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26.1 Greenhouse Effect & Global Warming
26.1.1 What is the Greenhouse Effect?
Definition: The Greenhouse Effect is the process by which certain gases in Earth’s atmosphere absorb and re-emit infrared (longwave) radiation, thereby warming the Earth’s surface and lower atmosphere beyond what solar radiation alone would produce.
Basic Mechanism:
26.1.2 Importance of the Natural Greenhouse Effect
- Without any greenhouse effect, Earth’s average temperature would be approximately −18°C (currently ~+15°C)
- The natural GHE keeps Earth 33°C warmer than it would otherwise be
- It makes liquid water, and therefore life, possible
- It moderates temperature difference between day and night
26.1.3 Natural vs. Human-Induced Greenhouse Effect
| Feature | Natural Greenhouse Effect | Human-Induced (Enhanced) Effect |
|---|---|---|
| Cause | Natural processes (volcanoes, wetlands, ocean) | Fossil fuels, deforestation, industry, agriculture |
| Status | Always existed; necessary for life | Enhanced since Industrial Revolution (~1750) |
| Main gases | H₂O vapour, CO₂, CH₄, N₂O (natural) | CO₂ ↑, CH₄ ↑, N₂O ↑ + HFCs, PFCs, SF₆ |
| Temperature | Maintains ~15°C average | Added ~1.55°C above pre-industrial (2024) |
| Equilibrium? | Yes — in natural balance | No — imbalance growing |
| Carbon cycle | Balanced (sources = sinks) | Unbalanced — emissions >> absorption |
26.1.4 How Scientists Study Past Climate Change?
🧊 Proxy Methods
- Ice Cores — air bubbles trap ancient atmosphere; CO₂ data up to 800,000 years ago (Antarctica, Greenland)
- Tree Rings (Dendrochronology) — wider rings = warmer/wetter years
- Sediment Cores — pollen, fossils, isotope ratios reveal past climates
- Coral Records — isotopic signatures of temperature
- Stalactites/Stalagmites — cave growth rates indicate past moisture
🌡️ Direct Instrumental Records
- Systematic records since ~1850 (175-year observational record)
- Keeling Curve — continuous CO₂ measurement at Mauna Loa, Hawaii since 1958
- Satellite data since 1979 for sea ice, temperature
- Argo floats — 4,000 autonomous floats measure ocean temperature/salinity
- Atmospheric radiosondes (weather balloons)
🔬 Key Finding from Ice Core Records
CO₂ has not exceeded 300 ppm in the past 800,000 years — until now. In 2024 it reached 423.9 ppm. This is a level not seen in at least 2 million years (IPCC AR6). AR6 2023
26.2 Greenhouse Gases (GHGs) & Global Warming
26.2.1 Global Warming Potential (GWP) & Atmospheric Lifetime
| Gas | Atmospheric Lifetime | GWP (20-yr) | GWP₁₀₀ AR6 | Pre-industrial (ppb) | 2024 Level | % Rise |
|---|---|---|---|---|---|---|
| CO₂ | Centuries–millennia | 1 | 1 | 278 ppm | 423.9 ppm | +52% |
| CH₄ (biogenic) | ~12 years | ~82 | 27 | 729 ppb | 1942 ppb | +166% |
| N₂O | ~109 years | ~273 | 273 | 270 ppb | 338 ppb | +25% |
| HFCs | 1.5–270 years | — | 148–14,800 | ~0 | Rising | — |
| SF₆ | 3,200 years | — | 25,200 | ~0 | Rising | — |
| Black Carbon | Days–weeks | ~900 (GWP₂₀) — Short-lived | ~0 | Rising | — | |
📌 GWP — Key Exam Points
- GWP is always measured relative to CO₂ = 1
- Standard time horizon: 100 years (GWP₁₀₀) — used in UNFCCC, NDCs, Paris Agreement
- GWP₂₀ (20-year) is used for short-lived climate pollutants — methane appears far more potent
- Higher GWP = more dangerous per molecule, but quantity in atmosphere also matters
- IPCC AR6 updated GWP values — methane biogenic = 27 (was 28 in AR5)
- Methane fossil = 29.8 (includes CO₂ from oxidation)
- N₂O GWP₁₀₀ = 273 (unchanged from AR5)
Individual GHG Profiles
💡 Sources: Fossil fuel combustion (75%), deforestation, cement (limestone calcination), land-use change
📊 2024: 423.9 ppm — record high. +52% above pre-industrial 278 ppm
🌍 Responsible for ~76% of total GHG radiative forcing due to volume
⏳ Long atmospheric lifetime — persists for centuries; some portion for 100,000 years
💡 Sources: 60% human (livestock, rice paddies, landfills, fossil fuels, biomass burning); 40% natural (wetlands)
📊 2024: 1,942 ppb — +166% above pre-industrial
🌍 Responsible for ~16% of warming by long-lived GHGs
⏳ Lifetime ~12 years; breaks down to H₂O + CO₂ in stratosphere
🇮🇳 India: Rice paddies + cattle are major sources
💡 Sources: Nitrogen fertiliser use, animal manure, biomass burning, industrial processes
📊 2024: 338.0 ppb — +25% above pre-industrial 270 ppb
⏳ Atmospheric lifetime ~109 years; also destroys stratospheric ozone
🌿 Called “the forgotten greenhouse gas” — often overlooked but very potent
💡 Tropospheric ozone acts as a GHG (unlike stratospheric ozone which protects us)
📊 Formed by photochemical reactions: NOx + VOCs + sunlight → O₃
⚠️ Both a GHG and an air pollutant (crop damage, health harm)
🌫️ Contributes ~0.4 W/m² positive radiative forcing — 3rd most important GHG by forcing
💡 Most abundant GHG by volume; responsible for ~50% of natural GHE
🔄 Not directly controlled by humans — acts as a positive feedback
Warmer air → more evaporation → more water vapour → more warming → cycle continues
⏳ Short lifetime (days to weeks) — concentration controlled by temperature, not emissions
💡 Sources: Diesel engines, coal combustion, biomass burning, cooking fires
🔬 Not strictly a “gas” — fine particles (~0.1–1 µm) that absorb sunlight directly
⏳ Very short lifetime (days–weeks) but high warming impact, especially over ice/snow
🇮🇳 India is a major source of black carbon — cookstoves a big contributor
✅ Short-Lived Climate Pollutant (SLCP) — fastest way to slow near-term warming
Fluorinated Gases (F-gases)
Fluorinated gases are the most powerful and longest-lasting GHGs. They are entirely human-made — zero natural sources.
| F-Gas Type | Examples | GWP₁₀₀ | Uses |
|---|---|---|---|
| HFCs (Hydrofluorocarbons) | HFC-134a, HFC-23 | 148 – 14,800 | Refrigerants, AC, aerosols (replace CFCs) |
| PFCs (Perfluorocarbons) | CF₄, C₂F₆ | 6,630 – 11,100 | Aluminium smelting, semiconductor |
| SF₆ | Sulphur hexafluoride | 25,200 | Electrical insulation, circuit breakers |
| NF₃ | Nitrogen trifluoride | 17,400 | Electronics manufacturing |
🌍 Kigali Amendment (2016) to Montreal Protocol mandates phase-down of HFCs — could avoid 0.5°C of warming by 2100. India ratified in September 2021.
Carbon Monoxide (CO) — Indirect GHG
- CO is not a direct GHG but an indirect one — it reacts with OH radicals, reducing their availability to break down methane, thereby increasing methane’s atmospheric lifetime
- Sources: Incomplete combustion, vehicle exhaust, biomass burning, industrial processes
- Also a primary air pollutant and health hazard
- Lifetime: ~1–3 months in atmosphere
26.3 Carbon Footprint
Definition
Carbon footprint is the total amount of greenhouse gases (in CO₂ equivalent) emitted directly or indirectly by an individual, organisation, product, event, or country over a given time period.
It includes:
- Direct emissions: Burning fuel in your car, cooking with LPG
- Indirect emissions: Electricity consumption (power plant emissions), goods you purchase (embedded carbon)
Unit: tonnes of CO₂ equivalent (tCO₂e)
26.3.1 Carbon Footprint of Various Sectors — India & Global
| Sector | India Share (2022) | Global Share | Key Sources |
|---|---|---|---|
| Energy (Power) | ~32% | ~34% | Coal power plants (>95% of power sector emissions in India) |
| Agriculture | ~14% | ~11% | Rice paddies (CH₄), livestock (CH₄), fertilisers (N₂O) |
| Industry | ~22% | ~24% | Steel, cement (process CO₂), chemicals |
| Transport | ~14% | ~16% | Road, aviation, shipping |
| Buildings | ~7% | ~6% | Electricity for cooling/heating, cooking |
| Waste | ~4% | ~3% | Landfills (CH₄), wastewater |
| LULUCF | Carbon sink (−ve) | +ve (source) | Forest cover acts as sink for India |
📌 India’s Emission Story — Key Points for Exam
- India: 3rd largest emitter globally (after China and USA) in absolute terms
- But per capita: India (~2.4 tCO₂) vs USA (~15 tCO₂) vs global average (~5 tCO₂)
- India contributed 31% of global energy-sector emission growth in the decade 2014–2024
- Coal = >50% of India’s total CO₂ (electricity + heat generation)
- Positive: India surpassed 50% non-fossil installed electricity capacity in 2024 — achieved NDC target 5 years early!
- India’s 4th Biennial Update Report (BUR-4) submitted to UNFCCC in December 2024 — showed 7.93% reduction in GHGs in 2020 vs 2019 (COVID effect)
- India committed to net-zero by 2070 (announced at COP26, Glasgow)
26.3.2 State-wise Carbon Emissions in India
Based on CEEW (Council on Energy, Environment and Water) study — manufacturing GHG emissions by state:
Top Emitting States (Manufacturing + Energy)
- Gujarat — 14% (largest; natural gas, liquid fuels)
- Odisha — 13%
- Chhattisgarh — 10% (metals/coal)
- Jharkhand — 9%
- Karnataka — 8%
- Maharashtra — 8% (chemicals leader)
- Andhra Pradesh — 7%
- Rajasthan — 5% (cement/minerals)
Sector-Specific Leaders
- Metal industry emissions: Chhattisgarh (22%)
- Mineral (cement): Rajasthan (17%)
- Chemical industry: Maharashtra (40%)
- IPPU emissions top 5: Gujarat (16%), Maharashtra (14%), Rajasthan (11%), Andhra Pradesh (9%)
IPPU = Industrial Processes and Product Use
🌍 Global Top Emitters (Energy Sector CO₂)
- China — ~31% of global CO₂
- USA — ~15%
- India — ~8%
- EU — ~7%
- Russia — ~5%
Top 10% richest households emit 45%+ of global GHGs; bottom 50% emit only 15% — IPCC AR6
26.4 Climate Sensitivity
Key Definitions
- Equilibrium Climate Sensitivity (ECS): Long-term global warming after CO₂ doubling and full climate system adjustment. IPCC AR6 Best Estimate: 3°C (likely range: 2.5°C–4°C)
- Transient Climate Response (TCR): Warming at the time of CO₂ doubling (70-year period). AR6: ~1.8°C (1.2–2.4°C likely range)
- Earth Energy Imbalance (EEI): Difference between incoming solar energy and outgoing energy. Currently +0.79 W/m² (2006–2018) — Earth is absorbing more than it emits
26.4.1 Feedbacks Drive Uncertainty in Climate Sensitivity
Climate feedbacks are processes that amplify or dampen the initial warming caused by GHGs.
🔺 Positive Feedbacks (Amplify Warming)
- Water Vapour Feedback: Warming → more evaporation → more WV → more GHE (strongest feedback)
- Ice-Albedo Feedback: Warming → ice melts → darker ocean/land exposed → less reflection → more absorption → more warming
- Permafrost/Methane Feedback: Warming → permafrost thaws → releases trapped CH₄ and CO₂ → more warming (tipping point risk!)
- Forest Dieback: Extreme heat/drought → forests die → less CO₂ absorption + decomposition releases CO₂
- Cloud Feedback (low clouds): Low-altitude clouds decrease → less reflection → more warming
🔻 Negative Feedbacks (Dampen Warming)
- Planck/Blackbody Feedback: Warmer Earth radiates more energy (Stefan-Boltzmann law) — main stabiliser
- Lapse Rate Feedback: Higher altitude warming increases outgoing radiation
- Chemical Weathering: CO₂ dissolves in rain → reacts with rocks → forms carbonates (very slow, millions of years)
- Ocean Carbon Sink: Oceans absorb ~25% of CO₂ emitted (though weakening)
- Vegetation Greening: More CO₂ → some plants grow more → absorb more CO₂
⚠️ IPCC AR6 Warning: Tipping Points 2023
Beyond certain temperature thresholds, positive feedbacks become self-reinforcing and irreversible. These are called tipping points:
- Greenland Ice Sheet collapse (possible at 1.5–2°C)
- West Antarctic Ice Sheet destabilisation
- Amazon rainforest dieback (>3–4°C)
- Permafrost carbon bomb (large CH₄ release above ~2°C)
- Atlantic Meridional Overturning Circulation (AMOC) collapse
Key finding: Each additional 0.5°C of warming significantly increases the likelihood of crossing these irreversible thresholds.
📊 IPCC AR6 Synthesis Report (March 2023) — Key Numbers
📋 IPCC AR6 Working Group Structure (2021–2023)
“The Physical Science Basis” — 234 scientists, 66 countries. Confirmed human causation as “unequivocal”. Extended the Hockey Stick graph. Attributed specific extreme weather events to climate change.
“Impacts, Adaptation and Vulnerability” — Documented current devastating impacts, assessed adaptive limits for ecosystems and communities, warned about loss & damage.
“Mitigation of Climate Change” — Identified viable pathways: solar, wind, electrification, demand management. Cost of inaction exceeds cost of mitigation.
Final synthesis. UN Secretary-General called it “survival manual for humanity.” Finalised at Interlaken, Switzerland. Basis for COP28 (Dubai, 2023). AR7 cycle began July 2023 — expected ~2030.
🔴 Current Affairs: GHG & Climate Records (2024–2025)
WMO State of the Global Climate 2024 (March 2025) Must Know
- 2024 was the warmest year in 175 years of records at 1.55 ± 0.13°C above 1850–1900 baseline
- First likely calendar year where global temperature crossed the 1.5°C threshold
- 2015–2024: All 10 years are the 10 warmest on record — “an extraordinary streak” (WMO)
- Driven by rising GHG concentrations + El Niño (persisted mid-2023 to May 2024)
- Ocean heat content: 16 zettajoules increase in 2024 alone — ~140× world’s total electricity generation
- Antarctic sea ice: 2nd lowest extent ever (2024), after 2023
- The Paris Agreement’s 1.5°C goal “not yet dead but in grave danger” — WMO
WMO Greenhouse Gas Bulletin No. 21 (October 2025) — 2024 Data Most Recent
- CO₂: 423.9 ppm — Record high. 52% above pre-industrial (278 ppm). +3.5 ppm in one year — largest annual rise since 1957
- CH₄: 1,942 ppb — Record high. 166% above pre-industrial
- N₂O: 338.0 ppb — Record high. 25% above pre-industrial
- Reasons for record CO₂ jump: Continued fossil fuel emissions + weakened land and ocean carbon sinks + exceptional wildfires (El Niño effect)
- CO₂ growth rate tripled: 0.8 ppm/year (1960s) → 2.4 ppm/year (2011–2020) → 3.5 ppm (2023–24)
- Radiative forcing from long-lived GHGs increased by 54% between 1990 and 2024
- Concern: Ocean CO₂ sinks weakening as oceans warm (lower solubility at higher temperature)
India’s GHG Position 2024 India-Specific
- India registered the largest absolute GHG increase in 2024 — +165 MtCO₂e (China: +126 MtCO₂e)
- India’s per capita growth rate: +3.7% (vs global average +0.04%)
- India = 3rd largest emitter globally; per capita (~2.4 tCO₂e) still below global average
- Power sector: India responsible for 8% of global energy CO₂ despite having 18% of world population
- Coal production hit record 1 billion tonnes in FY 2024–25
- Green milestone: Non-fossil sources supplied majority of new electricity generation in 2024–25 — first time ever
- India achieved 50% non-fossil installed capacity NDC target 5 years early
- India’s Carbon Credit Trading Scheme (CCTS) to begin by mid-2026
- BUR-4 submitted to UNFCCC (December 2024) — showed 2020 emissions 7.93% below 2019
COP30 — Belém, Brazil (November 2025) Upcoming
- Full name: UNFCCC Conference of the Parties (COP30)
- Venue: Belém, Amazon region, Brazil — symbolically important for forest/climate justice
- Key agenda: Nations must submit updated NDCs (Nationally Determined Contributions) for 2030 and 2035
- IPCC data presented at COP30 shows countries’ current pledges are insufficient to limit to 1.5°C or even 2°C
- Climate finance for developing nations: Key demand — $100 Bn/year commitment was fulfilled in 2022 (2 years late)
🎯 Practice MCQs
AR6 SYR (March 2023) recorded a 1.1°C rise for the 2011–2020 average. Human causation was stated as “unequivocal” (not “very likely” — that was AR5). CO₂ in 2019 was highest in at least 2 million years. AR7 cycle will conclude around 2030, not 2025.
This represents a record high and was 52% above the pre-industrial level of 278 ppm. The jump of 3.5 ppm from 2023 to 2024 was the largest annual increase since measurements began in 1957 at Mauna Loa.
GWP₁₀₀ (100-year Global Warming Potential) is the standard metric used in UNFCCC and Paris Agreement reporting. CO₂ = 1 (reference). CH₄ biogenic = 27, N₂O = 273 (IPCC AR6 values). GWP₂₀ (20-year) is also used for short-lived climate pollutants.
SF₆ is used in electrical equipment (circuit breakers, transformers). It has an atmospheric lifetime of ~3,200 years and a GWP of 25,200 — the highest of any gas per kilogram. Even small leaks represent massive CO₂-equivalent emissions.
A positive feedback AMPLIFIES the initial change. Ice reflects ~80% of sunlight (high albedo). When ice melts, it reveals dark ocean or land (~6% albedo), causing more absorption, more warming, more melting — a dangerous self-reinforcing loop. Options (a), (c), and (d) are all negative feedbacks that dampen warming.
BUR-4 showed a 7.93% reduction in total GHG emissions in 2020 versus 2019, primarily due to COVID-19 lockdowns reducing economic activity. Excluding LULUCF, India’s emissions were 2,959 MtCO₂e in 2020.
❓ Frequently Asked Questions
Climate Change is broader — it refers to long-term shifts in global or regional climate patterns, including changes in temperature, precipitation patterns, sea levels, storm frequency, wind patterns, and more. Climate change includes global warming but also encompasses all its cascading effects.
Think of it this way: Global Warming is the fever; Climate Change is the illness that the fever causes throughout the body.
However, WMO stated the goal is “not yet dead but in grave danger.” The long-term trend is around 1.34–1.41°C (2024), and if current policies continue, we are heading for ~3°C by 2100 (IPCC AR6).
This is why targeting methane is considered the fastest way to slow near-term warming. The Global Methane Pledge (COP26, 2021) committed to cut methane by 30% by 2030 — India has not signed it.
What’s happening: In 2024, land and ocean sinks were significantly weaker than normal due to El Niño, drought, and record wildfires. This caused the record 3.5 ppm CO₂ jump in 2024. WMO warns that as temperatures rise, sinks may become less effective — creating a dangerous positive feedback loop (more CO₂ → warming → weaker sinks → even more CO₂ in atmosphere).
IPCC AR6 narrowed the ECS range to 2.5°C–4°C (likely), with a best estimate of 3°C (compared to 1.5°C–4.5°C in previous reports — a big improvement in scientific certainty).
Why it matters for policy: If ECS is 4°C, we have much less room to emit before hitting dangerous levels. If it’s 2.5°C, we have slightly more flexibility. The narrower range in AR6 means policymakers can plan with more confidence — unfortunately, the evidence points to significant warming requiring rapid action regardless.
India’s NDC (updated 2022): Reduce emissions intensity of GDP by 45% by 2030 (vs 2005 levels); achieve 50% cumulative electric power from non-fossil fuel by 2030 (achieved early in 2024); net-zero by 2070.
India argues its per capita emissions are far below developed nations and it deserves space to grow. It has NOT signed the Global Methane Pledge and continues to expand coal production, while simultaneously rapidly scaling renewables.
📚 Previous Year Questions (UPSC)
Q: With reference to ‘Global Climate Change Alliance’, which of the following statements is/are correct?
1. It is an initiative of the European Union.
2. It provides technical and financial support to targeted developing countries to integrate climate change into their development policies.
(a) 1 only (b) 2 only (c) Both 1 and 2 (d) Neither 1 nor 2
Answer: (c) — Both statements are correct. The GCCA was launched by the EU in 2007 and supports ACP (Africa, Caribbean, Pacific) countries vulnerable to climate change.
Q: “Methane hydrates” are often in the news. Which of the following correctly explains why?
(a) They can be used as fuel
(b) They are a potential source of methane for climate change
(c) Both (a) and (b)
(d) Neither (a) nor (b)
Answer: (c) — Methane hydrates (ice-like structures in ocean floors and permafrost) are both a potential energy source AND a climate tipping point — if destabilised by warming, they release vast amounts of methane.
Q: With reference to the Indian Ocean Dipole (IOD) — how does a positive IOD affect Indian monsoon and global GHG levels? [Paraphrased from Mains 2022]
Relevance: Climate sensitivity section — the IOD is a feedback mechanism that affects both ocean temperature patterns and monsoon behaviour. Positive IOD (warmer western Indian Ocean) generally correlates with stronger Indian monsoons but drier conditions in Australia, which can increase wildfire CO₂.
Q: “Discuss the main objectives of the Loss and Damage fund established at COP27. What are the challenges in its operationalisation?”
Relevance to this chapter: Loss & Damage is the consequence of climate change impacts exceeding adaptation capacity. IPCC AR6 (WG2) extensively documented L&D — developing nations need $127 Bn/year by 2030 for adaptation + significant L&D finance (AR6). The fund was announced at COP27 (Sharm el-Sheikh, 2022) and operationalised at COP28 (Dubai, 2023).
🏁 Quick Revision — Must Remember for Exam
Greenhouse Effect: Short-wave in → long-wave out → GHGs trap → warming. Natural GHE = +33°C. Human-enhanced GHE = extra 1.55°C (2024).
Top 3 GHGs by warming contribution: CO₂ (76% of forcing), CH₄ (16%), N₂O
GWP₁₀₀: CO₂=1 | CH₄ biogenic=27 | N₂O=273 | SF₆=25,200 (highest)
2024 Records (WMO): CO₂=423.9 ppm (+52% pre-industrial) | CH₄=1942 ppb (+166%) | N₂O=338 ppb (+25%) | Temperature=1.55°C above baseline
IPCC AR6 SYR (2023): Human causation “unequivocal” | 1.5°C by early 2030s | Need -43% emissions by 2030 | 3.5 billion face water stress
India: 3rd largest emitter | Low per capita (2.4 tCO₂) | Largest absolute increase in 2024 (+165 MtCO₂e) | Net-zero by 2070 | 50% non-fossil capacity NDC achieved early
Climate Sensitivity ECS: AR6 best estimate = 3°C for CO₂ doubling (range 2.5–4°C)
Feedbacks: Positive (amplify) = water vapour, ice-albedo, permafrost | Negative (dampen) = Planck radiation, ocean sink
