Earth as a System treats our planet as five linked spheres: geosphere, hydrosphere, cryosphere, atmosphere and biosphere. It follows energy from the Sun as electromagnetic radiation, explains insolation, the solar constant and albedo, and shows how latitude and the atmosphere make heating uneven. That uneven heating drives valley and mountain breezes, planetary winds and ocean currents. The chapter then traces the water, carbon, nitrogen and oxygen cycles and how human activity disturbs them, which is the base for later work on life processes and the environment.
Key Concepts
1. The Earth as One System: Five Interacting Spheres
Life on Earth runs on a constant flow of energy and matter. The Sun is the main source of energy; the Earth’s hot interior and chemical reactions in air, water and rocks also drive this flow. This chapter joins ideas you met separately in earlier classes into one Earth system of five interacting spheres.
| Sphere | What it contains | Indian examples from the chapter |
|---|---|---|
| Geosphere | Solid rocks, soil, landforms and the Earth’s interior | Deccan plateau, Thar desert |
| Hydrosphere | Liquid water: oceans, rivers, lakes and groundwater | Ganga-Brahmaputra river system |
| Cryosphere | Solid water: ice and snow | Himalayan glaciers, snow in Ladakh, polar ice caps |
| Atmosphere | The air surrounding the Earth that we breathe | Cleaner air in mountains and forests |
| Biosphere | All living organisms and their habitats | Mangroves, forests, farms, ocean plankton, coral reefs |
Solar heating, movement of air and water, and nutrient cycling connect these spheres in a delicate balance. The key idea: a disturbance in one sphere leads to changes in the others.
Activity 13.1 in words: a mountain scene has snow on the peaks, a lake in the valley, grass and grazing sheep. Snowmelt (cryosphere) feeds the lake (hydrosphere), which waters grass (biosphere) growing on soil (geosphere). If winters bring less snow for a few years, the lake level falls, less grass grows and the sheep have less to eat.
A large-scale chain from the chapter:
- Warmer Arabian Sea water leads to more evaporation from the sea.
- This causes fluctuations in the southwest monsoon, so rainfall becomes variable: floods in some regions of India and drought in others. The hydrosphere is disrupted.
- A rise in atmospheric temperature can speed up the melting of glaciers and polar ice (cryosphere).
- Melting can flood low-lying regions and, in the long run, raise sea levels that threaten coastal cities.
- Ecosystems in the biosphere are disturbed because habitats are lost.
In “interrelationship” answers, start in one sphere, name the change, then name each sphere it passes into.
2. Solar Radiation, Insolation and the Solar Constant
Solar radiation is the main source of energy on the Earth. It reaches us as electromagnetic (EM) waves, which travel through a vacuum at the speed of light, 3 × 108 m s-1. Sound waves are different: they are mechanical waves and need a medium, which is why sound cannot cross the empty space between the Sun and the Earth while light can.
The electromagnetic spectrum
The full range of EM radiation is called the electromagnetic spectrum. In order from high frequency (short wavelength) to low frequency (long wavelength):
Gamma rays → X-rays → Ultraviolet (UV) → Visible → Infrared (IR) → Microwaves → Radio waves
Gamma rays and X-rays carry very high energy and are mostly filtered out by the upper atmosphere. Microwaves and radio waves carry too little energy to warm the Earth. About 99 per cent of the Sun’s energy falls in the UV, visible and IR range.
Short wavelength UV is mostly absorbed by the ozone layer, protecting life. Visible light reaches the surface and powers photosynthesis, the primary source of food for most organisms. Infrared warms the surface, which re-radiates heat that greenhouse gases partly trap.
Ready to Go Beyond box: UV rays lie between 100 nm and 400 nm (1 nm = 10-9 m) and carry more energy than visible light. Long exposure damages eyes and skin and raises cancer risk; UV is also used to kill germs in water purifiers.
Insolation and the solar constant
Insolation is the amount of the Sun’s radiation that reaches the Earth’s surface. It warms the Earth’s surface and its atmosphere.
The solar constant is the average amount of solar energy received per unit time per unit area that is perpendicular to the Sun’s rays at the top of the Earth’s atmosphere. Its value is about 1.4 kW m-2, or about 1400 J s-1 m-2. It is the Sun’s energy available before any absorption, scattering or reflection by the atmosphere.
Gases, clouds and dust absorb and scatter some of this energy on the way down. So the maximum insolation at the surface is lower, about 1 kW m-2 under clear sky conditions.
| Quantity | Where it is measured | Approximate value |
|---|---|---|
| Solar constant | Top of the atmosphere, surface held perpendicular to the rays | 1.4 kW m-2 |
| Maximum insolation | Earth’s surface, clear sky | 1 kW m-2 |
The solar constant helps scientists understand the Earth’s energy balance, climate and weather. India, in the tropical and sub-tropical regions, receives abundant sunlight all year. This insolation drives the southwest monsoon and gives India large potential for solar energy.
Energy from insolation: E = intensity × area × time. With intensity in J s-1 m-2, area in m2 and time in s, E is in joules.
Anna Mani, India’s pioneering atmospheric scientist, mapped insolation across India and, with S. Rangarajan, published Solar Radiation Over India (1982), the country’s first insolation atlas.
3. Why the Earth Heats Unevenly: Albedo, Materials and Latitude
Land heats up faster than water. Dark surfaces absorb more sunlight and heat up; light-coloured surfaces reflect more and stay cooler, which is why dark roads and dark clothes get hot in summer.
Albedo
The fraction of solar radiation reflected by a surface is called its albedo. The word comes from Latin and means whiteness. Albedo is a fraction, so it has no unit and lies between 0 and 1.
- High albedo surfaces reflect more light and stay cool.
- Low albedo surfaces reflect less, absorb more and heat up quickly.
| Material (NCERT Table 13.1) | Albedo given in the textbook | Meaning |
|---|---|---|
| Snow | 0.80 to 0.90 | Reflects 80 to 90% of incoming sunlight |
| Ice | 0.50 to 0.70 | Reflects 50 to 70% |
| Crushed rock | 0.25 to 0.30 | Reflects 25 to 30% |
The textbook leaves the rows for light-coloured soil, black soil and ocean water for you to fill in (Activity 13.2). It states the pattern: snow and ice have high albedo, keeping polar regions very cold, while black soil and ocean water have lower albedo and are relatively warmer.
All objects also radiate heat. A concrete house feels hot at night in summer because the concrete re-radiates the heat it absorbed in the day. Traditional houses with thick mud and wooden walls stay cool in hot summers because they re-radiate less.
Urban heat island effect
Cities are warmer than surrounding rural areas, especially in summer and at night. Steel, concrete, brick and asphalt absorb solar radiation, retain it and re-radiate it, warming the city and raising the demand for air conditioning. Villages and forests stay cooler through shade and transpiration. Human land use can change local climate.
Latitude and the Earth’s shape
The Earth is spherical, so the Sun’s rays strike different latitudes at different angles.
- Near the equator, the radiation is concentrated over a smaller area, so each square metre gets more energy. Equatorial regions stay relatively warm all year.
- Near the poles, the same radiation is spread over a larger area, so each square metre gets less. Polar regions are much colder.
The spherical shape and the tilt of the Earth’s axis also cause seasons and changing day length. The chapter summary names the shape of the Earth, latitude and tilt of the axis as mainly responsible for uneven heating, which drives global winds and ocean currents.
4. The Atmosphere: Layers, Ozone and the Greenhouse Effect
The atmosphere is the air surrounding the Earth, held in place by the Earth’s gravity. It consists mainly of nitrogen (78%) and oxygen (21%), with small amounts of argon, carbon dioxide, water vapour and other gases.
Layers of the atmosphere
The atmosphere is layered. From the ground up: troposphere, stratosphere, mesosphere, thermosphere, exosphere.
| Layer | Approximate altitude | Features |
|---|---|---|
| Troposphere | 0 to 12 km (average height about 12 km) | Weather forms here; heated from the Earth’s surface; temperature decreases with height (about 6.5 °C per km) |
| Stratosphere | 12 to 50 km | Ozone layer absorbs UV; temperature increases with height; calm, little vertical mixing |
| Mesosphere, thermosphere, exosphere | Above 50 km | Minor role in regulating climate at the surface |
- Troposphere: nearly all weather phenomena happen here. It is heated from the ground up, so warm air near the surface rises and drives winds and storms. Its height is greatest above the equator and lowest above the poles.
- Stratosphere: ozone absorbs UV rays and heats this layer, so temperature rises with height. Warmer air sitting above cooler air does not mix vertically, which keeps the stratosphere calm and confines weather to the troposphere.
- At about 100 km above the Earth begins the region we call outer space.
Two protective roles of the atmosphere
- It partly absorbs incoming solar radiation. The ozone layer blocks harmful UV rays. Clouds and other gases absorb some sunlight before it reaches the surface.
- It traps outgoing heat. The surface absorbs sunlight and re-radiates it as infrared. Greenhouse gases such as CO2, CH4 and water vapour absorb this re-radiated heat and stop it escaping into space.
Without the atmosphere, the Earth would be too cold for life. The trapping of heat by greenhouse gases is the greenhouse effect.
Excess CO2 from human activities enhances the greenhouse effect and causes global warming, which if left unchecked could make the Earth uninhabitable. Venus is hotter than Mercury, even though Mercury is closer to the Sun, because Venus has an atmosphere that has led to an uncontrolled greenhouse effect.
Ozone layer and the ozone hole
The ozone layer is a protective shield that absorbs harmful UV radiation. When ozone molecules are destroyed faster than they form, the layer thins. In the late 20th century, human-made chlorofluorocarbons (CFCs), used in refrigerators and aerosols, caused severe ozone loss over Antarctica, known as the ozone hole. More UV reaching the Earth harms organisms and ecosystems. A global agreement, the Montreal Protocol, reduced the use of CFCs, and the ozone layer is now slowly recovering.
5. Uneven Heating Drives Winds and Ocean Currents
Wind is the movement of air from a region of high pressure to a region of low pressure. These pressure differences come mainly from uneven heating of the Earth’s surface. The same idea explains land and sea breezes and much larger winds and currents.
Local winds: valley and mountain breezes
In mountains, the slopes and the valley floor heat and cool at different rates.
| Feature | Valley breeze | Mountain breeze |
|---|---|---|
| When | Daytime | After sunset (night) |
| Which part is warmer | Sun-facing slopes heat faster than the valley floor | Valley floor stays warmer; slopes lose heat faster |
| What the air does | Warm air over the slopes rises, making low pressure; cooler valley air moves up the slopes | Cool, denser air over the slopes flows down into the valley |
| Direction | Up the slope | Down the slope |
Such daily wind changes are common in Shimla, Dehradun and other Himalayan valleys, and they help regulate temperature and moisture for soil and crops.
Planetary winds and pressure belts
Uneven heating between the equator and the poles creates belts of high and low pressure. The large-scale pressure differences move air over long distances as planetary winds.
| Latitude | Pressure belt | What the air does |
|---|---|---|
| 0° (equator) | Equatorial low pressure belt | Intense heating makes warm air rise; it moves poleward at height |
| About 30° N and S | Sub-tropical high pressure belts | Air cools, becomes denser and sinks; part flows back to the equator along the surface |
| About 60° N and S | Sub-polar low pressure belts | Surface air from 30° moving poleward meets cold polar air and rises |
| About 90° N and S (poles) | Polar high pressure belts | Very cold, dense air sinks and flows towards the sub-polar belts |
The rotation of the Earth deflects these winds from straight paths, so planetary winds follow curved paths. Winds turn to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Ocean currents
Ocean currents are the continuous movement of large masses of ocean water. Factors that drive and shape them:
- Winds: strong planetary winds drag surface water through friction, setting surface currents in motion.
- Temperature: warm equatorial water travels over the surface towards the poles; colder, denser water slowly flows back towards the equator at deeper levels.
- Salinity: lower-salinity water is less dense and stays near the surface; higher-salinity, denser water sinks and moves at depth.
- Earth’s rotation: deflects moving water into large circular patterns called gyres, clockwise in the Northern Hemisphere and counter clockwise in the Southern Hemisphere.
- Land masses: continents block and redirect currents.
Why currents matter: they carry heat from the equator towards the poles and reduce temperature differences. The warm North Atlantic Drift, an extension of the Gulf Stream, keeps many ports on the northwestern coast of Europe ice-free in winter, supporting trade. Currents also carry nutrients that support marine ecosystems.
6. Biogeochemical Cycles: Water, Carbon, Nitrogen and Oxygen
Living organisms constantly exchange matter and energy with the air, water, soil and rocks. The cyclic movement of matter and energy between the abiotic (non-living) and biotic (living) components is called a biogeochemical cycle. These cycles keep essential nutrients such as carbon, nitrogen and oxygen recycled and available to life.
a) Water cycle
Water evaporates from water bodies and is transpired by plants, condenses into clouds, and returns as rain, hail or snow. It runs off to the ocean or seeps underground as groundwater. On the way it dissolves minerals and carries nutrients to the oceans for marine life.
How climate change affects the water cycle:
- A warmer atmosphere holds more moisture, causing heavier rain in some areas (such as intensified monsoons) and droughts elsewhere.
- Melting glaciers add water to rivers and, in the long run, raise sea levels, threatening coastal cities such as Mumbai and Chennai.
- Sudden bursts of intense rain cause more runoff, which erodes soil. Less water infiltrates, so groundwater recharge falls and farming in dry months becomes harder.
b) Carbon cycle
Carbon forms the backbone of life: every protein, carbohydrate, fat and DNA molecule contains carbon. Carbon circulates between the atmosphere (CO2), biosphere (plants and animals), geosphere (carbonate rocks and fossil fuels such as coal and oil) and hydrosphere (dissolved CO2 and marine shells).
| Fast carbon cycle | Slow carbon cycle | |
|---|---|---|
| Time scale | Days to years | Millions of years |
| Processes | Photosynthesis takes CO2 into glucose; respiration releases CO2; animals eat plants or other animals; decomposition of dead organisms returns CO2 to the air | Dead plants and animals are buried and turned into fossil fuels (coal, oil, gas); dead marine organisms sink and store carbon on the ocean floor |
Burning fossil fuels returns carbon stored over millions of years to the air as CO2 on a very short time scale. Ocean water absorbs CO2 to form carbonate and bicarbonate ions, used by phytoplankton for photosynthesis and by some marine organisms to build shells.
Carbon numbers from the chapter: carbon is about 49% of the dry weight of living organisms; 71% of global carbon is in the oceans, and this oceanic reservoir regulates atmospheric CO2; the atmosphere holds only about 1% of global carbon.
Rising CO2: burning fossil fuels and deforestation have raised atmospheric CO2 from 315 ppm to 420 ppm since 1960 (ppm = parts per million), which the textbook calls about 35% and unprecedented in human history. This record is the Keeling curve; its yearly sawtooth dips come from Northern Hemisphere plant growth absorbing CO2. Excess CO2 strengthens the greenhouse effect, causing global warming, melting ice, rising sea level and extreme weather; for India this may mean more intense monsoons and threats to agriculture.
c) Nitrogen cycle
Nitrogen is essential for making proteins and nucleic acids. The largest reservoir of nitrogen is the atmosphere, but N2 gas is rather non-reactive and plants and animals cannot use it directly. It must first be converted into soluble compounds.
| Step | Change | Done by |
|---|---|---|
| Nitrogen fixation | Atmospheric N2 → ammonia (NH3) | Nitrogen-fixing bacteria: Rhizobium in root nodules of legumes, Azotobacter in soil |
| Nitrification (step 1) | Ammonia (NH3) → nitrite (NO2-) | Nitrosomonas |
| Nitrification (step 2) | Nitrite (NO2-) → nitrate (NO3-) | Nitrobacter |
| Assimilation | Plants absorb nitrogen compounds from soil; animals get nitrogen by eating plants or other animals | Plants, animals |
| Ammonification | Dead organisms and wastes → ammonia returned to soil | Decomposers (bacteria and fungi) |
| Denitrification | Some nitrates → nitrogen gas (N2) | Denitrifying bacteria such as Pseudomonas |
Denitrification completes the cycle and keeps nitrogen balanced in ecosystems. Lightning also fixes a tiny amount of atmospheric nitrogen as nitrogen oxides.
Haber-Bosch process (Ready to Go Beyond box): most nitrogen today is fixed artificially by this process, which makes ammonia from atmospheric nitrogen for fertilisers and enabled India’s Green Revolution. It uses about 1 to 2% of global energy, and overuse of fertilisers has degraded soil and water.
d) Oxygen cycle
About 21% of the atmosphere is free oxygen (O2). Oxygen is part of most biological molecules (carbohydrates, proteins, nucleic acids, fats). It also exists in combined forms: metal oxides and minerals in the crust, and carbon dioxide in the air.
- Removal of O2: respiration by plants and animals (releases CO2), combustion of fuels (releases CO2), and oxide formation.
- Restoration of O2: photosynthesis, in which plants use sunlight, water and CO2 to form glucose and release O2.
The balance between consumption (respiration and combustion) and production (photosynthesis) circulates oxygen between the atmosphere, land, oceans and living organisms. The oxygen and carbon cycles are linked: photosynthesis takes in CO2 and gives out O2, while respiration and combustion do the reverse.
7. Human Impact on the Earth’s Processes
Human actions disturb the biogeochemical cycles across all the spheres.
| Human activity | Cycle or sphere affected | Result |
|---|---|---|
| Burning fossil fuels | Carbon cycle, atmosphere | Excess CO2, stronger greenhouse warming; natural sinks (forests, oceans) become saturated |
| Excess CO2 absorbed by oceans | Hydrosphere, biosphere | Sea water becomes more acidic, threatening plankton and coral reefs |
| Warming of oceans | Carbon cycle | Warmer water absorbs less CO2, so the ocean works less well as a carbon sink |
| Overuse of fertilisers | Nitrogen cycle, hydrosphere | Nitrates run into rivers and lakes, causing algal blooms that deplete oxygen and kill fish (eutrophication) |
| Deforestation | Carbon, oxygen and water cycles; geosphere; biosphere | Less photosynthesis and transpiration, possible fall in local rainfall, changed surface albedo, more soil erosion, habitat and biodiversity loss |
| Vehicular emissions | Atmosphere | React with sunlight to form smog and harmful ground-level ozone |
| CFCs (past use) | Atmosphere | Ozone hole over Antarctica |
The Montreal Protocol has started the recovery of the ozone layer through global cooperation, while the Kyoto Protocol and the Paris Agreement on CO2 emissions have been less successful.
Restoring the balance: conserve energy, switch to renewables such as solar and wind, plant trees, save water, practise sustainable farming, and reduce, reuse and recycle.
Mission LiFE (Lifestyle for Environment), an India-led initiative introduced at the United Nations Climate Change Conference in 2021, encourages eco-friendly lifestyles.
Formula and Reaction Sheet
| Item | Formula, reaction or value | Note |
|---|---|---|
| Energy received from sunlight | E = intensity × area × time | Intensity in J s-1 m-2, area in m2, time in s gives E in J |
| Solar constant | About 1.4 kW m-2 = 1400 J s-1 m-2 | Top of the atmosphere, perpendicular to the rays |
| Maximum surface insolation | About 1 kW m-2 | Clear sky |
| Unit conversion | 1 kW = 1000 W = 1000 J s-1; 1 kWh = 3.6 × 106 J | 1 kWh is one unit on an electricity bill |
| Albedo | Albedo = reflected radiation / incoming radiation | No unit; fraction absorbed = 1 - albedo (ignoring transmission) |
| Troposphere temperature fall | About 6.5 °C per km of height | Temperature at height h ≈ surface temperature - 6.5 × h (h in km) |
| Photosynthesis | Carbon dioxide + water → glucose + oxygen (in sunlight) | Takes in CO2, restores O2 |
| Respiration | Glucose + oxygen → carbon dioxide + water + energy | Uses O2, releases CO2 |
| Combustion of fuels | Fuel + oxygen → carbon dioxide + heat | Burning fossil fuels returns stored carbon to air |
| Nitrogen fixation | N2 → NH3 | Rhizobium, Azotobacter |
| Nitrification | NH3 → NO2- → NO3- | Nitrosomonas, then Nitrobacter |
| Denitrification | NO3- → N2 | Pseudomonas |
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Important Definitions
- Earth system: the Earth viewed as one system of five interacting spheres (geosphere, hydrosphere, cryosphere, atmosphere, biosphere) connected by flows of energy and matter.
- Insolation: the amount of the Sun’s radiation that reaches the Earth’s surface.
- Solar constant: the average amount of solar energy received per unit time per unit area perpendicular to the Sun’s rays at the top of the Earth’s atmosphere; about 1.4 kW m-2.
- Albedo: the fraction of solar radiation reflected by a surface.
- Greenhouse gases: gases such as CO2, CH4 and water vapour that absorb heat re-radiated by the Earth’s surface.
- Planetary winds: large-scale winds set up by pressure belts formed through uneven heating between the equator and the poles.
- Ocean current: continuous movement of large masses of ocean water.
- Gyre: a large circular pattern of ocean currents formed by the Earth’s rotation.
- Biogeochemical cycle: the cyclic movement of matter and energy between the abiotic and biotic components of the Earth.
- Nitrogen fixation: conversion of atmospheric N2 into ammonia.
- Nitrification: conversion of ammonia into nitrite and then nitrate by nitrifying bacteria.
- Ammonification: return of nitrogen compounds such as ammonia to the soil when decomposers break down dead matter and wastes.
- Denitrification: conversion of some nitrates back into nitrogen gas by denitrifying bacteria.
- Eutrophication: excess nutrients (nitrates from fertilisers) in water bodies causing algal blooms that deplete oxygen and kill fish.
Solved Examples (NCERT-Based)
Example 1: Solar energy on 1 m2 in one hour (NCERT Example 13.1)
How much solar energy will be received by a 1 m2 area in one hour, if the insolation on the surface of the Earth were 1 kW m-2?
Solution: E = intensity × area × time.
Intensity = 1 kW m-2 = 1000 J s-1 m-2; area = 1 m2; time = 1 h = 3600 s.
E = 1000 × 1 × 3600 = 3600000 J = 3.6 × 106 J.
The textbook notes that this equals one unit of household electricity, and is roughly the energy needed to melt 5 kg of ice and heat the water to 100 °C.
Example 2: Energy at the top of the atmosphere
Using the solar constant, find the solar energy received in one minute by a 2 m2 panel held perpendicular to the Sun’s rays at the top of the atmosphere.
Solution: Solar constant = 1400 J s-1 m-2; area = 2 m2; time = 60 s.
E = 1400 × 2 × 60 = 168000 J = 1.68 × 105 J.
Example 3: Rooftop sunlight in a day
A flat roof of 10 m2 receives an average insolation of 1 kW m-2 for 5 hours. Find the solar energy falling on it in kWh and in joules.
Solution: Power falling on the roof = 1 kW m-2 × 10 m2 = 10 kW.
Energy = power × time = 10 kW × 5 h = 50 kWh = 50 × 3.6 × 106 J = 1.8 × 108 J.
Solar panels convert only a fraction of this into electricity.
Example 4: Albedo and absorbed energy
Sunlight of 1000 W falls on each square metre of two surfaces: fresh snow with albedo 0.80 and crushed rock with albedo 0.25. How much power does each square metre absorb, and which surface warms faster?
Solution: Reflected = albedo × incoming. Absorbed = incoming - reflected.
Snow: reflected = 0.80 × 1000 = 800 W; absorbed = 1000 - 800 = 200 W.
Crushed rock: reflected = 0.25 × 1000 = 250 W; absorbed = 1000 - 250 = 750 W.
The rock absorbs almost four times as much, so it warms faster. Low albedo means more absorption and more heating, which is why polar snow stays cold.
Example 5: Temperature on a hill
On a summer afternoon the temperature at a plain near sea level is 30 °C. Estimate the temperature at a hill station 2 km higher, using the textbook’s rate of fall in the troposphere.
Solution: Temperature falls about 6.5 °C per km in the troposphere.
Fall over 2 km = 6.5 × 2 = 13 °C. Estimated temperature = 30 - 13 = 17 °C.
This is an estimate, since 6.5 °C per km is an average rate.
Example 6: Reading the CO2 record
Atmospheric CO2 rose from 315 ppm to 420 ppm since 1960. Find the increase in ppm and as a percentage of the 1960 value.
Solution: Increase = 420 - 315 = 105 ppm.
Percentage increase = (105 / 315) × 100 = 33.3%.
These two rounded readings give about 33%. The textbook states the rise as about 35%, so quote 35% for a direct question and show 105 / 315 when asked to calculate. The chapter calls this rise unprecedented in the history of human civilisation.
Example 7: Grass-covered and barren mountains (NCERT Exercise 5)
How are mountain and valley breezes formed? Suppose there are two mountains, one covered with grass and another covered with barren rocks; would the temperature of the two mountain breezes be different? If so, how?
Solution: By day, sun-facing slopes heat faster than the valley floor. Warm air over the slopes rises, creating low pressure, and cooler valley air moves up the slopes: the valley breeze. After sunset the slopes lose heat faster than the valley floor. Air over the slopes becomes cool and dense and flows down into the valley: the mountain breeze.
The two mountain breezes would likely differ. Bare rock heats strongly by day and keeps re-radiating heat after sunset, warming the air above it. Grass shades the ground and cools it through transpiration. So the grassy mountain would be expected to give a cooler mountain breeze.
Example 8: Tracing a carbon atom (NCERT Exercise 9)
Explain with suitable diagram the path that carbon takes to go back to the atmosphere. You may start from plants using CO2 from the atmosphere.
Solution: Draw “CO2 in atmosphere” at the top with arrows to and from plants, animals, decomposers, fossil fuels and the ocean:
- Plants take in CO2 and make glucose by photosynthesis.
- Path A: plants respire and release CO2 back to the air.
- Path B: animals eat plants, respire and release CO2.
- Path C: plants and animals die; decomposers break down their bodies and release CO2.
- Path D (slow): buried remains become coal, oil and gas over millions of years; burning these fuels releases CO2.
- Path E: oceans absorb CO2 and exchange it with the air; marine organisms store some carbon in shells and on the ocean floor.
Competency-Based Questions (with answers)
1. Case-based: The shrinking lake
A village in a high valley depends on a lake fed by snowmelt. The lake waters pastures where shepherds graze sheep. For three winters, snowfall has been much lower than usual.
(a) Name the spheres involved in this chain. Cryosphere (snow), hydrosphere (lake), geosphere (soil of the pasture), biosphere (grass, sheep, people), atmosphere (snowfall, temperature).
(b) Predict the effect on the sheep. Less snow means less meltwater in summer, a lower lake level, less water for grass and so less fodder for the sheep.
(c) Which idea of the chapter does this case show? A disturbance in one sphere leads to changes in the others.
2. Case-based: Heat in the city
A student records night temperatures in a city centre and in a village 30 km away on the same summer nights. The city is warmer every night. The city has concrete buildings and asphalt roads; the village has fields and trees.
(a) Name the effect. The urban heat island effect.
(b) Explain why the city stays warmer at night. Concrete, brick and asphalt absorb solar radiation by day and retain it. At night they re-radiate the stored heat and keep the city air warm. Vegetation in the village cools the air through shade and transpiration.
3. Assertion-Reason
Options for Q3 to Q5: (a) Both A and R are true and R is the correct explanation of A. (b) Both A and R are true but R is not the correct explanation of A. (c) A is true but R is false. (d) A is false but R is true.
Assertion (A): Weather phenomena are confined mainly to the troposphere. Reason (R): In the stratosphere, temperature increases with height, so there is little vertical mixing of air.
Answer: (a). The warm-over-cool arrangement in the stratosphere keeps it calm and holds weather in the troposphere.
4. Assertion-Reason
Assertion (A): Polar regions are very cold. Reason (R): Snow and ice have a low albedo.
Answer: (c). A is true. R is false: snow and ice have a high albedo, which is why they reflect most sunlight and keep the poles cold. The spreading of sunlight over a larger area at high latitudes also contributes.
5. Assertion-Reason
Assertion (A): Plants cannot directly use atmospheric nitrogen. Reason (R): N2 gas is rather non-reactive and must first be converted into soluble compounds.
Answer: (a). Both true, and R explains A.
6. Data reasoning: The Keeling curve
The textbook’s graph of atmospheric CO2 from 1960 to 2025 rises steadily with a small sawtooth wiggle every year. It starts near 315 ppm and ends near 420 ppm.
(a) What causes the overall rise? Burning fossil fuels and deforestation.
(b) What causes the yearly dips in the sawtooth? Plant growth in the Northern Hemisphere each year absorbs CO2 through photosynthesis.
(c) Give two effects of this rise named in the chapter. Any two of: global warming from a stronger greenhouse effect, melting of glaciers and Arctic sea ice, rising sea level, more extreme weather, more acidic sea water.
7. Competency MCQ: Planetary winds
Air sinks around 30° N. Which statement is correct?
(i) It forms a low pressure belt and all of it flows to the pole. (ii) It forms a high pressure belt; part flows to the equator and part towards the poles along the surface. (iii) It forms a high pressure belt and all of it rises again at the equator without moving. (iv) It forms a low pressure belt deflected to the left.
Answer: (ii). Sinking air makes the sub-tropical high pressure belt. Part returns to the equator; part moves poleward and rises near 60° N.
8. Case-based: A green lake
Farmers near a lake apply large amounts of nitrate fertiliser. After heavy rain, the lake turns green and many fish die.
(a) Name the process. Eutrophication.
(b) Explain the chain from field to dead fish. Excess nitrates wash into the lake, algae grow rapidly into an algal bloom, the bloom depletes oxygen in the water, and fish die.
(c) Which cycle has been disturbed, and one way to prevent this? The nitrogen cycle. Use fertilisers in the right amount and practise sustainable farming.
Important Questions for Exams
1-Mark Questions
Q1. Choose the most appropriate option to describe the role of biogeochemical cycles in an ecosystem. (i) To provide food directly to all organisms. (ii) To recycle essential nutrients between biotic and abiotic components. (iii) To create new elements for use by living things. (iv) To remove pollutants and toxins from the organism. (NCERT Exercise 1)
Answer: (ii).
Q2. Which of the following is primarily responsible for warming of the Earth? (i) Solar radiation is immediately absorbed by carbon dioxide, which then releases it as heat. (ii) The atmosphere’s tiny particles absorb incoming solar radiation, which directly heats the Earth. (iii) The Earth’s surface absorbs solar radiation, which is then re-radiated and trapped by greenhouse gases. (iv) The Earth’s environment is heated only by the solar radiation reflected by the clouds. (NCERT Exercise 2)
Answer: (iii).
Q3. What is the approximate value of the solar constant?
Answer: About 1.4 kW m-2 (1400 J s-1 m-2).
Q4. Name the bacterium that converts nitrite into nitrate.
Answer: Nitrobacter.
2-Mark Questions
Q5. Distinguish between insolation and solar constant.
Answer: Insolation is the Sun’s radiation that reaches the Earth’s surface; its maximum is about 1 kW m-2 under clear sky. The solar constant is the average solar energy per unit time per unit area perpendicular to the rays at the top of the atmosphere, before any absorption, scattering or reflection; it is about 1.4 kW m-2.
Q6. Why is an excess of CO2 in the atmosphere considered undesirable even though it is required by plants? (NCERT Exercise 10)
Answer: Plants need CO2 for photosynthesis, and some CO2 keeps the Earth warm. Excess CO2 intensifies the greenhouse effect, causing global warming, melting ice, rising sea levels and extreme weather, and it makes sea water more acidic, threatening plankton and coral reefs.
Q7. You have witnessed weather phenomena, such as winds, storms, rainfall, etc. Which atmospheric layer is mainly responsible for such phenomena and what is the primary reason for its occurrence? (NCERT Exercise 6)
Answer: The troposphere. It is heated from the Earth’s surface, so temperature falls with height (about 6.5 °C per km). Warm air near the ground rises, and this rising air drives winds and storms and carries moisture up to form clouds and rain.
3-Mark Questions
Q8. Describe how albedo affects the Earth’s surface temperature and its climate. (NCERT Exercise 4)
Answer: Albedo is the fraction of solar radiation a surface reflects. High-albedo surfaces such as snow (0.80 to 0.90) and ice (0.50 to 0.70) reflect most sunlight and stay cool, which keeps polar regions very cold. Low-albedo surfaces such as black soil and ocean water absorb more and are relatively warmer. Changes in land cover change albedo: deforestation alters surface albedo, and cities of concrete and asphalt absorb and re-radiate heat, forming urban heat islands. So albedo helps set both local temperatures and the global energy balance.
Q9. Explain how climate change affects the water cycle. Illustrate with examples. (NCERT Exercise 3)
Answer: A warmer atmosphere holds more moisture, giving heavier rain in some areas, such as intensified monsoons, and droughts elsewhere. Melting glaciers add water to rivers and raise sea levels over time, threatening coastal cities such as Mumbai and Chennai. Sudden intense rain increases runoff and soil erosion, and less infiltration reduces groundwater recharge, making farming hard in dry months.
Q10. How is heat lost from the surface of the Earth? What is its significance? (NCERT Exercise 11)
Answer: The surface absorbs sunlight and re-radiates heat as infrared radiation into the atmosphere. Part of this outgoing heat escapes to space, and part is absorbed by greenhouse gases (CO2, CH4, water vapour). This trapping keeps the Earth warm enough for life. Re-radiation also explains the urban heat island effect and hot concrete houses at night. Too much trapping from excess greenhouse gases causes global warming.
Q11. If the Earth were a flat disc instead of a sphere, how would the patterns of solar radiation and temperature be different? (NCERT Exercise 12)
Answer: On a sphere, rays fall concentrated near the equator and spread over larger areas near the poles, giving warm equatorial and cold polar regions. On a flat disc facing the Sun, rays would strike every part at nearly the same angle, so insolation would be nearly uniform. Temperature differences between equator and poles would largely disappear, the pressure belts they create would weaken, and planetary winds and ocean currents driven by uneven heating would be much weaker.
5-Mark Questions
Q12. Explain the processes involved in the nitrogen cycle. How would life on Earth be affected if nitrogen were not cycled? (NCERT Exercise 7)
Answer: Atmospheric N2 is the largest reservoir but is non-reactive. (1) Nitrogen fixation: Rhizobium in root nodules of legumes and Azotobacter in soil convert N2 into ammonia; lightning also fixes some nitrogen as nitrogen oxides. (2) Nitrification: Nitrosomonas converts ammonia to nitrite, and Nitrobacter converts nitrite to nitrate. (3) Assimilation: plants absorb these compounds; animals get nitrogen by eating plants or other animals. (4) Ammonification: decomposers (bacteria, fungi) break down dead matter and wastes, returning ammonia to soil. (5) Denitrification: Pseudomonas converts some nitrates back to N2, completing the cycle.
If nitrogen were not cycled, fixed nitrogen in the soil would run out, plants could not make proteins and nucleic acids, food chains would collapse and dead matter would not return its nutrients.
Q13. What are the impacts of deforestation on the Earth’s oxygen and carbon cycles? What are the other consequences of deforestation? (NCERT Exercise 8)
Answer: Oxygen and carbon cycles: fewer trees means less photosynthesis, so less CO2 is removed from the air and less O2 is released. Burning or decay of cleared trees adds CO2. Forests act as carbon sinks, and deforestation saturates and weakens these sinks, adding to greenhouse warming. Other consequences: reduced transpiration can lower local rainfall (water cycle); surface albedo changes; without roots to hold the soil, erosion increases; habitats are destroyed and biodiversity declines as species lose their homes.
Q14. Describe the interrelationship between different spheres of the Earth. Illustrate with example how these spheres function in a delicate balance. (NCERT Exercise 15)
Answer: The five spheres (geosphere, hydrosphere, cryosphere, atmosphere, biosphere) are connected by solar heating, movement of air and water, and nutrient cycling, so a disturbance in one changes the others. Example: warmer Arabian Sea water increases evaporation and makes the southwest monsoon variable (atmosphere, hydrosphere), bringing floods to some regions and drought to others. Rising temperature melts glaciers and polar ice (cryosphere), flooding low-lying areas and raising sea level, and the resulting habitat loss disturbs ecosystems (biosphere). A smaller example: less winter snow lowers a lake, which reduces grass on the soil for grazing sheep.
Common Mistakes and Examiner Tips
- Mixing up insolation and solar constant. Fix: the solar constant is measured at the top of the atmosphere (about 1.4 kW m-2); insolation is what reaches the surface (maximum about 1 kW m-2).
- Saying snow has low albedo because it is cold. Fix: snow is cold because it has high albedo and reflects most sunlight. High albedo means more reflection.
- Swapping valley and mountain breezes. Fix: valley breeze by day, blowing up the slope; mountain breeze at night, blowing down the slope. Each is named after where the air comes from.
- Writing that the stratosphere has weather. Fix: nearly all weather is in the troposphere. The stratosphere is calm because temperature rises with height there.
- Treating all ozone as good. Fix: stratospheric ozone blocks UV; ground-level ozone from vehicle emissions and sunlight is harmful.
- Getting the direction of deflection wrong. Fix: winds deflect to the right in the Northern Hemisphere and to the left in the Southern. Gyres turn clockwise in the north and counter clockwise in the south.
- Mixing up Nitrosomonas and Nitrobacter. Fix: follow the path ammonia → nitrite → nitrate. Nitrosomonas does the first step (ammonia to nitrite); Nitrobacter does the second (nitrite to nitrate).
- Confusing ammonification with nitrification. Fix: ammonification returns ammonia from dead matter (decomposers); nitrification oxidises ammonia to nitrite and nitrate (nitrifying bacteria).
- Calling the greenhouse effect harmful in itself. Fix: the natural greenhouse effect keeps the Earth warm enough for life. The problem is the enhanced effect from excess CO2.
- Forgetting units in energy numericals. Fix: convert kW to J s-1 (× 1000) and hours to seconds (× 3600) before multiplying. Write the final unit.
- Listing spheres without linking them. Fix: in “interrelationship” answers, show a chain: change in sphere A → effect in sphere B → effect in sphere C.
Quick Revision Points
- The Sun is the main source of energy; the Earth’s hot interior and chemical reactions also drive energy and matter flows.
- About 99% of the Sun’s energy is in the UV, visible and IR range.
- Solar constant about 1.4 kW m-2; maximum surface insolation about 1 kW m-2.
- 1 m2 at 1 kW m-2 for 1 hour receives 3.6 × 106 J, one unit of electricity.
- Albedo is the fraction of radiation reflected. Snow 0.80 to 0.90, ice 0.50 to 0.70, crushed rock 0.25 to 0.30.
- Rays are concentrated at the equator and spread at the poles; shape, latitude and tilt cause uneven heating.
- Air: nitrogen 78%, oxygen 21%.
- Troposphere 0 to about 12 km, weather, temperature falls about 6.5 °C per km; thickest over the equator.
- Stratosphere 12 to 50 km, ozone layer, temperature rises with height.
- Greenhouse gases: CO2, CH4, water vapour. Venus is hotter than Mercury due to an uncontrolled greenhouse effect.
- CFCs caused the ozone hole; the Montreal Protocol is helping it recover.
- Wind blows from high to low pressure. Valley breeze by day, mountain breeze at night.
- Pressure belts: equatorial low (0°), sub-tropical high (30°), sub-polar low (60°), polar high (90°).
- Winds deflect right in the north, left in the south; gyres clockwise in the north, counter clockwise in the south.
- Ocean currents are driven by winds, temperature, salinity, rotation and land masses; the North Atlantic Drift keeps European ports ice-free.
- Carbon: fast cycle days to years, slow cycle millions of years; CO2 rose from 315 to 420 ppm since 1960.
- Nitrogen: fixation (Rhizobium, Azotobacter), nitrification (Nitrosomonas, Nitrobacter), assimilation, ammonification, denitrification (Pseudomonas).
- Oxygen is used in respiration, combustion and oxide formation, and restored mainly by photosynthesis.
Weightage and Exam Pattern
Class 9 has no board exam. This chapter is assessed in school exams (periodic tests, half-yearly and annual papers), and the NCERT source gives no fixed mark allocation for it. It is mostly descriptive, with a few simple energy calculations, so questions draw on definitions, sequences, cause-and-effect chains and short numericals.
| Question type | What this chapter typically supplies |
|---|---|
| MCQ and one-word | Solar constant value, layer names and heights, albedo meaning, bacteria in the nitrogen cycle, NCERT Exercise 1 and 2 |
| Assertion-Reason | Weather in the troposphere, albedo of snow, non-reactive N2, direction of deflection |
| Short answer (2 to 3 marks) | Insolation versus solar constant, valley and mountain breezes, effects of excess CO2, climate change and the water cycle |
| Long answer (5 marks) | Nitrogen cycle with its bacteria, deforestation and the carbon and oxygen cycles, interrelationship of the spheres |
| Case-based and numerical | Energy from insolation (E = intensity × area × time), urban heat islands, eutrophication, the Keeling curve |
This chapter is the foundation for Class 10 Science, where the textbook says you will explore how life sustains itself. Photosynthesis, respiration and the flow of energy and matter through living things are the ideas you carry forward.
Class 9 Science · Chapter 13 – swipe through all 10 cards to understand the whole chapter.
One Earth, five linked spheres
Energy and matter flow between the geosphere, hydrosphere, cryosphere, atmosphere and biosphere, so a change in one spreads to the others.
Chain example: warmer Arabian Sea → more evaporation → variable monsoon → floods and droughts.
- Geosphere: rocks, soil, landforms, interior
- Cryosphere: ice and snow; hydrosphere: liquid water
- Less winter snow → lower lake → less grass for sheep
Sunlight arrives as EM waves
Solar radiation travels through vacuum at the speed of light. About 99% of the Sun’s energy lies in the UV, visible and infrared range.
UV lies between 100 nm and 400 nm and can harm eyes and skin.
- UV: mostly absorbed by the ozone layer
- Visible: reaches the surface, powers photosynthesis
- IR: warms the surface, partly trapped by greenhouse gases
Insolation and the solar constant
The solar constant is measured at the top of the atmosphere; insolation is what reaches the ground after absorption and scattering.
1 m2 at 1 kW m⁻2 for 1 hour gets 3.6 × 106 J, one unit of electricity.
- Solar constant ≈ 1.4 kW m⁻2 = 1400 J s⁻1 m⁻2
- Maximum surface insolation ≈ 1 kW m⁻2 (clear sky)
- Anna Mani made India’s first insolation atlas (1982)
Albedo: how much a surface reflects
Albedo is the fraction of solar radiation reflected. High albedo keeps a surface cool; low albedo makes it absorb more and warm up.
Cities of concrete and asphalt re-radiate heat at night: the urban heat island effect.
- Snow 0.80 to 0.90, ice 0.50 to 0.70
- Crushed rock 0.25 to 0.30
- Black soil and ocean water: lower albedo, warmer
Why the equator is hot and the poles cold
On a spherical Earth, rays are concentrated over a small area at the equator and spread over a large area near the poles.
Shape, latitude and axial tilt together cause uneven heating and seasons.
- Equator: more energy per square metre, warm all year
- Poles: same energy spread wider, very cold
- Uneven heating drives global winds and ocean currents
Layers and the greenhouse effect
Air is 78% nitrogen and 21% oxygen. Weather stays in the troposphere; the ozone layer sits in the stratosphere.
CFCs caused the ozone hole; the Montreal Protocol is helping it recover.
- Troposphere 0 to 12 km; stratosphere 12 to 50 km
- CO2, CH4, water vapour trap re-radiated heat
- Venus is hotter than Mercury: uncontrolled greenhouse effect
Breezes and planetary winds
Wind blows from high to low pressure. Uneven heating of slopes and valleys, and of the equator and poles, creates those pressure differences.
Earth’s rotation deflects winds right in the north, left in the south.
- Valley breeze by day (up slope), mountain breeze at night (down slope)
- Low at 0° and 60°, high at 30° and 90°
- Rising air means low pressure; sinking air means high
Currents carry heat poleward
Winds drag surface water; temperature, salinity, rotation and continents shape the flow into gyres.
The North Atlantic Drift keeps many European ports ice-free in winter.
- Warm water moves poleward at the surface
- Cold, salty, dense water returns at depth
- Gyres: clockwise in north, counter clockwise in south
Carbon, nitrogen and oxygen cycles
Biogeochemical cycles move matter between abiotic and biotic parts of the Earth so nutrients stay available to life.
CO2 rose from 315 ppm to 420 ppm since 1960 (Keeling curve).
- Fixation: Rhizobium, Azotobacter; nitrification: Nitrosomonas, Nitrobacter
- Denitrification: Pseudomonas returns N2 to air
- O2 used in respiration and combustion, restored by photosynthesis
How people disturb the cycles
Fossil fuels, fertilisers, deforestation and vehicle emissions push the Earth’s cycles out of balance.
Mission LiFE (2021) promotes eco-friendly lifestyles to restore the balance.
- Excess CO2 warms the planet and acidifies the sea
- Excess nitrates cause algal blooms: eutrophication
- Deforestation: less photosynthesis, rainfall, more erosion
📝 Practice Earth as a System: Energy, Matter, and Life - 10 questions
Concept-check questions · with answers & explanations
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Why B: The chapter stresses that a disturbance in one sphere leads to changes in others. Removing forest destroys habitat (biosphere), removes roots that hold soil and so increases erosion (geosphere), alters local water flow and the flow of nearby rivers (hydrosphere), and reduces transpiration, which can reduce local rainfall (atmosphere). All four are affected.
Why not A: Living things exchange matter and energy with the non-living spheres, so a biosphere change spreads to the others.
Why not C: Mangroves link water, soil, air and life; limiting the effect to water ignores erosion, habitat loss and rainfall.
Why not D: The chapter says interconnectedness helps ecosystems recover from disturbances, but recovery is neither instant nor guaranteed; the balance is described as delicate.
Remember: Pull one thread of the Earth system and every sphere moves.
Why A: Step 1 (energy = intensity × area × time): incident energy = 1000 J s⁻¹ m⁻² × 2 m² × (30 × 60 s) = 1000 × 2 × 1800 = 3.6 × 10⁶ J. Step 2 (efficiency as a fraction): electrical energy = 20 per cent of 3.6 × 10⁶ J = 0.20 × 3.6 × 10⁶ = 7.2 × 10⁵ J.
Why not B: 3.6 × 10⁶ J is the total incident energy before applying the 20 per cent conversion.
Why not C: 1.8 × 10⁶ J is the incident energy for 1 m² only, or half the total, with efficiency ignored.
Why not D: 3.6 × 10⁵ J is 20 per cent of the energy on 1 m² instead of 2 m²; the panel area was left out.
Remember: Incident energy first (intensity × area × time), then multiply by the efficiency fraction.
Why D: Dark bare rock has a lower albedo than grass, so it reflects less and absorbs more of the incoming radiation, heating up faster and more. The air over its slopes becomes warmer. Grass also cools its surface through transpiration and shade, so the grassy hill stays cooler.
Why not A: Plants use only a small part of the incoming light for photosynthesis and lose heat by transpiration; they do not absorb all sunlight as heat.
Why not B: The rock in the stem is dark, so the grass is not the darker surface; besides, vegetation cools by transpiration whereas bare rock absorbs and re-radiates heat.
Why not C: Receiving the same sunlight is not enough; what matters is how much each surface absorbs versus reflects, which differs with albedo and cover.
Remember: Low albedo plus no transpiration: bare dark rock makes the hotter slope.
Why D: Venus has an atmosphere that trapped re-radiated heat so strongly that it produced an uncontrolled greenhouse effect. Mercury, though nearer the Sun, has essentially no atmosphere to trap heat. NCERT cites this to show what excess greenhouse gases can do.
Why not A: Distance does matter for the sunlight received; the point is that an atmosphere can outweigh the distance advantage, not that distance is irrelevant.
Why not B: Mercury has essentially no atmosphere, so it has no ozone layer to block sunlight; the chapter explains the difference through the atmosphere of Venus.
Why not C: A higher albedo would make a planet COOLER by reflecting more sunlight, not hotter.
Remember: Venus is the runaway greenhouse; a thick atmosphere beats a closer orbit.
Why D: NCERT says water with lower salinity, being less dense, tends to remain near the surface, while higher-salinity, denser water sinks and moves at deeper levels. With temperature equal, salinity alone decides: the saltier sample X sinks.
Why not A: Dissolved salt makes water denser, not lighter; salty water sinks rather than floats.
Why not B: Fresher water is less dense and stays up; it does not sink.
Why not C: Both temperature AND salinity affect density; equal temperature does not remove the salinity difference.
Remember: Saltier or colder means denser, and denser water sinks.
Why A: Clearing forests results in decreased photosynthesis, so less CO2 is taken up and less oxygen restored; reduced transpiration can lead to a decline in local rainfall; surface albedo is altered; without tree roots to hold the soil, erosion increases; and habitats are destroyed, reducing biodiversity. Deforestation also saturates natural carbon sinks, intensifying greenhouse warming.
Why not B: Removing trees REDUCES photosynthesis and transpiration; the direction of every effect is reversed.
Why not C: Trees are central to both the carbon cycle (CO2 uptake) and the oxygen cycle (O2 release), so their loss changes both.
Why not D: Bare soil does not absorb the CO2 that lost trees would have; forests are the sink that is lost.
Remember: Cut the forest and you cut photosynthesis, transpiration, rainfall and soil-holding roots all at once.
Why A: Less snow (cryosphere) means less melt-water feeding the lake, so its level falls in summer (hydrosphere). Less water supports less grass growth, so the sheep that graze on it have less food (biosphere). This is the exact inference NCERT draws from Activity 13.1.
Why not B: Less snowfall cannot give MORE melt-water; the direction of the effect is reversed.
Why not C: The chapter does not link reduced snowfall to lake evaporation or sheep migration; it links it to water supply for grass.
Why not D: Snowfall is not described as a cause of erosion, and a deeper lake is the opposite of what less inflow would produce.
Remember: Cryosphere to hydrosphere to biosphere: less snow, less lake water, less grass.
Why C: Step 1 (energy = intensity × area × time): intensity 1 kW/m² = 1000 J s⁻¹ m⁻². Step 2 (unit conversion, hours to seconds): 1 hour = 60 × 60 = 3600 s. Step 3 (multiply): E = 1000 × 1 × 3600 = 3,600,000 J = 3.6 × 10⁶ J, which NCERT notes is one unit of household electricity.
Why not A: 3600 J uses 1 W/m² instead of 1 kW/m²; the kilo prefix (× 1000) was dropped.
Why not B: 1000 J is the energy in ONE second, not one hour; the time factor was left out.
Why not D: 6 × 10⁴ J comes from using 60 s (one minute) instead of 3600 s (one hour).
Remember: 1 kW for 1 hour = 1 kWh = 3.6 × 10⁶ J = one unit on the electricity bill.
Why B: Compared with rural areas, cities have more built-up surfaces of steel, concrete, brick and asphalt. These materials absorb solar radiation, retain the heat and re-radiate it, warming the city more, particularly at night. Rural areas and forests have vegetation that stays cool through shade and plant transpiration.
Why not A: A city and the villages around it are at practically the same latitude, so they receive the same insolation.
Why not C: Human breath is negligible compared with the heat stored in building materials and roads.
Why not D: The ozone layer is in the stratosphere and is not thinner over cities; it is unrelated to the heat island.
Remember: Concrete stores the day’s sun and gives it back at night; trees shade and transpire.
Why A: NCERT says human-made CFCs used in refrigerators and aerosols destroyed ozone faster than it was naturally formed, creating the ozone hole. The Montreal Protocol, a global agreement, reduced CFC use, and the ozone layer is now slowly recovering. So both statements are true and R directly explains A.
Why not B: The recovery is a direct result of cutting CFCs, so R IS the explanation.
Why not C: R is true: the Montreal Protocol targeted CFCs and is described as a success of international scientific cooperation.
Why not D: A is true: the chapter states the layer is slowly recovering.
Remember: CFCs made the hole, Montreal Protocol is closing it.
Chapter Navigation
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Frequently Asked Questions
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 kW per square metre. Insolation is the radiation that actually reaches the Earth’s surface. Gases, clouds and dust absorb and scatter part of the energy, so maximum insolation is only about 1 kW per square metre under clear sky.
Albedo is the fraction of solar radiation a surface reflects. Snow (0.80 to 0.90) and ice (0.50 to 0.70) have high albedo, reflect most sunlight and stay cold, which keeps polar regions very cold. Black soil and ocean water have lower albedo, absorb more and are warmer. Changing land cover, such as clearing forests or building cities, changes albedo and local temperature.
The troposphere, about 12 km high on average, is heated from the Earth’s surface, so temperature falls with height by about 6.5 °C per km. Warm air near the ground rises, driving winds, storms and cloud formation. In the stratosphere above, ozone absorbs UV and temperature rises with height, so there is little vertical mixing and that layer stays calm.
By day, sun-facing slopes heat faster than the valley floor. Warm air over the slopes rises and cooler valley air moves up the slopes: the valley breeze. After sunset the slopes cool faster, the air over them becomes cool and dense, and it flows down into the valley: the mountain breeze. These are common in places like Shimla and Dehradun.
Nitrogen fixation by Rhizobium and Azotobacter turns N₂ into ammonia. Nitrification follows: Nitrosomonas turns ammonia into nitrite and Nitrobacter turns nitrite into nitrate. Plants assimilate these compounds and animals eat plants. Decomposers return ammonia to the soil by ammonification. Denitrifying bacteria such as Pseudomonas turn some nitrate back into N₂ gas. Lightning also fixes a little nitrogen.
Some CO₂ is needed for photosynthesis and to keep the Earth warm. Excess CO₂ from burning fossil fuels and deforestation, which rose from 315 ppm to 420 ppm since 1960, strengthens the greenhouse effect. This leads to global warming, melting glaciers and Arctic sea ice, rising sea level, extreme weather, and more acidic sea water that threatens plankton and coral reefs.
Eutrophication happens when too much fertiliser is used and nitrates wash into rivers and lakes. The extra nutrients cause a rapid growth of algae called an algal bloom. The bloom depletes oxygen in the water and fish die. It shows how a human change to the nitrogen cycle spreads into the hydrosphere and biosphere and threatens water bodies and coastal fisheries.
