Environment

Environment: Everything surrounding an organism — the air, water, soil, other living beings, and human-made structures. The word derives from the French environ (around). More precisely, it is the sum total of all conditions (biotic + abiotic + social) that affect the existence, growth, and development of an organism.

Two major types:

• Natural environment: All elements created by nature — lithosphere, hydrosphere, atmosphere, biosphere; exists independent of human action
• Human-made (built/anthropogenic) environment: Structures and conditions created by humans — cities, roads, factories, farms, canals; increasingly dominates Earth's surface

Why the distinction matters: Many environmental problems occur when the human-made environment disrupts natural systems — pollution enters the hydrosphere; land use change destroys the biosphere; emissions alter the atmosphere.

Lithosphere — the land domain:

• Composed of Earth's crust (continental: ~30–50 km thick; oceanic: ~6–7 km thick) + the solid uppermost mantle
• Total lithosphere thickness: averages ~100 km (varies from 15 km at mid-ocean ridges to 300 km under old continental cratons)
• Divided into tectonic plates that move → earthquakes, volcanoes, mountain building
• Functions: reservoir of minerals and soil; foundation for terrestrial life; stores fossil fuels, groundwater

Hydrosphere — the water domain:

• ~71% of Earth's surface is covered by water
• Distribution: ~96.5% in oceans (saline); ~2.5% freshwater; of freshwater — ~69% locked in glaciers/ice caps, ~30% groundwater, <1% in lakes/rivers accessible to humans
• India's water challenge: 4% of world's freshwater but 18% of world's population

Atmosphere — the air domain:

• Composition (dry air): Nitrogen 78.09%, Oxygen 20.95%, Argon 0.93%, CO₂ ~0.0430% (~430 ppm; NOAA 2025 seasonal peak exceeded 430 ppm for first time; 2024 annual avg: ~424.6 ppm)
• Layers from surface upward: Troposphere (0–~13 km avg; all weather occurs here) → Stratosphere (13–50 km; ozone layer at 15–35 km) → Mesosphere → Thermosphere → Exosphere
• Greenhouse effect: CO₂, CH₄, N₂O, H₂O, O₃ trap outgoing infrared radiation → warm Earth ~33°C above what it would otherwise be (without greenhouse effect, Earth's avg temp = -18°C)

Biosphere — the life domain:

• Extends from deep ocean trenches (organisms at ~11 km below surface in Mariana Trench) to high altitudes (~6 km above sea level for most active life; microbial spores detected up to ~41 km)
• All life exists here; no life has ever been confirmed beyond Earth's biosphere
• The biosphere is the thinnest sphere — a film of life on a planet, proportionally as thin as the skin of an apple

Ecosystem: A functional unit in nature comprising biotic components (all living organisms) + abiotic components (non-living physical environment) that interact with each other through energy flow and nutrient cycling.

Scale: Ecosystems exist at every scale — a dew drop, a leaf, a pond, a forest, an ocean, the entire biosphere (sometimes called the global ecosystem).

Trophic structure (energy flow):

• Producers (autotrophs): Plants, algae, cyanobacteria — convert sunlight into food via photosynthesis; base of all food chains
• Primary consumers (herbivores): Eat producers — deer, cattle, caterpillars
• Secondary consumers (carnivores): Eat herbivores — frogs, foxes, smaller fish
• Tertiary consumers (apex predators): Tigers, sharks, eagles — at top of food chain
• Decomposers: Bacteria and fungi — break down dead organic matter → return nutrients to soil/water; the "recyclers" of the ecosystem

10% energy rule (Lindemann's Law): Only ~10% of energy transfers from one trophic level to the next → explains why fewer top predators can be supported; why vegetarian diets use less land/energy.

Nutrient cycling: Unlike energy (which flows through and is lost as heat), nutrients (N, P, C, S) cycle within the ecosystem — decomposers are critical for returning them to soil for plant uptake.

Anthropocene: A proposed geological epoch (not yet officially ratified by IUGS) starting ~1950 in which human activity has become the dominant influence on Earth's geology and ecosystems. Human impact is now equivalent to a geological force.

Key principle: Humans are not separate from nature — they are a part of the biosphere. What we do to the environment, we ultimately do to ourselves. Environmental justice = human justice.

Sustainable Development (Brundtland Definition, 1987): "Development that meets the needs of the present without compromising the ability of future generations to meet their own needs." — Our Common Future, World Commission on Environment and Development (WCED), October 1987.

Three pillars (Triple Bottom Line):

• Economic: Growth and poverty alleviation
• Social: Equity and human well-being
• Environmental: Ecological integrity and resource conservation

These three must be balanced — development that is economically profitable but environmentally destructive is not sustainable.

Key milestones:

India and sustainable development:

• India's GDP growth vs. environmental cost: Fastest-growing major economy but ranked 176/180 in Environmental Performance Index (EPI 2022)
• India's commitment: Net-zero by 2070; 500 GW renewable energy by 2030; 50% energy from non-fossil sources by 2030 (updated NDC)
• Green GDP: Government exploring natural capital accounting to reflect true cost of growth