These Earth as a System Energy Matter and Life Class 9 notes compress Chapter 13 of the rationalised NCERT Science textbook (Exploration, Grade 9) for the current academic session into a revision-ready format: the five interacting spheres, why the Sun heats the Earth unevenly, how that drives winds and ocean currents, and the four biogeochemical cycles.
Skim the revision summary first, then work through the sections you find hardest. Page references are to the NCERT textbook, which you can verify at ncert.nic.in.
The Five Spheres That Make Up the Earth System
The chapter’s central idea is that the Earth behaves as one connected system: energy and matter flow between interacting ‘spheres’, so a disturbance in one sphere ripples into the others (NCERT, p. 253).
The chapter opens with four framing questions you should be able to answer by the end:
- How does warming of Arabian Sea water affect the southwest monsoon in India?
- If a large forest is cleared, how can that affect the flow of a river in that area?
- What might happen to coastal cities if glaciers and polar ice keep melting faster?
- How would rising carbon dioxide levels affect ocean plankton?
The five spheres are:
- Geosphere: solid rocks, soil, landforms and the Earth’s interior — the Deccan plateau and the Thar desert.
- Hydrosphere: liquid water — oceans, rivers like the Ganga–Brahmaputra system, lakes and groundwater.
- Cryosphere: frozen water — Himalayan glaciers, snow in Ladakh and polar ice caps.
- Atmosphere: the air we breathe — cleaner in mountains and forests.
- Biosphere: all living organisms and their habitats — mangroves, forests, farms, ocean plankton and coral reefs. (See our notes on patterns in life and diversity for how this variety of life is classified.)
Activity 13.1 asks you to spot each sphere in a landscape like Fig. 13.1 and trace how snow becomes lake water. The inference is the chapter’s key skill: a disturbance in one sphere changes the others. Less snowfall in winter → less lake water in summer → less grass for the sheep that graze there.
The same chain works on a national scale for the southwest monsoon: warmer Arabian Sea water → more evaporation → fluctuating monsoon → floods in some regions of India while others face drought. Rising atmospheric temperature also accelerates glacier and polar-ice melting, which can raise sea levels and threaten coastal cities through habitat loss in the biosphere.
Why the Sun Heats the Earth Unevenly
Solar radiation is the primary source of energy for the Earth. It arrives as electromagnetic (EM) waves, which travel through a vacuum at the speed of light, \( 3 \times 10^8\ \text{ms}^{-1} \). This is the key difference from sound waves, which are mechanical and need a medium to travel.
The full range is the electromagnetic spectrum (Fig. 13.2). About 99 per cent of the Sun’s energy falls in the ultraviolet (UV), visible and infrared (IR) range. Each region plays a distinct role:
- UV: mostly absorbed by the ozone layer — protects life and adds some atmospheric heating.
- Visible light: reaches the surface, drives photosynthesis (the primary food source for most organisms), and partly warms land and water.
- Infrared: warms the surface, which re-radiates heat; greenhouse gases (\( \text{CO}_2 \), \( \text{CH}_4 \), water vapour) trap part of this outgoing heat and keep the Earth warm enough for life.
Gamma rays and X-rays carry very high energy and are filtered by the upper atmosphere; microwaves and radio waves carry too little energy to warm the Earth significantly.
Insolation is the amount of the Sun’s radiation that actually reaches the Earth’s surface. The solar constant is the average solar energy received per unit time per unit area perpendicular to the Sun’s rays at the top of the atmosphere — about \( 1.4\ \text{kWm}^{-2} \) or \( 1400\ \text{J s}^{-1}\text{m}^{-2} \) (NCERT, p. 254).
Because gases, clouds and dust absorb and scatter some energy, the maximum surface insolation is lower, about \( 1\ \text{kWm}^{-2} \) under a clear sky (NCERT, p. 255).
India’s tropical and sub-tropical location means abundant sunlight year-round — this drives the southwest monsoon and gives India huge solar-energy potential. Anna Mani, India’s pioneering atmospheric scientist, mapped this resource in the 1950s and co-published Solar Radiation Over India (1982), the country’s first insolation atlas.
That potential is now being realised through large-scale solar deployment. The textbook’s estimation problem (Fig. 13.4) suggests that even a fraction of the Thar desert covered with solar panels could supply India’s electricity needs.
Albedo, Latitude, and the Atmosphere’s Role
Albedo is the fraction of solar radiation a surface reflects (from the Latin for ‘whiteness’). High-albedo surfaces reflect more and stay cool; low-albedo surfaces absorb more and heat up faster (NCERT, p. 256).
| Material | Albedo | Effect |
|---|---|---|
| Snow | 0.80–0.90 | Reflects most sunlight — keeps polar regions very cold |
| Ice | 0.50–0.70 | Strong reflector |
| Crushed rock | 0.25–0.30 | Moderate reflector |
| Black soil / ocean water | Lower | Absorb more radiation — relatively warmer |
This is why polar regions stay cold: snow and ice reflect a large share of incoming radiation, while dark surfaces like black soil and ocean water absorb more and warm up.
Urban heat island effect: cities are warmer than surrounding rural areas, especially at night in summer. Concrete, brick, steel and asphalt absorb and re-radiate heat; vegetation cools rural areas through shade and transpiration (NCERT, p. 257).
Real-life fix: this is why white roofs and light-coloured roads reduce the urban heat island effect in Indian cities. Light surfaces have higher albedo — they reflect more sunlight instead of absorbing and re-radiating it at night, so the city stays cooler and air-conditioning demand drops.
Latitude and the Earth’s shape: because the Earth is spherical, the Sun’s rays strike different latitudes at different angles. Near the equator the rays concentrate over a smaller area, so the region stays warm all year; near the poles the same energy spreads over a larger area, so it stays cold.
This equator-to-pole temperature difference is the engine of global winds and ocean currents.
Atmosphere composition: nitrogen (78%) and oxygen (21%), with small amounts of argon, \( \text{CO}_2 \), water vapour and other gases (NCERT, p. 258). Two layers matter most for climate:
| Layer | Altitude | Key feature |
|---|---|---|
| Troposphere | 0–12 km | Nearly all weather happens here; temperature decreases with height (~6.5 °C/km) |
| Stratosphere | 12–50 km | Ozone layer absorbs UV; temperature increases with height, which calms the layer and keeps weather confined below |
The atmosphere protects life twice over: it partly absorbs incoming radiation (ozone blocks UV), and it traps outgoing heat — the greenhouse effect. Without it the Earth would be too cold for life; with too much \( \text{CO}_2 \), global warming could make it uninhabitable.
Ozone misconception autopsy: ozone is not ‘good’ or ‘bad’ by itself — location decides. Stratospheric ozone (12–50 km up) is protective: it absorbs harmful UV radiation. Ground-level ozone, formed when vehicular emissions react with sunlight in smog, is a harmful pollutant that makes city air unhealthy (NCERT, p. 265).
In the late 20th century, chlorofluorocarbons (CFCs) from refrigerators and aerosols destroyed stratospheric ozone over Antarctica — the ‘ozone hole’. The Montreal Protocol, a global agreement, cut CFC use and the layer is now slowly recovering (NCERT, p. 259).
From Uneven Heating to Winds and Ocean Currents
Wind is air moving from high to low pressure. Uneven heating creates those pressure differences at every scale — from a mountain valley to the whole planet.
Local winds — valley and mountain breezes: mountain slopes and the valley floor heat and cool at different rates (NCERT, p. 259).
| Valley breeze (day) | Mountain breeze (night) | |
|---|---|---|
| Timing | During the day | After sunset / at night |
| Cause | Sun-facing slopes heat faster than the valley floor; warm air rises, creating low pressure | Slopes lose heat faster and cool below the valley floor |
| Air movement | Cooler valley air moves up the slopes | Cooler, denser air flows down into the valley |
| Example | Daily wind reversal in hilly regions like Shimla, Dehradun and other Himalayan valleys | |
Planetary winds: uneven heating between the equator and the poles sets up global pressure belts (NCERT, p. 260):
- Equatorial low pressure belt: intense heating makes warm air rise.
- Sub-tropical high pressure belts (~30° N/S): the rising air cools, becomes denser and sinks.
- Sub-polar low pressure belts (~60° N/S): surface air moving poleward meets cold polar air and rises.
- Polar high pressure belts (~90° N/S): very cold, dense air sinks.
The Earth’s rotation deflects these winds from straight paths: to the right in the Northern Hemisphere, to the left in the Southern Hemisphere (the Coriolis effect).
Ocean currents are continuous movements of large masses of ocean water. Strong planetary winds drag surface water by friction; temperature and salinity differences, the Earth’s rotation and the distribution of land masses also steer them (NCERT, p. 261).
Warm equatorial water travels over the surface toward the poles while colder, denser water returns at depth. The Earth’s rotation deflects these flows into large circular patterns called gyres — clockwise in the Northern Hemisphere, counter-clockwise in the Southern Hemisphere.
By transporting heat from the equator toward the poles, currents reduce global temperature differences. The North Atlantic Drift (an extension of the Gulf Stream) keeps many northwestern European ports ice-free in winter even at high latitudes, and currents also transport nutrients that support marine ecosystems.
The Four Biogeochemical Cycles
A biogeochemical cycle is the cyclic movement of matter and energy between the abiotic (non-living) and biotic (living) components of the Earth. It keeps essential nutrients like carbon, nitrogen and oxygen available to life (NCERT, p. 262).
Water cycle: evaporation from rivers, lakes and oceans → condensation into clouds → precipitation as rain, hail or snow → flow back to the ocean. Some water seeps through soil and rocks (infiltration) to become groundwater, dissolving minerals and carrying nutrients to the sea.
Climate change is disrupting this cycle: a warmer atmosphere holds more moisture → heavier rains in some areas (intensified monsoons) and droughts elsewhere. Melting glaciers raise sea levels, threatening coastal cities like Mumbai and Chennai; intense rainfall causes runoff that erodes soil, and less infiltration reduces groundwater recharge (NCERT, p. 262).
Carbon cycle: carbon forms the backbone of life — every protein, carbohydrate, fat and DNA molecule contains it. It circulates between the atmosphere (\( \text{CO}_2 \)), biosphere (plants and animals), geosphere (carbonate rocks, fossil fuels) and hydrosphere (dissolved \( \text{CO}_2 \), marine shells) (NCERT, p. 263).
- Fast cycle (days to years): photosynthesis converts atmospheric \( \text{CO}_2 \) into glucose; respiration releases \( \text{CO}_2 \) back; decomposition returns it after organisms die.
- Slow cycle (millions of years): dead organisms get buried and become fossil fuels (coal, oil, gas); burning them releases carbon as \( \text{CO}_2 \) on a very short time scale.
Human activities — burning fossil fuels and deforestation — have raised atmospheric \( \text{CO}_2 \) by about 35% since 1960, from 315 ppm to 420 ppm (ppm = parts per million), an unprecedented rise (NCERT, p. 263).
Carbon facts worth remembering: carbon makes up ~49% of the dry weight of living organisms, and 71% of global carbon is stored in the oceans — this oceanic reservoir regulates atmospheric \( \text{CO}_2 \). The atmosphere holds only about 1% of total global carbon (NCERT, p. 263).
Nitrogen cycle: nitrogen is essential for proteins and nucleic acids. The atmosphere is the largest reservoir, but nitrogen gas (\( \text{N}_2 \)) is non-reactive and cannot be used directly by plants and animals — it must first be converted to soluble compounds (NCERT, p. 264). The five steps:
- Nitrogen fixation: \( \text{N}_2 \rightarrow \text{NH}_3 \), done by Rhizobium (root nodules of legumes) and Azotobacter (soil); lightning and the Haber-Bosch process also fix nitrogen.
- Assimilation: plants absorb nitrogen compounds from soil; animals get nitrogen by eating plants or other animals.
- Ammonification: decomposers (bacteria and fungi) break down dead matter and waste, returning ammonia to the soil.
- Nitrification: Nitrosomonas converts ammonia to nitrite (\( \text{NO}_2^- \)), then Nitrobacter converts nitrite to nitrate (\( \text{NO}_3^- \)).
- Denitrification: Pseudomonas converts some nitrates back to nitrogen gas, completing the cycle.
Mnemonic: keep the order with Fix, Assimilate, Ammonify, Nitrify, Denitrify — initials F-A-A-N-D. Hook: “Fix And Ammonify, Nitrify, Denitrify.” Say the chain once and the sequence sticks.
Oxygen cycle: oxygen is about 21% of the atmosphere and part of most biological molecules. Respiration and combustion consume \( \text{O}_2 \) and release \( \text{CO}_2 \); photosynthesis restores \( \text{O}_2 \) using sunlight, water and \( \text{CO}_2 \). This balance between consumption and production circulates oxygen across all spheres (NCERT, p. 264).
How Human Activities Disrupt the Cycles
Excess \( \text{CO}_2 \) and ocean acidification: more atmospheric \( \text{CO}_2 \) means more dissolves in the ocean, making seawater more acidic. This threatens plankton and coral reefs. Warmer ocean water also absorbs less \( \text{CO}_2 \), weakening the ocean’s role as a carbon sink (NCERT, p. 265).
Eutrophication: overuse of fertilisers adds excess nitrates to rivers and lakes → widespread algal blooms → algae deplete oxygen when they decay → fish die. This threatens water bodies and coastal fisheries (NCERT, p. 265).
Deforestation hits several spheres at once: less photosynthesis and transpiration → reduced local rainfall; altered surface albedo; soil erosion without tree roots; habitat loss → declining biodiversity.
Global cooperation and individual action:
- Montreal Protocol: cut CFCs; the ozone layer is slowly recovering — a success story.
- Kyoto Protocol and Paris Agreement: meant to reduce \( \text{CO}_2 \) emissions; so far less successful.
- Mission LiFE (Lifestyle for Environment), an India-led initiative launched at the UN Climate Change Conference in 2021, promotes mindful eco-friendly lifestyles.
- Individual actions: save water and energy, reduce waste, reuse and recycle, plant trees, switch to solar and wind.
Key Terms and Definitions at a Glance
Quickest way to revise definitions — term, meaning, example:
| Term | Meaning | Example |
|---|---|---|
| Geosphere | Solid part of the Earth — rocks, soil, landforms, interior | Deccan plateau, Thar desert |
| Hydrosphere | All liquid water on the surface and underground | Ganga–Brahmaputra system, oceans, lakes, groundwater |
| Cryosphere | Frozen water — ice and snow | Himalayan glaciers, snow in Ladakh, polar ice caps |
| Atmosphere | The layer of air around the Earth | Air we breathe; cleaner in mountains and forests |
| Biosphere | All living organisms and their habitats | Mangroves, forests, farms, ocean plankton, coral reefs |
| Insolation | Solar radiation that reaches the Earth’s surface | ~\( 1\ \text{kWm}^{-2} \) under a clear sky |
| Solar constant | Solar energy per unit time per unit area at the top of the atmosphere, perpendicular to the rays | \( 1.4\ \text{kWm}^{-2} = 1400\ \text{J s}^{-1}\text{m}^{-2} \) |
| Albedo | Fraction of solar radiation reflected by a surface | Snow 0.80–0.90; crushed rock 0.25–0.30 |
| Electromagnetic spectrum | Full range of EM radiation from gamma rays to radio waves | UV, visible, IR carry 99% of the Sun’s energy |
| Greenhouse effect | Trapping of outgoing heat by \( \text{CO}_2 \), \( \text{CH}_4 \), water vapour | Keeps Earth warm enough for life |
| Valley breeze | Daytime upslope flow of cool valley air | Shimla, Dehradun |
| Mountain breeze | Night-time downslope flow of cold dense air | Himalayan valleys |
| Planetary winds | Large-scale winds from global pressure belts | Equatorial low → sub-tropical high circulation |
| Ocean current | Continuous movement of large masses of ocean water | North Atlantic Drift |
| Gyre | Large circular ocean current pattern | Clockwise in the Northern Hemisphere |
| Biogeochemical cycle | Cyclic movement of matter between biotic and abiotic components | Water, carbon, nitrogen, oxygen cycles |
| Nitrogen fixation | Conversion of \( \text{N}_2 \) into ammonia | Rhizobium, Azotobacter, lightning |
| Nitrification | Conversion of ammonia → nitrite → nitrate | Nitrosomonas, Nitrobacter |
| Ammonification | Decomposers return ammonia to soil | Bacteria and fungi |
| Denitrification | Conversion of nitrates back to \( \text{N}_2 \) gas | Pseudomonas |
| Eutrophication | Algal blooms from excess nitrates deplete oxygen, killing fish | Fertiliser runoff into lakes |
Formulas and Energy Calculations
The chapter’s quantitative tools are few but exam-relevant. Know each with its units:
- Speed of light in vacuum: \( c = 3 \times 10^8\ \text{ms}^{-1} \) (NCERT, p. 254)
- Solar constant: \( 1.4\ \text{kWm}^{-2} = 1400\ \text{J s}^{-1}\text{m}^{-2} \) (NCERT, p. 254)
- Unit conversion: \( 1\ \text{nm} = 10^{-9}\ \text{m} \) (NCERT, p. 255)
- Energy received by a surface: \( E = \text{Intensity} \times \text{area} \times \text{time} \) (NCERT, p. 255)
Why is surface insolation lower than the solar constant? The solar constant is measured at the top of the atmosphere, before any absorption, scattering or reflection. Gases, clouds and dust remove some energy on the way down, so the maximum surface value is about \( 1\ \text{kWm}^{-2} \) under clear skies.
Worked Example: Solar Energy Received by a Surface
Method: use \( E = \text{Intensity} \times \text{area} \times \text{time} \), converting all units to SI first. Unit conversion is the step that earns the mark.
Problem: A 2 m² solar panel receives insolation of 0.8 kWm⁻² for 30 minutes. How much solar energy does it receive?
Step 1: Convert insolation to SI units.
\( 0.8\ \text{kWm}^{-2} = 800\ \text{Wm}^{-2} = 800\ \text{J s}^{-1}\text{m}^{-2} \).
Step 2: Convert time to seconds.
\( 30\ \text{min} = 30 \times 60 = 1800\ \text{s} \).
Step 3: Substitute into \( E = \text{Intensity} \times \text{area} \times \text{time} \):
\[ E = 800\ \text{J s}^{-1}\text{m}^{-2} \times 2\ \text{m}^2 \times 1800\ \text{s} \]
Step 4: Compute:
\[ E = 2{,}880{,}000\ \text{J} = 2.88 \times 10^6\ \text{J} \]
Step 5: Convert to kilowatt-hours using \( 1\ \text{kWh} = 3.6 \times 10^6\ \text{J} \):
\[ E = \frac{2.88 \times 10^6}{3.6 \times 10^6}\ \text{kWh} = 0.8\ \text{kWh} \]
Final answer: the panel receives \( 2.88 \times 10^6\ \text{J} \) of energy, equal to \( 0.8\ \text{kWh} \).
Check: the textbook’s Example 13.1 shows a 1 m² area at \( 1\ \text{kWm}^{-2} \) for one hour gives \( 3.6 \times 10^6\ \text{J} \) — one unit of electricity. Our panel is twice the area but lower intensity and half the time, so 0.8 of that unit makes sense.
Common Mistakes Students Make
| Students write… | Correct rule… | How to check |
|---|---|---|
| “Insolation and the solar constant are the same.” | Insolation is what reaches the surface (~\( 1\ \text{kWm}^{-2} \)); the solar constant (\( 1.4\ \text{kWm}^{-2} \)) is measured at the top of the atmosphere. | Ask: where is it measured? Top of atmosphere → solar constant; surface → insolation. |
| “Ozone is always good for us.” | Stratospheric ozone blocks UV (protective); ground-level ozone from smog is harmful. | Location decides: 12–50 km up = protective; city air = pollutant. |
| “The valley breeze blows at night.” | Valley breeze is daytime (warm air rises up slopes); mountain breeze is night (cold dense air sinks). | Day → air moves up; night → air moves down. |
| “Plants use nitrogen gas directly from air.” | \( \text{N}_2 \) is non-reactive; it must be fixed into ammonia/nitrates first. | Look for the fixation step before assimilation. |
| “Nitrification is the same as nitrogen fixation.” | Fixation converts \( \text{N}_2 \rightarrow \text{NH}_3 \); nitrification converts \( \text{NH}_3 \rightarrow \text{NO}_2^- \rightarrow \text{NO}_3^- \). | Check the starting material: \( \text{N}_2 \) = fixation; ammonia = nitrification. |
| “Winds deflect left in the Northern Hemisphere.” | Coriolis deflects winds right in the Northern Hemisphere, left in the Southern. | Northern = right; Southern = left. |
Exam Notes: What Examiners Look For
Exam questions on this chapter come from the Revise, Reflect, Refine set (NCERT, p. 267). The pattern:
- Q2 (warming mechanism): classic MCQ — the Earth’s surface absorbs solar radiation, re-radiates it, and greenhouse gases trap it. Option (iii) states this mechanism exactly.
- Q5 (mountain vs valley breeze): application question — add the albedo twist: grass-covered slopes and barren rock slopes cool at different rates, so the two mountain breezes differ in temperature.
- Q7 (nitrogen cycle) and Q9 (carbon path): diagram-and-explain favourites. Name the process first, then the organism/bacteria, then the product — this sequence earns the mark.
- Q12 (flat disc Earth) and Q13 (temperature rise): systems-thinking questions — connect insolation pattern to temperature, then temperature to the cryosphere, hydrosphere and biosphere.
The ‘At a Glance’ points (NCERT, p. 266) are the safest revision checklist — every bullet is a potential 1-mark question. For full marks on process questions, always state: process name → organism → product → why it matters.
For the complete set of chapter resources, browse the Class 9 Science notes hub, or go up to all Class 9 notes and the main CBSE notes library.
Earth as a System Energy Matter and Life Class 9 Notes: Quick Revision Summary
- The Sun’s electromagnetic radiation is the primary source of energy on Earth.
- Most weather processes (evaporation, condensation, precipitation) occur in the troposphere.
- The Earth’s shape, latitude and axial tilt drive variations in insolation → uneven heating.
- Uneven heating generates winds and ocean currents.
- Matter and energy cycle continuously between biotic and abiotic systems.
- Atmospheric oxygen is consumed by combustion, respiration and oxide formation, and restored mainly by photosynthesis.
- Water, carbon, nitrogen and oxygen cycle continuously between the atmosphere, oceans, land and living organisms.
- Biogeochemical cycles make nutrients available, sustain life, regulate climate and balance ecosystems.
One-line takeaway: energy flows, matter cycles.
Frequently Asked Questions
Why is the solar constant higher than the insolation reaching the Earth’s surface?
The solar constant (\( 1.4\ \text{kWm}^{-2} \)) is measured at the top of the atmosphere before any absorption. Gases, clouds and dust absorb and scatter some energy on the way down, so maximum surface insolation is about \( 1\ \text{kWm}^{-2} \) under clear skies.
What is the difference between a valley breeze and a mountain breeze?
Timing and direction. During the day, sun-heated slopes make warm air rise and cooler valley air flows upslope — a valley breeze. At night, slopes cool faster, and cold dense air flows down into the valley — a mountain breeze.
Why is ozone protective in the stratosphere but harmful at ground level?
Location decides the effect. Stratospheric ozone (12–50 km up) absorbs harmful UV radiation and protects life. Ground-level ozone forms when vehicular emissions react with sunlight in smog and is a health-damaging pollutant.
How does the carbon cycle connect the atmosphere, biosphere, and geosphere?
Plants take \( \text{CO}_2 \) from the atmosphere for photosynthesis (biosphere). When organisms die and get buried, their carbon becomes fossil fuels in the geosphere over millions of years. Burning those fuels returns \( \text{CO}_2 \) to the atmosphere on a short time scale.
What is eutrophication and how does it harm water bodies?
Excess nitrates from fertiliser overuse enter rivers and lakes, causing algal blooms. When the algae decay, they deplete oxygen and kill fish, threatening water bodies and coastal fisheries.
Why are winds deflected to the right in the Northern Hemisphere?
Because of the Earth’s rotation on its axis (the Coriolis effect). The rotation deflects moving air from a straight path — to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Reference: NCERT Class 9 Science textbook, Chapter 13 — Earth as a System: Energy, Matter, and Life.
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