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CBSE · Class 9 · Science

Chapter 13: Earth as a System: Energy, Matter, and Life

Spheres of the Earth, uneven heating, albedo, the atmosphere, winds and ocean currents, biogeochemical cycles and human impact.

Notes

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Earth as a system of spheres
  • Life on Earth is powered by a constant flow of energy and matter. The Sun is the main source of energy; the Earth's hot interior and chemical reactions also drive flows.
  • Geosphere: solid rocks, soil and landforms (Deccan plateau, Thar desert) and the Earth's interior.
  • Hydrosphere: liquid water: oceans, rivers (Ganga–Brahmaputra), lakes, groundwater.
  • Cryosphere: ice and snow (Himalayan glaciers, snow in Ladakh, polar ice caps).
  • Atmosphere: the air surrounding the Earth.
  • Biosphere: all living organisms and their habitats (mangroves, forests, farms, plankton, coral reefs).
  • A disturbance in one sphere changes the others. Example: less snowfall (cryosphere) → less water in the lake (hydrosphere) → less grass for sheep (biosphere). Warmer Arabian Sea water → more evaporation → variable monsoon rainfall → floods in some regions and drought in others; atmospheric warming melts glaciers, raises sea level and threatens coastal habitats.
Solar radiation and the electromagnetic spectrum
  • Solar radiation reaches the Earth as electromagnetic (EM) waves, which travel through vacuum at 3 × 10⁸ m s⁻¹. Unlike sound, EM waves do not need a medium.
  • Electromagnetic spectrum from high frequency (short wavelength) to low frequency: gamma rays, X-rays, ultraviolet (UV), visible, infrared (IR), microwaves, radio waves.
  • About 99 per cent of the Sun's energy falls in the UV, visible and IR range. Gamma rays and X-rays are mostly filtered by the upper atmosphere.
  • UV (100–400 nm) is mostly absorbed by the ozone layer. Visible light gives energy for photosynthesis and partly warms land and water. Infrared warms the Earth's surface, which re-radiates heat; part is trapped by greenhouse gases (CO₂, CH₄, water vapour).
  • Insolation: the Sun's radiation reaching the Earth's surface. Solar constant: average solar energy received per unit time per unit area perpendicular to the rays at the top of the atmosphere, about 1.4 kW m⁻² (1400 J s⁻¹ m⁻²). Maximum insolation at the surface under a clear sky is about 1 kW m⁻², because of absorption and scattering.
Interaction with the surface: albedo and heat
  • Land heats faster than water. Dark surfaces absorb more sunlight; light surfaces reflect more and stay cooler.
  • Albedo: the fraction of solar radiation reflected by a surface. Snow 0.80–0.90, ice 0.50–0.70, crushed rock 0.25–0.30. High albedo stays cool; low albedo (black soil, ocean water) warms quickly.
  • All objects re-radiate heat; a concrete house stays hot at night, while thick mud and wood walls re-radiate less.
  • Urban heat island effect: cities are warmer than rural areas because steel, concrete, brick and asphalt absorb and retain heat, while rural areas and forests have vegetation that cools through shade and transpiration.
Latitude, Earth's shape and the atmosphere
  • The Earth is spherical, so the Sun's rays strike different latitudes at different angles. At the equator the radiation is concentrated over a smaller area; near the poles it is spread over a larger area. So the equator is warm and the poles are cold. Together with the tilt of the axis, this causes uneven heating and seasons, and drives winds and ocean currents.
  • Atmosphere (held by gravity): nitrogen 78%, oxygen 21%, small amounts of argon, CO₂, water vapour and other gases.
  • Troposphere (0–12 km): nearly all weather; temperature decreases with height (about 6.5 °C per km); warm air rises and drives winds and storms.
  • Stratosphere (12–50 km): ozone layer absorbs UV, so temperature rises with height; lack of vertical mixing keeps weather in the troposphere. Above are the mesosphere, thermosphere and exosphere.
  • The atmosphere (i) absorbs part of the incoming radiation (ozone absorbs UV; clouds and gases absorb some) and (ii) traps outgoing heat through greenhouse gases. Without it the Earth would be too cold for life; excess CO₂ causes global warming. Venus is hotter than Mercury because of an uncontrolled greenhouse effect.
  • Ozone loss: CFCs (from refrigerators) caused the Antarctic ozone hole; the Montreal Protocol reduced CFC use and the layer is slowly recovering.
Winds and ocean currents
  • Wind: movement of air from high pressure to low pressure; pressure differences come from uneven heating.
  • Local winds: valley breeze by day (slopes heat faster, warm air rises, cooler air moves up from the valley); mountain breeze at night (slopes cool faster, cool dense air flows down into the valley). They help regulate temperature and moisture and support crops and soil.
  • Planetary winds: air rises at the equator (low pressure belt), moves poleward, sinks near 30° N and S (sub-tropical high), returns along the surface to the equator. Some flows to the poles, rises near 60° (sub-polar low). At the poles cold dense air forms polar highs. The Earth's rotation deflects winds to the right in the Northern Hemisphere and to the left in the Southern.
  • Ocean currents: continuous movement of large masses of water, driven by winds (friction), temperature and salinity differences, the Earth's rotation and the shape of continents. Warm water flows from the equator towards the poles at the surface; colder, denser water returns at depth. Circular patterns called gyres rotate clockwise in the Northern Hemisphere and anticlockwise in the Southern.
  • Currents regulate climate by moving heat from the equator towards the poles. The Gulf Stream and North Atlantic Drift keep many ports of north-west Europe ice-free in winter. They also carry nutrients.
Water cycle
  • Evaporation from water bodies and transpiration from plants → condensation to form clouds → precipitation (rain, hail, snow) → run-off and infiltration into groundwater → flows back to the oceans.
  • Water dissolves minerals and carries nutrients to oceans.
  • Climate change: a warmer atmosphere holds more moisture, giving heavier rain in some areas and drought elsewhere; melting glaciers raise sea levels (threatening cities such as Mumbai and Chennai); intense bursts of rain cause run-off and soil erosion and reduce groundwater recharge. The water cycle links all four spheres.
Carbon cycle
  • Carbon forms the backbone of life: every protein, carbohydrate, fat and DNA molecule contains it. It cycles between the atmosphere (CO₂), biosphere, geosphere (carbonate rocks, fossil fuels) and hydrosphere (dissolved CO₂, shells).
  • Fast cycle (days to years): photosynthesis takes CO₂ from the air to make glucose; respiration and decomposition return CO₂.
  • Slow cycle (millions of years): dead plants and animals are buried and form fossil fuels (coal, oil, gas); burning them returns CO₂ to the air quickly.
  • The ocean absorbs CO₂ (forming carbonate and bicarbonate ions); phytoplankton use it; shell-forming organisms make shells; dead organisms sink and store carbon for a long time.
  • Facts: carbon is about 49% of dry weight of living organisms; 71% of the global carbon is in oceans; the atmosphere holds only about 1%. Atmospheric CO₂ has risen about 35% since 1960 (315 ppm to 420 ppm).
Nitrogen and oxygen cycles
  • Nitrogen (N₂) makes up most of the air but is non-reactive; living things need soluble compounds. Steps of the nitrogen cycle:
  • Nitrogen fixation: N₂ → ammonia (NH₃) by Rhizobium (root nodules of legumes) and Azotobacter; lightning forms nitrogen oxides; the Haber-Bosch process makes ammonia for fertilisers.
  • Nitrification: ammonia → nitrite (Nitrosomonas) → nitrate (Nitrobacter).
  • Assimilation: plants take up nitrogen compounds; animals eat plants or animals.
  • Ammonification: decomposers (bacteria, fungi) break organic matter into ammonia.
  • Denitrification: Pseudomonas converts nitrates back to N₂.
  • Oxygen cycle: about 21% of the air is O₂. Respiration, combustion and oxide formation use oxygen and release CO₂; photosynthesis restores O₂.
Human impact and what we can do
  • Burning fossil fuels and deforestation raise CO₂; excess CO₂ intensifies the greenhouse effect (global warming, melting ice, sea-level rise, extreme weather) and makes the oceans more acidic, threatening plankton and coral reefs. Warmer ocean water absorbs less CO₂.
  • Eutrophication: excess fertiliser (nitrates) washes into rivers and lakes and causes algal blooms that use up oxygen and kill fish.
  • Deforestation: less photosynthesis and transpiration, less local rainfall, altered albedo, soil erosion, loss of habitat and biodiversity.
  • Vehicle emissions react with sunlight to form smog and harmful ground-level ozone.
  • Actions: Montreal Protocol (ozone recovery); Kyoto Protocol and Paris Agreement (less successful); renewable energy (solar, wind), planting trees, saving water and energy, sustainable farming, reducing, reusing and recycling. Mission LiFE (2021) encourages mindful, eco-friendly lifestyles.