BharatNotes Why this chapter matters for UPSC: The four domains — lithosphere, hydrosphere, atmosphere, and biosphere — are the conceptual pillars of physical geography and environmental science. UPSC Prelims tests atmospheric layers, ocean names and sizes, ozone layer location, greenhouse gases, and biodiversity hotspots. UPSC GS III (Environment) draws heavily on atmosphere and biosphere concepts for climate change, ozone recovery, and conservation questions.
Critical correction from standard textbooks: Many NCERT-based notes incorrectly state India has 2 biodiversity hotspots. The correct figure, per Conservation International (the defining authority), is 4 biodiversity hotspots in India — Himalaya, Western Ghats & Sri Lanka, Indo-Burma, and Sundaland (which includes the Nicobar Islands).
PART 1 — Quick Reference Tables
Four Domains of the Earth
| Domain | Definition | Key Components | % of Earth |
|---|---|---|---|
| Lithosphere | The solid rocky outer shell — Earth's crust and uppermost mantle | Continents, islands, ocean floor, mountains, plains, plateaus | ~29% land surface |
| Hydrosphere | All water on Earth in all forms | Oceans, seas, rivers, lakes, glaciers, groundwater, water vapour | ~71% ocean surface |
| Atmosphere | Layer of gases held by gravity surrounding Earth | Nitrogen 78%, Oxygen 21%, Argon 0.93%, CO₂ ~0.0423% (2024 global annual mean 422.8 ppm per NOAA GML; Mauna Loa 424.61 ppm; May 2025 seasonal peak 430.5 ppm), water vapour, trace gases | Extends to ~10,000 km |
| Biosphere | The zone of life — where the other three domains interact | All living organisms; ecosystems; from deep ocean vents to high atmosphere | Thin zone ~20 km total |
Updated CO₂ figure (May 2026): The standard textbook figure of 0.04% (400 ppm) for CO₂ is outdated. The 2024 global annual mean was 422.8 ppm (NOAA GML); Mauna Loa station recorded 424.61 ppm for 2024. The May 2025 seasonal peak reached 430.5 ppm — the first monthly average exceeding 430 ppm. Source: NOAA Global Monitoring Laboratory / Scripps Institution of Oceanography.
Seven Continents (by Area)
| Continent | Area (million km²) | % of Land | Notable Features |
|---|---|---|---|
| Asia | 44.6 | 29.5% | Largest continent; Himalayas (highest peaks), Siberia, Gobi Desert, largest population |
| Africa | 30.3 | 20.4% | Second largest; Sahara (largest hot desert), equatorial rainforests, Great Rift Valley |
| North America | 24.7 | 16.5% | Rocky Mountains, Great Plains, Mississippi-Missouri river system, Great Lakes |
| South America | 17.8 | 12.0% | Amazon Basin (largest tropical rainforest), Andes (longest mountain range), Angel Falls |
| Antarctica | 14.0 | 9.2% | Southernmost; 98% covered by ice sheet averaging ~2 km thick (over 1.8 km per NSIDC; thickest point ~4.9 km); no permanent human population; governed by Antarctic Treaty System |
| Europe | 10.5 | 7.0% | Second smallest; Alps, River Rhine; most industrialised continent historically |
| Australia | 7.7 | 5.2% | Smallest continent; entirely in Southern Hemisphere; also called "island continent" |
Memory aid (decreasing order of area): Asia, Africa, North America, South America, Antarctica, Europe, Australia — "All Animals Need Special Attention Every Afternoon"
Five Oceans (by Area)
| Ocean | Area (million km²) | Key Features |
|---|---|---|
| Pacific | ~165 | Largest (~30% of Earth's total surface); Ring of Fire volcanic arc; deepest point (Mariana Trench, ~10,935 m per NOAA 2020 measurement) |
| Atlantic | ~106 | S-shaped; Mid-Atlantic Ridge (the largest segment of the global mid-ocean ridge system — the full system at ~65,000 km is the longest mountain chain on Earth); separates Americas from Europe/Africa |
| Indian | ~70 | Third largest; warm waters; monsoon-driven circulation; critical for India's trade and security |
| Southern (Antarctic) | ~21 | Recognised by National Geographic (June 8, 2021) as 5th ocean; encircles Antarctica from 60°S to the continent; unique Antarctic Circumpolar Current |
| Arctic | ~15 | Smallest; mostly frozen (sea ice); rapidly shrinking due to climate change; Arctic warming at ~4× global average rate |
Southern Ocean — important nuance for UPSC: National Geographic officially recognised the Southern Ocean as the 5th ocean on World Oceans Day, 8 June 2021. However, the International Hydrographic Organization (IHO) — the global authority for ocean boundaries — has not yet achieved full member consensus on recognising it as a separate ocean (it was recognised in 1937 but delisted in 1953). The IHO is still deliberating. For UPSC: the answer "5 oceans" is now accepted; the Southern Ocean exists; note the IHO nuance for Mains.
Layers of the Atmosphere
| Layer | Altitude | Temperature Trend | Key Features |
|---|---|---|---|
| Troposphere | 0–~13 km average (8 km at poles, 16–18 km at equator) | Decreases with altitude (~6.5°C per km) | All weather occurs here; contains ~75% of atmospheric mass and ~99% of water vapour; tropopause is the upper boundary |
| Stratosphere | ~13–50 km | Increases with altitude | Ozone layer (~20–35 km) absorbs UV; aircraft (commercial jets) cruise in lower stratosphere; no weather turbulence; very dry |
| Mesosphere | ~50–80 km | Decreases with altitude | Coldest layer (−90°C at top); meteors burn up here (shooting stars); difficult to study (too high for aircraft, too low for satellites) |
| Thermosphere | ~80–600 km | Increases dramatically (up to 2,000°C) | Auroras (Aurora Borealis/Australis) occur here; ISS orbits at ~400 km within this layer; air is extremely thin |
| Exosphere | ~600–10,000 km | Effectively no temperature | Merges with outer space; most satellites orbit here; hydrogen and helium atoms escape to space |
Troposphere height — corrected: The commonly cited "0–12 km" is an approximation. The correct figures are: ~8 km over the poles and ~16–18 km over the equator (due to thermal expansion of warm air at the tropics). The average is approximately 13 km.
PART 2 — Detailed Notes
The Lithosphere — Earth's Rocky Shell
Lithosphere: The rigid outer shell of Earth, comprising the crust and the uppermost solid part of the mantle. It is broken into large fragments called tectonic plates that float on the semi-molten asthenosphere beneath.
Structure of the Earth (inside out):
| Layer | Depth | State | Composition |
|---|---|---|---|
| Crust | 0–30 km (continental), 0–10 km (oceanic) | Solid | Continental crust: granite (SIAL — silica + aluminium); Oceanic crust: basalt (SIMA — silica + magnesium); oceanic crust is denser but thinner |
| Mantle | 30–2,900 km | Solid but flows slowly (plastic) | Silicate rock; convection currents in the mantle drive plate movement |
| Outer Core | 2,900–5,100 km | Liquid | Iron-nickel; convection of liquid iron generates Earth's magnetic field (magnetosphere) |
| Inner Core | 5,100–6,371 km | Solid | Iron-nickel; solid despite extreme heat (~5,000–6,000°C) because of immense pressure |
Continental Drift and Plate Tectonics:
The continents were once joined as a single supercontinent called Pangaea (from Greek: pan = all; gaia = Earth). Pangaea was fully assembled approximately 300 million years ago (Early Permian, ~299–273 Mya) and began breaking up approximately 200–225 million years ago (Early Jurassic) — not 250 million years ago as sometimes cited in older textbooks.
The breakup sequence:
- Pangaea splits into Laurasia (north — today's North America, Europe, Asia) and Gondwana (south — today's South America, Africa, Antarctica, Australia, India)
- Gondwana further fragments — India broke away and drifted northward, colliding with Asia ~40–50 million years ago to form the Himalayas
- Separation is ongoing — the Atlantic Ocean continues to widen by ~2.5 cm per year
Evidence for continental drift (UPSC Mains):
- Coastlines of South America and Africa fit like puzzle pieces
- Same fossil species (Glossopteris fern; Mesosaurus reptile) found on both sides of the Atlantic
- Matching rock types and mountain chains across currently separated continents
- Paleomagnetism — ancient rocks preserve records of past magnetic pole positions
The next supercontinent: Within approximately 250 million years, the continents will reassemble into a new supercontinent (sometimes called "Amasia" or "Pangaea Proxima") as the Atlantic closes and Africa-Americas merge with Eurasia.
The Hydrosphere — Earth's Water System
Distribution of Earth's water:
| Category | % of Total Water | Detail |
|---|---|---|
| Oceans (salt water) | ~96.5% | Average salinity ~35 parts per thousand (USGS figure; older sources say 97.5% but that includes other saline sources) |
| Fresh water | ~2.5% | All remaining water |
| — Glaciers and ice caps | ~1.74% (~69% of fresh water) | Mostly Antarctic ice sheet and Greenland |
| — Groundwater | ~0.76% (~30% of fresh water) | Aquifers; recharged by rainfall |
| — Surface water (rivers, lakes) | ~0.01% (<1% of fresh water) | All rivers, lakes, swamps |
| — Atmosphere (water vapour) | Trace | Drives the water cycle |
Critical point for UPSC: Of all Earth's water, less than 1% is accessible fresh surface water (rivers, lakes) — the water that humans and most ecosystems depend on. This is why water is a critical resource and why glacial melt from the Himalayas threatens river flows across South Asia.
The Indian Ocean — strategic significance:
- Third largest ocean but most strategically important for India
- ~80% of the world's seaborne oil trade passes through the Indian Ocean
- India's trade: ~90% by volume and ~70% by value moves by sea
- India's Exclusive Economic Zone (EEZ): 2.37 million km² — rich in fisheries, minerals, energy
- Indo-Pacific concept: Links the Indian Ocean to the Western Pacific as a single strategic space — India's extended neighbourhood under SAGAR (Security and Growth for All in the Region) doctrine
The Atmosphere — Earth's Protective Blanket
Why the atmosphere is essential for life:
- Oxygen: For respiration by all aerobic organisms
- CO₂: Raw material for photosynthesis; without it, all plant life collapses
- Greenhouse effect: Traps heat → maintains average surface temperature of +15°C (without atmosphere, Earth would average −18°C — too cold for liquid water)
- Ozone layer: Absorbs UV-B radiation → prevents DNA damage, skin cancer, cataracts
- Water cycle: Evaporation, condensation, precipitation — the atmosphere is the medium through which the water cycle operates
- Protection from meteors: Most space debris burns up in the mesosphere
Atmospheric composition (updated to May 2026):
- Nitrogen (N₂): 78.09%
- Oxygen (O₂): 20.95%
- Argon (Ar): 0.93%
- Carbon dioxide (CO₂): ~422.8 ppm (2024 global annual mean, NOAA GML); 424.61 ppm (Mauna Loa 2024 annual mean); 430.5 ppm (May 2025 seasonal peak) — up from pre-industrial 280 ppm; rising at ~2.5–3 ppm/year
- Water vapour (H₂O): 0–4% (variable)
- Methane (CH₄), Nitrous oxide (N₂O), ozone (O₃), and other trace gases: remainder
Greenhouse gases and their sources:
| Gas | Main Human Sources | Global Warming Potential (100-yr) |
|---|---|---|
| CO₂ | Fossil fuels, deforestation | 1 (baseline) |
| CH₄ (Methane) | Livestock, rice paddies, landfills, natural gas leaks | ~28–34 |
| N₂O (Nitrous Oxide) | Fertilisers, combustion | ~265–298 |
| HFCs | Refrigerants (replacement for CFCs) | Up to 14,800 |
| SF₆ | Electrical switchgear | ~23,500 |
UPSC GS III: India is the 3rd largest emitter of CO₂ in absolute terms (after China and the USA), but has per-capita emissions far below the global average. India's NDC (Nationally Determined Contribution) under the Paris Agreement commits to: 45% reduction in emission intensity of GDP by 2030 (vs 2005); 50% of cumulative electric power from non-fossil sources by 2030.
The Ozone Layer — Latest Status (May 2026):
Ozone (O₃): An allotrope of oxygen — three oxygen atoms bonded together. Concentrated mainly in the stratosphere (15–35 km) where it forms the "ozone layer" — Earth's UV shield, with peak concentration at 20–25 km altitude.
Ozone depletion — mechanism:
- Chlorofluorocarbons (CFCs) and other halocarbons drift into the stratosphere
- UV breaks them down, releasing chlorine atoms
- Each chlorine atom can destroy up to 100,000 ozone molecules catalytically
- This thinning is most severe over Antarctica each spring (September–November) — the "ozone hole"
Montreal Protocol (1987):
- International treaty to phase out ozone-depleting substances (ODS)
- Has now achieved the phase-out of over 99% of controlled ODS production globally
- Considered the most successful international environmental treaty ever
Recovery status (2025):
- The 2025 Antarctic ozone hole was the 5th smallest since 1992 (when Montreal Protocol began taking effect)
- 2025 hole closed on 1 December — earliest closure since 2019
- Ozone layer projected to recover to 1980 levels:
- Tropics and mid-latitudes: by ~2040
- Arctic: by ~2045
- Antarctica: by ~2066
- March 2025 MIT study confirmed: ozone recovery is primarily attributable to CFC reductions — direct proof the Montreal Protocol worked
Kigali Amendment (2016) to Montreal Protocol:
- Extended the Protocol to phase down HFCs (Hydrofluorocarbons) — used as CFC replacements in refrigeration/air conditioning
- HFCs don't deplete ozone but are powerful greenhouse gases (up to 14,800× CO₂)
- India ratified the Kigali Amendment in 2021
Tropospheric ozone vs stratospheric ozone — an important distinction:
- Stratospheric ozone: Beneficial — shields life from UV-B; being recovered under Montreal Protocol
- Tropospheric ozone (ground-level): Harmful — a pollutant formed when NOₓ and VOCs react in sunlight; damages lungs and crops; a major air quality problem in Indian cities
The Biosphere — The Zone of Life
Biosphere: The global ecological system encompassing all living organisms and their interactions with the other three domains. It extends from the deepest ocean trenches (~11 km below sea level — home to chemosynthetic bacteria at hydrothermal vents) to the upper atmosphere (~10 km — where spores and microorganisms are found).
India's Biodiversity — corrected facts for UPSC:
India's 4 Biodiversity Hotspots (Conservation International definition — a hotspot must have ≥1,500 endemic vascular plant species AND have lost ≥70% of its original habitat):
| Hotspot | India's Portion | Key Species |
|---|---|---|
| Himalaya | Entire Indian Himalayan region (north of Indo-Gangetic Plain) | Snow leopard, red panda, Himalayan brown bear; 10,000+ plant species, ~3,160 endemic |
| Western Ghats & Sri Lanka | Western Ghats mountain chain (~160,000 km²) | Lion-tailed macaque, Nilgiri tahr, purple frog; 508 bird species, 131 amphibian species (87% endemic) |
| Indo-Burma | Northeast India south of Brahmaputra (Meghalaya, Manipur, Mizoram, etc.) | Hoolock gibbon (India's only ape), Sangai deer (brow-antlered deer); large freshwater fish diversity |
| Sundaland | Nicobar Islands (the Andaman Islands are NOT part of this hotspot) | Nicobar megapode, Nicobar flying fox; shared with SE Asia |
Critical correction: Many textbooks and coaching notes still cite only 2 hotspots for India (Western Ghats and Eastern Himalayas). The correct, current answer per Conservation International is 4 hotspots. This distinction has appeared in UPSC mains.
India's overall biodiversity status:
- One of 17 megadiverse countries — together these 17 countries hold ~70% of the world's biodiversity
- India has 2.4% of Earth's land area but accounts for ~7–8% of all known species
- ~1,04,561 species of animals documented (per MoEF's 2023 fauna checklist — the world's first complete national fauna inventory) and ~45,500 species of plants
- High endemism: 55.8% of amphibians, 45.8% of reptiles, 33% of plants found nowhere else on Earth
- India ranks 3rd globally for reptile diversity (889 species)
PART 3 — UPSC Enrichment
Analytical Dimensions — Mains Answer Writing
Q: "The atmosphere acts as both a shield and a blanket for life on Earth. Critically examine how human activities are altering both these functions."
Structure:
Shield function (ozone layer):
- Stratospheric ozone absorbs UV-B → protects DNA, prevents skin cancer, cataracts, immune suppression
- CFCs (refrigerants, aerosols) depleted the ozone layer → ozone hole over Antarctica
- Montreal Protocol (1987) + Kigali Amendment (2016) → ODS phase-out → ozone layer is recovering
- India's role: ratified Kigali Amendment 2021; HFC phase-down under implementation
Blanket function (greenhouse effect):
- Natural greenhouse gases (CO₂, CH₄, H₂O vapour) maintain Earth's temperature at +15°C
- Human fossil fuel burning → CO₂ rose from 280 ppm (pre-industrial) to 427 ppm (2025) — a 52% increase
- Enhanced greenhouse effect → global warming → 1.1°C above pre-industrial average (as of 2023–24)
- Consequences for India: monsoon disruption, Himalayan glacial melt, sea level rise (threatens Mumbai, Kolkata, Chennai), extreme weather events
Conclusion: The Montreal Protocol shows international cooperation can fix an atmospheric problem. The Paris Agreement attempts the same for climate change — but is less binding, less comprehensive, and the CO₂ trend shows it is insufficient so far.
Q: "India's vast biodiversity is both a strength and a responsibility. Discuss with reference to India's international commitments."
Structure:
- India's biodiversity strength: 4 hotspots, megadiverse country, 7–8% of global species in 2.4% of land
- Threats: habitat destruction, invasive species, climate change, human-wildlife conflict
- Protected area network: 107 National Parks (April 2025), 573 Wildlife Sanctuaries, 18 Biosphere Reserves (13 in UNESCO MAB)
- International commitments: CBD (Convention on Biological Diversity), Nagoya Protocol on access and benefit-sharing, Kunming-Montreal Global Biodiversity Framework (2022) — "30×30" target (protect 30% of land and ocean by 2030)
- India's targets under Kunming-Montreal: restore 30% of degraded ecosystems; reduce invasive species by 50%; align biodiversity into sectoral policies
Key Treaties, Bodies & Policies
| Item | Relevance |
|---|---|
| Montreal Protocol (1987) | Phase-out of ODS; most successful environmental treaty; ozone layer recovering |
| Kigali Amendment (2016) | Phase-down of HFCs under Montreal Protocol; India ratified 2021 |
| Paris Agreement (2015) | Limit global warming to 1.5–2°C; India's NDC: 45% emission intensity reduction by 2030, 50% non-fossil power by 2030 |
| CBD (Convention on Biological Diversity) | International treaty on biodiversity conservation; India is a party |
| Kunming-Montreal Framework (2022) | "30×30" — protect 30% of land and ocean by 2030; successor to Aichi Targets |
| Antarctic Treaty System (1959) | Governs Antarctica; bans military activity and mineral extraction; India is a consultative party (since 1983); India has two research stations: Maitri and Bharati |
| SAGAR doctrine | India's maritime security framework for the Indian Ocean — Security and Growth for All in the Region; PM Modi, 2015 |
High-Yield Prelims Facts Checklist
| Fact | Answer |
|---|---|
| Largest continent | Asia (44.6 million km²) |
| Smallest continent | Australia (7.7 million km²) |
| Largest ocean | Pacific |
| Smallest ocean | Arctic |
| Ocean most important for India | Indian Ocean |
| Deepest point on Earth | Mariana Trench, Pacific (~10,935 m per NOAA 2020; older figure of 11,034 m is superseded) |
| Southern Ocean recognised by National Geographic | June 8, 2021 (World Oceans Day) |
| Southern Ocean IHO status | Not yet officially recognised by IHO |
| Fresh water % of total | ~2.5% |
| Fresh water in glaciers/ice | ~69% of all fresh water |
| Accessible surface fresh water | <1% of all water |
| All weather occurs in | Troposphere |
| Ozone layer located in | Stratosphere (15–35 km; peak concentration at 20–25 km) |
| Meteors burn up in | Mesosphere |
| Auroras occur in | Thermosphere |
| ISS altitude | ~400 km (Thermosphere) |
| Troposphere height at equator | 16–18 km |
| Troposphere height at poles | ~8 km |
| Troposphere height average | ~13 km |
| CO₂ concentration (2024 annual mean) | 422.8 ppm (NOAA global); 424.61 ppm (Mauna Loa); May 2025 seasonal peak: 430.5 ppm (first monthly avg >430 ppm) |
| Pre-industrial CO₂ | ~280 ppm |
| N₂ in atmosphere | 78.09% |
| O₂ in atmosphere | 20.95% |
| Pangaea fully assembled | ~300 million years ago (Early Permian) |
| Pangaea began breaking up | ~200–225 million years ago (Early Jurassic) |
| India's biodiversity hotspots | 4 — Himalaya, Western Ghats & Sri Lanka, Indo-Burma, Sundaland |
| Global biodiversity hotspots total | 36 |
| India as megadiverse country | One of 17 megadiverse countries |
| India's % of global species | ~7–8% (in 2.4% of land) |
| Ozone hole location | Over Antarctica (South Pole) |
| Montreal Protocol year | 1987 |
| Ozone full recovery projected | 2040 (tropics/mid-lat), 2045 (Arctic), 2066 (Antarctica) |
| India's research stations in Antarctica | Maitri and Bharati |
| India joined Antarctic Treaty as consultative party | 1983 |
Exam Strategy
Prelims traps:
- India has 4 biodiversity hotspots, NOT 2 — Western Ghats, Himalaya, Indo-Burma, Sundaland
- Ozone layer is in the stratosphere — NOT the troposphere; ground-level ozone is a pollutant in the troposphere (two different things)
- Meteors burn in the mesosphere — NOT thermosphere or stratosphere
- ISS is in the thermosphere — NOT the exosphere
- Southern Ocean — National Geographic recognises it as 5th; IHO has not given full official status
- Pangaea broke up ~200–225 million years ago — not 250 million years ago
- CO₂ 2024 annual mean: 422.8 ppm (NOAA global) / 424.61 ppm (Mauna Loa); May 2025 peak 430.5 ppm — the textbook figure of "0.04%" (400 ppm) is outdated
- Troposphere average height is ~13 km — not 12 km; varies 8 km (poles) to 18 km (equator)
- Smallest continent is Australia — Europe is the second smallest; a common confusion
- Mariana Trench (Pacific) is the deepest — NOT in the Indian Ocean
Mains topics from this chapter:
- Ozone layer depletion and recovery — Montreal Protocol and Kigali Amendment
- Greenhouse effect and climate change — India's NDC and Paris Agreement
- India's biodiversity — hotspots, megadiversity, Kunming-Montreal Framework
- Indian Ocean — strategic importance, Indo-Pacific, SAGAR doctrine
- Continental drift and plate tectonics — formation of the Himalayas
Practice Questions
Prelims:
The ozone layer that protects Earth from UV radiation is located in which layer of the atmosphere? (a) Troposphere (b) Stratosphere (c) Mesosphere (d) Thermosphere
Which ocean is the largest in the world? (a) Pacific Ocean (b) Atlantic Ocean (c) Indian Ocean (d) Arctic Ocean
Approximately what percentage of Earth's total water is fresh water? (a) 30% (b) 10% (c) ~2.5% (d) 50%
All weather phenomena like rain, storms, and cyclones occur in which layer of the atmosphere? (a) Troposphere (b) Stratosphere (c) Mesosphere (d) Thermosphere
How many biodiversity hotspots does India have, according to Conservation International? (a) 2 (b) 3 (c) 4 (d) 6
The International Space Station (ISS) orbits in which layer of the atmosphere? (a) Stratosphere (b) Mesosphere (c) Thermosphere (d) Exosphere
The Kigali Amendment (2016) to the Montreal Protocol addresses which substance? (a) CFCs (Chlorofluorocarbons) (b) HFCs (Hydrofluorocarbons) (c) Carbon dioxide (d) Nitrous oxide
Which of the following is NOT one of India's four biodiversity hotspots? (a) Western Ghats (b) Indo-Burma (c) Thar Desert (d) Himalaya
