NCERT Solutions for Class 9 Science Chapter 13: Earth as a System: Energy, Matter and Life – Free PDF Download

Chapter 13, “Earth as a System: Energy, Matter and Life,” is the final chapter of the new NCERT “Exploration” 2026-27 edition of Class 9 Science, and it pulls together physics, chemistry and biology to explain how the Earth functions as one connected system of energy flow and matter recycling. These solutions were cross-checked against LearnCBSE, Careers360 and Vedantu’s Chapter 13 solution pages, with facts and terminology independently verified against standard Earth-science concepts (uneven solar heating, albedo, atmospheric layers, and the carbon, nitrogen, oxygen and water cycles); a factual correction was made where one source undercounted the Earth’s spheres.

Last Updated: September 23, 2026

NCERT Solutions for Class 9 Science Chapter 13: Earth as a System: Energy, Matter and Life

Revise, Reflect, Refine (NCERT Textbook, Page No. 267)

1. 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.
(ii) To recycle essential nutrients between biotic and abiotic components. Biogeochemical cycles move nutrients such as carbon, nitrogen, oxygen and water back and forth between living organisms and the non-living environment (air, water, soil), keeping the ecosystem’s nutrient supply balanced instead of depleting it.

2. 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.
(iii) The Earth’s surface absorbs solar radiation, which is then re-radiated and trapped by greenhouse gases. Most incoming sunlight passes through the atmosphere and is absorbed by land and water. The warmed surface re-emits this energy as infrared (heat) radiation, and greenhouse gases such as CO2, methane and water vapour trap part of it, which is what actually warms the atmosphere (the natural greenhouse effect).

3. Explain how climate change affects the water cycle. Illustrate with examples.
Rising global temperature speeds up evaporation from oceans, rivers and lakes, so the atmosphere holds more moisture. This intensifies the water cycle: some regions get heavier, more concentrated rainfall leading to floods, while others experience longer dry spells and drought as rainfall patterns shift. Faster melting of glaciers and polar ice adds extra water to rivers and oceans, raising sea levels and threatening low-lying coastal areas. Intense, short bursts of rain also increase surface runoff rather than letting water soak into the ground, which reduces groundwater recharge and affects irrigation and drinking-water supplies. For example, parts of India have seen both unusually heavy monsoon flooding and prolonged drought years in the same decade — both linked to a climate-disturbed water cycle.

4. Describe how albedo affects the Earth’s surface temperature and its climate.
Albedo is the fraction of incoming solar radiation that a surface reflects back rather than absorbs. High-albedo surfaces such as fresh snow and ice reflect most sunlight and stay cool, while low-albedo surfaces such as oceans, forests and dark soil absorb more energy and warm up. This uneven absorption creates temperature differences between regions, which in turn drive winds and ocean currents. Albedo change can also reinforce warming: as rising temperatures melt reflective snow and ice, the darker land or ocean exposed underneath absorbs even more heat, accelerating further warming — a feedback loop that affects regional and global climate.

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?
Mountain and valley breezes form because slopes and valley floors heat up and cool down at different rates. During the day, the mountain slopes are heated by the sun faster than the valley air. The warm air over the slopes rises, creating low pressure that draws cooler air up from the valley — this upslope flow is the valley breeze. At night, the slopes lose heat quickly and the air above them becomes cool and dense; this cold air sinks and flows down into the valley as the mountain breeze. Yes, the two breezes would differ in temperature. The barren, rocky mountain has low albedo and heats up (and cools down) rapidly, so its daytime valley breeze would be warmer and its night-time mountain breeze would be colder. The grass-covered mountain has higher albedo and retains moisture through transpiration, so it heats and cools more gradually, giving milder breezes in both directions.

6. 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?
The troposphere, the lowest layer of the atmosphere, is responsible for weather phenomena such as winds, storms and rainfall. It holds nearly all of the atmosphere’s water vapour and dust particles and is heated directly from below by the Earth’s surface, so its temperature falls with increasing height. This makes warm surface air rise and cooler air sink, driving convection currents and wind patterns. The abundant moisture in the troposphere condenses around dust particles to form clouds, which produce precipitation — making the troposphere the most dynamic, weather-active layer of the atmosphere.

7. Explain the processes involved in the nitrogen cycle. How would life on Earth be affected if nitrogen were not cycled?
The nitrogen cycle moves nitrogen between the atmosphere, soil, plants, animals and microorganisms in five main steps. In nitrogen fixation, nitrogen-fixing bacteria such as Rhizobium (in root nodules of legumes) and free-living bacteria such as Azotobacter, along with lightning, convert unusable atmospheric nitrogen gas into ammonia/ammonium compounds. In nitrification, nitrifying bacteria such as Nitrosomonas convert ammonia into nitrites, and Nitrobacter converts nitrites into nitrates. In assimilation, plants absorb these nitrates from the soil and use them to build proteins and nucleic acids; animals then obtain nitrogen by eating plants or other animals. In ammonification, decomposers break down dead organisms and waste products, releasing nitrogen back into the soil as ammonia. Finally, in denitrification, denitrifying bacteria such as Pseudomonas convert soil nitrates back into nitrogen gas, which returns to the atmosphere, completing the cycle. If nitrogen were not cycled, plants would be unable to obtain usable nitrogen to build proteins and nucleic acids, stunting their growth; this would cut off the nitrogen supply to animals that depend on plants, disrupt food chains, and could eventually cause ecosystems to collapse.

8. What are the impacts of deforestation on the Earth’s oxygen and carbon cycles? What are the other consequences of deforestation?
Trees are central to both cycles: during photosynthesis they absorb carbon dioxide and release oxygen. When forests are cleared, less CO2 is absorbed and less oxygen is released, disturbing the natural balance of both gases in the atmosphere. If the felled trees are burned or left to decay, the carbon they had stored is released back into the atmosphere as CO2, adding to greenhouse gas levels and accelerating global warming. Beyond the two cycles, deforestation causes soil erosion (because tree roots no longer bind the soil), reduces transpiration and can lower local rainfall, destroys habitats and reduces biodiversity, and increases the risk of floods and landslides on bare slopes.

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.
Plants take in atmospheric CO2 during photosynthesis and convert it into glucose, which becomes part of plant tissue. When animals eat plants (or other animals), this carbon passes along the food chain. Both plants and animals return some carbon to the atmosphere as CO2 through respiration. When plants and animals die, decomposers (bacteria and fungi) break down their remains, releasing the stored carbon back into the atmosphere as CO2. Over millions of years, some dead organic matter escapes complete decomposition and gets buried under sediment, eventually forming fossil fuels such as coal and petroleum; when these fuels are burned (combustion) by humans, that long-stored carbon is released rapidly back into the atmosphere as CO2, completing the cycle.
Simple flow: Atmospheric CO2 → absorbed by plants (photosynthesis) → passed to animals (feeding) → released back as CO2 through respiration and decomposition; buried carbon → fossil fuels → combustion → CO2 returns to the atmosphere.

10. Why is an excess of CO2 in the atmosphere considered undesirable even though it is required by plants?
Plants need CO2 for photosynthesis, but only in the small, naturally balanced amount normally present in the atmosphere. When human activities such as burning fossil fuels and deforestation add CO2 faster than natural sinks (plants, oceans) can absorb it, the excess strengthens the greenhouse effect, trapping more heat and driving global warming. This leads to melting glaciers, rising sea levels, more extreme and unpredictable weather (floods, droughts, heatwaves), and disrupted rainfall patterns that reduce agricultural productivity — so while CO2 is essential in the right quantity, an excess destabilises climate and ecosystems.

11. How is heat lost from the surface of the Earth? What is its significance?
The Earth loses heat mainly through radiation: after absorbing solar energy, the warmed surface re-emits it as infrared (long-wave) radiation. Some of this radiated heat escapes directly into space, while a portion is intercepted, absorbed and partly re-radiated back down by greenhouse gases in the atmosphere. This controlled loss of heat is essential for maintaining the Earth’s energy balance — if all the absorbed heat were retained, the planet would overheat; if too much escaped, it would become too cold to sustain life. A steady balance between incoming solar energy and outgoing heat loss keeps Earth’s climate within the range needed to support life.

12. If the Earth were a flat disc instead of a sphere, how would the patterns of solar radiation and temperature be different?
On the real, spherical Earth, sunlight strikes different latitudes at different angles — nearly straight down at the equator and at a slanting angle near the poles — which is why the equator receives more concentrated energy and stays warmer while polar regions receive spread-out, weaker radiation and stay cold. This latitude-based temperature difference is what drives winds, ocean currents and distinct climate zones. If the Earth were a flat disc facing the Sun, sunlight would fall almost uniformly across the surface, so there would be little temperature difference between regions. Without that temperature contrast, the pressure belts, wind systems, ocean currents and seasonal patterns we rely on would not form, and the climate system would be far less dynamic (and far less capable of supporting the diversity of ecosystems Earth has today).

13. Suppose there is a rise in atmospheric temperature on Earth. How would this affect the cryosphere, hydrosphere and biosphere?
A rise in atmospheric temperature would affect all three spheres. In the cryosphere, glaciers, polar ice caps and seasonal snow would melt faster, shrinking the planet’s ice cover. This meltwater feeds into the hydrosphere, raising sea levels, altering ocean salinity and circulation, and increasing the risk of coastal flooding; warmer conditions also intensify evaporation and disturb rainfall patterns. In the biosphere, habitats shift or shrink — species adapted to cold conditions (such as polar bears and mountain-glacier-fed ecosystems) lose living space, coral reefs face bleaching from warmer, more acidic oceans, and changed rainfall and water availability affect agriculture, food chains and overall biodiversity.

14. Explain how the Earth’s atmosphere helps in maintaining a suitable temperature for life to survive on the Earth.
The atmosphere regulates Earth’s temperature in two main ways. First, it filters incoming solar radiation — the ozone layer absorbs harmful ultraviolet rays, while clouds, dust and gases absorb and scatter part of the visible and other radiation before it reaches the surface. Second, once the surface absorbs the remaining solar energy and re-radiates it as infrared heat, greenhouse gases (CO2, methane, water vapour) in the atmosphere absorb a portion of this outgoing heat and re-radiate some of it back toward the surface, which is the natural greenhouse effect. Without this insulating blanket, Earth would lose heat rapidly at night and become far too cold, much like the Moon, which has almost no atmosphere. By balancing incoming solar energy against outgoing heat loss, the atmosphere keeps surface temperatures within the narrow range needed to sustain life.

15. Describe the interrelationship between different spheres of the Earth. Illustrate with example how these spheres function in a delicate balance.
The Earth system is made up of five major, constantly interacting spheres: the geosphere (land, rocks, soil), the hydrosphere (oceans, rivers, lakes, groundwater), the cryosphere (glaciers, ice caps, snow, permafrost), the atmosphere (the envelope of air), and the biosphere (all living organisms). Energy and matter flow continuously between them through biogeochemical cycles such as the water, carbon and nitrogen cycles, and a change in one sphere ripples through the others. For example, in the water cycle, ocean water (hydrosphere) evaporates into the atmosphere, forms clouds, and falls as precipitation; some of it falls as snow that becomes glacial ice (cryosphere), while the rest flows over land (geosphere), recharges rivers and groundwater, and supports plant and animal life (biosphere). If one link is disturbed — say, reduced snowfall shrinks the cryosphere — it lowers meltwater supply to rivers and lakes (hydrosphere), which in turn reduces soil moisture and vegetation available to the biosphere. This tightly linked, delicately balanced system is why even small disturbances, such as excess greenhouse gas emissions or large-scale deforestation, can cascade into changes across the entire planet. (Note: some solution sources list only four Earth spheres, omitting the cryosphere — this is incorrect for this chapter, which explicitly treats the cryosphere as a distinct fifth sphere, as shown in Activity 13.1 on page 253 of the NCERT textbook.)

In-Text Questions (Think It Over, Pause and Ponder, What If…, and Activities)

Think It Over — How does the warming of Arabian Sea water affect the southwest monsoon in India? (Page 252)
Warmer Arabian Sea water increases evaporation, adding more moisture to the monsoon winds. This can intensify the southwest monsoon, causing heavier and more erratic rainfall, sudden cloudbursts, and even cyclones in some years, while other areas may see irregular or delayed rainfall — making the monsoon less predictable overall.

Think It Over — If a large forest is cleared, how can that affect the flow of a river in that area? (Page 252)
Forests slow down rainwater and allow it to soak into the soil, feeding rivers steadily over time. When a forest is cleared, tree roots no longer hold the soil or absorb rainwater, so more water runs off the surface quickly. This causes sudden, heavy flow (and flooding) during the rains but a much reduced, drier flow between rainfall events, making the river’s flow irregular.

Think It Over — What might happen to coastal cities in India if glaciers and polar ice keep melting faster? (Page 252)
Faster melting of glaciers and polar ice raises global sea levels. Coastal cities in India could face increased flooding of low-lying areas, damage to buildings and infrastructure, saltwater intrusion into freshwater sources, more destructive storm surges, and eventual displacement of people living near the coast.

Think It Over — How would increasing carbon dioxide levels in the atmosphere affect the ocean plankton? (Page 252)
Oceans absorb a large share of atmospheric CO2, and rising CO2 levels make seawater more acidic. This ocean acidification makes it harder for plankton and other shell-forming organisms to build and maintain their calcium-based shells, threatening their survival — and since plankton form the base of the marine food chain, their decline can disrupt the entire marine ecosystem.

Pause and Ponder — Visit the PhET simulation and study the effect of greenhouse gas concentration on surface temperature. (Page 258)
This is a hands-on simulation activity (phet.colorado.edu, “Greenhouse Effect”). Students should observe that as the concentration of greenhouse gases in the simulated atmosphere increases, more outgoing infrared radiation is trapped, and the simulated surface temperature rises — directly demonstrating the greenhouse effect described in the chapter.

Pause and Ponder — How does the cool mountain breeze benefit agricultural activity, particularly the crops and soil? (Page 261)
The cool mountain breeze lowers field temperature, reducing heat stress on crops, and slows down evaporation from the soil so that moisture is retained for longer, reducing the need for frequent irrigation. It also improves air circulation around plants, which helps prevent fungal diseases, creating overall more favourable conditions for healthy crop growth.

Pause and Ponder — What happens to the warm surface water from the equator as it travels toward the poles? What impact does this movement have on the area? (Page 261)
Warm equatorial surface water is carried toward the poles by ocean currents. As it travels, it gradually loses heat to the surrounding air, warming nearby coastal regions and giving them a milder climate than their latitude would otherwise suggest. This large-scale heat transport by ocean currents plays a key role in regulating global temperature distribution.

Pause and Ponder — The CO2 dissolved in the ocean is disturbed when the global temperature increases. What will happen to marine life? (Page 263)
As global temperature rises, the ocean’s capacity to absorb and hold dissolved CO2 changes, and increased CO2 uptake makes seawater more acidic. This harms plankton and other shell- and skeleton-forming organisms, damages coral reefs (leading to coral bleaching), and — since plankton anchor the marine food web — can reduce fish populations and destabilise marine ecosystems overall.

Pause and Ponder — What would happen to plants and animals on Earth if the biogeochemical cycles were disrupted and stopped? Explain by giving a few examples. (Page 265)
Biogeochemical cycles keep essential nutrients such as water, carbon, nitrogen and oxygen continuously available to living things. If these cycles stopped, plants and animals would lose access to nutrients they depend on, and food chains would break down. For example, a disrupted water cycle would cause irregular rainfall, droughts or floods that damage crops and reduce drinking water availability; a disrupted carbon cycle would let CO2 build up unchecked, intensifying global warming; a stalled nitrogen cycle would deprive plants of usable nitrogen, stunting growth and weakening food chains; and a disrupted oxygen cycle would upset the atmospheric oxygen balance needed for respiration.

Pause and Ponder — Discuss how human activities increase the concentration of greenhouse gases in the atmosphere. What would you do as an individual to reduce the emission of greenhouse gas? (Page 266)
Human activities such as burning fossil fuels (coal, petrol, diesel) for transport, electricity and industry release large amounts of CO2; deforestation removes trees that would otherwise absorb CO2; and livestock farming, waste burning and certain industrial processes release methane and nitrous oxide — all of which strengthen the greenhouse effect and drive global warming. As an individual, one can help by using public transport, cycling or walking instead of private vehicles, saving electricity and using energy-efficient appliances, planting trees, reducing, reusing and recycling waste, and avoiding unnecessary burning of waste or crop residue.

What If… — If photosynthesis stopped, what would happen to the Earth? (Page 265)
If photosynthesis stopped, plants would no longer be able to make their own food or release oxygen. Plants would die first, and this would cut off the primary food source for herbivores, which would in turn affect carnivores — collapsing food chains across ecosystems. Atmospheric CO2 levels would rise steadily since it would no longer be absorbed by plants, while oxygen levels would fall as it stopped being replenished, disturbing the atmosphere’s gas balance. Ultimately, without photosynthesis renewing food and oxygen supplies, life on Earth as we know it would not be sustainable.

Activity 13.1: Let Us Explore — Identify examples of the geosphere, hydrosphere, cryosphere, atmosphere and biosphere in the given landscape figure. (Page 253)
Geosphere is represented by land, soil, rocks and mountains; hydrosphere by the lake or water body; cryosphere by snow and ice on the mountain peaks; atmosphere by the surrounding air; and biosphere by all living organisms in the scene, such as the herder, the sheep and the grass or plants.

Activity 13.1: Let Us Explore — How does snow (cryosphere) eventually become part of the lake (hydrosphere)? (Page 253)
When temperatures rise, snow on the mountain slopes (cryosphere) melts. This meltwater flows downhill over the land (geosphere) and eventually drains into the lake, becoming part of the hydrosphere — showing how the cryosphere and hydrosphere are directly linked.

Activity 13.1: Let Us Explore — If there is less snowfall during winters for a few years, how would this affect the lake’s level and the grass available for the sheep? (Page 253)
Less winter snowfall means the cryosphere stores less frozen water. With less snow to melt in spring and summer, the lake would receive less meltwater and its level would drop. Lower lake levels and reduced soil moisture would in turn mean less grass growth for the sheep to graze on, showing how a change in one sphere (cryosphere) cascades through the hydrosphere and biosphere.

Activity 13.1: Let Us Explore — Discuss how all the spheres are interconnected, and how a disturbance in one can lead to changes in others. (Page 253)
All five spheres are linked in a chain of cause and effect: for instance, reduced snowfall (cryosphere) lowers lake water levels (hydrosphere), which reduces soil moisture and grass growth (biosphere) available for grazing animals. Similarly, rising atmospheric temperatures can accelerate snowmelt, altering water levels and affecting the plants and animals that depend on that ecosystem. This shows that even a small disturbance in one sphere can trigger a chain of changes across the entire Earth system.

Activity 13.2: Let Us Find Out — Complete the table on the reflection (albedo) of solar radiation by different surfaces: light coloured soil, black soil, and ocean water. (Page 256)
Using reference data for surface albedo: light coloured soil reflects roughly 30–40% of incoming solar radiation (albedo ≈ 0.30–0.40); black soil, being darker, reflects far less and absorbs more heat, with an albedo of roughly 0.10–0.20; and ocean water has one of the lowest albedos of all common surfaces, around 0.05–0.10, meaning it absorbs most of the solar radiation that reaches it. (These are approximate reference ranges — exact values found from different authentic sources may vary slightly, and the surrounding snow, ice and crushed rock values given in the textbook table, 0.80–0.90, 0.50–0.70 and 0.25–0.30 respectively, confirm the general pattern that lighter/whiter surfaces have much higher albedo than darker surfaces.)

Why This Chapter Matters

Chapter 13 serves as a fitting capstone to Class 9 Science because it draws together ideas from across the year’s syllabus — heat and energy transfer from the physics chapters, atoms and chemical cycles from the chemistry chapters, and classification, ecosystems and life processes from the biology chapters — and shows how they combine to keep the entire planet habitable. Understanding uneven solar heating, albedo, atmospheric layers, and the water, carbon, nitrogen and oxygen cycles gives students the foundation they will build on in Class 10 with topics like “Our Environment” and management of natural resources, and it grounds real-world issues such as climate change, deforestation and sustainability in solid scientific reasoning rather than just current-affairs awareness.

More on This Chapter

Extra Questions | Revision Notes | Formulas Handbook

Frequently Asked Questions

How many spheres make up the Earth system according to this chapter?
Five: the geosphere (land and rocks), hydrosphere (water bodies), cryosphere (ice and snow), atmosphere (air), and biosphere (all living organisms). All five are interconnected, and Activity 13.1 in the textbook (page 253) specifically asks students to identify examples of each from a single landscape.

What is albedo, and why is it important in this chapter?
Albedo is the fraction of solar radiation a surface reflects rather than absorbs. High-albedo surfaces like snow and ice stay cool because they reflect most sunlight, while low-albedo surfaces like oceans and dark soil absorb more heat and warm up faster. This difference drives uneven heating of the Earth, which in turn causes winds, ocean currents and regional climate variation — a core idea tested in Q4 and Q5 of the exercise.

Which four biogeochemical cycles are covered in Chapter 13?
The water cycle, the carbon cycle, the nitrogen cycle and the oxygen cycle. Together they explain how essential elements move continuously between living organisms (biotic) and the non-living environment (abiotic), which is the concept tested directly in exercise Question 1.

Why does the troposphere get most of the attention when discussing weather?
Because the troposphere is the lowest atmospheric layer, closest to the Earth’s surface, and contains almost all of the atmosphere’s water vapour and dust. It is heated from below, which drives rising and sinking air currents that create wind, clouds, storms and rainfall — making it the layer responsible for nearly all weather phenomena, as explained in exercise Question 6.

Is this chapter important for board exams even though it’s the last chapter?
Yes. As the concluding chapter of Class 9 Science, it integrates concepts from earlier physics, chemistry and biology chapters and is a common source of long-answer and diagram-based questions (such as the carbon and nitrogen cycle diagrams) in school exams, in addition to laying groundwork for environment-related topics in Class 10.

Written by Satish

NCERTBooks.org is an independent educational resource run by a small team focused on making official NCERT textbooks easy to find, read, and download for students, parents, and teachers across India. We are not affiliated with NCERT or the Ministry of Education -- we organise publicly available NCERT content by class and subject, verify links against official sources, and build tools (like our in-browser reader) that make studying more convenient. Every guide we publish is written and reviewed by our team based on the actual NCERT curriculum and syllabus.