Class 9 › Science › Chapter 13

Earth as a System: Energy, Matter, and Life

The five spheres of the Earth, solar radiation and uneven heating, the atmosphere, winds and ocean currents, the water, carbon, nitrogen and oxygen cycles, and human impact on them.

1. Earth as a System: Five Spheres

Earth system: life on Earth is powered by a constant flow of energy and matter. The Sun is the main energy source; the Earth's hot interior and chemical reactions in air, water and rocks also drive flows. The Earth is one system of interacting 'spheres'.

A change in one sphere changes the others.

Sphere	What it includes	Examples
Geosphere	Solid rocks, soil, landforms, Earth's interior	Deccan plateau, Thar desert
Hydrosphere	Liquid water: surface water and groundwater	Oceans, Ganga–Brahmaputra river system, lakes
Cryosphere	Solid water: ice and snow	Himalayan glaciers, polar ice caps
Atmosphere	The air surrounding the Earth	Air in mountains and forests
Biosphere	All living organisms and their habitats	Mangroves, forests, coral reefs, plankton

Interconnected example: less winter snowfall (cryosphere) → less lake water in summer (hydrosphere) → less grass for sheep (biosphere).
Large-scale example: warmer Arabian Sea → more evaporation → variable southwest monsoon (floods in some places, drought in others); warmer air → melting glaciers and polar ice → sea level rise → coastal cities threatened and habitat loss.

2. Solar Radiation and Insolation

Solar radiation reaches the Earth as electromagnetic (EM) waves, which travel through a vacuum at the speed of light, 3 × 10⁸ m s⁻¹. (Sound waves are mechanical and need a medium.)
High-frequency / short-wavelength EM waves (gamma rays, X-rays) have very high energy and are harmful to life; they are mostly filtered by the upper atmosphere.

Electromagnetic spectrum; solar radiation is mainly UV, visible and IR.

Radiation	What happens to it
UV (100–400 nm)	Mostly absorbed by the ozone layer, which protects life and gives some atmospheric heating. Overexposure damages eyes and skin and raises cancer risk; useful for killing germs in water purifiers.
Visible	Reaches the surface; gives energy for photosynthesis; partly warms land and water.
Infrared	Warms the surface, which re-radiates heat back; part is trapped by greenhouse gases (CO₂, CH₄, water vapour), keeping Earth warm enough for life.
Gamma, X-rays	Mostly filtered by the upper atmosphere.
Microwaves, radio	Carry very little energy to warm the Earth significantly.

Insolation

Incoming solar radiation

The amount of the Sun's radiation that reaches the Earth's surface; it warms the surface and atmosphere. Maximum about 1 kW m⁻² under clear sky.

Solar constant

At the top of the atmosphere

Average solar energy per unit time per unit area, perpendicular to the Sun's rays, at the top of the atmosphere: about 1.4 kW m⁻² (1400 J s⁻¹ m⁻²), before any absorption, scattering or reflection.

Solar energy received E = Intensity × Area × Time

Example 13.1

Q. Solar energy received by a 1 m² area in one hour if insolation is 1 kW m⁻²?

Solution: E = 1000 J s⁻¹ m⁻² × 1 m² × 3600 s = 3,600,000 J = 3.6 × 10⁶ J (equal to one household electricity 'unit').

Insolation is lower than the solar constant because gases, clouds and dust absorb and scatter some energy.
India is in tropical and sub-tropical latitudes, so it gets abundant sunlight, which drives the southwest monsoon and offers great solar-power potential (even part of the Thar desert could meet India's electricity needs).
Anna Mani mapped India's insolation and co-published Solar Radiation Over India (1982), the first insolation atlas.

3. Albedo and Surface Heating

Albedo: the fraction of solar radiation reflected by a surface (Latin albedo = whiteness). High albedo → reflects more, stays cool. Low albedo → absorbs more, heats quickly.

Surface	Albedo	Effect
Snow	0.80–0.90	Reflects most radiation; polar regions very cold
Ice	0.50–0.70	High albedo
Crushed rock	0.25–0.30	Moderate
Black soil, ocean water	Low	Absorb more, relatively warmer

Land heats faster than water; dark roads and dark clothes get hotter than light-coloured ones.
All objects re-radiate heat: a concrete house stays hot at night, while thick mud-and-wood houses stay cooler due to less re-radiation.
Urban heat island effect: cities are warmer than nearby rural areas (especially in summer and at night) because steel, concrete, brick and asphalt absorb and retain heat; rural areas and forests stay cool through shade and transpiration. It raises air-conditioning demand.

4. Latitude and the Earth's Shape

The Earth is spherical, so the Sun's rays strike different latitudes at different angles.
Equator: radiation is concentrated over a smaller area → warm all year. Poles: the same radiation is spread over a larger area → much colder.
Earth's tilt together with its shape causes seasons and changing day length.
This uneven heating creates temperature differences that drive global winds and ocean currents.
If the Earth were a flat disc, rays would strike every place at the same angle, so there would be no equator-to-pole temperature gradient of this kind.

5. Role of the Atmosphere

Held by gravity; composition: nitrogen 78%, oxygen 21%, plus small amounts of argon, carbon dioxide, water vapour and other gases.

Layers of the atmosphere (not to scale). Outer space begins at about 100 km.

Troposphere

0–12 km

Nearly all weather happens here; heated from the Earth's surface. Temperature decreases with height (~6.5 °C/km); rising warm air drives winds and storms. Tallest over the equator, lowest over the poles.

Stratosphere

12–50 km

Contains the ozone layer, which absorbs UV and heats the air, so temperature increases with height. This stops vertical mixing and keeps weather confined to the troposphere.

Two crucial roles of the atmosphere

1. Absorbs part of the incoming radiation: the ozone layer blocks harmful UV; clouds and gases absorb some sunlight before it reaches the surface.

2. Traps outgoing heat: the surface re-radiates in the infrared; greenhouse gases (CO₂, CH₄, water vapour) absorb it. Without the atmosphere Earth would be too cold for life.

Excess CO₂ from human activities strengthens the greenhouse effect and causes global warming.
Venus is hotter than Mercury (though farther from the Sun) because its atmosphere causes an uncontrolled greenhouse effect.
Ozone layer: when ozone is destroyed faster than formed the layer thins. CFCs (refrigerators, aerosols) caused the Antarctic ozone hole; the Montreal Protocol cut CFC use and the layer is slowly recovering.
K.R. Ramanathan measured ozone from 18,000 ft in the Himalayas (1934) and found it lower than expected; later led early monsoon forecasting.

6. Uneven Heating Causes Winds

Wind: movement of air from a region of high pressure to a region of low pressure. Pressure differences are mainly caused by uneven heating of the Earth's surface.

Valley breeze (day)

Up the slope

Slopes facing the Sun heat faster than the valley floor; air over the slopes warms and rises (low pressure) and cooler valley air moves up the slope.

Mountain breeze (night)

Down the slope

After sunset slopes cool faster; cooler, denser air flows down into the valley. Seen in Shimla, Dehradun and other Himalayan valleys.

Local winds affect weather, agriculture and daily life, and help regulate temperature and moisture in mountains.
Barren rock slopes heat and cool more strongly than grass-covered slopes (lower albedo of vegetation helps moderate it), so breeze temperatures can differ between the two mountains.

Planetary winds

Latitude	Pressure belt	Air motion
0° (equator)	Equatorial low	Intense heating, warm air rises, then moves poleward at height
About 30° N and S	Sub-tropical high	Cooled air becomes denser and sinks; surface air flows back to the equator (one circulation cycle); some moves toward the poles
About 60° N and S	Sub-polar low	Air from the sub-tropics meets cold polar air and rises
About 90° N and S	Polar high	Very cold, dense air sinks and flows toward the sub-polar belts

Earth's rotation deflects winds: right in the Northern Hemisphere, left in the Southern Hemisphere, so planetary winds follow curved paths.

7. Ocean Currents

Ocean currents: continuous movement of large masses of ocean water.

Drivers: strong planetary winds drag surface water by friction; differences in temperature and salinity; Earth's rotation; the distribution of land masses.
Vertical pattern: warm equatorial water flows over the surface toward the poles; colder, denser water flows back toward the equator at depth. Low-salinity water stays near the surface; high-salinity, denser water sinks.
Gyres: large circular patterns formed as rotation deflects moving water: clockwise in the Northern Hemisphere, counter-clockwise in the Southern Hemisphere. Continents block and redirect currents.
Importance: carry heat from the equator to the poles (reducing temperature differences) and transport nutrients that support marine ecosystems. The warm North Atlantic Drift (extension of the Gulf Stream) keeps many ports of north-western Europe ice-free in winter.
IITM, Pune models the Indian monsoon by coupling energy flows between atmosphere, ocean, land and ice.

8. Biogeochemical Cycles: Water Cycle

Biogeochemical cycle: the cyclic movement of matter and energy between the abiotic and biotic components of the Earth, so that nutrients such as carbon, nitrogen and oxygen are recycled and stay available for life. It helps ecosystems recover from disturbance.

Water cycle

1. Evaporation from oceans, lakes and rivers, and transpiration from plants.

2. Condensation to form clouds.

3. Precipitation (rain, hail, snow); water flows back to the ocean as run-off, or seeps in (infiltration) to become groundwater.

Water dissolves minerals from soil and rocks and carries nutrients to the oceans to support marine life.
Climate change effects: warmer air holds more moisture, giving heavier rain in some places (intensified monsoon) and drought elsewhere; melting glaciers raise sea level (threat to Mumbai, Chennai); intense bursts of rain increase run-off and soil erosion and reduce infiltration, so groundwater recharge falls and farming becomes harder in dry months.
The water cycle links the cryosphere, hydrosphere, atmosphere, geosphere and biosphere.

9. Carbon Cycle

Carbon is the backbone of life (every protein, carbohydrate, fat and DNA molecule). It circulates among the atmosphere (CO₂), biosphere, geosphere (carbonate rocks, fossil fuels) and hydrosphere (dissolved CO₂, marine shells).

	Fast cycle (days to years)	Slow cycle (millions of years)
Process	Plants convert atmospheric CO₂ to glucose by photosynthesis; CO₂ returns by respiration and decomposition	Dead plants and animals are buried and become fossil fuels (coal, oil, gas)
Return to air	Respiration, decay	Burning fossil fuels releases CO₂ in a very short time

Oceans: exchange CO₂ with air; absorbed CO₂ forms carbonate and bicarbonate ions, used by phytoplankton for photosynthesis and by some organisms to make shells. When organisms die and sink, carbon is stored for a long time.
Numbers: carbon is ~49% of the dry weight of living organisms; 71% of global carbon is in the oceans; the atmosphere holds only ~1%.
Human impact: burning fossil fuels and deforestation raised atmospheric CO₂ by about 35% since 1960 (315 ppm to 420 ppm), intensifying the greenhouse effect: global warming, melting of glaciers and Arctic sea ice, sea level rise and extreme weather.
Some CO₂ is necessary to keep Earth warm, but the balance is critical.

10. Nitrogen Cycle

Nitrogen is essential for proteins and nucleic acids. The largest reservoir is the atmosphere (N₂), but N₂ is non-reactive and cannot be used directly by plants and animals; it must first be converted to soluble compounds.

Nitrogen cycle: fixation, nitrification, assimilation, ammonification, denitrification.

Step	What happens	Organisms
Nitrogen fixation	Atmospheric N₂ → ammonia (NH₃); lightning also fixes nitrogen oxides	Rhizobium (root nodules of legumes), Azotobacter (soil)
Nitrification	Ammonia → nitrite (NO₂⁻) → nitrate (NO₃⁻)	Nitrosomonas (to nitrite), Nitrobacter (to nitrate)
Assimilation	Plants take up nitrogen compounds from soil; animals get nitrogen by eating plants or other animals	Plants, animals
Ammonification	Dead matter and wastes are broken down, returning ammonia to the soil	Decomposers: bacteria, fungi
Denitrification	Nitrates → nitrogen gas, completing the cycle	Pseudomonas

Haber–Bosch process (early 1900s): makes ammonia artificially from atmospheric nitrogen; most fertilisers are made this way ('Bread from Air'); enabled India's Green Revolution. It uses about 1–2% of global energy, and fertiliser overuse degrades soil and water.
If nitrogen were not cycled, plants could not get usable nitrogen, so proteins and nucleic acids could not be made and life would be affected.

11. Oxygen Cycle

Oxygen is one of the most abundant elements; 21% of the atmosphere is free O₂. It is part of carbohydrates, proteins, nucleic acids and fats, and also occurs in metal oxides and minerals in the crust and as CO₂ in air.

Uses oxygen

Consumption

Respiration (plants and animals) and combustion of fuels use O₂ and release CO₂; oxide formation also uses O₂.

Restores oxygen

Production

Photosynthesis uses sunlight, water and CO₂ to form glucose and release O₂.

The balance between consumption and production circulates oxygen between atmosphere, land, oceans and organisms. If photosynthesis stopped, oxygen would run out and food chains would collapse.

12. Human Impact on Earth's Processes

Human activity	Effect
Burning fossil fuels, deforestation	More CO₂; stronger greenhouse effect; global warming; saturates natural carbon sinks (forests, oceans)
Excess CO₂ absorbed by oceans	Ocean becomes more acidic; threatens plankton and coral reefs. Warmer water also absorbs less CO₂ (weaker carbon sink)
Overuse of fertilisers	Nitrates reach rivers and lakes; algal blooms deplete oxygen and kill fish: eutrophication
Deforestation	Less photosynthesis and transpiration (less local rainfall), altered albedo, soil erosion, habitat and biodiversity loss
Vehicle emissions	React with sunlight to form smog and harmful ground-level ozone

Stratospheric ozone protects life; ground-level ozone is harmful to health.
Global cooperation: the Montreal Protocol began the recovery of the ozone layer; the Kyoto Protocol and Paris Agreement (CO₂ cuts) have been less successful.
What helps: conserve energy, switch to renewables (solar, wind), plant trees, save water, practise sustainable farming, reduce, reuse and recycle. India has planted billions of trees and expanded solar and renewable energy.
Mission LiFE (Lifestyle for Environment, introduced in 2021) encourages mindful, eco-friendly lifestyles.

13. Quick Revision

Concept	Key point
Five spheres	Geo, Hydro, Cryo, Atmo, Bio
Insolation vs solar constant	Reaches the surface (~1 kW m⁻²) vs top of atmosphere (~1.4 kW m⁻²)
Albedo	Fraction reflected; snow high, ocean and black soil low
Troposphere / stratosphere	0–12 km, temperature falls / 12–50 km, ozone layer, temperature rises
Greenhouse gases	CO₂, CH₄, water vapour trap outgoing infrared heat
Planetary wind deflection	Right in Northern, left in Southern Hemisphere
Gyres	Clockwise (N), counter-clockwise (S)
Nitrogen cycle steps	Fixation, nitrification, assimilation, ammonification, denitrification
Eutrophication	Fertiliser nitrates → algal bloom → oxygen depletion → fish die

Textbook MCQs

Answer key

Q1 (role of biogeochemical cycles): (ii) recycle essential nutrients between biotic and abiotic components. Q2 (what warms the Earth): (iii) surface absorbs solar radiation, which is re-radiated and trapped by greenhouse gases.

Remember

Key numbers

Speed of light 3 × 10⁸ m s⁻¹ · UV 100–400 nm · N₂ 78%, O₂ 21% · 6.5 °C/km · CO₂ 315 → 420 ppm · 71% of carbon in oceans.

Common mistakes:
• Saying the atmosphere heats the Earth directly; the surface absorbs sunlight and re-radiates infrared that greenhouse gases trap.
• Mixing up insolation (at the surface) and solar constant (top of atmosphere).
• Saying temperature always falls with height; it rises in the stratosphere.
• Saying planetary winds blow straight from high to low pressure; rotation deflects them.
• Confusing nitrification (ammonia → nitrite → nitrate) with denitrification (nitrate → N₂).
• Saying CO₂ is harmful in any amount; the balance is what matters.
• Confusing stratospheric (protective) ozone with ground-level (harmful) ozone.