Earth as a System: Energy, Matter, and Life — Detailed Notes
Life on the Earth is powered by a constant flow of energy and matter. The main source of energy is the Sun; the Earth’s hot interior and chemical reactions also contribute. The Earth works as one system made of interacting “spheres” that constantly exchange energy and matter — a disturbance in one sphere changes the others.
The five spheres
• Geosphere — solid rocks, soil, landforms (Deccan plateau, Thar desert) and the Earth’s interior. • Hydrosphere — liquid water: oceans, rivers (Ganga–Brahmaputra), lakes, groundwater. • Cryosphere — solid water: ice and snow (Himalayan glaciers, polar ice caps). • Atmosphere — the air surrounding the Earth. • Biosphere — all living organisms and their habitats (mangroves, forests, plankton, coral reefs). Example of linkage: warmer Arabian Sea → more evaporation → monsoon fluctuations (floods/droughts) and faster glacier melt → rising sea level → threat to coastal cities and habitats.
13.1 Uneven heating of the Earth
Solar radiation reaches the Earth as electromagnetic (EM) waves travelling through vacuum at the speed of light (3 × 10⁸ m/s). About 99% of the Sun’s energy is in the UV, visible and infrared (IR) range. Short-wavelength UV is mostly absorbed by the ozone layer; visible light drives photosynthesis and warms land/water; IR warms the surface, which re-radiates heat that greenhouse gases (CO₂, CH₄, water vapour) trap — keeping the Earth warm enough for life.
Insolation = amount of the Sun’s radiation reaching the Earth’s surface. Solar constant = average solar energy per unit time per unit area perpendicular to the Sun’s rays at the top of the atmosphere ≈ 1.4 kW/m² (1400 J s⁻¹ m⁻²); at the surface under clear sky it is about 1 kW/m². (Anna Mani mapped India’s solar insolation.) Example: energy on 1 m² in 1 hour at 1 kW/m² = 1000 × 1 × 3600 = 3.6 × 10⁶ J (≈ 1 unit of household electricity).
13.1.1–13.1.2 Albedo & latitude
Albedo = fraction of solar radiation reflected by a surface. High-albedo surfaces (snow 0.80–0.90, ice 0.50–0.70) stay cool; low-albedo surfaces (black soil, ocean water) absorb more and warm up. The Urban Heat Island effect: concrete/asphalt cities absorb and re-radiate heat, staying warmer than green rural areas. Because the Earth is spherical, the Sun’s rays hit the equator directly (concentrated → warm) but the poles at a slant (spread out → cold). The Earth’s tilt causes seasons. This uneven heating drives winds and ocean currents.
13.1.3 The atmosphere
Composition: nitrogen 78%, oxygen 21%, plus argon, CO₂, water vapour. Layers: Troposphere (0–12 km) — all weather; temperature decreases with height (~6.5°C/km). Stratosphere (12–50 km) — holds the ozone layer that absorbs UV, so temperature increases with height (calm, no weather). Then mesosphere, thermosphere, exosphere (minor climate role). The atmosphere protects life by (1) absorbing incoming UV (ozone) and (2) trapping outgoing IR heat (greenhouse effect). Excess CO₂ → enhanced greenhouse effect → global warming. (CFCs caused the ozone hole; the Montreal Protocol is helping it recover.)
13.2 Winds & ocean currents
Wind = air moving from high to low pressure, caused by uneven heating. Local winds: valley breeze (day — warm air rises up sunlit slopes), mountain breeze (night — cool dense air sinks into valley). Planetary winds: pressure belts — equatorial low, sub-tropical high (30°), sub-polar low (60°), polar high (90°); winds deflect right in the Northern Hemisphere, left in the Southern (Earth’s rotation). Ocean currents: large moving water masses driven by winds, temperature and salinity differences and Earth’s rotation, forming circular gyres (clockwise in N, anticlockwise in S). They transport heat (e.g. North Atlantic Drift / Gulf Stream keeps European ports ice-free) and nutrients.
13.3 Biogeochemical cycles
The cyclic movement of matter and energy between abiotic (non-living) and biotic (living) components is a biogeochemical cycle; it recycles nutrients (carbon, nitrogen, oxygen, water) and keeps them available for life.
• Water cycle — evaporation, transpiration, condensation, precipitation, infiltration, runoff, groundwater; links all five spheres. Climate change intensifies rains in some areas and droughts in others. • Carbon cycle — fast cycle (days–years): photosynthesis takes CO₂, respiration & decomposition return it; slow cycle (millions of years): burial → fossil fuels → combustion returns CO₂. Oceans hold ~71% of global carbon. Human activity raised atmospheric CO₂ ~35% since 1960 (the Keeling curve). • Nitrogen cycle — steps: nitrogen fixation (Rhizobium, Azotobacter → ammonia; also lightning), nitrification (Nitrosomonas → nitrite, Nitrobacter → nitrate), assimilation by plants, ammonification (decomposers return ammonia), denitrification (Pseudomonas → N₂). The Haber-Bosch process fixes N₂ into fertiliser (“bread from air”). • Oxygen cycle — respiration and combustion use O₂ and release CO₂; photosynthesis restores O₂.
13.4 Human impact
Excess CO₂ → ocean acidification (threatens plankton & coral) and warming reduces the ocean’s ability to absorb CO₂. Overuse of fertilisers → eutrophication (algal blooms deplete oxygen, kill fish). Deforestation → less photosynthesis/transpiration, less rainfall, soil erosion, altered albedo, biodiversity loss. Vehicle emissions → ground-level smog and harmful ground-level ozone. Solutions: renewable energy (solar, wind), tree planting, water saving, sustainable farming, and global cooperation (Montreal Protocol, Paris Agreement; India’s Mission LiFE).
Key Terms
geosphere, hydrosphere, cryosphere, atmosphere, biosphere, EM spectrum, insolation, solar constant, albedo, urban heat island, troposphere, stratosphere, ozone layer, greenhouse effect, planetary winds, valley/mountain breeze, ocean currents, gyres, biogeochemical cycle, water/carbon/nitrogen/oxygen cycle, nitrogen fixation, nitrification, denitrification, Haber-Bosch, eutrophication.
Revise, Reflect, Refine — All NCERT Exercise Answers
1Choose the correct option for the role of biogeochemical cycles in an ecosystem.
(ii) To recycle essential nutrients between biotic and abiotic components. Biogeochemical cycles move elements like carbon, nitrogen and oxygen back and forth between living organisms and the non-living environment (air, water, soil), keeping them continuously available for life. They do not create new elements, nor directly feed organisms, nor simply remove toxins.
2Which of the following is primarily responsible for warming of the Earth?
(iii) The Earth’s surface absorbs solar radiation, which is then re-radiated and trapped by greenhouse gases. Sunlight (mainly visible) passes through the atmosphere and is absorbed by the surface; the warm surface re-radiates this energy as infrared (heat), and greenhouse gases (CO₂, CH₄, water vapour) absorb and trap this outgoing IR — this is what keeps the Earth warm. CO₂ does not absorb the incoming radiation directly.
3Explain how climate change affects the water cycle. Illustrate with examples.
A warmer atmosphere holds more moisture, so the water cycle becomes more intense and uneven. Examples: heavier, more intense rainfall and floods in some regions (intensified monsoons) while others face droughts; faster melting of glaciers adds water to rivers and, in the long run, raises sea level (threatening Mumbai, Chennai); sudden heavy downpours cause more runoff and soil erosion but less infiltration, lowering groundwater recharge and making agriculture harder in dry months. Thus climate change disturbs evaporation, precipitation and storage across all spheres.
4Describe how albedo affects the Earth’s surface temperature and its climate.
Albedo is the fraction of solar radiation a surface reflects. High-albedo surfaces (snow, ice ~0.8) reflect most sunlight and stay cold, keeping polar regions cool. Low-albedo surfaces (black soil, ocean, dark roads) absorb more and warm up. This controls local and global temperature: e.g. melting of bright ice exposes darker water/land, which absorbs more heat and causes further warming (a feedback). So albedo strongly influences how much the Earth heats up and shapes its climate.
5How are mountain and valley breezes formed? If one mountain is grass-covered and another barren rock, would their mountain breezes differ in temperature? How?
Valley breeze (day): sunlit slopes heat faster than the valley floor; warm air over the slopes rises (low pressure), so cooler valley air flows up the slope. Mountain breeze (night): slopes cool faster, the air over them becomes cold and dense and flows down into the valley. Yes, they would differ. The barren-rock mountain absorbs and re-radiates heat strongly (low albedo, no transpiration), so its breeze would be relatively warmer; the grass-covered mountain stays cooler (shade + transpiration + higher albedo), so its mountain breeze would be cooler.
6You have seen winds, storms and rainfall. Which atmospheric layer is mainly responsible, and why?
The troposphere (0–12 km) — nearly all weather phenomena occur here. The primary reason is that this layer is heated from below by the Earth’s surface, so temperature decreases with height; the warm surface air rises, creating convection currents, winds, clouds and storms. (In the stratosphere above, temperature increases with height, so air is stable and weather does not form there.)
7Explain the processes of the nitrogen cycle. How would life on Earth be affected if nitrogen were not cycled?
Steps: Nitrogen fixation — bacteria (Rhizobium in legume root nodules, Azotobacter) and lightning convert inert atmospheric N₂ into ammonia. Nitrification — Nitrosomonas turns ammonia into nitrite, Nitrobacter into nitrate. Assimilation — plants absorb nitrates to make proteins and nucleic acids; animals get nitrogen by eating plants. Ammonification — decomposers break down dead matter/waste, returning ammonia to soil. Denitrification — Pseudomonas converts nitrates back to N₂, completing the cycle. If nitrogen were not cycled, usable nitrogen in soil would run out; plants could not make proteins or DNA, so they would die, and the animals that depend on them would starve — life could not continue.
8What are the impacts of deforestation on the Earth’s oxygen and carbon cycles? What are other consequences?
Carbon cycle: fewer trees means less CO₂ is removed by photosynthesis, and burning/decay of the cleared trees releases stored carbon — so atmospheric CO₂ rises, strengthening global warming. Oxygen cycle: less photosynthesis means less oxygen is released into the atmosphere. Other consequences: reduced transpiration lowers local rainfall; loss of roots causes soil erosion; surface albedo changes; and destruction of habitats leads to loss of biodiversity.
9Explain with a suitable diagram the path carbon takes to return to the atmosphere, starting from plants using CO₂.
Path: Atmospheric CO₂ → plants (photosynthesis) → glucose/organic matter. From there carbon returns to the air by several routes: (a) plants respire and release CO₂; (b) animals eat plants, respire, and release CO₂; (c) when plants/animals die, decomposers break them down, releasing CO₂; (d) over millions of years buried remains form fossil fuels, and burning (combustion) of coal, oil and gas releases CO₂ quickly. Diagram (cycle): CO₂ (air) → photosynthesis → plants → (respiration / eaten by animals → respiration / death → decomposition / fossil fuels → combustion) → CO₂ (air).
10Why is excess CO₂ in the atmosphere undesirable even though plants need it?
Plants use CO₂ for photosynthesis, and a certain amount keeps the Earth warm enough for life — so CO₂ is necessary. But excess CO₂ strongly intensifies the greenhouse effect, trapping too much heat and causing global warming: melting glaciers and sea ice, rising sea levels, more extreme weather and disturbed monsoons, plus ocean acidification that harms plankton and coral. The problem is one of balance — beyond the needed amount, more CO₂ does far more harm than good.
11How is heat lost from the Earth’s surface? What is its significance?
The Earth’s surface absorbs incoming sunlight and then re-radiates it as infrared (heat) radiation back towards space. Significance: part of this outgoing IR is absorbed by greenhouse gases (CO₂, CH₄, water vapour) and radiated back, which keeps the surface warm enough to support life — without it the Earth would be far too cold. This heat loss and partial trapping is what sets the Earth’s energy balance and temperature; too much trapping (excess greenhouse gases) causes global warming.
12If the Earth were a flat disc instead of a sphere, how would solar radiation and temperature patterns differ?
On a sphere, the Sun’s rays strike different latitudes at different angles — direct (concentrated) at the equator and slanting (spread out) at the poles — producing the equator-to-pole temperature difference that drives winds and currents. If the Earth were a flat disc facing the Sun, the rays would hit the whole surface at nearly the same angle, so heating would be far more uniform — little temperature difference between ‘equator’ and ‘edges’. That would greatly weaken or remove the pressure differences that cause planetary winds, ocean currents and the seasonal/climate patterns we see.
13Suppose atmospheric temperature on Earth rises. How would this affect the cryosphere, hydrosphere and biosphere?
Cryosphere: glaciers, snow and polar ice would melt faster, shrinking the ice cover. Hydrosphere: melted ice and thermal expansion raise sea levels and add water to rivers, causing flooding of low-lying/coastal areas; evaporation and rainfall patterns become more erratic (floods and droughts). Biosphere: habitats are lost (coastal and polar species, coral reefs via warming/acidification), species migrate or decline, and biodiversity falls as ecosystems are disrupted. Thus warming in the atmosphere cascades through all the spheres.
14Explain how the Earth’s atmosphere helps maintain a suitable temperature for life.
The atmosphere plays two protective roles. (1) It filters incoming radiation — the ozone layer absorbs harmful UV, and clouds/gases scatter and absorb part of the sunlight, so the surface is not over-heated or over-exposed. (2) It traps outgoing heat — the surface re-radiates infrared, and greenhouse gases (CO₂, CH₄, water vapour) absorb and send some of it back, keeping the surface warm at night and overall warm enough for life. Without the atmosphere the Earth would be too cold; this balance keeps temperatures in the range life needs (excess greenhouse gas upsets it).
15Describe the interrelationship between the Earth’s spheres. Illustrate with an example of their delicate balance.
The five spheres — geosphere, hydrosphere, cryosphere, atmosphere and biosphere — continuously exchange energy and matter, so a change in one affects the others. Example: rising atmospheric temperature (atmosphere) melts Himalayan glaciers (cryosphere), adding water to rivers and raising sea level (hydrosphere), which floods coastal land and destroys mangroves and crops (biosphere) and erodes/soaks the soil (geosphere). Another: warmer Arabian Sea → stronger evaporation → erratic monsoon → floods and droughts affecting farms and fisheries. These show the spheres work in a delicate balance — disturbing one ripples through all, so the Earth must be understood as a single connected system.