NCERT Solutions for Class 9 Science Chapter 13 Earth as a System: Energy, Matter, and Life (PDF) – 2027

Warmer sea water evaporates faster, so more water vapour is pushed into the monsoon winds — and the monsoon becomes stronger but far more erratic.

The river becomes flashy — it floods heavily during rain and shrinks or dries up between rains — and it carries much more silt.

• More run-off, less infiltration: without leaf litter and roots, rain runs quickly off the bare ground, so the river floods after heavy rain.
• Less groundwater recharge: less water soaks in to feed the river slowly, so in dry months its flow falls or it dries up.
• More soil erosion: soil washed off the slopes silts up the river bed and makes floods worse.
• Less local rainfall: fewer trees mean less transpiration, which can reduce rain in the area.

Sea level rises, and low-lying coastal cities such as Mumbai and Chennai face flooding, salt water entering their groundwater, and loss of land.

The extra CO₂ dissolves in sea water and makes it more acidic, which harms plankton — especially the tiny organisms that build shells of calcium carbonate.

Fig. 13.1 shows a high mountain valley with a lake, a snow-covered range behind it, and a shepherd grazing his flock on the grass at the water’s edge. Every sphere is present in that one picture:

By melting. When the air and the ground warm up in spring and summer, snow and glacier ice absorb heat, change from the solid state to the liquid state, and the melt water runs down the slope as streams into the lake.

The lake level would fall in summer, and with less water the grass would grow thinner — so there would be less fodder for the sheep.

The five spheres are joined by two things that never stop moving between them — energy (mainly from the Sun) and matter (water, carbon, nitrogen, oxygen and minerals). Because they share the same energy and the same matter, a change forced on one sphere must appear in the others.

No. Solar radiation is spread very unevenly over the Earth, and this unevenness is what drives the winds, the ocean currents and the water cycle.

1. Shape of the Earth (latitude): the Earth is a sphere, so near the equator the Sun’s rays fall almost vertically and their energy is concentrated on a small area; near the poles the rays are slanting and the same energy is spread over a larger area.
2. Tilt of the Earth’s axis: as the Earth revolves, the tilt changes the angle of the rays and the length of the day, giving seasons.
3. Type of surface (albedo): snow and ice reflect most sunlight and stay cold, while dark soil and water absorb more and become warmer; land also heats faster than water.

Estimate: about 4600 km² of solar panels — a square of side about 68 km — which is only about 2 per cent of the Thar desert.

The Sun’s radiation is partly reflected back to space by the atmosphere and clouds, partly absorbed by the atmosphere and clouds, partly reflected by the Earth’s surface (depending on its albedo), and the rest is absorbed by the surface, which warms up. The warm surface then re-radiates heat as infrared radiation. Greenhouse gases (CO₂, CH₄, water vapour) absorb part of this heat and stop it escaping, keeping the Earth warm.

The three missing rows, taken from standard published albedo tables:

High albedo (snow, ice) → the surface stays cool; low albedo (black soil, ocean water) → the surface absorbs more and becomes warmer.

Observation: when the concentration of greenhouse gases is increased, more of the infrared radiated by the Earth’s surface is absorbed and sent back, so the surface temperature rises. With no greenhouse gases the surface becomes very cold. The incoming sunlight does not change — only the escape of heat is slowed.

What to change in the simulation, and what you will see:

The ozone layer in the stratosphere acts as a protective shield: it absorbs most of the Sun’s harmful ultraviolet (UV) radiation before it reaches the ground.

• UV rays have high energy; prolonged exposure damages the eyes and skin and increases the risk of cancer.
• Increased UV can harm living organisms and whole ecosystems.
• When ozone is destroyed faster than it forms, the layer thins. CFCs from refrigerators and aerosols caused the ozone hole over Antarctica; the Montreal Protocol cut the use of CFCs, and the layer is now slowly recovering.

The night-time mountain breeze lowers the temperature in the valley, keeps the air moist and stirring, and slows the loss of water from soil and leaves — all of which help crops and protect soil.

• It lowers the night temperature in the valley, so crops recover from the heat stress of the day.
• Cooler air slows evaporation from the soil and transpiration from leaves, so the soil keeps its moisture.
• Warm days and cool nights suit hill crops such as apples, plums and tea.
• A gentle breeze dries leaf surfaces (less fungal disease), and moist soil is less likely to dry out and be eroded.

The warm surface water gives up its heat to the air as it moves poleward, gradually cools, becomes denser, sinks at high latitudes and returns to the equator through the deep ocean. The heat it delivers on the way makes the coasts it passes far warmer than their latitude would suggest.

Impact on the area:

• Heat is carried from the equator towards the poles, reducing temperature differences across the planet.
• Coasts it reaches get milder winters — the North Atlantic Drift (an extension of the Gulf Stream) keeps many ports of north-western Europe ice-free in winter, which helps trade.
• Nutrients are transported, supporting large marine ecosystems and fisheries.

Marine life is hit from two directions at once — the sea becomes more acidic, which attacks shells and coral, and the warmer water holds less dissolved oxygen and less dissolved CO₂, which stresses fish and weakens the ocean as a carbon sink.

Plants and animals could not survive, because the Earth gets no new matter from outside — the same water, carbon, nitrogen and oxygen must be used again and again. For example:

• Water cycle: no rain — rivers and soil would dry up; plants could not make food and animals would have no water.
• Carbon cycle: respiration and decomposition would not return CO₂, so photosynthesis would stop, food chains would collapse and dead matter would pile up.
• Nitrogen cycle: no nitrates in the soil — plants could not make proteins and nucleic acids, so animals would get no protein either.
• Oxygen cycle: O₂ used in respiration and combustion would not be replaced, so aerobic life would die.

Two supplies would fail almost at once — food and oxygen — and the whole living world would collapse from the bottom of the food chain upwards.

• No new food: producers could not make glucose, so plants would die, followed by herbivores and then carnivores.
• No new oxygen: respiration and combustion would keep using O₂, so its level in the air would fall and aerobic organisms would die.
• More CO₂: nothing would remove CO₂ from the air, so the greenhouse effect and warming would increase.
• The carbon and oxygen cycles would break down.

Human beings add greenhouse gases in two ways — by releasing carbon that was locked away, and by destroying the systems that used to absorb it.

Sample answer — what I can do: switch off lights and fans when not in use and use LED bulbs; walk, cycle or use public transport; reduce, reuse and recycle; avoid wasting food, water and electricity; plant and protect trees; use solar or other renewable energy where possible.

Correct option: (ii) To recycle essential nutrients between biotic and abiotic components.

Biogeochemical cycles move essential nutrients such as carbon, nitrogen and oxygen between the living (biotic) and non-living (abiotic) components, so that they remain available to support life.

Correct option: (iii) The Earth’s surface absorbs solar radiation, which is then re-radiated and trapped by greenhouse gases.

A warmer atmosphere holds more water vapour, so the water cycle runs faster and more violently — heavier downpours in some places, longer droughts in others, and a store of frozen water that is draining away.

Albedo decides what fraction of the arriving sunlight is thrown straight back. A high-albedo surface reflects most of it and stays cold; a low-albedo surface absorbs most of it and becomes warm. Because different regions of the Earth have very different albedos, albedo helps set the climate of each region.

• High albedo surfaces (snow 0.80 – 0.90, ice) reflect most sunlight, so polar and snow-covered regions stay very cold.
• Low albedo surfaces (black soil, ocean water, dark roads) absorb more sunlight and become warmer; cities of concrete and asphalt form urban heat islands.
• Melting ice or cleared forests change the albedo, and so change the climate — e.g. melting ice exposes dark water, which absorbs more heat.

Both breezes form because the mountain slopes and the valley floor heat up and cool down at different rates. Yes, the two mountain breezes would differ: the breeze from the barren rocky mountain would be warmer, and the breeze from the grass-covered mountain would be cooler.

Part 1 — how the two breezes form

Part 2 — grassy slope versus barren rock

Barren rock has no shade or transpiration, so by day it absorbs and stores much more heat; at night it re-radiates this heat (like concrete) and keeps the air above it warmer. The grassy slope stores less heat and re-radiates less, so the air over it cools more — its mountain breeze is cooler and moister.

The troposphere — the lowest layer, 0 – 12 km, in which “nearly, all weather phenomena take place”. The primary reason is that the troposphere is heated from the Earth’s surface below, so its temperature falls with height (about 6.5 °C per km), and warm air near the ground can therefore rise.

The atmosphere is the largest reservoir of nitrogen, but N₂ gas is non-reactive and cannot be used directly by plants and animals. It is converted into soluble compounds, and back again, by these processes:

If nitrogen were not cycled: atmospheric N₂ could not be used, and soil nitrates would be used up and never replaced. Plants could not make proteins and nucleic acids, so they could not grow; animals, which get nitrogen only by eating plants or other animals, would lack protein too. Life on Earth could not continue.

Deforestation attacks both cycles from the same side: it removes the machinery that takes CO₂ out of the air and puts O₂ in, and at the same time releases the carbon the trees had stored.

The other consequences the chapter lists:

• Less transpiration, so less local rainfall.
• Change in surface albedo, so the local temperature changes.
• No roots to hold the soil, so more soil erosion, silted rivers and floods.
• Loss of habitats, so a decline in biodiversity.

Plants take CO₂ from the air by photosynthesis; the carbon then returns to the atmosphere by respiration, decomposition and combustion, and CO₂ is also exchanged between the ocean and the air.

Plants need CO₂ for photosynthesis, but they can use only a limited amount. CO₂ is also a greenhouse gas: excess CO₂ traps more of the heat radiated by the Earth and intensifies the greenhouse effect. This leads to:

The surface loses heat in four ways — by radiating infrared, by conduction to the air touching it, by convection as warm air rises, and by evaporation, which carries away latent heat. Of these, only radiation can take energy right out to space.

Significance: this heat loss balances the energy received from the Sun, so the Earth does not keep on heating up. Greenhouse gases trap part of the outgoing infrared, keeping the Earth warm enough for life. Heat carried upward by convection and evaporation drives winds, clouds and rain, and the loss of heat at night makes nights cooler than days.

On a flat disc held face-on to the Sun, every point would receive the rays at the same angle, so the insolation would be the same everywhere. There would be no equator-to-pole temperature difference — and therefore no pressure belts, no planetary winds and no heat-carrying ocean currents. Temperature would differ only because of the kind of surface (land, water or ice).

The rise passes through the three spheres in sequence: ice melts, the melt water raises the sea and speeds the water cycle, and the changed water and temperature disturb every living community.

The atmosphere works like a two-way filter. On the way in it removes the radiation that would harm life; on the way out it holds back part of the heat that would otherwise escape. Both are needed for the Earth’s temperature to sit in the narrow range life can survive.

The five spheres are linked because they share the same energy and the same matter. Energy flows from the Sun through all of them; matter cycles round and round between them. Neither can move through one sphere without changing the others.

They are joined by solar heating, winds and ocean currents, the water cycle, and the carbon, nitrogen and oxygen cycles, which move energy and matter between rocks, water, ice, air and living things.

Example of the delicate balance: less snowfall in winter (cryosphere) → less melt water, so the lake level falls in summer (hydrosphere) → less water for grass (biosphere) → less fodder for sheep. Similarly, a warmer Arabian Sea (hydrosphere) → more evaporation → erratic monsoon (atmosphere) → floods and droughts that damage crops (biosphere).

Both planets would still have the great equator-to-pole circulation, because that is caused by latitude and by the Earth’s rotation. What they would lose is everything that depends on land and sea lying side by side — sea breezes, land breezes, and above all the monsoon.

Sample answer — a meal of roti and dal

Other activities that add CO₂ or nitrogen: making urea by the energy-intensive Haber–Bosch process; diesel tractors, threshers and irrigation pumps; fertiliser run-off (nitrates reach rivers and cause eutrophication); transporting and milling the grain; cooking on LPG or firewood; packaging, and food waste rotting in landfills.

Method: choose the IMD station nearest your city or district. For 5 years in the 1980s and 5 years in the 2020s, record the June – September rainfall and the number of days with more than 50 mm of rain. Average each block and compare them in a table like this:

Connecting to the chapter: if heavy-rain days have increased, link it to a warmer Arabian Sea — more evaporation puts more moisture into the monsoon winds, giving fewer but heavier downpours. Worse flooding can also come from land-use change: when forests and farms are turned into concrete, less water soaks in and run-off increases.

The single biggest change is that the Earth is now being watched continuously instead of being sampled occasionally — so scientists can see a change while it is happening, not years later.