Earth as a System: Energy, Matter, and Life
Earth is not merely a collection of air, water, rocks, ice and living organisms. It behaves as an interconnected system in which energy moves through natural processes and matter is continuously transferred among different parts of the planet.
1. Earth as One Interconnected System
Life on Earth depends on a constant movement of energy and matter. The chapter identifies the Sun as the main source of energy. In addition, the hot interior of Earth and chemical reactions in air, water and rocks also contribute to the movement and transformation of energy and matter.
Earlier, you may have studied winds, the water cycle, seasons, microorganisms, soil, climate and photosynthesis as separate topics. This chapter brings them together. Sunlight can heat an ocean; ocean heating can alter evaporation; evaporation can influence atmospheric moisture and rainfall; rainfall affects rivers, soil, crops and ecosystems. One event can therefore travel through several parts of Earth’s system.
Follow the Energy
Solar radiation warms the land and water, creates temperature differences, helps generate winds and currents, powers evaporation, and supplies visible light for photosynthesis.
Follow the Matter
Water, carbon, nitrogen and oxygen move repeatedly among air, water, land and living organisms. Snow can become lake water, lake water can evaporate, and atmospheric water can later return as precipitation.
For scientific understanding, remember that matter is repeatedly recycled, whereas incoming energy is transferred through Earth-system processes and is eventually re-radiated as heat. The textbook itself uses the expression “cyclic movement of matter and energy” while introducing biogeochemical cycles; if a direct textbook definition is asked in an examination, use the textbook wording.
The arrows show exchanges of matter and energy among the five spheres. No sphere can be completely understood in isolation.
2. The Five Spheres of Earth
Geosphere
The geosphere is the solid part of Earth. It includes rocks, soil, landforms and Earth’s interior. Textbook examples include the Deccan Plateau and Thar Desert.
It provides soil and minerals, shapes drainage, stores geological carbon and supports terrestrial habitats.
Hydrosphere
The hydrosphere includes liquid water in oceans, rivers, lakes and groundwater. The chapter names the Ganga–Brahmaputra river system as an example.
It redistributes heat, transports dissolved substances and provides the water needed by living organisms.
Cryosphere
The cryosphere is water in its solid form: snow and ice. Examples include Himalayan glaciers, snow in Ladakh and polar ice caps.
Atmosphere
The atmosphere is the air surrounding Earth. It provides essential gases, produces weather, absorbs some harmful radiation and helps regulate temperature.
Biosphere
The biosphere includes all living organisms and their habitats. Textbook examples include mangroves, forests, farms, ocean plankton and coral reefs.
The five spheres exchange water, gases, minerals, nutrients and energy. A change in one may therefore alter the others.
| Sphere | Main component | Textbook examples | Typical interaction |
|---|---|---|---|
| Geosphere | Rocks, soil, landforms and interior | Deccan Plateau, Thar Desert | Minerals dissolve into water; soil supports vegetation. |
| Hydrosphere | Liquid water | Oceans, Ganga–Brahmaputra, lakes, groundwater | Evaporation, runoff and nutrient transport. |
| Cryosphere | Snow and ice | Himalayan glaciers, Ladakh snow, polar ice caps | Melting supplies rivers and lakes and influences sea level. |
| Atmosphere | Air | Air surrounding Earth | Weather, gas exchange and heat transfer. |
| Biosphere | Living organisms and habitats | Mangroves, farms, plankton, coral reefs | Photosynthesis, respiration and decomposition. |
3. Interaction Between Earth’s Spheres
Activity 13.1 introduces the central systems idea using a landscape containing snow, a lake, rocks, vegetation and animals. Snow in the cryosphere melts and becomes liquid water in the hydrosphere. This water supports grass in the biosphere, which in turn provides food for sheep.
What if snowfall decreases?
The chapter then scales the same idea up to the Arabian Sea. Warmer Arabian Sea water produces more evaporation. This can contribute to fluctuations in the southwest monsoon and variability in rainfall. Some regions may experience flooding while others experience drought.
At the same time, higher atmospheric temperatures can accelerate melting of glaciers and polar ice. Over time this can contribute to flooding of low-lying areas and sea-level rise, threatening coastal cities and habitats.
Systems thinking means tracing the original disturbance, the transfer mechanism and the later consequences in other spheres.
4. Uneven Heating of Earth
Solar radiation is the main energy source for Earth. It reaches Earth as electromagnetic waves. Unlike sound waves, which require a material medium, electromagnetic waves can travel through the vacuum of space.
Earth is not heated equally everywhere. Heating depends on several factors, including:
- latitude;
- angle at which sunlight strikes the surface;
- Earth’s spherical shape;
- axial tilt and changing length of day;
- land versus water;
- colour and material of the surface;
- clouds and atmospheric particles;
- reflection, absorption and scattering.
5. Electromagnetic Spectrum
The entire range of electromagnetic radiation is called the electromagnetic spectrum.
In order from relatively short wavelength and high frequency toward longer wavelength and lower frequency, the chapter shows:
About 99% of the Sun’s energy reaching Earth is concentrated mainly in the UV, visible and infrared regions. These are therefore especially important for climate and life.
Ultraviolet Radiation
The textbook gives the UV wavelength range as 100–400 nm, where 1 nm = 10−9 m.
Much short-wavelength UV is absorbed by the ozone layer. Prolonged exposure can damage eyes and skin and increase cancer risk.
Useful applications mentioned include killing germs in water purifiers and helping power fluorescent lights.
Visible Radiation
Visible light reaches Earth’s surface and supplies the energy needed for photosynthesis, the primary source of food for most organisms.
It also illuminates the environment and contributes to warming land and water.
Infrared Radiation
Infrared radiation contributes strongly to surface heating. A warmed Earth then re-radiates energy mainly as infrared radiation.
Greenhouse gases absorb some of this outgoing heat.
Wavelength increases toward radio waves; frequency and energy increase toward gamma rays.
6. Insolation and the Solar Constant
Insolation warms Earth’s surface and atmosphere and helps drive major atmospheric and oceanic processes.
= 1400 J s−1 m−2.
The solar constant represents available solar energy before absorption, scattering or reflection by the atmosphere.
Clouds, gases and dust remove or redirect part of the incoming solar radiation. Therefore, the maximum insolation reaching Earth’s surface under clear-sky conditions is lower—about 1 kW m−2 according to the chapter.
Why Insolation Matters to India
India lies largely in tropical and subtropical regions and receives abundant sunlight. The chapter connects this solar heating with the southwest monsoon, agriculture and India’s considerable potential for renewable solar energy.
Bridging Science and Society — Anna Mani
Anna Mani, one of India’s pioneering atmospheric scientists, mapped solar insolation across India during the 1950s.
With S. Rangarajan, she later published Solar Radiation Over India in 1982, producing the country’s first insolation atlas. These measurements helped demonstrate India’s large solar-energy potential.
Textbook Example 13.1
Question: How much solar energy will be received by a 1 m² area in one hour if the insolation at Earth’s surface is 1 kW m⁻²?
Intensity = 1 kW m⁻²
= 1000 J s⁻¹ m⁻²
Area = 1 m²
Time = 1 hour
= 3600 s
E = 1000 × 1 × 3600
E = 3,600,000 J
E = 3.6 × 10⁶ J
The textbook also points out that this equals one unit of household electrical energy, or 1 kWh.
Think as a Scientist — How Much Land Would Solar Panels Need?
The textbook asks students to estimate how much land covered with solar panels would be needed to supply India’s electrical power. A scientific estimate would require:
- India’s electrical power demand;
- an assumed surface insolation;
- the area of panels;
- the fraction of incoming solar energy converted into electricity.
The chapter’s qualitative conclusion is that even a fraction of the Thar Desert covered with solar panels could potentially provide an enormous amount of electrical energy.
The textbook explicitly asks students to obtain current figures from external sources, so no present-day numerical estimate is inserted here and presented as though it came from the textbook.
Incoming sunlight can be reflected or absorbed by the atmosphere and clouds, reflected from the surface, or absorbed by the surface and later re-radiated as heat.
7. Interaction of Solar Radiation with Earth’s Surface
Different materials do not absorb and heat equally. Land generally heats faster than water, and even different land surfaces behave differently.
Dark surfaces absorb a greater fraction of sunlight, while light-coloured surfaces reflect more and remain comparatively cooler.
This explains familiar experiences:
- dark roads become very hot in sunshine;
- light-coloured surfaces remain comparatively cooler;
- dark clothes often feel hotter than white clothes in summer.
Albedo
High Albedo
Reflects a large fraction of radiation and absorbs less. The surface therefore tends to remain cooler.
Low Albedo
Reflects less radiation and absorbs more. The surface tends to warm more strongly.
| Material | Albedo given in textbook | Interpretation |
|---|---|---|
| Snow | 0.80–0.90 | Very high reflection. |
| Ice | 0.50–0.70 | High reflection. |
| Crushed rock | 0.25–0.30 | Lower than snow and ice. |
| Light-coloured soil | Not supplied — Activity 13.2 asks students to research it | Reflects more than darker soil. |
| Black soil | Not supplied — Activity 13.2 research entry | Lower albedo; stronger absorption. |
| Ocean water | Not supplied — Activity 13.2 research entry | Discussed as a relatively low-albedo surface. |
Snow reflects a large fraction of incoming radiation, while darker surfaces absorb a larger fraction.
Heat Absorption and Re-radiation
All objects radiate heat. A concrete structure may remain warm at night during summer because concrete absorbed heat during the day and re-radiates it after sunset.
The chapter contrasts this with traditional houses having thick mud and wooden walls, which can remain cooler because they re-radiate less heat into the indoor environment.
8. Urban Heat Island Effect
Cities can be warmer than nearby rural areas, particularly during hot summer conditions and at night. This is called the urban heat island effect.
Urban areas contain large quantities of:
- concrete;
- steel;
- brick;
- asphalt;
- roads and buildings.
These materials absorb solar radiation, retain heat and later re-radiate it. Rural and forested areas usually have more vegetation, which produces cooling through shade and transpiration.
Higher city temperatures can also increase demand for air-conditioning, placing additional stress on urban energy systems and ecosystems.
Built surfaces retain and re-radiate more heat, while vegetation provides shade and transpiration cooling.
9. Latitude and Earth’s Shape
Earth is approximately spherical. Therefore, the Sun’s rays do not strike every latitude at the same angle.
Near the Equator
Solar radiation strikes more directly and is concentrated over a smaller surface area. This produces stronger heating.
Toward the Poles
Solar radiation strikes at a more slanting angle and the same amount of energy is spread over a larger area. This contributes to lower temperatures.
energy spread over a larger area → weaker heating.
Earth’s axial tilt, its revolution around the Sun and changing day length also cause seasonal and latitudinal differences in solar heating.
The resulting global temperature differences help drive planetary winds and ocean currents.
Direct equatorial rays concentrate solar energy; oblique polar rays spread it across a larger surface.
| Feature | Equatorial Region | Polar Region |
|---|---|---|
| Angle of sunlight | More direct | More oblique |
| Area over which energy spreads | Smaller | Larger |
| Typical heating | Stronger | Weaker |
10. Role of the Atmosphere
Atmospheric Composition
Nitrogen
≈78%
Oxygen
≈21%
Other gases
Small amounts of argon, carbon dioxide, water vapour and other gases.
Why the Atmosphere Is Essential
- It partly absorbs incoming solar radiation.
- The ozone layer absorbs harmful UV radiation.
- Clouds and gases absorb or reflect some incoming sunlight.
- Greenhouse gases absorb some of Earth’s outgoing infrared heat.
- It helps produce winds, clouds, storms and rainfall.
- It reduces extreme changes in surface temperature.
The chapter also notes that interactions involving solar radiation and cosmic rays influence atmospheric processes and can affect communication systems.
Layers of the Atmosphere
| Layer | Altitude stated in chapter | Main Features |
|---|---|---|
| Troposphere | 0–12 km average | Nearly all weather; heated from Earth’s surface; temperature decreases with height at about 6.5 °C km⁻¹. |
| Stratosphere | 12–50 km | Contains the ozone layer; UV absorption warms the layer and temperature increases with altitude. |
| Mesosphere | Exact range not tabulated in this chapter | Introduced as a higher atmospheric layer. |
| Thermosphere | Exact range not tabulated in this chapter | Higher atmospheric layer; only a minor role in surface climate is discussed here. |
| Exosphere | Outermost named atmospheric layer | Transition toward outer space. |
Troposphere
Almost all weather occurs in the troposphere. It is heated mainly from Earth’s surface. Temperature decreases with height at approximately 6.5 °C per kilometre.
Warm air rises, helping generate winds and storms. The troposphere is thickest above the equator and thinnest above the polar regions.
Stratosphere
The stratosphere extends from about 12–50 km. The ozone layer absorbs UV radiation and warms this layer, causing temperature to increase with altitude.
This temperature structure reduces vertical mixing, which is one reason weather remains largely confined to the troposphere.
The chapter notes that around 100 km above Earth is the beginning of the region commonly called outer space.
The chapter provides numerical altitude ranges specifically for the troposphere and stratosphere and introduces the higher layers qualitatively.
Meet a Scientist — K. R. Ramanathan
Indian atmospheric scientist K. R. Ramanathan climbed to about 18,000 feet in the Himalayas in 1934 to measure ozone levels.
He found ozone levels lower than expected. The chapter states that his work helped lay the foundation for understanding how UV absorption varies with altitude and pollution. Later, he led early monsoon-forecasting efforts.
11. Greenhouse Effect
- Solar radiation reaches Earth.
- Earth’s surface absorbs part of it.
- The surface warms.
- The warm surface re-radiates energy mainly as infrared radiation.
- Greenhouse gases absorb part of this outgoing infrared radiation.
- This reduces the rate at which heat escapes to space and keeps Earth warmer.
Greenhouse Gases Mentioned
Natural Greenhouse Effect
Necessary for life. Without the atmosphere trapping some outgoing heat, Earth would be too cold for life as we know it.
Enhanced Greenhouse Effect
Excess greenhouse gases, especially additional CO₂ from human activities, strengthen heat retention and contribute to global warming.
Venus and Mercury
The chapter reminds students that Venus is hotter than Mercury even though Mercury is closer to the Sun. Venus has a dense atmosphere associated with an uncontrolled greenhouse effect.
Greenhouse gases absorb part of the infrared energy emitted by Earth’s warmed surface.
| Feature | Natural Greenhouse Effect | Enhanced Greenhouse Effect |
|---|---|---|
| Cause | Normal amounts of atmospheric greenhouse gases | Additional greenhouse gases, especially excess CO₂ |
| Role | Keeps Earth warm enough for life | Produces additional warming |
| Environmental meaning | Essential natural process | Climate concern |
12. Ozone Layer
The ozone layer is located in the stratosphere. It absorbs harmful UV radiation and acts as a protective shield for life.
If ozone molecules are destroyed faster than they are produced naturally, the ozone layer becomes thinner and less effective.
CFCs and the Ozone Hole
In the late twentieth century, chlorofluorocarbons (CFCs), formerly used in refrigerators and aerosols, caused severe ozone loss over Antarctica. This became known as the ozone hole.
More UV reaching Earth can harm organisms and ecosystems.
Montreal Protocol
The Montreal Protocol reduced the use of CFCs. The textbook describes the ozone layer as slowly recovering and presents this as an important example of international scientific cooperation.
Very Important Distinction
Ozone high in the stratosphere = protective.
Ozone near ground level = harmful pollutant.
Never write in an examination that “ozone is always harmful” or that “all ozone is beneficial.”
Location determines ozone’s environmental role: protective in the stratosphere, harmful near ground level.
13. Uneven Heating Causes Winds
Pressure differences are mainly created by uneven heating of Earth’s surface. The same basic mechanism produces local winds and contributes to global circulation.
Valley Breeze — Day
- Sun-facing mountain slopes heat rapidly.
- Air touching the slopes becomes warmer.
- Warm air rises.
- A low-pressure region develops over the slopes.
- Cooler air from the valley moves upward along the slopes.
This daytime upslope wind is called a valley breeze.
Mountain Breeze — Night
- After sunset the mountain slopes cool rapidly.
- Air above them becomes colder.
- Cold air becomes denser.
- Dense air flows down the mountain slopes toward the valley.
This nighttime downslope wind is called a mountain breeze.
The chapter mentions Shimla, Dehradun and other Himalayan valleys. These local winds affect temperature, moisture, agriculture, soil and everyday life in mountainous regions.
Day: cooler valley air moves upslope. Night: cold dense mountain air flows downslope.
| Feature | Valley Breeze | Mountain Breeze |
|---|---|---|
| Time | Daytime | Night |
| Slope condition | Slopes heat rapidly | Slopes cool rapidly |
| Air movement | Upslope | Downslope |
| Main cause | Warm rising air creates lower pressure | Cold dense air drains downhill |
14. Planetary Winds and Pressure Belts
Uneven heating between the equator and poles creates large belts of atmospheric pressure. Air movement between these belts produces planetary winds.
0° — Equatorial Low Pressure
Strong solar heating warms the air. Warm air rises, producing an equatorial low-pressure belt.
≈30° N and S — Subtropical High Pressure
Air that rose near the equator moves poleward at higher altitude. It cools, becomes denser and sinks near 30° north and south, forming subtropical high-pressure belts.
≈60° N and S — Subpolar Low Pressure
Some surface air from the subtropical belts moves toward the poles. Around 60°, it meets cold polar air and rises, producing subpolar low-pressure belts.
≈90° N and S — Polar High Pressure
Extremely cold, dense polar air sinks and forms polar high-pressure belts.
Northern Hemisphere → right
Southern Hemisphere → left
Rising air is associated with low pressure; sinking dense air produces high pressure. Earth’s rotation bends the surface wind paths.
| Latitude | Pressure Belt | Vertical Motion |
|---|---|---|
| 0° | Equatorial low | Warm air rises |
| ≈30° N/S | Subtropical high | Cooler air sinks |
| ≈60° N/S | Subpolar low | Air rises |
| ≈90° N/S | Polar high | Cold dense air sinks |
15. Ocean Currents
Strong planetary winds drag surface ocean water through friction, producing surface currents.
Ocean circulation is also influenced by:
- temperature;
- salinity;
- water density;
- Earth’s rotation;
- distribution of continents.
Warm and Cold Water
Warm equatorial water can travel toward higher latitudes near the surface. Colder, denser water can flow back toward lower latitudes through deeper levels of the ocean.
Salinity and Density
Water of lower salinity is generally less dense and tends to remain nearer the surface. Water with higher salinity is denser and tends to sink, contributing to deep-water movement.
Ocean Gyres
In the simplified pattern given in the chapter:
- Northern Hemisphere gyres rotate clockwise.
- Southern Hemisphere gyres rotate counter-clockwise.
Earth’s rotation and the positions of continents help redirect currents into broad circular systems.
Gulf Stream and North Atlantic Drift
The Gulf Stream carries warm ocean water from the southern part of the east coast of North America across the Atlantic. Its extension, the North Atlantic Drift, flows toward northwestern Europe.
This warm current moderates climate and helps keep many ports ice-free during winter even at high latitudes. This has important effects on human activity, including trade and commerce.
Ocean currents redistribute heat and can strongly influence regional climate.
Ocean Currents and Life
Ocean currents transport nutrients as well as heat. They therefore support marine ecosystems and help connect the hydrosphere, atmosphere and biosphere.
| Feature | Surface Currents | Deep Currents |
|---|---|---|
| Major influence emphasised | Planetary winds and friction | Density differences caused by temperature and salinity |
| Typical movement | Warm equatorial water can move toward poles | Cold dense water can move back toward lower latitudes |
India’s Scientific Contributions — IITM, Pune
Scientists at the Indian Institute of Tropical Meteorology (IITM), Pune use advanced computer models that couple atmosphere, ocean, land and ice in simulations of the Indian monsoon.
According to the chapter, these models use information from satellites, buoys in the Indian Ocean and stations in Antarctica. They improve seasonal forecasts and help scientists study how global warming may change monsoon rainfall patterns across India.
16. Biogeochemical Cycles
Living organisms continuously exchange materials with air, water, soil and rocks. These exchanges connect biotic components with abiotic components.
These cycles keep important nutrients available and contribute to environmental balance. The chapter studies:
- Water cycle
- Carbon cycle
- Nitrogen cycle
- Oxygen cycle
These cycles are interconnected. For example, a plant simultaneously:
- takes up water;
- absorbs usable nitrogen compounds;
- takes in carbon dioxide;
- releases oxygen;
- transpires water vapour.
The cycles share organisms and processes instead of operating as four independent systems.
17. Water Cycle
Evaporation
Liquid water from oceans, rivers, lakes and other water bodies changes into water vapour.
Transpiration
Plants release water vapour into the atmosphere.
Condensation
Water vapour cools and changes into tiny liquid droplets, helping form clouds.
Precipitation
Water returns from the atmosphere as rain, snow or hail.
Runoff
Some water flows over the surface through streams and rivers and eventually returns to the oceans.
Infiltration and Groundwater
Some water seeps through soil and rocks, becoming groundwater.
As water moves through the geosphere, it dissolves minerals and can transport nutrients. Water supports terrestrial life and eventually transports material toward the oceans, supporting marine organisms.
The water cycle transfers matter between atmosphere, hydrosphere, geosphere, cryosphere and biosphere.
Climate Change and the Water Cycle
A warmer atmosphere can contain more moisture. The chapter connects this with heavier rainfall in some regions, including more intense monsoon events, while other regions may experience drought.
Melting glaciers can add water to rivers and contribute to long-term sea-level rise. The chapter specifically mentions threats to coastal cities such as Mumbai and Chennai.
Very intense rainfall produces rapid runoff. This can:
- increase soil erosion;
- reduce infiltration;
- reduce groundwater recharge;
- make agriculture harder during dry months.
The chapter explicitly connects the cryosphere, hydrosphere, atmosphere, geosphere and biosphere through climate-driven changes in water.
18. Carbon Cycle
Carbon forms the backbone of life. It occurs in:
- carbohydrates;
- proteins;
- fats;
- DNA and other nucleic-acid molecules.
Major Carbon Reservoirs
Atmosphere
Carbon dioxide gas — CO₂.
Biosphere
Plants, animals and other organisms.
Geosphere
Carbonate rocks and fossil fuels such as coal and oil.
Hydrosphere
Dissolved CO₂ and carbon present in marine shells.
Fast Carbon Cycle
The fast cycle occurs over days to years.
- Plants absorb atmospheric CO₂.
- Through photosynthesis, they use sunlight to form glucose.
- Animals obtain carbon by eating plants or other animals.
- Respiration returns CO₂ to the atmosphere.
- When organisms die, decomposition returns carbon to the environment.
The fast cycle is dominated by biological processes operating over days to years.
Slow Carbon Cycle
The slow carbon cycle occurs over millions of years. Dead organisms can become buried and eventually form coal, oil and gas.
Fossil fuels are burned for heating, cooking, transport and industrial activity. Combustion returns the stored carbon to the atmosphere as CO₂ on a very short time scale.
millions of years of carbon storage versus rapid release through fossil-fuel combustion.
Oceans in the Carbon Cycle
Atmospheric CO₂ and ocean water continually exchange carbon. Seawater absorbs CO₂, producing dissolved forms including carbonate and bicarbonate ions.
Phytoplankton use carbon during photosynthesis. Some marine organisms use carbon-containing compounds in forming shells. When organisms die and sink to the seafloor, some carbon can remain stored for long periods.
Biological exchange, ocean exchange and geological storage operate on very different time scales.
Keeling Curve — Rising Atmospheric CO₂
The chapter shows atmospheric CO₂ concentration from approximately 1960 to 2025. The concentration rises from about 315 ppm to roughly 420 ppm, which the chapter describes as an increase of about 35%.
The graph has a long-term upward trend with a small saw-tooth pattern. The seasonal dips are linked with yearly plant growth in the Northern Hemisphere, when vegetation removes more CO₂ through photosynthesis.
The long-term rise is connected with fossil-fuel burning and deforestation.
Conceptual textbook-data recreation: the graph represents the approximate trend and seasonal pattern given in the source; it is not a pixel copy of the textbook artwork.
Threads of Curiosity — Carbon Facts
- Carbon constitutes about 49% of the dry weight of living organisms according to the chapter.
- The chapter states that around 71% of global carbon is found in oceans.
- The atmosphere holds only about 1% of total global carbon.
| Feature | Fast Carbon Cycle | Slow Carbon Cycle |
|---|---|---|
| Time scale | Days to years | Millions of years |
| Main processes | Photosynthesis, feeding, respiration and decomposition | Burial, fossil-fuel formation and later combustion |
| Main significance | Rapid biological exchange | Long-term geological storage |
19. Nitrogen Cycle
Nitrogen is needed for the formation of proteins and nucleic acids.
The atmosphere is Earth’s largest nitrogen reservoir, but atmospheric N₂ is relatively non-reactive. Most plants and animals cannot use atmospheric N₂ directly. It must first be converted into usable compounds.
1. Nitrogen Fixation
Nitrogen-fixing bacteria convert atmospheric nitrogen into ammonia.
- Rhizobium — found in root nodules of legumes.
- Azotobacter — found in soil.
These organisms convert atmospheric N₂ into ammonia (NH₃).
2. Nitrification
Nitrifying bacteria convert ammonia step by step:
- Nitrosomonas: ammonia → nitrite (NO₂⁻)
- Nitrobacter: nitrite → nitrate (NO₃⁻)
3. Assimilation
Plants absorb usable nitrogen compounds from the soil. Animals obtain nitrogen by consuming plants or other animals.
4. Ammonification
When organisms die or produce wastes, decomposer bacteria and fungi break down organic matter and return nitrogen compounds such as ammonia to the soil.
5. Denitrification
Denitrifying bacteria such as Pseudomonas convert some nitrate back into atmospheric nitrogen gas. This returns nitrogen to the atmosphere and completes the cycle.
Nitrogen fixation makes atmospheric nitrogen usable; denitrification returns nitrogen to the atmosphere.
Lightning and Nitrogen Fixation
During lightning, a small quantity of atmospheric nitrogen becomes fixed into nitrogen oxides. This is a non-biological nitrogen-fixation pathway.
| Process | Main Transformation | Organisms / Agents Mentioned |
|---|---|---|
| Nitrogen fixation | N₂ → fixed nitrogen/ammonia | Rhizobium, Azotobacter, lightning |
| Nitrification | Ammonia → nitrite → nitrate | Nitrosomonas and Nitrobacter |
| Assimilation | Soil nitrogen → plant tissues → animals | Plants and food chains |
| Ammonification | Organic nitrogen → ammonia | Decomposer bacteria and fungi |
| Denitrification | Nitrate → atmospheric N₂ | Pseudomonas |
Ready to Go Beyond — Haber–Bosch Process
The Haber–Bosch process, developed in the early twentieth century, artificially fixes atmospheric nitrogen to make ammonia.
Ammonia is used to make fertilisers. The chapter calls this development “Bread from Air” because it transformed agriculture and greatly increased food production.
The chapter connects synthetic fertilisers with India’s Green Revolution and states that more than half the nitrogen atoms in the human body today ultimately come from Haber–Bosch-derived nitrogen.
However, the process is very energy-intensive. The chapter gives an approximate requirement of 1–2% of global energy. Excessive fertiliser use can also damage soil and water.
20. Oxygen Cycle
Oxygen is one of Earth’s most abundant elements. Approximately 21% of the atmosphere consists of free oxygen gas, O₂.
Oxygen is also present in biological molecules such as:
- carbohydrates;
- proteins;
- nucleic acids;
- fats.
Oxygen also occurs in combined forms in minerals, metal oxides and carbon dioxide.
Processes Consuming Atmospheric Oxygen
- Respiration
- Combustion
Process Restoring Oxygen
Plants restore oxygen through photosynthesis. They use sunlight, water and CO₂ to form glucose and release O₂.
Oxygen and carbon cycles are linked through photosynthesis, respiration and combustion.
Interconnection of All Four Cycles
- Photosynthesis requires water and CO₂ and releases oxygen.
- Plants also require usable nitrogen compounds for growth.
- Respiration consumes oxygen and releases carbon dioxide.
- Decomposition transfers both carbon and nitrogen.
- Water transports dissolved nutrients through ecosystems.
Plant growth is a useful example of all four cycles operating at the same time.
21. Human Impact on Earth’s Processes
Human activities can alter natural cycles strongly enough to disturb balance across several Earth spheres at once.
Fossil-Fuel Burning
Burning coal, oil and gas releases carbon that had been stored for geological time scales. This raises atmospheric CO₂ and strengthens the enhanced greenhouse effect.
The chapter connects rising CO₂ with:
- global warming;
- extreme weather;
- glacier and sea-ice melting;
- sea-level rise;
- biodiversity loss;
- changes in rainfall.
Ocean Acidification
Excess atmospheric CO₂ can be absorbed by seawater. The chapter explains that greater absorption makes seawater more acidic and can threaten plankton, coral reefs and marine ecosystems.
Warmer ocean water also reduces the capacity of oceans to absorb CO₂ effectively.
Carbon Sinks
Forests remove CO₂ through photosynthesis. Oceans absorb atmospheric CO₂. Deforestation removes forest carbon uptake, while warming can reduce the ocean’s effectiveness as a carbon sink.
Eutrophication
Excessive fertiliser use can add large amounts of nitrate to rivers and lakes through runoff.
This process is called eutrophication. It can damage freshwater ecosystems and coastal fisheries.
Nutrient enrichment causes excessive algal growth, which can ultimately create oxygen-poor water.
Effects of Deforestation
Atmosphere
Less photosynthesis means less CO₂ uptake and less O₂ production. Reduced transpiration may also contribute to lower local rainfall.
Hydrosphere
Runoff and local water availability can change.
Geosphere
Without tree roots binding soil, erosion can increase and soil stability decreases.
Biosphere
Habitat destruction can reduce biodiversity as organisms lose their natural homes.
Energy Balance
Forest removal changes surface properties and therefore changes albedo.
One land-use change can alter gases, water, soil, ecosystems and surface-energy balance simultaneously.
Smog and Ground-Level Ozone
The chapter explains that vehicular emissions react in sunlight to produce ground-level smog and can lead to the formation of ground-level ozone.
Ground-level ozone is harmful to health and makes city air unhealthy.
Stratospheric ozone → protective
Ground-level ozone → pollutant and harmful
22. Restoring Earth’s Balance
Because human activities affect all five spheres, solutions also need to work at several levels.
Energy
- Conserve energy.
- Use resources efficiently.
- Increase renewable energy.
- Use solar and wind energy.
Land and Vegetation
- Plant trees.
- Protect forests.
- Use sustainable farming practices.
Water and Materials
- Save water.
- Reduce waste.
- Reuse materials.
- Recycle materials.
Individual Action vs Collective Action
Individual
Save water, food and electricity; avoid unnecessary waste; reuse and recycle; choose responsible consumption.
Collective / Global
Expand renewable energy, protect ecosystems, improve farming, coordinate national policies and cooperate internationally.
International Environmental Cooperation
Montreal Protocol
Reduced CFC use and began the process of ozone-layer recovery. The textbook presents this as an important example of successful international cooperation.
Kyoto Protocol
Intended to reduce greenhouse-gas emissions. The chapter describes the results of CO₂-reduction efforts under Kyoto and Paris as less successful than the Montreal Protocol.
Paris Agreement
Another global effort aimed at reducing greenhouse-gas emissions and limiting climate change. The statement above reflects the textbook’s own comparative framing.
Threads of Curiosity — Mission LiFE
Mission LiFE stands for Lifestyle for Environment.
The chapter describes it as an India-led global initiative introduced at the United Nations Climate Change Conference in 2021.
Mission LiFE encourages:
- mindful consumption;
- saving energy;
- conserving resources;
- eco-friendly lifestyles;
- individual and community action.
Its message fits the chapter perfectly: unsustainable consumption can disturb an interconnected Earth system, while many responsible actions together can help restore balance.
Solar energy drives physical processes; matter cycles through Earth’s spheres; human activities can disturb these processes; sustainable action can reduce disturbance.
23. Important Cause → Effect Chains
Warmer Arabian Sea
Warmer sea → evaporation increases → southwest monsoon fluctuates → rainfall variability → floods in some regions and drought in others.
Global Warming
Greenhouse gases increase → greenhouse effect strengthens → temperature rises → glaciers and polar ice melt faster → sea-level rise increases.
Deforestation
Forest removal → photosynthesis decreases + transpiration decreases + soil binding decreases → CO₂ uptake falls + rainfall patterns may change + erosion increases + habitat is lost.
Fertiliser Overuse
Excess nitrate → algal bloom → dissolved oxygen falls → fish and aquatic ecosystems are damaged.
Urbanisation
Concrete and asphalt increase → heat absorption and storage increase → re-radiation increases → urban heat island strengthens.
Ocean Warming
Ocean temperature rises → CO₂ absorption capacity decreases → ocean carbon sink becomes less effective → carbon-cycle balance is disturbed.
24. Master Comparison Tables
| Comparison | First Concept | Second Concept |
|---|---|---|
| Energy flow vs matter cycling | Energy enters mainly from the Sun, drives processes and is eventually re-radiated as heat. | Water, carbon, nitrogen and oxygen are repeatedly transferred and recycled. |
| High vs low albedo | High albedo reflects more and absorbs less. | Low albedo reflects less and absorbs more. |
| Equatorial vs polar heating | More direct sunlight; energy concentrated over smaller area. | More oblique sunlight; energy spread over larger area. |
| Troposphere vs stratosphere | 0–12 km average; weather; temperature decreases with altitude. | 12–50 km; ozone; temperature increases with altitude. |
| Natural vs enhanced greenhouse effect | Natural and necessary for a habitable temperature. | Additional warming due to excessive greenhouse gases. |
| Stratospheric vs ground-level ozone | Protective; absorbs harmful UV. | Harmful air pollutant associated with smog. |
| Valley vs mountain breeze | Daytime, upslope. | Nighttime, downslope. |
| Local vs planetary winds | Small-scale circulation caused by local heating differences. | Large-scale circulation produced by global pressure belts. |
| Warm vs cold ocean currents | Transfer warmer water toward higher latitudes. | Carry colder denser water, including deep return flows. |
| Surface vs deep currents | Strongly influenced by winds and friction. | Strongly influenced by density differences related to temperature and salinity. |
| Fast vs slow carbon cycle | Days to years; photosynthesis, respiration, feeding and decomposition. | Millions of years; burial, fossil-fuel formation and geological storage. |
| Nitrogen fixation vs nitrification | Atmospheric N₂ is converted to fixed nitrogen/ammonia. | Ammonia is converted to nitrite and then nitrate. |
| Ammonification vs denitrification | Decomposition returns ammonia to soil. | Nitrate is converted back to atmospheric N₂. |
| Photosynthesis vs respiration | Uses CO₂ and water; stores solar energy; releases O₂. | Uses O₂; releases energy from food and returns CO₂. |
| Renewable vs fossil-fuel energy | Solar/wind reduce the need to release stored fossil carbon. | Combustion rapidly releases geological carbon as CO₂. |
25. Textbook Activities Explained
Activity 13.1 — Let Us Explore
Aim: To identify the five Earth spheres in a landscape and understand how they interact.
Material/Data Required: Figure 13.1 from the chapter, notebook and class discussion.
Procedure:
- Identify one example each of the geosphere, hydrosphere, cryosphere, atmosphere and biosphere.
- Explain how snow eventually becomes part of the lake.
- Predict what several years of reduced snowfall would do to lake levels and grass.
- Discuss how a disturbance in one sphere can affect the others.
Observation: Snow can melt and enter lakes. Lake water supports vegetation, and vegetation supports grazing animals.
Explanation: Water transfers from cryosphere to hydrosphere and then supports the biosphere.
Inference: A change in one sphere can cause changes in others.
Conclusion: Earth’s spheres are interconnected and maintain a delicate balance.
Possible viva questions
1. Which sphere includes a glacier?
Cryosphere.
2. How does snow become lake water?
Snow melts into liquid water and flows into streams, rivers or
lakes.
3. Why can less snowfall affect sheep?
Less snowmelt may reduce lake water, which can reduce grass growth
and therefore food available for sheep.
Activity 13.2 — Let Us Find Out
Aim: To compare the albedo of common surfaces.
Material/Data Required: Textbook Table 13.1 and authentic books/websites.
Procedure:
- Record snow albedo: 0.80–0.90.
- Record ice albedo: 0.50–0.70.
- Record crushed-rock albedo: 0.25–0.30.
- Research values for light-coloured soil, black soil and ocean water.
- Compare reflection between surfaces.
Observation: Snow and ice have comparatively high albedo. Darker surfaces and ocean water are discussed as lower-albedo surfaces.
Explanation: A surface with high albedo reflects more incoming radiation; a low-albedo surface absorbs more.
Inference: Surface material and colour affect solar heating.
Conclusion: Differences in surface properties contribute to uneven heating of Earth.
The chapter includes an internet-safety reminder not to share personal information while researching online.
Possible viva questions
1. Define albedo.
The fraction of incoming solar radiation reflected by a surface.
2. Which has higher albedo: snow or crushed rock?
Snow.
3. Why does a low-albedo surface heat more?
It reflects less and absorbs a greater fraction of the incoming
radiation.
26. Textbook Thinking Sections
Think It Over
1. How does warming of Arabian Sea water affect the southwest monsoon?
2. If a large forest is cleared, how can river flow be affected?
3. What might happen to coastal cities if glaciers and polar ice melt faster?
4. How could increasing atmospheric CO₂ affect ocean plankton?
Pause and Ponder
1. What happens to surface temperature as greenhouse-gas concentration increases?
2. How can a cool mountain breeze benefit crops and soil?
3. What happens to warm surface water moving from the equator toward the poles?
4. If global temperature rises and oceanic CO₂ balance is disturbed, what may happen to marine life?
5. What would happen if biogeochemical cycles were disrupted and stopped?
6. How do human activities increase greenhouse gases, and what can an individual do?
What If… Photosynthesis Stopped?
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Ready to Go Beyond — UV Radiation
Click to view key learning
Ready to Go Beyond — Haber–Bosch
Click to view key learning
The Journey Beyond — Model Approaches
Ocean-covered Earth versus land-covered Earth: how might winds differ?
Trace the carbon and nitrogen in a recent meal.
Compare monsoon rainfall in two different decades.
The Quest Continues
What new discoveries could real-time Earth-observation tools reveal?
27. Indian Scientific Contributions
Anna Mani
Mapped solar insolation across India during the 1950s and helped establish a scientific basis for studying India’s solar resource.
S. Rangarajan
Co-authored Solar Radiation Over India with Anna Mani in 1982.
India’s First Insolation Atlas
Solar Radiation Over India brought together measurements demonstrating India’s considerable solar-energy potential.
K. R. Ramanathan
Measured ozone in the Himalayas at 18,000 ft in 1934 and later contributed to early monsoon-forecasting work.
IITM, Pune
Uses coupled atmosphere–ocean–land–ice models together with satellite, buoy and Antarctic observations to improve monsoon forecasting and climate research.
Mission LiFE
India-led initiative promoting environmentally responsible lifestyles and resource conservation.
28. Important Definitions
Earth System
The interacting whole formed by Earth’s spheres and the movement of energy and matter among them.
Geosphere
Solid rocks, soil, landforms and Earth’s interior.
Hydrosphere
Liquid water in oceans, rivers, lakes and groundwater.
Cryosphere
Solid water in the form of ice and snow.
Atmosphere
The air surrounding Earth and held by gravity.
Biosphere
All living organisms and their habitats.
Electromagnetic Radiation
Energy travelling as electromagnetic waves, which can move through a vacuum.
Electromagnetic Spectrum
The complete range of electromagnetic radiation.
Ultraviolet Radiation
High-energy electromagnetic radiation; the chapter gives a range of 100–400 nm.
Visible Radiation
Solar radiation visible to humans and important for photosynthesis.
Infrared Radiation
Radiation associated strongly with heating and with Earth’s outgoing thermal radiation.
Insolation
The Sun’s radiation reaching Earth’s surface.
Solar Constant
Average solar energy received per unit time per unit area perpendicular to the Sun’s rays at the top of the atmosphere: approximately 1.4 kW m⁻².
Albedo
Fraction of incoming solar radiation reflected by a surface.
Re-radiation
Radiation of stored heat energy from a warmed surface.
Urban Heat Island
Condition in which a city becomes warmer than surrounding rural areas because built surfaces absorb and retain heat and vegetation is reduced.
Greenhouse Effect
Warming produced when greenhouse gases absorb part of Earth’s outgoing infrared radiation.
Greenhouse Gas
A gas such as CO₂, CH₄ or water vapour that absorbs outgoing infrared radiation.
Ozone Layer
Ozone-containing region of the stratosphere that absorbs harmful UV radiation.
Valley Breeze
Daytime upslope wind produced when warm air rises over heated mountain slopes.
Mountain Breeze
Nighttime downslope flow of cold dense air from cooled mountain slopes.
Planetary Winds
Large-scale winds produced by global pressure differences caused by uneven heating.
Ocean Current
Continuous movement of a large mass of ocean water.
Gyre
Large circular pattern of ocean circulation influenced by Earth’s rotation and continents.
Biogeochemical Cycle
Cyclic movement of matter and energy between biotic and abiotic components, using the chapter’s definition.
Evaporation
Conversion of liquid water into water vapour.
Transpiration
Release of water vapour from plants.
Condensation
Conversion of water vapour into liquid droplets.
Precipitation
Water falling from clouds as rain, snow or hail.
Runoff
Water flowing over the land surface.
Infiltration
Seepage of water through soil and rocks.
Groundwater
Water stored beneath Earth’s surface in soil and rock spaces.
Carbon Cycle
Movement of carbon among atmosphere, organisms, rocks, fossil fuels and oceans.
Carbon Sink
A system such as a forest or ocean that absorbs and stores carbon.
Nitrogen Fixation
Conversion of atmospheric N₂ into usable nitrogen compounds.
Nitrification
Conversion of ammonia into nitrite and then nitrate.
Assimilation
Uptake of usable nitrogen compounds by plants and their transfer through food chains.
Ammonification
Decomposition process that returns nitrogen compounds such as ammonia to soil.
Denitrification
Conversion of nitrate back into atmospheric nitrogen gas.
Oxygen Cycle
Cycling of oxygen through photosynthesis, respiration, combustion and related processes.
Eutrophication
Nutrient enrichment of water causing excessive algal growth and oxygen depletion.
Algal Bloom
Rapid excessive growth of algae in nutrient-rich water.
Ground-Level Ozone
Harmful ozone pollutant formed near Earth’s surface in sunlight-driven air-pollution processes.
29. Formula & Numerical Practice
E = I × A × t
1 W = 1 J s⁻¹
1 kW = 1000 W
1 hour = 3600 s
1 minute = 60 s
1 kWh = 3.6 × 10⁶ J
Numerical 1 is the textbook example. The remaining problems are additional source-consistent Class 9 practice.
Click to view answer
A = 1 m²
t = 1 h = 3600 s
E = IAt
= 1000 × 1 × 3600
= 3,600,000 J
= 3.6 × 10⁶ J.
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E = 800 × 2 × 1800
= 2,880,000 J
= 2.88 × 10⁶ J.
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E = 900 × 0.5 × 1200
= 540,000 J
= 5.4 × 10⁵ J.
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t = 15 × 60 = 900 s
E = 1000 × 4 × 900
= 3.6 × 10⁶ J.
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E = 700 × 3 × 7200
= 15,120,000 J
= 1.512 × 10⁷ J.
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E = 500 × 1.2 × 2400
= 1.44 × 10⁶ J.
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I = 7.2 × 10⁶ ÷ (2 × 3600)
= 1000 W m⁻²
= 1 kW m⁻².
Click to view answer
t = 3.375 × 10⁶ ÷ (750 × 2.5)
= 1800 s
= 30 minutes.
30. Complete Question–Answer Bank
Every answer below uses native
<details> and
<summary>, so click-to-view answers work even when
JavaScript is unavailable.
A. MCQs — 30 Questions
(a) Hydrosphere (b) Geosphere (c) Cryosphere (d) Biosphere
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(a) Groundwater (b) Coral reef (c) Himalayan glacier (d) Troposphere
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(a) Moon (b) Sun (c) soil (d) ocean currents
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(a) 3 × 10⁸ m s⁻¹ (b) 3 × 10⁶ m s⁻¹ (c) 300 m s⁻¹ (d) 1.4 × 10³ m s⁻¹
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(a) 1–10 nm (b) 10–100 nm (c) 100–400 nm (d) 400–700 nm
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(a) gamma and X-rays (b) UV, visible and IR (c) microwaves only (d) radio waves only
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(a) 1.4 W m⁻² (b) 14 W m⁻² (c) 1.4 kW m⁻² (d) 14 kW m⁻²
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(a) 1 W m⁻² (b) 1 kW m⁻² (c) 10 kW m⁻² (d) 140 kW m⁻²
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(a) absorbs almost all radiation (b) reflects a large fraction (c) produces nitrogen (d) cannot heat
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(a) Crushed rock (b) Ice (c) Snow (d) Black soil
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(a) more forests (b) concrete and asphalt heat storage (c) less incoming sunlight (d) ozone recovery
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(a) 21% (b) 50% (c) 78% (d) 1%
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(a) thermosphere (b) troposphere (c) exosphere (d) mesosphere
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(a) 1.2 km (b) 12 km (c) 50 km (d) 100 km
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(a) increases 6.5°C/km (b) decreases about 6.5°C/km (c) remains constant (d) doubles
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(a) Soil heating (b) Ozone absorbs UV (c) Oceans boil (d) Nitrogen freezes
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(a) low to high pressure (b) high to low pressure (c) equator only (d) ocean only
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(a) day (b) night (c) winter only (d) storms only
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(a) equatorial low (b) subtropical high (c) polar low (d) no pressure belt
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(a) left (b) right (c) upward (d) downward
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(a) clockwise (b) counter-clockwise (c) vertically (d) randomly
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(a) Antarctic Current (b) Gulf Stream (c) Ganga (d) polar wind
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(a) condensation (b) nitrification (c) respiration (d) combustion
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(a) combustion (b) photosynthesis (c) denitrification (d) erosion
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(a) Rhizobium (b) Nitrosomonas (c) Nitrobacter (d) Pseudomonas
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(a) Nitrobacter (b) Rhizobium (c) Azotobacter (d) Pseudomonas
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(a) assimilation (b) denitrification (c) photosynthesis (d) infiltration
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(a) low sunlight (b) excess nitrate runoff (c) high albedo (d) ozone recovery
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(a) stratosphere (b) soil (c) ocean floor (d) biosphere
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(a) 3.6 × 10³ J (b) 3.6 × 10⁶ J (c) 360 J (d) 1.4 J
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B. Assertion–Reason — 10 Questions
A. Both Assertion and Reason are true and Reason correctly explains
Assertion.
B. Both are true but Reason does not correctly explain Assertion.
C. Assertion is true but Reason is false.
D. Assertion is false but Reason is true.
Reason: Snow has high albedo and reflects much incoming radiation.
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Reason: The troposphere is heated from Earth’s surface and rising warm air drives winds and storms.
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Reason: Ozone absorbs ultraviolet radiation and warms the layer.
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Reason: Sunlit mountain slopes warm rapidly and air over them rises.
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Reason: Earth’s rotation deflects moving air.
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Reason: They transport heat between latitudes.
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Reason: N₂ is relatively non-reactive and must first be converted into usable compounds.
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Reason: Nitrate enrichment can trigger algal blooms that deplete dissolved oxygen.
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Reason: Stratospheric ozone absorbs UV radiation.
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Reason: Carbon stored for millions of years can be released by combustion over a very short time.
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C. Very Short Answer — 15 Questions
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D. 2-Mark Questions — 15 Questions
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E. 3-Mark Questions — 15 Questions
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F. Long Answer / 5-Mark Questions — 10 Questions
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G. Give Reasons — 12 Questions
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H. Differentiate Between — 10 Questions
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Hydrosphere: liquid water in oceans, rivers, lakes and groundwater.
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I. Diagram-Based Questions — 10 Questions
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J. Numerical Questions — 8 Questions
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E = 800 × 2 × 1800 = 2.88 × 10⁶ J.
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E = 900 × 0.5 × 1200 = 5.4 × 10⁵ J.
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E = 1000 × 4 × 900 = 3.6 × 10⁶ J.
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E = 700 × 3 × 7200 = 1.512 × 10⁷ J.
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E = 500 × 1.2 × 2400 = 1.44 × 10⁶ J.
Click to view answer
= 7.2 × 10⁶/(2 × 3600)
= 1000 W m⁻².
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= 3.375 × 10⁶/(750 × 2.5)
= 1800 s
= 30 minutes.
K. Competency-Based Questions — 12 Questions
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L. HOTS — 10 Questions
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M. Case Studies — 5 Detailed Cases
Case Study 1 — Warmer Arabian Sea and the Monsoon
Sea-surface warming increases evaporation. The chapter links warmer Arabian Sea water with fluctuations in the southwest monsoon and greater variability in rainfall.
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Case Study 2 — Urban Heat Island
A city expands concrete roads and buildings while green areas decline.
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Case Study 3 — Himalayan Glacier Retreat
Rising atmospheric temperatures accelerate glacier melting and alter seasonal meltwater.
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Case Study 4 — Fertiliser Runoff
Following intense rain, nitrate-rich runoff enters a lake and a dense algal bloom develops.
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Case Study 5 — Ocean Warming and the Carbon Cycle
Atmospheric CO₂ rises while an ocean region becomes progressively warmer.
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31. Revise, Reflect, Refine — Complete Solutions
Question 1
Choose the most appropriate option describing the role of biogeochemical cycles.
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Question 2
Which option best describes the process primarily responsible for warming Earth?
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Question 3
Explain how climate change affects the water cycle with examples.
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Question 4
Describe how albedo affects surface temperature and climate.
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Question 5
How are mountain and valley breezes formed? Would mountain breezes from a grass-covered mountain and a barren rocky mountain differ in temperature?
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Question 6
Which atmospheric layer is mainly responsible for winds, storms and rainfall? Why?
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Question 7
Explain the processes of the nitrogen cycle. How would life be affected if nitrogen were not cycled?
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Question 8
What are the impacts of deforestation on Earth’s oxygen and carbon cycles? Give other consequences.
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Question 9
Explain with a suitable diagram the path carbon takes back to the atmosphere, starting from plants using atmospheric CO₂.
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Question 10
Why is excess CO₂ undesirable even though plants require carbon dioxide?
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Question 11
How is heat lost from Earth’s surface? What is its significance?
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Question 12
If Earth were a flat disc instead of a sphere, how would solar radiation and temperature patterns differ?
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Question 13
If atmospheric temperature rises, how would the cryosphere, hydrosphere and biosphere be affected?
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Hydrosphere: river flow, evaporation and sea level change; coastal flooding risk rises.
Biosphere: habitats, crops, fisheries and ecosystems experience stress and changing water availability.
Question 14
Explain how Earth’s atmosphere maintains a suitable temperature for life.
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Question 15
Describe the interrelationship between different Earth spheres and illustrate their delicate balance with an example.
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The Journey Beyond — Separate Model Responses
1. Compare atmospheric circulation on an ocean-only planet and a land-only planet.
2. Trace carbon and nitrogen from the atmosphere into a meal.
3. How should a student investigate long-term monsoon rainfall change?
32. Complete Exam Revision
Must Remember
- The Sun is Earth’s main energy source.
- Earth has five interacting spheres.
- Uneven heating drives winds and ocean currents.
- Matter moves through water, carbon, nitrogen and oxygen cycles.
- Changes in one sphere can affect several others.
Important Values
- Speed of light: 3 × 108 m s−1
- UV wavelength: 100–400 nm
- Solar constant: ≈1.4 kW m−2
- Maximum clear-sky surface insolation: ≈1 kW m−2
- Atmosphere: 78% nitrogen, 21% oxygen
- Troposphere: ≈0–12 km average
- Tropospheric temperature decrease: ≈6.5 °C km−1
- Stratosphere: ≈12–50 km
- Snow albedo: 0.80–0.90
- Ice albedo: 0.50–0.70
- Crushed rock albedo: 0.25–0.30
- Atmospheric CO₂ graph: approximately 315 ppm in 1960 → 420 ppm in 2025
Frequently Confused Concepts
Insolation reaches the surface; solar constant is specified at the top of the atmosphere.
Stratospheric ozone protects life; ground-level ozone is pollution.
Valley = day and upslope. Mountain = night and downslope.
Fixation starts with N₂. Nitrification starts with ammonia.
Biological exchange takes days to years; geological storage takes millions of years.
Common Mistakes
- Writing that all parts of Earth receive equal solar heating.
- Reversing wind direction: wind generally moves high → low pressure.
- Reversing hemispheric deflection: right in north, left in south.
- Writing that weather occurs mainly in the stratosphere.
- Writing that all ozone is harmful.
- Writing that all ozone is beneficial.
- Confusing nitrogen fixation with nitrification.
- Treating carbon, nitrogen, oxygen and water cycles as independent.
- Forgetting to convert kW → W and hours/minutes → seconds in numerical problems.
Cause-and-Effect Revision
Diagram Revision Checklist
At a Glance
- Electromagnetic radiation from the Sun is Earth’s primary energy source.
- Most evaporation, condensation, precipitation and weather occur in the troposphere.
- Earth’s shape, latitude and axial tilt help create variations in insolation.
- Uneven heating generates winds and ocean currents.
- Water, carbon, nitrogen and oxygen continuously move among air, ocean, land and organisms.
- Biogeochemical cycles keep nutrients available, support life and help maintain ecosystem balance.
Exam Tips
- In long answers, write processes as Cause → Mechanism → Result.
- Use the exact bacteria names in nitrogen-cycle questions.
- Label arrows clearly in cycle diagrams.
- Mention both biological and physical spheres when answering Earth-system questions.
- Include units at every step in solar-energy numericals.
- In climate questions, avoid giving only one consequence—trace the change through several spheres.
Last-Minute Revision Chart
Ready for New Horizons?
The chapter closes by reminding students that science is not merely a collection of facts. Science is a connected way of understanding nature: ask questions, look for evidence, recognise patterns, test ideas and investigate “what if?”
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