🌡️ Chapter 06 · Topic 01 · Climate Change
Greenhouse Effect & Global Warming
Natural vs enhanced greenhouse effect, greenhouse gases (CO₂, CH₄, N₂O, CFCs), GWP, CO₂e, 1.5°C & 2°C targets, feedback mechanisms, tipping points — complete UPSC & PSC notes.
🌿 Natural Greenhouse Effect
The greenhouse effect is the process by which certain gases in Earth’s atmosphere trap heat from the sun, warming the planet’s surface. Without it, Earth’s average temperature would be about −18°C instead of the current +15°C — making life as we know it impossible.
- Solar radiation (shortwave) passes through the atmosphere and warms Earth’s surface
- Earth re-emits this energy as longwave infrared (heat) radiation
- Greenhouse gases (GHGs) absorb this outgoing infrared radiation and re-emit it in all directions — including back toward Earth
- This trapping of heat is the natural greenhouse effect — essential for life
- The atmosphere acts like the glass of a greenhouse — transparent to incoming sunlight, but opaque to outgoing heat
📌 Key Analogy: The greenhouse effect is like a blanket around Earth. A thin blanket (natural GHGs) keeps us warm. But adding more blankets (enhanced GHGs from human activity) makes the planet dangerously hot.
🏭 Enhanced Greenhouse Effect
The enhanced (anthropogenic) greenhouse effect refers to the intensification of the natural greenhouse effect due to human activities that increase the concentration of GHGs in the atmosphere.
- Burning of fossil fuels (coal, oil, natural gas) — releases CO₂ and other GHGs
- Deforestation — reduces CO₂ absorption; burning forests releases stored carbon
- Agriculture — rice paddies and livestock (cattle) release CH₄; fertilisers release N₂O
- Industrial processes — cement production, steel making release CO₂; refrigerants release CFCs/HFCs
- Waste decomposition — landfills release CH₄
- Result: global mean temperature has risen by approximately 1.1°C above pre-industrial levels (as of 2023)
⭐ Exam Tip: The enhanced greenhouse effect is the cause of global warming and climate change. The natural greenhouse effect is beneficial. This distinction is frequently tested. Human activities have increased CO₂ from 280 ppm (pre-industrial) to over 420 ppm (current) — a 50% increase.
🧪 Greenhouse Gases — Comprehensive Table
| Gas | GWP (100-yr) | Main Sources | Atmospheric Lifetime | Current Conc. |
|---|---|---|---|---|
| Carbon Dioxide (CO₂) | 1 (reference) | Fossil fuel combustion, deforestation, cement production | Centuries to millennia | ~420 ppm |
| Methane (CH₄) | 27–30 | Livestock, rice paddies, natural gas leaks, landfills, wetlands | ~12 years | ~1,900 ppb |
| Nitrous Oxide (N₂O) | 273 | Agricultural fertilisers, livestock manure, industrial processes | ~109 years | ~336 ppb |
| CFCs (e.g., CFC-12) | 10,200–10,900 | Old refrigerants, aerosols, foam blowing (now banned) | 100+ years | Declining (Montreal Protocol) |
| HFCs (e.g., HFC-134a) | 1,430 | Refrigerants, air conditioners (replaced CFCs) | 14 years | Rising (Kigali Amendment targets) |
| Water Vapour (H₂O) | N/A (feedback) | Evaporation from oceans, lakes; not directly emitted by humans | Days to weeks | Variable |
| Ozone (O₃) | Variable | Formed from NOₓ + VOCs (tropospheric); also stratospheric | Hours to weeks | Variable |
| SF₆ (Sulphur hexafluoride) | 23,500 | Electrical switchgear, semiconductor manufacturing | ~3,200 years | Trace amounts |
📌 Most Important GHGs for Exams: CO₂ is the most abundant anthropogenic GHG. CH₄ is ~80× more potent than CO₂ over 20 years (GWP-20 = 80). N₂O is 273× more potent than CO₂. CFCs have the highest GWP but are being phased out. SF₆ has the highest GWP of all (23,500) but is present in tiny amounts.
📊 Global Warming Potential (GWP)
- GWP measures how much heat a greenhouse gas traps in the atmosphere over a specific time period (usually 100 years), relative to CO₂
- CO₂ has a GWP of 1 (the reference standard)
- A gas with GWP of 100 traps 100× more heat than CO₂ per unit mass over 100 years
- GWP depends on: (1) how effectively the gas absorbs infrared radiation, and (2) how long it stays in the atmosphere
- Used to compare the climate impact of different gases and convert them to CO₂ equivalent (CO₂e)
⭐ CO₂ Equivalent (CO₂e): A standardised unit to express the warming impact of all GHGs in terms of CO₂. Formula: CO₂e = mass of gas × GWP. For example, 1 tonne of CH₄ = 27–30 tonnes CO₂e. This allows comparison and aggregation of different GHGs in climate targets and carbon accounting.
📈 CO₂ Levels — Then and Now
| Period | CO₂ Level | Notes |
|---|---|---|
| Pre-industrial (before 1750) | ~280 ppm | Stable for thousands of years; natural carbon cycle in balance |
| 1958 (Keeling Curve begins) | ~315 ppm | First systematic measurements at Mauna Loa Observatory, Hawaii |
| 2000 | ~370 ppm | Rapid rise with industrialisation |
| 2023 (current) | ~420 ppm | Highest in at least 3 million years; 50% above pre-industrial |
📌 Keeling Curve: The continuous record of atmospheric CO₂ measured at Mauna Loa Observatory, Hawaii since 1958 by Charles David Keeling. It shows a steady upward trend with seasonal oscillations (CO₂ dips in summer when plants absorb more). It is the longest-running direct measurement of atmospheric CO₂.
🎯 Temperature Targets — 1.5°C and 2°C
- The Paris Agreement (2015) set the goal of limiting global warming to well below 2°C above pre-industrial levels, with efforts to limit it to 1.5°C
- Current warming: approximately 1.1°C above pre-industrial levels (IPCC AR6, 2021)
- At 1.5°C: coral reefs decline by 70–90%; sea level rise of ~0.26–0.77 m by 2100; extreme heat events more frequent
- At 2°C: coral reefs decline by >99%; sea level rise of ~0.36–0.87 m; significantly worse impacts across all sectors
- IPCC AR6 warns that 1.5°C may be reached as early as the early 2030s at current emission rates
- To limit warming to 1.5°C, global CO₂ emissions must reach net zero by around 2050
⭐ Why 1.5°C matters: The difference between 1.5°C and 2°C of warming may seem small but has enormous consequences — especially for small island states, coral reefs, and vulnerable communities. At 2°C, 37% of the world’s population would be exposed to severe heat at least once every 5 years (vs 14% at 1.5°C).
🔄 Feedback Mechanisms
Feedback mechanisms amplify or dampen the initial warming caused by GHGs. Positive feedbacks amplify warming; negative feedbacks dampen it.
| Feedback | Type | Mechanism |
|---|---|---|
| Ice-Albedo Feedback | Positive | Warming → ice/snow melts → darker ocean/land exposed → less sunlight reflected (lower albedo) → more heat absorbed → more warming |
| Water Vapour Feedback | Positive | Warming → more evaporation → more water vapour in atmosphere → water vapour is a GHG → traps more heat → more warming. Strongest positive feedback. |
| Permafrost Methane Feedback | Positive | Warming → permafrost thaws → organic matter decomposes → releases CO₂ and CH₄ → more warming. A major tipping point risk. |
| Cloud Feedback | Mixed | Complex — low clouds cool (reflect sunlight); high clouds warm (trap heat). Net effect uncertain but likely slightly positive. |
| Planck Feedback | Negative | Warmer Earth radiates more energy to space → stabilising effect. The primary negative feedback. |
| Lapse Rate Feedback | Negative (tropics) / Positive (poles) | Changes in how temperature varies with altitude affect outgoing radiation. |
📌 Albedo: The fraction of solar energy reflected by a surface. Fresh snow has albedo ~0.8–0.9 (reflects 80–90% of sunlight). Ocean has albedo ~0.06 (absorbs 94%). As ice melts, Earth’s overall albedo decreases, absorbing more heat — a powerful positive feedback.
⚠️ Tipping Points
A tipping point is a threshold in the climate system beyond which a change becomes self-sustaining and potentially irreversible, even if the initial forcing (e.g., GHG emissions) is reduced.
- West Antarctic Ice Sheet collapse — could raise sea levels by 3–5 metres; may be triggered at 1.5–2°C
- Greenland Ice Sheet collapse — could raise sea levels by ~7 metres over centuries
- Amazon rainforest dieback — deforestation + warming could convert Amazon from carbon sink to carbon source; “savannification”
- Permafrost thaw — Arctic permafrost contains ~1.5 trillion tonnes of carbon; thawing releases CO₂ and CH₄
- Atlantic Meridional Overturning Circulation (AMOC) slowdown — disrupts heat distribution; could cause cooling in Europe and sea level rise on US East Coast
- Coral reef die-off — mass bleaching and death of coral ecosystems at 1.5–2°C
- Boreal forest dieback — warming and drought could kill large areas of northern forests
⭐ Cascading Tipping Points: IPCC AR6 warns that tipping points can interact — triggering one may trigger others in a “cascade.” For example, permafrost thaw releases CH₄ → more warming → more ice melt → more albedo loss → more warming. This makes early action critical.
✅ Revision Checklist — Greenhouse Effect & Global Warming
✅ Natural greenhouse effect = essential for life = keeps Earth at +15°C (without it = −18°C)
✅ Enhanced greenhouse effect = human-caused = burning fossil fuels + deforestation + agriculture
✅ CO₂ = GWP 1 = reference gas = pre-industrial 280 ppm → current ~420 ppm
✅ CH₄ = GWP 27–30 (100-yr) = 80 (20-yr) = from livestock, rice paddies, landfills
✅ N₂O = GWP 273 = from fertilisers and livestock manure
✅ CFCs = highest GWP among common gases = banned under Montreal Protocol
✅ SF₆ = highest GWP of all = 23,500 = from electrical equipment
✅ Water vapour = most abundant GHG = feedback gas (not directly emitted by humans)
✅ GWP = heat trapped relative to CO₂ over 100 years
✅ CO₂e = mass × GWP = standardised unit for comparing GHGs
✅ Keeling Curve = CO₂ record since 1958 = Mauna Loa Observatory, Hawaii
✅ Paris Agreement = limit warming to well below 2°C; aim for 1.5°C
✅ Current warming = ~1.1°C above pre-industrial (IPCC AR6)
✅ 1.5°C likely reached by early 2030s at current rates
✅ Ice-albedo feedback = positive = ice melts → less reflection → more warming
✅ Water vapour feedback = strongest positive feedback
✅ Permafrost methane feedback = positive = thaw releases CO₂ + CH₄
✅ Tipping points = irreversible thresholds = WAIS, Greenland, Amazon, permafrost, AMOC
✅ Albedo = fraction of sunlight reflected = snow ~0.85; ocean ~0.06
✅ Enhanced greenhouse effect = human-caused = burning fossil fuels + deforestation + agriculture
✅ CO₂ = GWP 1 = reference gas = pre-industrial 280 ppm → current ~420 ppm
✅ CH₄ = GWP 27–30 (100-yr) = 80 (20-yr) = from livestock, rice paddies, landfills
✅ N₂O = GWP 273 = from fertilisers and livestock manure
✅ CFCs = highest GWP among common gases = banned under Montreal Protocol
✅ SF₆ = highest GWP of all = 23,500 = from electrical equipment
✅ Water vapour = most abundant GHG = feedback gas (not directly emitted by humans)
✅ GWP = heat trapped relative to CO₂ over 100 years
✅ CO₂e = mass × GWP = standardised unit for comparing GHGs
✅ Keeling Curve = CO₂ record since 1958 = Mauna Loa Observatory, Hawaii
✅ Paris Agreement = limit warming to well below 2°C; aim for 1.5°C
✅ Current warming = ~1.1°C above pre-industrial (IPCC AR6)
✅ 1.5°C likely reached by early 2030s at current rates
✅ Ice-albedo feedback = positive = ice melts → less reflection → more warming
✅ Water vapour feedback = strongest positive feedback
✅ Permafrost methane feedback = positive = thaw releases CO₂ + CH₄
✅ Tipping points = irreversible thresholds = WAIS, Greenland, Amazon, permafrost, AMOC
✅ Albedo = fraction of sunlight reflected = snow ~0.85; ocean ~0.06