If a large forest is cleared, how can that affect the flow of a river in that area?
Key idea
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
Key idea
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 scaleA 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).
Key idea
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
Key idea
Insolation and the Solar Constant
1
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.
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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.
Key idea
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
Key idea
The Urban Heat Island Effect
Concrete, steel, and asphalt absorb solar radiation and re-radiate heat, especially at night.
Cities run warmer than the countrysideRural 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.
Key idea
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.
Key idea
Layers of the Atmosphere
Layer | Altitude | Key feature
Troposphere0 – 12 km — Weather forms here; temperature falls with height
Stratosphere12 – 50 km — Ozone layer absorbs UV; temperature rises with height
Key idea
The Atmosphere's Greenhouse Effect
1
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.
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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.
Key idea
Valley and Mountain Breezes
1
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.
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Mountain breeze (night)
After sunset, slopes cool faster than the valley. Cool, dense air over the slopes sinks down into the valley.
Key idea
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.
Key idea
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 DriftThis 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.
Key idea
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.
Key idea
The Water Cycle, and Climate Change
The classic cycleEvaporation → condensation into clouds → precipitation (rain, hail, snow) → runoff and infiltration back to rivers, groundwater, and oceans.
A warmer atmosphere holds more moistureThis intensifies monsoons in some areas and droughts elsewhere; melting glaciers add water to rivers and raise sea levels, threatening cities like Mumbai and Chennai.
Key idea
The Carbon Cycle
CO₂ in atmosphere
Photosynthesis (plants)
Respiration, decay, combustion
Back to atmosphere
Key idea
The Nitrogen Cycle
Step | What happens
Nitrogen fixationBacteria (Rhizobium, Azotobacter) convert atmospheric N₂ into ammonia
NitrificationAmmonia is converted to nitrite, then nitrate, by soil bacteria
AssimilationPlants absorb nitrates; animals get nitrogen by eating plants or other animals
AmmonificationDecomposers break down waste and dead matter, returning ammonia to soil
DenitrificationBacteria convert nitrates back into nitrogen gas, completing the cycle
Key idea
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.
Key idea
Human Impact on Earth's Systems
Human activity | Effect
Burning fossil fuelsExcess CO₂ intensifies the greenhouse effect and acidifies oceans
Overusing fertilisersExcess nitrates cause eutrophication — algal blooms that deplete oxygen and kill fish
DeforestationLess photosynthesis and transpiration, altered albedo, more soil erosion, habitat loss
Key idea
Global Cooperation and Individual Action
1
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.
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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.
Chapter · Key terms to remember
Key Terms
Geosphere / Hydrosphere / Cryosphere
Solid earth / liquid water / frozen water.
Insolation
Solar radiation reaching the Earth's surface.
Albedo
Fraction of sunlight a surface reflects.
Chapter · Key terms to remember
More Key Terms
Greenhouse effect
Atmosphere trapping outgoing heat, warming Earth.
Planetary winds
Large-scale winds from global pressure belts.
Ocean gyre
Large circular ocean current pattern.
Chapter · Key terms to remember
More Key Terms
Biogeochemical cycle
Cyclic movement of matter between living and non-living systems.
Eutrophication
Excess nutrients causing algal blooms that deplete oxygen.
Questions for your notebook
Write these down, then discuss
1
Explain how climate change affects the water cycle. Illustrate with examples.
2
Describe how albedo affects the Earth's surface temperature and its climate.
3
How are mountain and valley breezes formed? Would the mountain breeze differ between a mountain covered with grass and one covered with barren rock?
Project as-is — students copy the questions, then the class discusses answers together.