Science · Class 9 · Chapter 13

Earth as a System: Energy, Matter, and Life

• Geosphere / Hydrosphere / Cryosphere — Solid earth / liquid water / frozen water.
• Insolation — Solar radiation reaching the Earth's surface.
• Albedo — Fraction of sunlight a surface reflects.
• Greenhouse effect — Atmosphere trapping outgoing heat, warming Earth.
• Planetary winds — Large-scale winds from global pressure belts.
• Ocean gyre — Large circular ocean current pattern.
• Biogeochemical cycle — Cyclic movement of matter between living and non-living systems.
• Eutrophication — Excess nutrients causing algal blooms that deplete oxygen.

Chapter 13 · Concept 1 of 19

The Earth's Five Spheres

• Sphere | What it is
• Geosphere — Solid rocks, soil, and landforms — and the Earth's interior
• Hydrosphere — Liquid water — oceans, rivers, lakes, and groundwater
• Cryosphere — Frozen water — glaciers, snow, and polar ice caps
• Atmosphere — The air surrounding the Earth
• Biosphere — All living organisms and their habitats

Chapter 13 · Concept 2 of 19

The Spheres Are Interconnected

• Less snowfall (cryosphere) means less water reaches the lake in summer (hydrosphere) — leaving less grass for grazing animals (biosphere).
• At a larger scale — A warmer Arabian Sea (hydrosphere) increases evaporation, disrupting the monsoon (atmosphere) — bringing floods to some regions and drought to others, while warming accelerates glacier melt (cryosphere), threatening coastal cities and habitats (biosphere).

Chapter 13 · Concept 3 of 19

Solar Radiation and the EM Spectrum

• About 99% of the Sun's energy reaching Earth falls within the ultraviolet, visible, and infrared range.
• Band | What it does
• Ultraviolet (UV) — Mostly absorbed by the ozone layer, protecting life
• Visible light — Powers photosynthesis; partly warms land and water
• Infrared (IR) — Warms the surface, which re-radiates heat — some trapped by greenhouse gases

Chapter 13 · Concept 4 of 19

Insolation and the Solar Constant

• Solar constant ≈ 1.4 kW/m² — The Sun's energy reaching the top of Earth's atmosphere, before any absorption or scattering. At the surface, under clear skies, insolation is closer to 1 kW/m².
• Example 13.1 — Energy on 1 m² in 1 hour at 1 kW/m²: E = 1000 J/s/m² × 1 m² × 3600 s E = 3.6 × 10⁶ J — enough to melt 5 kg of ice and heat the water to 100°C.

Chapter 13 · Concept 5 of 19

Albedo

• Albedo is the fraction of solar radiation a surface reflects. High albedo means cooler; low albedo means warmer.
• Surface | Approximate albedo
• Snow — 0.80 – 0.90 (reflects most light — stays cold)
• Ice — 0.50 – 0.70
• Black soil / ocean water — Low — absorbs more, warms up faster

Chapter 13 · Concept 6 of 19

The Urban Heat Island Effect

• Concrete, steel, and asphalt absorb solar radiation and re-radiate heat, especially at night.
• Cities run warmer than the countryside — Rural areas and forests stay cooler through shade and plant transpiration. Cities, with more built-up surfaces and less vegetation, trap more heat — raising energy demand for cooling.

Chapter 13 · Concept 7 of 19

Latitude and the Earth's Shape

• Because Earth is a sphere, sunlight hits different latitudes at different angles.
• Concentrated at the equator, spread at the poles — Equatorial sunlight is concentrated over a small area, keeping it warm year-round. Polar sunlight spreads over a much larger area, keeping it cold — this temperature difference drives global winds and ocean currents.

Chapter 13 · Concept 8 of 19

Layers of the Atmosphere

• Layer | Altitude | Key feature
• Troposphere — 0 – 12 km — Weather forms here; temperature falls with height
• Stratosphere — 12 – 50 km — Ozone layer absorbs UV; temperature rises with height

Chapter 13 · Concept 9 of 19

The Atmosphere's Greenhouse Effect

• A protective blanket — Greenhouse gases — CO₂, methane, water vapour — trap outgoing infrared heat, keeping Earth warm enough for life. Without an atmosphere, Earth would be far too cold.
• Too much of a good thing — Excess CO₂ from human activity intensifies this effect beyond balance, causing global warming. Venus shows the extreme case — hotter than Mercury, despite being farther from the Sun.

Chapter 13 · Concept 10 of 19

Valley and Mountain Breezes

• Valley breeze (daytime) — Sun-facing slopes heat faster than the valley floor. Warm air over the slopes rises, and cooler valley air flows up to replace it.
• Mountain breeze (night) — After sunset, slopes cool faster than the valley. Cool, dense air over the slopes sinks down into the valley.

Chapter 13 · Concept 11 of 19

Planetary Winds

• Intense equatorial heating makes warm air rise, creating a low-pressure belt; it sinks again near 30° latitude, forming high-pressure belts.
• Curved, not straight — Earth's rotation deflects these winds — to the right in the Northern Hemisphere, to the left in the Southern — so planetary winds trace curved paths rather than heading straight from high to low pressure.

Chapter 13 · Concept 12 of 19

Ocean Currents and Gyres

• Wind, temperature, salinity, and Earth's rotation combine to drive continuous, large-scale ocean circulation.
• The Gulf Stream and North Atlantic Drift — This warm current carries heat from the Atlantic's warm waters to northwestern Europe — keeping its ports ice-free in winter, and moderating climate across the ocean.

Chapter 13 · Concept 13 of 19

Biogeochemical Cycles

• Living (biotic) and non-living (abiotic) parts of the Earth constantly exchange matter and energy.
• Keeping nutrients in circulation — Carbon, nitrogen, oxygen, and water cycle continuously between the atmosphere, oceans, land, and living organisms — keeping essential nutrients available to sustain life.

Chapter 13 · Concept 14 of 19

The Water Cycle, and Climate Change

• The classic cycle — Evaporation → condensation into clouds → precipitation (rain, hail, snow) → runoff and infiltration back to rivers, groundwater, and oceans.
• A warmer atmosphere holds more moisture — This intensifies monsoons in some areas and droughts elsewhere; melting glaciers add water to rivers and raise sea levels, threatening cities like Mumbai and Chennai.

Chapter 13 · Concept 15 of 19

The Carbon Cycle

• CO₂ in atmosphere
• Photosynthesis (plants)
• Respiration, decay, combustion
• Back to atmosphere

Chapter 13 · Concept 16 of 19

The Nitrogen Cycle

• Step | What happens
• Nitrogen fixation — Bacteria (Rhizobium, Azotobacter) convert atmospheric N₂ into ammonia
• Nitrification — Ammonia is converted to nitrite, then nitrate, by soil bacteria
• Assimilation — Plants absorb nitrates; animals get nitrogen by eating plants or other animals
• Ammonification — Decomposers break down waste and dead matter, returning ammonia to soil
• Denitrification — Bacteria convert nitrates back into nitrogen gas, completing the cycle

Chapter 13 · Concept 17 of 19

The Oxygen Cycle

• Respiration and combustion use oxygen and release CO₂; photosynthesis restores it.
• A planet-wide balance — Plants use sunlight, water, and CO₂ to make glucose and release O₂ — balancing what respiration and combustion consume, and circulating oxygen between the atmosphere, land, oceans, and living things.

Chapter 13 · Concept 18 of 19

Human Impact on Earth's Systems

• Human activity | Effect
• Burning fossil fuels — Excess CO₂ intensifies the greenhouse effect and acidifies oceans
• Overusing fertilisers — Excess nitrates cause eutrophication — algal blooms that deplete oxygen and kill fish
• Deforestation — Less photosynthesis and transpiration, altered albedo, more soil erosion, habitat loss

Chapter 13 · Concept 19 of 19

Global Cooperation and Individual Action

• It can work — the ozone layer proves it — The Montreal Protocol phased out ozone-damaging CFCs through global cooperation, and the ozone layer is now slowly recovering.
• Everyday choices add up — Switching to renewable energy, planting trees, conserving water, and sustainable farming all help restore balance — alongside individual habits like reducing, reusing, and recycling.

Quick Recap

Check Your Understanding

1. 1Explain how climate change affects the water cycle. Illustrate with examples.
2. 2Describe how albedo affects the Earth's surface temperature and its climate.
3. 3How are mountain and valley breezes formed? Would the mountain breeze differ between a mountain covered with grass and one covered with barren rock?