Chapter 13: Earth as a System: Energy, Matter, and Life (Environmental Science)
Step-by-step answers to all 15 "Revise, Reflect, Refine" questions of Chapter 13, Earth as a System: Energy, Matter, and Life (NCERT Class 9 Science, Exploration, 2026-27): solar radiation, albedo, greenhouse effect, layers of the atmosphere, local winds, the water, nitrogen and carbon cycles, climate change and the interaction of Earth's spheres, with a carbon cycle diagram. All 15 questions are answered, with the key answer highlighted.
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.
Solution
Answer: (ii)
Biogeochemical cycles (water, carbon, oxygen, nitrogen cycles) move essential substances again and again between living things (biotic) and the air, water and soil (abiotic), so that the limited amount of matter on the Earth can be reused. They do not provide food directly (i), cannot create new elements (iii) (matter is only recycled), and removing toxins from an organism (iv) is the job of excretion, not of these cycles.
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.
Solution
Answer: (iii)
Most incoming sunlight passes through the atmosphere and is absorbed by the land and water, which get warm. The warm surface then re-radiates heat as infrared radiation. Greenhouse gases such as CO₂, methane and water vapour absorb this outgoing heat and send part of it back, keeping the Earth warm (the greenhouse effect). Incoming sunlight is not mainly absorbed by CO₂ or by particles (i, ii), and radiation reflected by clouds goes back to space rather than heating the Earth (iv).
Explain how climate change affects the water cycle. Illustrate with examples.
Solution
Climate change (global warming) adds more heat energy to the water cycle and makes it more intense and less predictable:
More evaporation: warmer oceans, lakes and soil lose more water as vapour; soils dry out faster, worsening droughts (e.g. longer dry spells in parts of Rajasthan and Marathwada).
Heavier rainfall: warm air holds more water vapour, so when it rains it can pour; extreme rainfall events, cloudbursts and floods become more frequent (e.g. cloudbursts in Himachal Pradesh and Uttarakhand, urban floods in Mumbai and Chennai).
Changed rain patterns: the monsoon becomes more erratic, with some regions getting too much rain and others too little, and the timing changes, affecting farming.
Melting ice and snow: glaciers (for example in the Himalayas) and polar ice melt faster. Rivers fed by glaciers may first flood and then shrink in the long run; less snow is stored for summer.
Rising sea levels: water from melting ice and the expansion of warm sea water raise the sea level, flooding low coastal areas such as the Sundarbans.
Stronger cyclones: warmer oceans supply more energy and moisture to tropical cyclones.
Warming increases evaporation and the moisture in the air, causing more droughts in some places and heavier rainfall, floods and cloudbursts in others; it makes the monsoon erratic, melts glaciers and polar ice, raises sea level and strengthens cyclones.
Describe how albedo affects the Earth's surface temperature and its climate.
Solution
Albedo is the fraction of incoming solar radiation that a surface reflects.
High albedo surfaces (snow and ice, about 0.8–0.9; light-coloured sand) reflect most sunlight, absorb little and stay cool. This is one reason polar regions are very cold.
Low albedo surfaces (ocean water, black soil, forests, asphalt) absorb most sunlight and get warmer.
Effect on climate:
The overall albedo of the Earth controls how much solar energy it keeps, and so its average temperature.
Melting ice speeds up warming: when ice and snow melt, the darker land or sea beneath is exposed. It absorbs more heat, which melts even more ice (a feedback loop), so the Arctic is warming fastest.
Changes in land use also matter: replacing forests or fields with roads and buildings, or the spread of deserts, changes the local albedo and temperature (e.g. urban heat islands).
Clouds and dust have high albedo and reflect sunlight, which cools the surface.
Surfaces with high albedo (snow, ice) reflect more sunlight and stay cool; low albedo surfaces (oceans, dark soil, forests, roads) absorb more and get warmer. So albedo controls how much solar energy the Earth retains; melting ice lowers albedo and speeds up warming.
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
Valley breeze (day): during the day, the mountain slopes facing the Sun heat up faster than the valley floor. The air over the slopes becomes warm and rises, creating low pressure. Cooler air from the valley moves up the slopes to replace it; this is the valley breeze.
Mountain breeze (night): after sunset, the slopes lose heat faster and become cooler than the valley floor. The air over the slopes becomes cold and dense and flows down into the valley; this is the mountain breeze.
Grassy versus rocky mountain: yes, the breezes would differ.
Barren rocks absorb a lot of solar radiation during the day (they are darker, with lower albedo, and have no shade or moisture) and store heat. At night they re-radiate this stored heat, so the air above them does not cool as much. The mountain breeze from the rocky mountain would therefore be warmer (and drier).
Grass keeps the surface cooler during the day through shade and transpiration (evaporation of water uses heat), and stores little heat, so there is little re-radiation at night. The mountain breeze from the grassy mountain would be cooler (and more humid).
(This is similar to why a concrete house feels hot at night while a mud house stays cool.)
Valley breeze: by day warm air rises from the sun-heated slopes and cool valley air moves up. Mountain breeze: at night cooled, dense air flows down the slopes. Yes: the rocky mountain stores more heat and re-radiates it at night, so its breeze is warmer; the grassy mountain stays cooler (shade, transpiration), giving a cooler breeze.
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?
Solution
The troposphere, the lowest layer of the atmosphere (about 0–12 km above the surface; highest over the equator).
Primary reason: the troposphere is heated from below by the Earth's surface, and the surface is heated unevenly by the Sun (land and water, equator and poles, day and night heat differently). This causes:
Differences in temperature and pressure, so air moves from high to low pressure: winds and storms.
Warm air rising and cooling, so water vapour condenses: clouds and rain.
Almost all the water vapour and most of the air are in this layer, so clouds, rain, snow and storms form here.
The troposphere. It contains almost all the water vapour and is unevenly heated from below by the Earth's surface, so temperature and pressure differences drive winds, storms, clouds and rain.
Explain the processes involved in the nitrogen cycle. How would life on Earth be affected if nitrogen were not cycled?
Solution
About 78% of the air is nitrogen (N₂), but plants and animals cannot use this gas directly. The nitrogen cycle converts it into usable forms and back:
Nitrogen fixation: nitrogen-fixing bacteria (Rhizobium in root nodules of legumes, Azotobacter in soil) convert atmospheric N₂ into ammonia (NH₃). Lightning also combines N₂ with oxygen to form nitrogen oxides, which reach the soil with rain.
Nitrification: nitrifying bacteria convert ammonia into nitrite (Nitrosomonas), then nitrite into nitrate (Nitrobacter).
Assimilation: plants absorb nitrates (and ammonium) from the soil and make proteins and nucleic acids; animals get nitrogen by eating plants or other animals.
Ammonification: when plants and animals die or excrete wastes, decomposers (bacteria and fungi) break down the organic matter and release ammonia into the soil.
Denitrification: denitrifying bacteria (Pseudomonas) convert nitrates back into nitrogen gas, which returns to the atmosphere.
If nitrogen were not cycled: nitrogen is needed to make proteins, DNA and chlorophyll. Without fixation and nitrification, plants would not get usable nitrogen, so they could not grow; then animals (which depend on plants) would also starve. Without ammonification, nitrogen would stay locked in dead bodies and wastes; without denitrification, nitrates would pile up in soil and water. Life as we know it would gradually stop.
The cycle has nitrogen fixation (bacteria, lightning), nitrification (ammonia → nitrite → nitrate), assimilation (by plants, then animals), ammonification (decomposers release ammonia) and denitrification (nitrate → N₂). Without it, plants could not make proteins and DNA, so plants and then animals could not survive.
What are the impacts of deforestation on the Earth's oxygen and carbon cycles? What are the other consequences of deforestation?
Solution
Effect on the oxygen and carbon cycles:
Fewer trees means less photosynthesis, so less CO₂ is taken out of the air and less oxygen is released.
Carbon stored in the trees is released as CO₂ when the wood is burnt or decays. Forests change from carbon stores (sinks) into sources of carbon.
So CO₂ in the atmosphere increases, which strengthens the greenhouse effect and adds to global warming and climate change.
Other consequences:
Soil erosion and loss of fertile topsoil, since roots no longer hold the soil; more landslides on slopes.
Floods and droughts: less water soaks into the ground and less groundwater is recharged; rivers silt up.
Changes in the water cycle and rainfall: fewer trees means less transpiration, so local rainfall decreases and the climate becomes hotter and drier.
Loss of biodiversity: habitats are destroyed and many plants and animals become endangered or extinct; human–wildlife conflict increases.
Desertification, and loss of livelihood for forest-dependent people (food, fuel, medicines).
Less photosynthesis removes less CO₂ and releases less O₂, and burning or decay of trees adds CO₂, increasing global warming. It also causes soil erosion, floods and droughts, less rainfall, loss of biodiversity and habitats, and desertification.
Explain with a suitable diagram the path that carbon takes to go back to the atmosphere. You may start from plants using CO₂ from the atmosphere.
Solution
Photosynthesis: green plants take in CO₂ from the atmosphere and, using sunlight, convert it into glucose and other carbon compounds (food).
Food chain: animals get carbon by eating plants (and other animals).
Respiration: plants and animals break down food for energy and release CO₂ back into the atmosphere.
Death and decomposition: when plants and animals die (and from their wastes), decomposers such as bacteria and fungi break down the remains; their respiration releases CO₂.
Fossil fuels and combustion: some dead remains get buried and, over millions of years, change into coal, petroleum and natural gas. When these fuels (or wood) are burnt, the stored carbon returns to the atmosphere as CO₂.
(Oceans also dissolve CO₂ from the air and release it back; some carbon is stored in shells and limestone.)
The carbon cycle: carbon fixed by photosynthesis returns to the atmosphere as CO₂ through respiration, decomposition and combustion
CO₂ → plants (photosynthesis) → animals (food) → back to the air by respiration of plants and animals, decomposition of dead remains, and burning of fossil fuels formed from buried remains.
Why is an excess of CO₂ in the atmosphere considered undesirable even though it is required by plants?
Solution
Plants need only a small amount of CO₂ for photosynthesis, and the natural carbon cycle keeps it in balance. But human activities (burning fossil fuels, deforestation) are adding CO₂ faster than plants and oceans can remove it. The excess is harmful because:
CO₂ is a greenhouse gas: more CO₂ traps more of the heat re-radiated by the Earth, causing global warming and climate change (heat waves, melting glaciers, rising sea level, erratic rainfall, stronger storms).
Oceans absorb extra CO₂ and become more acidic, which harms corals and shelled sea animals.
Plants cannot use all of it; the extra growth is limited by water, nutrients and heat stress, so it does not balance the excess.
Because the excess cannot be absorbed by plants and oceans; as a greenhouse gas it traps more heat, causing global warming and climate change, and it also makes oceans acidic.
How is heat lost from the surface of the Earth? What is its significance?
Solution
How heat is lost:
Radiation: the warm surface re-radiates energy as infrared radiation (long-wave heat). Part of it escapes into space; part is absorbed by greenhouse gases and clouds and sent back.
Conduction: heat passes from the surface to the air in contact with it.
Convection: the warmed air expands, rises and carries heat upward; cooler air takes its place (this produces winds).
Evaporation: water from oceans, lakes, soil and plants (transpiration) evaporates, taking heat with it; the heat is released higher up when the vapour condenses into clouds.
Significance:
It balances the heat received from the Sun, so the Earth does not keep getting hotter; together with the greenhouse effect, it keeps the average temperature suitable for life (about 15 °C).
It cools the Earth at night and drives winds, clouds, rain and the water cycle, which spread heat from warm to cold regions.
By infrared radiation from the surface, conduction to the air, convection (rising warm air) and evaporation. It balances the heat received from the Sun so the Earth does not overheat, keeps a life-friendly temperature, and drives weather and the water cycle.
If the Earth were a flat disc instead of a sphere, how would the patterns of solar radiation and temperature be different?
Solution
On the spherical Earth, the Sun's rays fall vertically near the equator and more and more slanted towards the poles. Slanting rays spread over a larger area and travel through more atmosphere, so the poles are cold and the equator is hot. This uneven heating creates temperature zones (tropical, temperate, polar), pressure belts, winds and ocean currents.
If the Earth were a flat disc facing the Sun:
The rays would fall at almost the same angle on every part of the sunlit face, so all places would receive nearly equal radiation and have about the same temperature.
There would be no latitude-based zones: no hot equator or cold poles, so no polar ice caps of the present kind.
With little difference in heating from place to place, there would be weaker pressure differences, and much weaker global winds and ocean currents. (Day and night, and the side facing away from the Sun, would still be very different: one face hot, the other dark and very cold.)
Weather and climate would be much more uniform, and the distribution of life would also change.
Rays would strike almost the whole sunlit face at the same angle, so heating and temperature would be nearly uniform, with no hot equator or cold poles, no climate zones and much weaker global winds and ocean currents.
Suppose there is a rise in atmospheric temperature on Earth. How would this affect the cryosphere, hydrosphere and biosphere?
Solution
Sphere
Effects of a rise in temperature
Cryosphere (ice and snow)
Glaciers, polar ice sheets and sea ice melt faster; snow cover and permafrost shrink. Less ice means lower albedo, so more heat is absorbed, which speeds up warming.
Hydrosphere (water)
Sea level rises (meltwater and expansion of warm water); more evaporation, so heavier rain in some places and droughts in others; glacier-fed rivers first flood then shrink; oceans become warmer and absorb more CO₂ (become acidic); stronger cyclones.
Biosphere (living things)
Habitats change or are lost (e.g. polar bears and penguins lose ice; corals bleach in warm water); species shift to cooler places or higher altitudes, and some become extinct; crop yields fall due to heat and erratic rain; insect-borne diseases spread; forest fires increase.
These effects are linked: melting ice (cryosphere) raises sea level (hydrosphere), which floods coasts and destroys habitats (biosphere).
Cryosphere: glaciers and polar ice melt, lowering albedo. Hydrosphere: sea level rises, rainfall becomes extreme (floods and droughts), oceans warm and acidify. Biosphere: habitats are lost, species migrate or die out, corals bleach, crops and health are affected.
Explain how the Earth's atmosphere helps in maintaining a suitable temperature for life to survive on the Earth.
Solution
Greenhouse effect: gases like CO₂, water vapour and methane let sunlight in but trap part of the heat re-radiated by the surface. Without them, the Earth's average temperature would be about −18 °C instead of about 15 °C, and water would be frozen.
Reduces day–night differences: the atmosphere acts like a blanket, so the surface does not get extremely hot in the day or extremely cold at night (unlike the Moon, which has no atmosphere and ranges from about 120 °C to −170 °C).
Reflects and filters sunlight: clouds and dust reflect part of the incoming sunlight, preventing overheating; the ozone layer in the stratosphere absorbs harmful UV rays.
Spreads heat: winds and moving clouds carry heat from hot to cold regions, reducing extremes across the planet.
It also holds water vapour for the water cycle, which moderates temperature through evaporation and condensation.
Greenhouse gases trap part of the outgoing heat (keeping the average near 15 °C instead of −18 °C), the atmosphere blankets the Earth to reduce day–night extremes, clouds reflect excess sunlight, ozone blocks UV, and winds spread heat around the globe.
Biosphere: all living things, which live where the other three meet.
They constantly exchange energy and matter, so a change in one affects the others.
Example: the water cycle and a forest. The Sun heats the oceans (hydrosphere); water evaporates into the atmosphere, forms clouds and falls as rain on land. Rain soaks into the soil (lithosphere), weathers rocks and carries minerals to rivers. Trees (biosphere) absorb this water and minerals, release water vapour by transpiration and oxygen by photosynthesis, and take CO₂ from the air. Their roots bind the soil and their leaves add humus. Thus each sphere supports the others.
Delicate balance: if the forest is cut down (biosphere), rainfall and transpiration decrease (atmosphere), soil is eroded (lithosphere), rivers silt up and floods increase (hydrosphere), and wildlife loses its home. Similarly, burning fossil fuels (taken from the lithosphere) adds CO₂ to the atmosphere, warming the Earth, melting ice (cryosphere), raising sea levels (hydrosphere) and threatening coastal life (biosphere).
The lithosphere, hydrosphere (with cryosphere), atmosphere and biosphere continuously exchange matter and energy, e.g. in the water and carbon cycles. In a forest, water, air, soil and plants support each other; disturbing one (like deforestation or burning fossil fuels) upsets all the others.
