation, which increases water vapour, which further enhances warming.

Methane (CH₄)

Methane is the second most important greenhouse gas after CO₂.

Why is methane dangerous?

• Over a 20-year period, methane has a GWP i.e. Global Warming Potential of 84
• This means it traps 84 times more heat per unit mass than CO₂

However:

• Methane is short-lived in the atmosphere (around 12 years)
• This makes it a key target for near-term climate mitigation

Physical Characteristics

• Main component of natural gas
• Colourless, odourless, tasteless
• Lighter than air
• Burns with a blue flame (complete combustion)
• Combustion produces CO₂ and H₂O

Excess Isoprene and Rising Methane Levels

What is Isoprene?

• A colourless, volatile hydrocarbon
• Most abundant non-methane VOC in the atmosphere

Sources

• Primarily emitted by:
  • Plants (especially oaks and poplars)
  • Some algae and bacteria
• Emissions increase with rising temperatures
• Also released from:
  • Fossil fuel combustion
  • Petroleum products
  • Synthetic rubber production

Why Excess Isoprene is a Problem

1. Ground-Level Ozone Formation
   • Sunlight + NOx + VOCs → Ozone
2. Secondary Organic Aerosols
   • Reduces air quality and visibility
3. Climate Change Feedback
   • Raises tropospheric ozone
   • Increases methane levels
   • Both are powerful greenhouse gases

Methane Emissions from Global Food Systems

This is a highly relevant UPSC topic linking environment, agriculture, and sustainability.

• The global food system accounts for one-third of global GHG emissions
• About one-third of food produced is wasted
• Food waste → rotting → methane release
• Food production consumes:
  • Large amounts of groundwater
  • Coal-based electricity

Livestock and Methane

Livestock emissions include:

• CO₂ (from urea)
• N₂O (from dung and urine)
• Methane (mainly from digestion)

An IPCC assessment shows:

• Methane accounts for at least 25% of current global warming

A 2021 UNEP–Climate and Clean Air Coalition report concluded:

• Cutting human-induced methane emissions by 45% this decade is crucial to slow climate change.

Cow Belching vs Flatulence: Clearing the Myth

• Popular belief: methane comes mainly from cow flatulence
• Scientific reality (NASA):
  • Cow belching is the dominant source
  • Due to enteric fermentation, where microbes break down complex sugars, releasing methane as a by-product

Nitrous Oxide (N₂O)

Nitrous Oxide is one of the most underestimated yet dangerous greenhouse gases.

Why is N₂O so critical?

• It is a greenhouse gas nearly 300 times more potent than CO₂ (in terms of GWP100).
• It has a very long atmospheric lifetime—up to 125 years, comparable to CO₂.
• Importantly, N₂O is now the single largest remaining threat to the ozone layer, after the successful phase-out of CFCs.

Trends and Sources

• Global N₂O emissions increased by ~30% between 1980 and 2016.
• It has the third-highest concentration among greenhouse gases, after CO₂ and methane.

Human Contribution

• 43% of total N₂O emissions are from anthropogenic sources.
• The agricultural sector is the dominant contributor due to nitrogen-based fertilisers.

India and Emerging Economies

• Most recent increases in N₂O emissions come from India, China, and Brazil.
• In India:
  • Agriculture accounts for over 70% of total N₂O emissions.
  • Fertilisers (mainly urea) contribute about 77% of agricultural N₂O emissions.

👉 This makes fertiliser efficiency and sustainable agriculture climate priorities, not just economic ones.

Clearing a Common Confusion: NOx Gases

All the following are oxides of nitrogen (NOx), but their climate roles differ:

• Nitrogen Oxide (NO) → Global cooling gas
• Nitrogen Dioxide (NO₂) → Global cooling gas
• Nitrous Oxide (N₂O) → Greenhouse gas

⚠️ UPSC often tests this distinction.

Black Carbon (Soot)

Black carbon is not a gas, but a particulate pollutant, yet its climate impact is enormous.

How does black carbon warm the Earth?

1. Reduces albedo when deposited on snow and ice
   → Less sunlight reflected, more absorbed
2. Strongly absorbs sunlight
   → Directly heats the surrounding air

Black carbon is considered one of the largest contributors to climate change after CO₂.

Key Difference from CO₂

• CO₂: Long-lived (decades to centuries)
• Black carbon: Short-lived (days to weeks)

👉 This means reducing black carbon emissions can yield rapid climate and health benefits, especially in regions like the Himalayas and Arctic.

Fluorinated Gases

Fluorinated gases are entirely anthropogenic and extremely powerful greenhouse gases.

A. Chlorofluorocarbons (CFCs)

• Previously used in:
  • Refrigeration
  • Aerosols
  • Foam insulation
• Phased out under the Montreal Protocol due to ozone depletion
• Also, very strong GHGs, with far higher warming potential than CO₂

B. Hydrofluorocarbons (HFCs)

• Introduced as CFC substitutes
• Used in:
  • Refrigerants
  • Aerosol propellants
  • Solvents
  • Fire retardants
• Do not deplete ozone, but:
  • Have high GWP
  • Have long atmospheric lifetimes

👉 This led to the Kigali Amendment (2016) to phase down HFCs.

C. Perfluorocarbons (PFCs)

• Composed only of carbon and fluorine
• Emitted during:
  • Aluminium production
  • Semiconductor manufacturing
• Used as alternatives to CFCs
• Characterised by:
  • Extremely long lifetimes
  • Very high GWP

D. Sulphur Hexafluoride (SF₆)

• One of the most potent greenhouse gases known
• Used in:
  • Magnesium processing
  • Semiconductor manufacturing
  • Electrical transmission equipment (circuit breakers, insulation)
• Also used as a tracer gas for leak detection

👉 Despite its limited use, its enormous GWP and long lifetime make it a major climate concern.

Carbon Monoxide (CO)

Carbon monoxide behaves differently from most GHGs.

Key Characteristics

• Less dense than air
• Short-lived
• Very weak direct greenhouse gas

Why does CO still matter?

• In the atmosphere, CO is oxidised into CO₂
• It increases concentrations of:
  • Methane
  • Tropospheric ozone

Both methane and tropospheric ozone are strong greenhouse gases, making CO an indirect contributor to global warming through radiative forcing.