NCERT Solutions Class 9 Science Chapter 13 Earth As a System Energy Matter and Life

F R E E S T U D Y M AT E R I A L F O R E V E R Y S T U D E N T

CLASS 9 · SCIENCE

NCERT Solutions

Chapter 13: Earth as a System:
Energy, Matter, and Life

NCERT Textbook — Exploration

BOOK PAGES SECTIONS QUESTIONS MEDIUM

252 – 269 16 39 English

Solutions, notes, sample papers & more at 56 pages

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

CLASS 9 · SCIENCE · EXPLORATION

NCERT Solutions — Chapter 13: Earth as a System:
Energy, Matter, and Life
Winds, monsoon rain, glaciers, ocean currents and the food on your plate are all driven by one thing — a
steady flow of energy from the Sun and an endless recycling of matter. This closing chapter of Grade 9 stops
treating these as separate topics and looks at the Earth as one system of five interacting spheres, in which
uneven solar heating sets the air and the oceans moving, and water, carbon, nitrogen and oxygen keep going
round and round.

TEXTBOOK BOOK PAGES

Exploration (Class 9) 252 – 269

SECTIONS QUESTIONS

16 39

MEDIUM

English

Think It Over — Page 252
Chapter opener

THINK IT OVER

Q1 How does the warming of Arabian Sea water affect the southwest monsoon in
India?

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

Warmer Arabian Sea surface → more evaporation

More evaporation → more water vapour carried by the southwest monsoon

More vapour + rising air → heavier bursts of rain in some regions

Same total moisture, unevenly delivered → dry spells and drought elsewhere

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

Why it happens: the rate at which water evaporates from a sea surface rises steeply
with temperature, and warm air can also hold more water vapour before it
condenses. So a warmer Arabian Sea loads the monsoon winds with extra moisture.
When that moist air is forced to rise — over the Western Ghats, or in a low-pressure
system — it cools, condenses and dumps the moisture in short, violent spells rather
than in steady rain. The chapter states this outcome directly: fluctuations in the
southwest monsoon give variability in rainfall, bringing floods to some regions
of India while leaving others in drought.

Note: this is a good example of the chapter's main idea. The change begins in the
hydrosphere (sea temperature), travels through the atmosphere (monsoon winds),
and lands in the biosphere (crops and farmers). One sphere is never disturbed alone.

Q2 If a large forest is cleared, how can that affect the flow of a river in that area?

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

Why it happens: a forest works like a sponge placed on a slope. Three things
change when it is removed.

Infiltration collapses. Leaf litter and root channels normally let rain soak into
the soil. On bare ground the same rain runs straight off the surface. The chapter
calls this "more run off … and less infiltration".
Groundwater recharge falls. The water that used to seep down and feed the
river slowly, all through the dry months, is no longer stored. So the river's lean-
season flow drops.
Soil erosion rises. "Without tree roots to hold the soil together, soil erosion could
increase." The eroded soil ends up as silt in the river bed, which raises the bed
and makes flooding worse.

There is also an effect on how much rain falls at all: clearing forests means
decreased photosynthesis and reduced transpiration, which can lead to decline
in the local rainfall.

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

Tip: remember the pattern as high peak, low base. The same annual rainfall is
delivered to the river as sharp floods instead of a steady supply.

Q3 What might happen to coastal cities in India if glaciers and polar ice keep melting
faster?

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

Rise in atmospheric temperature → melting of glaciers and polar ice (cryosphere)

Melt water flows into the ocean → volume of ocean water increases

Increased volume → sea level rises

Higher sea level + storm surge → flooding of low-lying coastal land

Why it happens: ice locked up on land is water that has been taken out of the
ocean. When it melts, that water is returned, so the ocean gains volume and the sea
stands higher against the coast. India is especially exposed because a large share of
its population, ports and industry sit within a few metres of sea level. The chapter
says this "can raise sea levels that may threaten coastal cities", and adds the further
consequence — the "ecosystems within the biosphere" are disturbed "by causing a
habitat loss", which for a coast means mangroves and fisheries.

Did you know? Melting sea ice (ice already floating in the ocean) does not by itself
raise sea level much, because floating ice already displaces its own weight of water.
It is the ice sitting on land — Himalayan glaciers, the Greenland and Antarctic ice
sheets — that adds new water to the ocean.

Q4 How would increasing carbon dioxide levels in the atmosphere affect the ocean
plankton?

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.

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

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More CO2 in air → more CO2 dissolves in the ocean

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Why it matters so much: the chapter states that "excess atmospheric CO2 increases
ocean absorption, making sea water more acidic. This could threaten tiny plankton

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sem 13.1: Let us explore — Page 253
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Class 9 Science Chapter 13 Earth as a System: Energy, Matter, and Life AglaSem · NCERT Solutions

Introduction — the spheres of the Earth

ACTIVITY 13.1

Q1 Observe the features of the Earth as shown in Fig. 13.1. Identify and circle one
example representing each of the geosphere, hydrosphere, cryosphere,
atmosphere, and biosphere.

Fig. 13.1, page 253 — some features of the Earth’s surface, redrawn sketch. Nothing in it
is named; that is what the activity asks you to do.

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:

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

SPHERE WHAT TO CIRCLE IN FIG. 13.1 WHY IT BELONGS THERE

Geosphere The bare rock and boulders on the valley floor and the Solid rock, soil and landforms
mountain slopes

Hydrosphere The lake Liquid water at the surface

Cryosphere The snow and ice capping the mountains Water in the solid state

Atmosphere The sky and clouds above the valley The layer of air held by the Earth's
gravity

Biosphere The sheep, the shepherd and the grass Living organisms and their
habitat

Check it yourself: the picture is chosen so that all five spheres touch each other. The
snow rests on rock, the melt water fills the lake, the lake water evaporates into the
air, and the grass — which needs soil, water and air together — feeds the sheep.
That overlap is the whole point of the activity.

Q2 How does snow (cryosphere) eventually become part of the lake (hydrosphere)?

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.

Snow / ice (solid, cryosphere)

+ heat from solar radiation and warmer air

→ melting at 0 °C

→ melt water runs down the slope
→ streams → lake (liquid, hydrosphere)

Page 6 of 56

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

Why it happens: snow is the same substance as lake water — only its state is
different. Melting needs energy, and that energy comes from the Sun. Once the
temperature at the snow surface reaches 0 °C, further heat goes into breaking the
bonds of the ice crystal (latent heat of fusion) rather than raising the temperature,
and the solid turns to liquid. Gravity then does the rest: the water flows downhill
until it reaches the lowest point in the valley, which is the lake.

Tip: a small part of the snow does not go through the liquid stage at all — it
sublimes, changing straight from ice to water vapour in the dry mountain air. That
water joins the atmosphere instead, and returns as precipitation elsewhere.

Q3 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?

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.

Less snowfall in winter → smaller store of snow and ice
Smaller store → less melt water in summer

Less melt water → lake level falls

Lower lake and drier soil → poorer growth of grass

Less grass → less food for the sheep

Why it happens: the snowpack is a natural reservoir. It collects the winter
precipitation as a solid and releases it slowly as liquid through the summer, exactly
when there is no rain and the grass needs water most. Cut the winter deposit and
the summer withdrawal must fall too. The chapter puts it in the same words: "less
snowfall in winters may lead to less water in the lake in summers, resulting in less
water to support the growth of grass."

Follow it one step further: fewer sheep can be grazed, so the shepherd's income
falls. A change that began in the cryosphere has ended in a human household —
which is why the Earth's spheres must be studied together.

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

Q4 Discuss with your classmates and write down how all the spheres are
interconnected, and how a disturbance in one can lead to changes in others.

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.

Atmosphere

Geosphere Hydrosphere

Energy
& Matter

Biosphere Cryosphere

The five spheres are not stacked layers — they overlap and exchange energy and matter along every
line. That is why a disturbance travels.

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

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Less snowfall in winter Cryosphere Lake level falls (hydrosphere) → grass grows poorly → sheep

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Arabian Sea warms Hydrosphere More evaporation → erratic monsoon (atmosphere) → floods

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. o Uneven Heating of the Earth
In-text Questions
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Q1 Does the solar radiation heat the Earth's surface evenly?
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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.

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

Three separate reasons, all worth knowing:

1. The Earth is a sphere, so latitude matters. Near the equator the Sun's rays
strike almost perpendicular and a beam is concentrated on a small area. Near the
poles the same beam strikes at a slanting angle and is spread over a much larger
area, so each square metre receives far less energy.
2. Different surfaces respond differently — albedo. Snow reflects 0.80 – 0.90 of
the radiation falling on it and stays cold; black soil and ocean water reflect very
little, absorb most of it, and become relatively warm. Land also heats faster than
water.
3. The axis is tilted, and the Earth revolves. This changes which hemisphere leans
towards the Sun, giving the seasons and changing the length of the day — so
even one place does not receive the same insolation through the year.

Intensity received on a horizontal surface, I = I0 cos θ

(θ = angle between the Sun's rays and the vertical at that place)

At the equator with the Sun overhead, θ = 0°: I = 1 kW m–2 × 1 = 1 kW m–2

At 60° latitude, θ = 60°: I = 1 kW m–2 × 0.50 = 0.5 kW m–2

Near the pole, θ → 90°: I → almost 0

Why this matters: uneven heating means uneven air pressure, and air always
moves from high pressure to low pressure. So this single fact is the starting point for
everything in Section 13.2 — local winds, planetary winds and ocean currents.

Think as a Scientist — Page 255

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

13.1 Uneven Heating of the Earth — an estimation problem

THINK AS A SCIENTIST

Q1 An interesting estimation problem that helps us to appreciate the enormous
amount of energy that we get from the Sun is to estimate how much of the Earth's
surface would be needed to be covered with solar panels to supply all the electric
power that our country uses today (Fig. 13.4). To make this estimate, you can find
these numbers on the internet, assume some insolation on the Earth's surface and
consider that some fraction of this energy is converted into electricity. You will
probably find that even a fraction of the area of the Thar desert, if covered with
solar panels, could supply India's electricity needs.

rows of solar panels

desert sand and scrub

Fig. 13.4, page 255 — solar panels in the desert, redrawn sketch.

Working it out below, about 4500 km2 of panels would do it — roughly a square 67 km × 67
km, which is only about 2 per cent of the Thar desert. Here is the full estimate with every unit
carried through.
Step 1 — how much electric power does India use?

Page 11 of 56

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

India's annual electricity consumption ≈ 1.6 × 1012 kW h (about 1600 billion units)

Number of hours in a year = 365 × 24 h = 8760 h

Average power, P = energy ÷ time

P = (1.6 × 1012 kW h) ÷ (8760 h)

P = 1.83 × 108 kW = 1.83 × 1011 W (about 180 GW)

Step 2 — how much electric power does one square metre of panel give?

Peak insolation at the surface ≈ 1 kW m–2 = 1000 W m–2 (clear sky, Sun high)
But averaged over day and night, cloud and low Sun, India receives about

5 kW h m–2 per day → 5000 W h m–2 ÷ 24 h ≈ 200 W m–2 as a round-the-clock average

Efficiency of a solar panel, η ≈ 20% = 0.20

Electric power per square metre = 0.20 × 200 W m–2 = 40 W m–2

Step 3 — divide.

Area required, A = power needed ÷ power per square metre

A = (1.83 × 1011 W) ÷ (40 W m–2)

A = 4.6 × 109 m2

1 km2 = 106 m2, so A = 4.6 × 109 ÷ 106 = ≈ 4600 km2

That is a square of side √4600 km2 ≈ 68 km

Step 4 — compare with the Thar desert.

Area of the Thar desert ≈ 2 × 105 km2

Fraction needed = 4600 km2 ÷ 2 × 105 km2 = 0.023 = about 2.3%

Page 12 of 56

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

Why the answer is so small: the Sun delivers energy at an enormous rate — 1400 J
every second on every square metre at the top of the atmosphere. India's entire
electrical demand, spread over a whole year, is tiny compared with what falls on
even a small patch of desert. The limit on solar power is therefore not the supply of
sunlight; it is the cost of panels, the land, and above all storage, because the Sun
does not shine at night or through the monsoon.

Check it yourself: your numbers will differ from these — use the latest
consumption figure and your own assumption for efficiency and average insolation.
As long as you write the units at every step and they cancel correctly (W ÷ W m–2 =
m2), your estimate is a good one. The purpose of an estimation problem is the order
of magnitude, not the last digit.

In-text Questions — Page 256
Lead-in to 13.1.1 Interaction of solar radiation on the Earth's surface

Q1 How does the Sun's radiation interact with the Earth's surface and atmosphere, and
warm the Earth?

The incoming radiation is split four ways — part is reflected by clouds and the
atmosphere, part is absorbed by the atmosphere, part is reflected by the ground, and the
rest is absorbed by the ground. Only the last part warms the surface, and the surface then
warms the air from below.

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

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Activity 13.2: Let us find
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Class 9 Science Chapter 13 Earth as a System: Energy, Matter, and Life AglaSem · NCERT Solutions

13.1.1 Interaction of solar radiation on the Earth's surface

ACTIVITY 13.2

Q1 Complete Table 13.1 using information from authentic sources like websites and
books. (Table 13.1: Reflection of solar radiation by surfaces of materials — Snow 0.80
– 0.90; Ice 0.50 – 0.70; Crushed rock 0.25 – 0.30; Light coloured soil; Black soil; Ocean
water)
Table 13.1: Reflection of solar radiation by surfaces of materials, as printed on page
256 — the last three albedo cells are blank.

S. NO. MATERIALS ALBEDO

1. Snow 0.80 – 0.90

2. Ice 0.50 – 0.70

3. Crushed rock 0.25 – 0.30

4. Light coloured soil

5. Black soil

6. Ocean water

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

Page 15 of 56

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

S. MATERIAL ALBEDO WHAT IT MEANS
NO.

1. Snow 0.80 – Reflects 80 – 90% — stays very cold (printed in the book)
0.90

2. Ice 0.50 – Reflects about half to two-thirds (printed in the book)
0.70

3. Crushed rock 0.25 – Reflects about a quarter (printed in the book)
0.30

4. Light coloured 0.30 – Pale, dry, sandy soil — reflects roughly a third
soil 0.40

5. Black soil 0.05 – Dark regur soil absorbs almost everything, so it heats fast
0.15

6. Ocean water 0.06 – With the Sun high, water reflects only about 6% and
0.10 absorbs the rest

Why the pattern goes this way: albedo is the fraction of solar radiation a surface
reflects. A surface that reflects little must absorb a lot, and absorbed radiation is
what raises temperature. So a high albedo surface stays cool and a low albedo
surface heats up. Whiteness and smoothness raise albedo; darkness and roughness
lower it. This is why snow-covered polar regions remain intensely cold — they turn
most of the incoming energy straight back to space — and why black cotton soil and
the open ocean are comparatively warm.

If insolation on a surface = 1000 W m–2,
Energy absorbed = (1 – albedo) × insolation

Snow (a = 0.85): absorbed = (1 – 0.85) × 1000 = 150 W m–2

Black soil (a = 0.10): absorbed = (1 – 0.10) × 1000 = 900 W m–2
Six times as much energy into the black soil, from exactly the same Sun.

Page 16 of 56

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

Did you know? Albedo makes a feedback loop in the Arctic. Sea ice (high albedo)
melts to expose ocean water (low albedo); the ocean absorbs more radiation,
warms, and melts more ice. This ice–albedo feedback is one reason the polar regions
are warming faster than the rest of the planet. Ranges are quoted because albedo
depends on the Sun's angle, on how wet the soil is, and on how fresh the snow is.

Pause and Ponder — Page 258
13.1.3 Role of the atmosphere

PAUSE AND PONDER

Q1 Visit the website given below and study the effect of the concentration of
greenhouse gas on greenhouse-effect

Note on the printed page: in the English edition (page 258) the web address itself is
missing — a blank white box is printed where the link should be, and only the last
part of the address, "greenhouse-effect", survives. The same question in the Hindi
edition Anveshan (page 258) prints the full address, so use that:
https://phet.colorado.edu/en/simulations/greenhouse-effect — the PhET
"Greenhouse Effect" simulation.

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

Page 17 of 56

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

WHAT YOU WHAT HAPPENS ON THE THE PHYSICS BEHIND IT
CHANGE SCREEN

Set the greenhouse gas Almost every infrared photon leaves; Nothing intercepts the outgoing IR, so
concentration to none the surface temperature falls sharply the Earth loses heat as fast as it gains it
— at a low temperature

Set it to ice age or 1750 Some IR photons bounce back; A modest greenhouse effect — the one
levels temperature settles at a moderate that made life possible
value

Slide it up to today and More IR photons are absorbed and More absorbers means a longer delay
beyond re-emitted downwards; the before heat escapes, so a higher steady
thermometer climbs temperature

Watch the photons Yellow (sunlight) passes down freely; Greenhouse gases are transparent to
view red (infrared) is caught by CO2 and visible light but opaque to infrared — the
CH4 molecules heart of the effect

The conclusion you should write down: raising the concentration of greenhouse
gas does not change how much sunlight comes in. It changes how easily heat gets
out. The Earth must keep radiating away as much energy as it receives, and if the
outgoing path is partly blocked, the only way to force enough energy through is for
the surface to become hotter. That is precisely why extra CO2 from human activity
raises global temperature.

Safety note printed in the book: "Ensure that you do not share any personal
information while using the internet."

Threads of Curiosity — Page 259
Why is the ozone layer so important?

THREADS OF CURIOSITY

Q1 Why is the ozone layer so important?

Because it is the only thing that absorbs the Sun's short-wavelength ultraviolet radiation
before it reaches the ground. Without it, UV would damage the eyes, skin and DNA of living
organisms and would break down whole ecosystems.

Page 18 of 56

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

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Ozone sits in the stratosphere (12 – 50 km)
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UV wavelength range = 100 nm to 400 nm, where 1 nm = 10–9 m
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The story the box tells, and why it matters:
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decided entirely by where it is.
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Pause and Ponder — Page

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The night-time mountain breeze
and stirring, and slows the
protect soil.

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

DAY NIGHT

mountain
breeze (down)

valley breeze (up)

valley floor (fields)

By day the sunlit slope heats, air rises and cool valley air flows up — the valley breeze. After sunset
the slope cools faster, and dense cold air slides down into the valley — the mountain breeze.

Why it helps, point by point:

It cuts heat stress. Plants close their stomata and stop photosynthesising
efficiently when they get too hot. A cool downslope wind at night brings the field
temperature down, so the plant recovers overnight.
It reduces water loss. Cooler air holds less water vapour before saturating, so
evaporation from the soil and transpiration from leaves both slow down. The soil
keeps its moisture for the next day. The chapter says these winds "help regulate
temperature, moisture conditions and support soil and crop health".
It gives a wide day–night temperature difference. Warm days and cool nights
are exactly what apples, plums, tea and many spices need for good flavour, colour
and sugar content. This is why hill agriculture around Shimla and Dehradun
grows crops the plains cannot.
It keeps the air moving and drains cold air away. Still, damp air over a crop
invites fungal disease; a gentle breeze dries the leaf surface. The downslope flow
also carries cold air off the slopes, reducing frost damage on the slope itself.
It protects the soil. Because the mountain breeze is gentle and does not blow in
gusts across bare ground, and because the soil stays moist, wind erosion and
drying-out of the topsoil are both reduced.

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

Tip: the mountain breeze forms because the slope loses heat faster than the valley
floor after sunset. The air on the slope cools, becomes denser, and gravity pulls it
down. It is dense cold air sinking — nothing pushes it.

Q3 What happens to the warm surface of water from the equator as it travels toward
the poles? What impact does this movement have on the area?

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.

Warm, less dense equatorial water flows poleward at the surface

Along the way it loses heat to the colder air above → water cools

Cooling (and evaporation, which raises salinity) → density increases

Denser water sinks at high latitudes

Cold, dense water returns towards the equator at deeper levels

→ a continuous conveyor of heat from equator to poles

The effect on the regions it reaches:

It reduces the temperature difference across the planet. The chapter says
ocean currents, "by transporting heat from the equator towards the poles …
reduce temperature differences across the planet". The equator would be hotter
and the poles colder without them.
It keeps high-latitude ports usable. The book's example is the North Atlantic
Drift, an extension of the Gulf Stream, which "flows toward the northwestern
coast of Europe and keeps many ports ice-free during winter, even at high
latitudes."
It supports trade and settlement. Ice-free ports all year mean shipping, trade
and commerce can continue — the chapter names this directly.
It feeds marine life. The same movement transports nutrients, so ocean
currents "support a massive ecosystem". Where cold, nutrient-rich deep water
rises again, fisheries are rich.

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

Note: the return flow at depth is driven by density, and density depends on both
temperature and salinity. "Water with lower salinity, being less dense, tends to
remain near the surface, while higher salinity, denser water sinks." Cooling and
evaporation together are what start the sinking.

Pause and Ponder — Page 263
13.3.2 Carbon cycle

PAUSE AND PONDER

Q4 The CO2 dissolved in the ocean is disturbed when the global temperature increases.
What will happen to marine life?

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.

Higher global temperature

(a) More CO2 in the air → more dissolves → carbonic acid → ocean acidification

→ carbonate shells of plankton, molluscs and corals dissolve / form poorly

(b) Warmer water dissolves less gas → less dissolved O2 → fish struggle to respire

(c) Warmer water absorbs less CO2 → weaker carbon sink → still more CO2 left in the air

Why gases behave this way: the solubility of a gas in water falls as the water gets
warmer — the same reason a warm fizzy drink goes flat faster than a cold one. So
warming does two contradictory-sounding things: the atmosphere pushes more CO2
in because there is far more of it up there, yet each litre of warm sea water can hold
less gas of any kind, including the oxygen fish need. The chapter states both halves:
excess CO2 "increases ocean absorption, making sea water more acidic", and
"warmer ocean water reduces the ocean's capacity to absorb CO2 as an effective
carbon sink."

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

GROUP OF ORGANISMS WHAT HAPPENS TO THEM

Phytoplankton Base of every marine food chain and a huge source of oxygen — damage
here starves everything above it

Shell-forming organisms (molluscs, Acidic water makes calcium carbonate shells thinner and harder to build
some plankton)

Coral reefs Bleach in warm water and dissolve in acidic water; reef fish lose their
habitat

Fish and coastal fisheries Less dissolved oxygen, less food, shifting migration routes — catches fall

Scale check: the chapter's Threads of Curiosity box tells you why this reservoir
matters so much — "out of the total quantity of global carbon, 71% carbon is found
in oceans", while the atmosphere holds only about 1%. Even a small change in the
ocean's chemistry therefore moves a very large amount of carbon.

Pause and Ponder — Page 265
13.3.4 Oxygen cycle

PAUSE AND PONDER

Q5 What would happen to plants and animals on Earth if the biogeochemical cycles
were disrupted and stopped? Explain by giving a few examples.

Life would end, because the Earth has no outside supply of matter. Every atom of water,
carbon, nitrogen and oxygen a plant or animal uses has been used before by some other
organism. Stop the recycling and the supply simply runs out.

Page 23 of 56

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

co m
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What happens cycle by cycle:

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Water cycle stops. No evaporation, no condensation, no rain. Rivers, lakes and
a g l
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soil moisture dry up. Plants cannot draw water for photosynthesis or transport

a gl minerals; animals have nothing to drink. Land life ends within days to weeks.
Carbon cycle stops. Plants keep drawing CO2 out of the air for photosynthesis

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.
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g l a
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long ago, N2 gas in the air. Rhizobium in the root nodules of the pulse crop fixed it

a gl
into ammonia, nitrifying bacteria turned it into nitrate, the plant assimilated it into
protein — and when you digest that protein and later return the waste to the soil,
decomposers ammonify it and hand it back. Break any one link in that chain and the
co m
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field stops feeding you.
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What if … — Page 265
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Class 9 Science Chapter 13 Earth as a System: Energy, Matter, and Life AglaSem · NCERT Solutions

13.4 Human Impact on Earth's Processes

WHAT IF …

Q1 What if … photosynthesis stopped, what would happen on the Earth?

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.

Photosynthesis: 6CO2 + 6H2O + sunlight → C6H12O6 + 6O2

Stop it and you remove, simultaneously:

• the only large-scale source of glucose — the food of nearly all life

• the only large-scale source of free O2 in the atmosphere

• the only large-scale removal of CO2 from the air

The sequence of events:

1. Producers die first. Green plants, algae and phytoplankton have no way to make
food and are exhausted within days.
2. Herbivores starve, then carnivores. Every food chain in the book begins at a
producer, so the loss travels upward through every trophic level.
3. Oxygen falls. Respiration by animals and microbes, and the combustion of fuels,
keep using O2. With no photosynthesis to restore it, the 21% oxygen in the
atmosphere is steadily drawn down. Aerobic life fails; only some anaerobic
microbes survive.
4. CO2 builds up. Respiration, decomposition and combustion keep releasing CO2
with nothing to absorb it, so the greenhouse effect intensifies and the Earth
grows hotter.
5. The carbon and oxygen cycles both break. The chapter defines the oxygen
cycle as the balance between consumption (respiration and combustion) and
production (photosynthesis). Remove production and there is no cycle left, only a
one-way loss.

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

Did you know? Most of this work is not done by forests. Microscopic phytoplankton
in the oceans carry out a huge share of the world's photosynthesis. That is why
ocean acidification and warming, which threaten plankton, matter to the air you
breathe on land.

Pause and Ponder — Page 266
13.4 Human Impact on Earth's Processes

PAUSE AND PONDER

Q6 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?

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.

HUMAN ACTIVITY GAS HOW IT WORKS
ADDED

Burning coal, oil and gas for CO2 Carbon buried over millions of years in the slow
electricity, transport and industry carbon cycle is returned to the air in seconds

Deforestation CO2 Removes a carbon sink, and burning or decay of the
trees releases their stored carbon

Paddy fields, livestock, landfills CH4 Anaerobic decay produces methane, a far stronger
greenhouse gas than CO2

Heavy use of nitrogen fertilisers Nitrogen Excess nitrogen in soil is converted by microbes into
oxides nitrogen oxides that escape to the air

Cement, steel and brick making CO2 Both the fuel burnt and the chemical reaction itself
release CO2

Page 26 of 56

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

Evidence from Fig. 13.14 (the Keeling curve):

CO2 in 1960 ≈ 315 ppm CO2 in 2025 ≈ 420 ppm

Increase = 420 – 315 = 105 ppm

Percentage rise = (105 ÷ 315) × 100 = 33% (the book rounds this to "about 35%")

Rate ≈ 105 ppm ÷ 65 years ≈ 1.6 ppm per year

Why an individual's actions still count: a single household's emissions are small,
but the total is simply the sum of hundreds of millions of households. That is the
reasoning behind Mission LiFE (Lifestyle for Environment), the India-led global
initiative introduced at the UN Climate Change Conference in 2021, which
"encourages people to adopt mindful, eco-friendly lifestyles".

Sample answer — what I would do as an individual:

Save electricity: switch off fans and lights when leaving a room, use LED bulbs, set the air
conditioner no lower than 24 °C. Every unit of electricity saved is CO2 not emitted at the
power station.
Change how I travel: walk or cycle for short distances, use a bus or the metro, and share a
car ride for school. Transport fuel is burnt carbon.
Reduce, reuse, recycle: the chapter names this directly. Manufacturing anything new costs
energy; a repaired or reused article costs almost none.
Cut food and water waste: wasted food means wasted fertiliser, wasted irrigation and
wasted fuel for cooking and transport.
Plant and protect trees: a growing tree removes CO2 from the air by photosynthesis and
stores its carbon in wood. India "has planted billions of trees".
Support renewable energy: solar water heating or a rooftop solar panel at home replaces
fossil-fuel electricity directly.

Tip: when you answer this in an examination, do not just list actions — say why each
one lowers emissions. "Use a bicycle" is a habit; "use a bicycle, because petrol is
fossil carbon and burning it puts CO2 into the air that the slow carbon cycle took
millions of years to remove" is science.

Revise, Reflect, Refine — Pages 267 – 268

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

End-of-chapter questions

REVISE, REFLECT, REFINE

Q1 Choose the most appropriate option to describe the role of biogeochemical cycles in
an ecosystem. (i) To provide food directly to all organisms. (ii) To recycle essential
nutrients between biotic and abiotic components. (iii) To create new elements for
use by living things. (iv) To remove pollutants and toxins from the organism.

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

Why the other options are wrong:

(i) is wrong — food is made by producers through photosynthesis. A cycle moves
the raw materials (carbon, nitrogen, water) around; it does not hand out food.
(iii) is wrong — elements cannot be created by any biological or chemical
process. The cycles use the same fixed stock of carbon, nitrogen and oxygen
atoms over and over.
(iv) is wrong — that is the job of excretory organs in an organism, not of a
planetary cycle.

The chapter's own definition settles it: "This cyclic movement of matter and energy
between the abiotic and biotic components is called the biogeochemical cycle", and
it "ensures that essential nutrients, such as carbon, nitrogen and oxygen are
recycled, and remain available to support life on the Earth."

Q2 Which of the following is primarily responsible for warming of the Earth? (i) Solar
radiation is immediately absorbed by carbon dioxide, which then releases it as heat.
(ii) The atmosphere's tiny particles absorb incoming solar radiation, which directly
heats the Earth. (iii) The Earth's surface absorbs solar radiation, which is then re-
radiated and trapped by greenhouse gases. (iv) The Earth's environment is heated
only by the solar radiation reflected by the clouds.

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

Page 28 of 56

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

co m
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Sunlight (mostly visible) passes through the atmosphere → absorbed by the ground
m l as
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Ground warms → radiates back as infrared (because the Earth is much cooler than the Sun)
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Greenhouse gases (CO2, CH4, water vapour) absorb that infrared
aPart of it is sent back down → surface stays warm enough for life

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(iv) Radiation reflected by clouds is energy lost to space — it cannot heat the

s
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Q3 Explain how climate change affects the water cycle. Illustrate with examples.

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

STEP OF THE HOW CLIMATE CHANGE EXAMPLE FROM THE CHAPTER
WATER CYCLE ALTERS IT

Evaporation Warmer seas and land Warmer Arabian Sea feeds the southwest
evaporate faster; warmer air monsoon
holds more moisture

Precipitation More moisture is released in "Heavier rains in some areas (like intensified
short, intense bursts; other monsoons) and droughts elsewhere"
regions get none

Melting Glaciers melt faster, adding "Melting glaciers add more water to rivers, raising
(cryosphere) water to rivers now and raising sea levels in the long run and threatening coastal
sea level later cities, such as Mumbai and Chennai"

Run off Intense rain runs off the "Sudden bursts of intense rainfall result in more
surface instead of soaking in, run off that erodes soil"
and erodes soil

Infiltration and Less water soaks in, so the "less infiltration reduces the recharge of
groundwater groundwater is not recharged groundwater, which in turn, makes sustaining
agriculture difficult, especially during dry months"

Why warmth speeds the cycle: evaporation needs energy, and a warmer surface
supplies it faster. Warm air can also hold more water vapour before it becomes
saturated, so it carries a bigger load before it rains. When that load is finally
released, it comes down as a heavier fall. The total water on the Earth has not
changed at all — the same water is simply being moved around faster and dumped
less evenly.

Note the linkage the chapter emphasises: "the water cycle links the cryosphere
(glaciers), hydrosphere (rivers and oceans), atmosphere (moisture), geosphere (soil
erosion and decreased infiltration), and biosphere (crops and fisheries), all of which
are affected by global warming." A single change in temperature reaches all five
spheres through this one cycle.

Q4 Describe how albedo affects the Earth's surface temperature and its climate.

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,

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

albedo helps set the climate of each region.

Albedo, a = radiation reflected ÷ radiation received (a pure number, 0 to 1)

Energy absorbed per unit area = (1 – a) × insolation

With insolation = 1000 W m–2:

Fresh snow, a = 0.85 → absorbed = (1 – 0.85) × 1000 = 150 W m–2

Ocean water, a = 0.06 → absorbed = (1 – 0.06) × 1000 = 940 W m–2

How this shapes climate:

Polar regions stay frozen. Snow and ice reflect 0.80 – 0.90 of the incoming
radiation, so very little energy is absorbed. The chapter says exactly this: high
albedo "makes polar regions very cold."
Oceans and black soil store heat. With low albedo they absorb almost
everything, so they are "relatively warmer" — and the ocean, having a huge heat
capacity, then acts as the planet's heat store.
Cities become heat islands. Dark asphalt roads and concrete have low albedo,
absorb strongly and re-radiate at night. That is the urban heat island effect, which
raises the demand for air conditioning.
Deforestation changes albedo. The chapter notes that clearing forest "alters
surface albedo", so land-use change alters the local energy balance directly.
Ice–albedo feedback. If ice melts, dark ocean is exposed; the ocean absorbs
more, warms more, and melts more ice. A small warming amplifies itself.

Everyday check: the chapter's own examples work at street level — dark roads heat
up faster than light-coloured pavements, and dark clothes feel hotter than white
ones in summer. Same Sun, different albedo.

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

Q5 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?

Both breezes are made by the slope and the valley floor heating and cooling at different
rates. And yes — the two mountain breezes would differ: the breeze coming down the
barren rocky mountain would be colder than the one coming down the grass-covered
mountain.
Part 1 — how the two breezes form

VALLEY BREEZE (DAY) MOUNTAIN BREEZE (NIGHT)

What heats/cools Sunlit slope heats faster than the Slope loses heat faster; valley floor stays
first valley floor warmer

Air on the slope Warms, expands, becomes less dense, Cools, becomes denser, sinks
rises

Pressure A low pressure region forms over the Denser cold air presses down the slope
slope

Direction of the Cooler valley air flows up the slope Cold slope air flows down into the valley
wind

Part 2 — grassy slope versus barren rock

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

Reasoning:

By day, bare rock heats more. Rock has a low specific heat capacity and no
vegetation to shade or transpire, so its surface temperature climbs quickly. Grass,
by contrast, is cooled by transpiration — evaporating water carries latent heat
away — and the shade of the vegetation keeps the soil cooler.
By night, bare rock also cools more. The chapter makes exactly this point about
building materials: concrete re-radiates its stored heat strongly at night, while
thick mud and wooden walls "offer cool conditions … due to less re-radiation." An
exposed rock surface radiates freely to the open sky and loses its heat rapidly.
The grassy slope holds its heat. Moist soil and plant tissue are largely water, and
water has a very high specific heat capacity (4200 J kg–1 °C–1), so the same loss of
heat produces a much smaller fall in temperature. The moisture the grass
transpires also puts water vapour — a greenhouse gas — into the air just above
the slope, which slows radiative cooling further.

Conclusion: the air resting on the barren rock cools to a lower temperature, so the
mountain breeze descending from it is colder. The grass-covered mountain gives a
milder, moister breeze. (By day the same difference makes the valley breeze on the
barren mountain the stronger one, because the temperature contrast driving it is
greater.)

Tip: the whole answer rests on one idea from earlier grades — a substance with
more water in it changes temperature more slowly. Grass and moist soil contain
water; bare rock does not.

Q6 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, 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.

Page 33 of 56

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

co m
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Surface absorbs sunlight → surface becomes warm
m l as
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Air in contact with it is heated → expands, becomes less dense
m a g
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Less dense air rises → convection currents form
aRising air cools → water vapour condenses → clouds → rainfall

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Air rushing in to replace the rising air = wind; if the rise is violent = storm
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Why the layer above is not stormy: in the stratosphere (12 – 50 km) the ozone layer
absorbs UV, so that layer is heated from above and its temperature rises with height.
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cruise near the top of the troposphere and in the lower stratosphere, above the

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Note: the troposphere is also where almost all the atmosphere's water vapour lives
— and without water vapourglthere is no cloud, no rain and no latent heat to power a
a
storm. Its height is greatest above the equator (where heating is strongest) and least
above the poles.
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agl Q7
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Class 9 Science Chapter 13 Earth as a System: Energy, Matter, and Life AglaSem · NCERT Solutions

Atmospheric N₂ (78%)

nitrogen fixation
(Rhizobium, Azotobacter, lightning)

Ammonia NH₃ denitrification
(Pseudomonas)
nitrification

Nitrite NO₂⁻ Plants → animals

nitrification death, waste
assimilation
Nitrate NO₃⁻ Decomposers
ammonification returns N to the soil as NH₃

The nitrogen cycle (compare Fig. 13.15). Nitrogen leaves the air only through fixation and returns to it
only through denitrification.

PROCESS WHAT HAPPENS WHO DOES IT

Nitrogen Atmospheric N2 → ammonia (NH3) Rhizobium in root nodules of legumes,
fixation Azotobacter in soil; also lightning, and the
Haber–Bosch process

Nitrification NH3 → nitrite (NO2–) → nitrate (NO3–) Nitrosomonas, then Nitrobacter

Assimilation Plants take up nitrate from soil and build Plants, then animals
proteins and nucleic acids; animals get
nitrogen by eating

Ammonification Dead bodies and waste are broken down, Decomposer bacteria and fungi
returning ammonia to the soil

Denitrification Some nitrate → N2 gas, back to the Pseudomonas
atmosphere — closing the cycle

Page 35 of 56

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

If nitrogen were not cycled: nitrogen is "an essential element for the synthesis of
proteins and nucleic acids in all living organisms". Proteins build enzymes, muscle
and every cell structure; nucleic acids (DNA and RNA) carry the instructions for life. If
the cycle stopped:

The 78% N2 in the air would be useless, because it "cannot be directly used by
plants and animals".
Soil nitrate would be used up by growing plants and never replaced, so plants
could not make protein — growth, repair and seed production would fail.
Animals, which obtain nitrogen only by eating plants or other animals, would
suffer protein starvation and die.
Decomposers would be unable to return locked-up nitrogen from dead matter, so
it would be permanently removed from circulation.

Life as we know it could not continue. Every organism's protein is borrowed
nitrogen that must eventually be given back.

Did you know? The chapter's Ready to Go Beyond box notes that "more than half the
nitrogen atoms in the human body come from the Haber–Bosch process" — the
industrial fixation of atmospheric nitrogen into ammonia, called "Bread from Air",
which made India's Green Revolution possible. Its cost is that it uses about 1 – 2% of
the world's energy, and over-use of the fertilisers it makes has degraded soil and
water.

Q8 What are the impacts of deforestation on the Earth's oxygen and carbon cycles?
What are the other consequences of deforestation?

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.

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

CYCLE EFFECT OF CLEARING FOREST

Oxygen Photosynthesis is the only large-scale process that restores atmospheric O2. Fewer trees means less O2
cycle produced, while respiration and combustion keep consuming it — so the balance of the cycle tilts
towards consumption.

Carbon Two blows at once. (a) A carbon sink is removed — trees no longer absorb CO2 for photosynthesis.
cycle (b) A carbon source is created — burning or decay of the felled trees releases their stored carbon as
CO2. The chapter says burning fossil fuels and deforestation together "saturate natural carbon sinks
like forests and oceans."

The other consequences the chapter lists:

Less rainfall locally. "Clearing forests results in decreased photosynthesis and reduced
transpiration, which can lead to decline in the local rainfall." Trees pump groundwater into
the air; remove them and that moisture supply stops.
Changed albedo. "It also alters surface albedo", so the local energy balance and
temperature change.
Soil erosion. "Without tree roots to hold the soil together, soil erosion could increase" —
topsoil is washed away, rivers silt up and floods worsen.
Loss of habitat and biodiversity. "Over time, habitats could be destroyed, leading to a
decline in biodiversity as many species lose their natural homes."

Why one act has so many effects: a forest is not just a stand of trees — it is a
working part of four spheres at once. It exchanges gases with the atmosphere,
transpires water into the hydrosphere, binds the geosphere with its roots, and
houses the biosphere. Cut it and you cut all four connections in one stroke.

Q9 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.

Carbon leaves the atmosphere by only one main door — photosynthesis — but it comes
back by four: respiration, decomposition, combustion of fossil fuels, and release from the
ocean.

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

CO₂ in the atmosphere

exchange
photosynthesis respiration
(the only way in)

CO₂ in ocean
combustion
Plants (glucose) respiration
decomposition
eaten

Fossil fuels
Animals
buried for millions of years
death

Dead matter

Green arrows: carbon leaving the atmosphere. Red arrows: the four return paths. Grey dashed
arrows: carbon moving between living and buried stores.

The path, written out:

CO2 in air → photosynthesis → glucose in the plant

→ (1) plant respiration → CO2 back to air

→ eaten by an animal → (2) animal respiration → CO2 back to air

→ plant/animal dies → (3) decomposition by microbes → CO2 back to air

→ buried for millions of years → coal, oil, gas → (4) combustion → CO2 back to air

→ dissolved in sea water as carbonate/bicarbonate → exchanged back with the air

Page 38 of 56

as e
a g l
Class 9 Science Chapter 13 Earth as a System: Energy, Matter, and Life AglaSem · NCERT Solutions

co m
m.
Two clocks, not one. The chapter is careful to separate them.

m as e
.co
The fast cycle runs "over days to years" — photosynthesis, respiration and
a g l
a s em
decomposition.

a gl The slow cycle runs "over millions of years" — burial of dead organisms and their
conversion into fossil fuels.

. com
This is the heart of the climate problem. Carbon that the slow cycle took millions of
ag
a s em
years to bury is being returned by combustion "on a very short time scale". We are
emptying a slow store through agalfast tap.

co m
e m.
om g l as
is.crequired by plants?
Why is an excess of CO2 in the atmosphere considered undesirable even though it
a
Q10

la sem
agANSWER
m a s
o agl
Because CO2's usefulness to plants and its effect on the Earth's temperature are two
. c
s e m
different jobs, and only one of them saturates. Plants can use only so much; the
a
agl
greenhouse effect keeps growing.

om
Why "required" and "excess" are not in conflict:
. c
m light,
A plant's demand has a ceiling. Photosynthesis is limited byewater,
s
. com
temperature
a gla factor becomes
and mineral supply as well as CO2. Once another

as em
l
limiting, extra CO2 adds nothing.
a g The greenhouse effect has no such ceiling. Every extra CO2 molecule intercepts

se m
com a
more outgoing infrared. The chapter's words: "While some amounts of carbon
. a g l
m
dioxide are necessary to keep the Earth warm enough to sustain life, the balance
is critical."
l a se
ag

co m
The consequences the chapter lists for excess CO2:
m .
m as e
.co
Intensified greenhouse effect → global warming
a g l
se m → melting of glaciers and Arctic sea ice
g l a
a c
→ rising sea level → threat to coastal cities
m .
m a s e
. co agl
→ more extreme weather conditions
e m
g l as
→ in India, more intense monsoons and changing rainfall patterns → threat to agriculture
a
→ more CO2 dissolving in the sea → ocean acidification → harm to plankton and coral

com
m .
m ase
.co


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

Evidence: Fig. 13.14, the Keeling curve, shows CO2 rising from about 315 ppm in
1960 to about 420 ppm today — a rise of 105 ppm, or 33% (the chapter rounds this
to "about 35%"), which it calls "an unprecedented rise in the history of human
civilisation." The little saw-teeth on the curve are the Northern Hemisphere's plants
breathing in each summer and releasing in each winter — proof that plants are
absorbing CO2, and equally proof that they are not absorbing enough to stop the
climb.

Q11 How is heat lost from the surface of the Earth? What is its significance?

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.

WAY HEAT HOW IT WORKS WHERE THE ENERGY GOES
IS LOST

Radiation The warm surface emits infrared radiation (it is Partly absorbed by greenhouse
far cooler than the Sun, so it radiates in the IR, gases; the rest escapes to space
not in visible light)

Conduction Heat passes to the thin layer of air in direct Into the lowest air layer
contact with the ground

Convection That warmed air expands, becomes less dense Carried upward through the
and rises; cooler air takes its place troposphere

Evaporation Water takes in latent heat to change from liquid Stored in the vapour; released high
to vapour, cooling the surface it left up when the vapour condenses into
cloud

Page 40 of 56

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

Why this loss matters — four reasons:

1. It keeps the Earth's temperature steady. Over a year the Earth must lose as
much energy as it gains from the Sun. If it lost less, it would go on heating
without limit.
2. It is the step the greenhouse effect acts on. Greenhouse gases do not block
sunlight coming in; they slow the infrared going out. Without any atmosphere
"the Earth would be too cold for life to survive"; with too much CO2 the loss is
slowed too far and the planet warms.
3. It drives the weather. Convection carries heat upward — that is what makes
winds and storms. Evaporation and condensation move water and latent heat
around the planet — that is the water cycle.
4. It explains the daily temperature swing. At night there is no incoming radiation
but the surface keeps radiating, which is why nights are cooler — and why a clear,
dry night is colder than a cloudy one, since clouds send some of the infrared
back.

Everyday check: the chapter's example of a concrete house that feels hot at night is
this idea in miniature — concrete stores heat by day and loses it by re-radiation at
night, into the room. Thick mud and wooden walls re-radiate less, so they stay cool.

Q12 If the Earth were a flat disc instead of a sphere, how would the patterns of solar
radiation and temperature be different?

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.

Page 41 of 56

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

FLAT DISC — same angle
SPHERE — same beam, different area everywhere

high latitude: cold

every strip gets
equator: hot
the same energy

strong equator–pole contrast no contrast → no driving force

On a sphere the same width of beam is spread over a larger surface at high latitude. On a flat disc
facing the Sun there is no such spreading.

On a sphere: intensity at a place, I = I0 cos θ

θ = 0° at the equator → I = I0 = 1 kW m–2

θ = 60° → I = 0.5 × 1 kW m–2 = 0.5 kW m–2
θ = 90° at the pole → I ≈ 0

On a flat disc facing the Sun: θ = 0° everywhere → I = I0 everywhere → uniform heating

Page 42 of 56

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

What would follow from uniform heating:

No pressure belts. The equatorial low, the sub-tropical highs at 30°, the sub-
polar lows at 60° and the polar highs all exist because heating differs with
latitude. Uniform heating means uniform pressure.
No planetary winds. Wind is air moving from high pressure to low pressure.
With no pressure difference there is no large-scale wind.
No heat-transporting ocean currents. Surface currents are dragged by
planetary winds and set going by differences in temperature and density. Both
driving forces vanish.
No equator-to-pole climate belts. There would be no tropics, no temperate
zone and no polar region — one climate everywhere on the lit face.
A dead far side. The face turned away from the Sun would receive nothing at all
and would be permanently, brutally cold — a far sharper contrast than anything
on the real Earth.

The point of the question: it is the Earth's shape — together with the tilt of its axis
— that makes the heating uneven, and it is the unevenness, not the amount of heat,
that drives the winds, the currents and the monsoon. A perfectly and evenly heated
planet would be a still one.

Q13 Suppose there is a rise in atmospheric temperature on Earth. How would this
affect the cryosphere, hydrosphere and biosphere?

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.

Page 43 of 56

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

co m
m.
SPHERE WHAT HAPPENS WHY

m snow cover and polar ice melt ase
Cryosphere
.cofaster; the ice sheets shrink
Glaciers,
a g l
More heat is supplied to ice already at 0 °C, so

a s em more of it changes state to liquid. Losing

ag l bright ice also lowers albedo, so still more
radiation is absorbed — the melting
accelerates itself

co m
m . ag
se
Hydrosphere Rivers first swell with melt water; sea Water stored on land as ice returns to the

l a
ag dissolves
level rises in the long run; evaporation ocean, adding volume. Warmer surfaces
increases; the sea warms and evaporate faster, so the water cycle runs
more CO2 harder — "heavier rains in some areas … and
droughts elsewhere"
co m
se m.
m l a
.co polar species), crop yields and rainfall
Biosphere Habitats are lost (mangroves, coral reefs, Every organism is adapted to a particular

m ag water supply and
range of temperature,

l a se patterns change, ocean acidification season. Change those faster than the species

a g threatens plankton and reefs, species shift can move or adapt, and it is lost — "causing a
or die out habitat loss"

m a s
m.co agl
Rise in atmospheric temperature
l a se
a g
→ cryosphere: accelerated melting of glaciers and polar ice

com
→ hydrosphere: flooding of low-lying regions now, rising sea level later, faster water cycle
.
m a s em
gl
→ biosphere: habitat loss, disrupted ecosystems, threatened agriculture and fisheries
. co a
m
ase
agl For India specifically: Himalayan glaciers feed the Ganga–Brahmaputra system, so
their retreat first raises river flow and later reduces it. Coastal cities such as Mumbai
se m
com g l a
. a
and Chennai face rising seas. And a warmer Arabian Sea makes the monsoon more
m
ase
intense and less reliable — which is a direct threat to agriculture.

agl

co m
Explain how the Earth's atmosphere helps in maintaining a suitable temperature
m .
e
Q14

com l as
for life to survive on the Earth.
. a g
e m
as ANSWER
agl c
The atmosphere works like a two-way filter. On the way in it removes the radiation that
m .
m a s e
agl
would harm life; on the way out it holds back part of the heat that would otherwise

. co
m
escape. Both are needed for the Earth's temperature to sit in the narrow range life can

as e
l
survive.
a g

co m
m .
m as e
.co


a g l Page 44 of 56

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

ROLE WHAT THE ATMOSPHERE DOES EFFECT ON TEMPERATURE

1. Absorbs part The ozone layer in the stratosphere absorbs Prevents overheating and protects
of the incoming harmful short-wavelength UV; clouds, dust and living tissue from UV damage
radiation gases reflect and absorb some sunlight before it
reaches the ground

2. Traps the The surface absorbs sunlight and re-radiates it Keeps the surface warm enough —
outgoing heat as infrared; greenhouse gases (CO2, CH4, water "without the atmosphere, the Earth
vapour) absorb that infrared and send part of it would be too cold for life to survive"
back down

3. Distributes Winds and convection carry warmth from hot Evens out the day–night and
heat regions to cold ones; water vapour carries latent equator–pole extremes
heat and releases it elsewhere

4. Acts as a Slows the escape of infrared once the Sun has set Stops the surface from freezing
blanket at night every night

The balance is the whole point. Too little greenhouse effect and the Earth would
freeze; too much and it would overheat. The chapter gives both extremes:

Too little: "Without the atmosphere, the Earth would be too cold for life to
survive."
Too much: "excess CO2 from human activities enhances the greenhouse effect,
causing global warming, which if left unchecked could make the Earth
uninhabitable."

The natural example the chapter offers is Venus — hotter than Mercury even though
Mercury is closer to the Sun, because Venus has a thick atmosphere with "an
uncontrolled greenhouse effect." Two planets, the same Sun; the atmosphere makes
the difference.

Composition to remember: nitrogen 78%, oxygen 21%, and small amounts of
argon, carbon dioxide and water vapour. It is that small amount of CO2 and water
vapour — not the abundant N2 and O2 — that does the greenhouse work.

Page 45 of 56

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

Q15 Describe the interrelationship between different spheres of the Earth. Illustrate
with example how these spheres function in a delicate balance.

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.

SPHERE WHAT IT IS EXAMPLE FROM THE CHAPTER

Geosphere Solid rock, soil, landforms and the Earth's The Deccan plateau, the Thar desert
interior

Hydrosphere Liquid water — oceans, rivers, lakes, The Ganga–Brahmaputra river system
groundwater

Cryosphere Water in the solid state — ice and snow Himalayan glaciers, snow in Ladakh, polar ice
caps

Atmosphere The air held around the Earth by gravity Cleaner air in the mountains and forests

Biosphere All living organisms and their habitats Mangroves, forests, farms, ocean plankton,
coral reefs

How they are joined:

Solar heating warms land, sea and ice unevenly — this alone connects the geosphere,
hydrosphere, cryosphere and atmosphere.
The water cycle takes water from ocean to air to rain to soil to river and back — through
every sphere in turn.
Biogeochemical cycles move carbon, nitrogen and oxygen between rock, water, air and
living things.
Winds and ocean currents carry heat and moisture from one region to another.

Three examples of the delicate balance:

Page 46 of 56

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

1. The mountain valley of Activity 13.1

Less snowfall (cryosphere) → less melt water → lake level falls (hydrosphere)

→ grass grows poorly (biosphere) → less fodder for the sheep

2. The Arabian Sea and the monsoon

Warmer sea (hydrosphere) → more evaporation → erratic monsoon (atmosphere)

→ floods in one region, drought in another → crops fail (biosphere)

3. Extra CO2 from burning fuels

More CO2 (atmosphere) → stronger greenhouse effect → glaciers melt (cryosphere)

→ sea level rises (hydrosphere) → coastal habitats lost (biosphere)

→ soil erosion where forests are cleared (geosphere)

Why "delicate": the balance is not held in place by anything rigid. It is a set of flows
— of energy and matter — that happen to be roughly equal in and out. Push hard on
any one flow, as human activity now does on the carbon flow, and the whole set
readjusts to a new state, which may be much less comfortable for life. That is the
closing lesson of the chapter, and the reason behind Mission LiFE: "unsustainable
consumption disturbs this balance."

The Journey Beyond — Page 268

Page 47 of 56

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

Project work

THE JOURNEY BEYOND

Q1 Consider two hypothetical Earth-sized planets that have an atmosphere. Assume
that one planet is entirely covered by oceans and the other is entirely by land.
Knowing that the Sun heats the equator more than the poles, how would the wind
patterns on these planets compare with the wind systems we observe on Earth,
with its combination of land and sea?

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.

ALL-OCEAN PLANET ALL-LAND PLANET THE REAL EARTH

Pressure belts and Present — smooth, Present — but far more Present, but broken up by
planetary winds unbroken belts right violent, because land heats continents
round the planet and cools fast

Daily and Very small — water has a Very large — land has a Intermediate; coasts mild,
seasonal huge heat capacity and small heat capacity, so it interiors extreme
temperature swing stores heat heats and cools quickly

Land and sea None — no coast exists None — no sea exists Present all along the
breezes coasts

Monsoon None None Present — driven by the
land heating faster than
the sea in summer

Water cycle and Abundant evaporation Almost no evaporation Uneven; wet coasts, dry
rainfall and rain everywhere source — a desert planet, interiors
dry and dusty

Ocean currents Strong, uninterrupted — None at all Bent into gyres by rotation
no continents to block or and blocked by continents
redirect them

Page 48 of 56

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

co m
m.
The reasoning:

m as e
.co
What survives on both planets. Unequal heating between equator and poles
a g l
a s em
still creates the equatorial low and the sub-tropical highs, and the planet's

agl rotation still deflects the winds — to the right in the Northern Hemisphere and to
the left in the Southern. So planetary winds exist on both.

o m
c ag
What disappears. The chapter says local winds and the land–sea breeze arise
.
m Remove one of the two and the
s e
from "uneven heating of land and water".

a g
mechanism has nothing to work la with. India's southwest monsoon is a giant
seasonal sea breeze — so an all-ocean or an all-land planet has no monsoon at
all.
co m
Why the all-land planet is stormier. Land has a much smaller heat capacity than
em.
m l as
.co g
water. It heats fiercely by day and cools sharply at night, so pressure differences

em a
s
build faster and winds are stronger and gustier — with dust storms rather than
a
l rain storms, since there is little water to evaporate.
ag Why the all-ocean planet is calmer but wetter. Water resists temperature

m a s
coand agl
change, so pressure gradients build slowly and the winds are steadier. But

m .
as e
evaporation is everywhere, so cloud rainfall are abundant and the currents

a g l
run unbroken around the globe.

co m
Sample conclusion to write: "Latitude and rotation set up the planetary wind belts
m .
o m l a se
on any rotating, heated planet. The contrast between land and sea sets up the
g systems are the most
.c and local winds. Earth has both, which is why its awind
m
asevaried of the three."
regional

agl
se m
com g l a
m . a
ase
Choose any one meal you ate recently (it could be anything, for example, roti and

gl
Q2

a
dal, rice and sambar, idli and chutney, and so on). For each main item in the meal,
find out and explain: (i) how the carbon in it originally came from carbon dioxide in

co
the air through photosynthesis, and (ii) how the nitrogen in it likely came from the
m
m .
e other human activities
atmosphere into the soil (for example, by bacteria or through the Haber-Bosch
process and fertilisers) and then into the plant. Further, s
o m l a
.c involved in producing or cooking this meal that addagextra carbon dioxide or
list

se m
g l a nitrogen to the environment.
a c
m .
m a s e
agl

. co
se m
Method: pick your meal, list its main items, and for each one trace two paths — the carbon path

l a
agfertiliser used along the way.
(air → leaf → your plate) and the nitrogen path (air → soil → root → protein → your plate). Then
list the fuel burnt and the

m
Sample answer — a meal of roti and dal

. co
e m
m l as
.co a g
Page 49 of 56

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

ITEM (I) WHERE ITS CARBON CAME FROM (II) WHERE ITS NITROGEN CAME
FROM

Roti Wheat leaves took CO2 from the air through Wheat is not a legume, so the farmer applies
(wheat) their stomata and, using sunlight and water, urea. That urea was made from atmospheric N2
made glucose by photosynthesis. The glucose by the Haber–Bosch process. In the soil it
was stored as starch in the grain. Grinding it becomes ammonia, nitrifying bacteria turn it
made the flour; the carbon in every chapati is into nitrate, and the wheat root assimilates the
atmospheric carbon. nitrate into the gluten proteins of the grain.

Dal (arhar / Same photosynthetic route — CO2 from the A pulse is a legume. Rhizobium living in nodules
moong — air became the carbohydrate of the seed. on its roots fixes N2 straight from the air into
a pulse) ammonia, which the plant uses to make the
protein that makes dal a protein food. Little or
no fertiliser is needed.

Cooking Carbon fixed by mustard or groundnut plants Fat contains almost no nitrogen; the crop's
oil / ghee (or by the grass a cow ate) and stored as fat. nitrogen came from soil nitrate as above.

Salt and Salt is mineral, not biological — no carbon —
spices path. Spices follow the plant route.

Human activities in this meal that add CO2 or nitrogen to the environment:

Making the fertiliser. The Haber–Bosch process is "energy intensive (uses ~ 1 – 2% of global
energy)" and that energy is mostly fossil fuel — so urea carries a CO2 cost before it reaches
the field.
Ploughing, sowing and harvesting. Diesel tractors and threshers burn fossil fuel.
Irrigation. Electric or diesel pumps, and much of India's electricity still comes from fossil
fuels.
Fertiliser run-off. Excess nitrate washes into rivers and lakes and causes eutrophication;
some soil nitrogen escapes as nitrogen oxides.
Transport and milling. Trucks to the mandi, the mill and the shop; the flour mill's motor.
Cooking. LPG or firewood — both release CO2. A pressure cooker cuts the fuel used, and so
cuts the emission.
Packaging and waste. Plastic packets are made from petroleum; food waste rotting in a
landfill releases methane.

The insight worth writing at the end: "the carbon in my roti was in the air a few
months ago; the carbon released to cook it was in the ground for millions of years."
The first is the fast cycle balancing itself; the second is a one-way addition. That
difference is the whole climate problem in one sentence.

Page 50 of 56

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

Q3 ndia Meteorological Department (IMD, or newspaper records, find the average
ptember, or the local season) for your city or district for 5 years during two decades,
such as the 1980s and 2020s. Note any trend you may find (increasing or decreasing,
or the total number of days) with heavy rain (>50 mm). How can this be connected
to the warmer Arabian Sea temperatures or changes in land use (forests to farms to
cities) as discussed in the chapter?

Note on the printed page: the first three lines of this bullet are cut off in the English
edition (page 268) — the sentence begins in the middle of a word. The same project
is printed complete in the Hindi edition Anveshan (page 268), and the full question
reads: "Using data from the India Meteorological Department (IMD,
https://mausam.imd.gov.in/) or newspaper records, find the average monsoon
rainfall (June – September, or the local season) for your city or district for 5
years during two decades, such as the 1980s and 2020s. Note any trend you
may find (increasing or decreasing, or the total number of days) with heavy
rain (>50 mm). How can this be connected to the warmer Arabian Sea
temperatures or changes in land use (forests to farms to cities) as discussed in
the chapter?"

Method — how to do this project properly:

1. Fix your station. Choose the IMD station nearest your city or district, so that you compare
like with like.
2. Fix your season. For most of India take June – September. For Tamil Nadu use the northeast
monsoon, October – December.
3. Collect two blocks of five years, for example 1981 – 1985 and 2019 – 2023, from
https://mausam.imd.gov.in/ or from old newspaper reports in a library.
4. Record two different things. (a) The total seasonal rainfall in mm. (b) The number of days on
which rainfall exceeded 50 mm. These can move in opposite directions, and that is the whole
point.
5. Average each block and set them side by side.

QUANTITY 1981 – 1985 AVERAGE 2019 – 2023 AVERAGE CHANGE

Total monsoon rainfall (mm) (fill in) (fill in) (+ / – …%)

Number of days with > 50 mm rain (fill in) (fill in) (+ / – … days)

Number of dry days in the season (fill in) (fill in) (+ / – … days)

Page 51 of 56

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

How to connect what you find to the chapter:

If heavy-rain days have increased even though the seasonal total has not
changed much, that is exactly the pattern the chapter predicts: a warmer Arabian
Sea gives "more evaporation from the sea", warm air holds more moisture, and
the same water arrives in fewer, fiercer bursts. Fewer rainy days, heavier each.
If flooding in your city has worsened faster than rainfall has, look at land use.
Forests and fields let rain infiltrate; concrete and asphalt do not. "Sudden bursts
of intense rainfall result in more run off … and less infiltration reduces the
recharge of groundwater."
If your city is hotter as well, add the urban heat island effect — cities warmed
by concrete and asphalt can strengthen the rising air that triggers local
downpours.
Be honest about the limits. Five years is a short sample and weather is naturally
variable. Say clearly that your result is a suggestion of a trend, not proof. A
scientist states the uncertainty; that is part of the answer.

Presentation tip: plot the two blocks as a bar chart — years on the x-axis, rainfall in
mm on the y-axis — and mark the >50 mm days with a second colour. A trend that is
hard to see in a table often jumps out of a graph.

The Quest Continues … — Page 268
An open question to carry forward

THE QUEST CONTINUES …

Q1 New tools are allowing scientists to observe the planet in real time and uncover
hidden connections between climate, ecosystems, and human activity. What new
discoveries will these tools reveal about the changing Earth, and how will they
improve our understanding of global warming and climate change?

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.

Page 52 of 56

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

TOOL WHAT IT CAN NOW SEE WHAT THAT COULD REVEAL

Earth-observation Sea-surface temperature, ice extent, forest Which individual sources are leaking
satellites cover, soil moisture, plumes of CO2 and methane; how fast a glacier is thinning;
CH4 — every few days, over the whole how quickly a forest recovers after a fire
planet

Ocean buoys and Temperature, salinity and dissolved CO2 How deep the ocean's heat and carbon
floats at many depths, reported automatically are being stored, and how currents are
shifting

Coupled computer Atmosphere, ocean, land and ice Better seasonal forecasts, and a clearer
models simulated together, as the scientists at view of how warming will change
IITM Pune do for the Indian monsoon monsoon rainfall across India

Ice cores, tree Past CO2 and temperature reaching back Whether today's rise really is
rings, sediments hundreds of thousands of years "unprecedented in the history of human
civilisation" — and it is

Automatic weather Dense local measurements of rain, heat Street-level maps of the urban heat
stations and citizen and air quality island and of flood risk
data

Why better observation changes the science, not just the record-keeping:

It exposes feedbacks. Loops such as ice–albedo, permafrost methane and the
weakening ocean carbon sink are what decide whether warming stays moderate
or runs away. You cannot measure a feedback from a single reading; you need
the whole system watched at once.
It turns "climate" into something local. A global average of 1.5 °C tells a farmer
in Vidarbha nothing. High-resolution models can say what will happen to that
district's sowing date.
It shortens the gap between damage and response. Real-time detection of a
methane leak, an illegal clearing or a marine heatwave means it can be acted on
this week rather than reported next year.
It makes the science checkable. Open satellite data lets anyone verify a
country's emission claims — which is what makes agreements like the Paris
Agreement possible to enforce.

Page 53 of 56

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

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

Quick revision

TERM WHAT IT MEANS VALUE / EXAMPLE FROM WHERE IT
THE CHAPTER APPEARS

Solar constant Average solar energy received per unit 1.4 kW m–2 = 1400 J s–1 m–2 Section 13.1
time on unit area held perpendicular to
the Sun's rays at the top of the
atmosphere

Insolation The solar radiation that actually reaches About 1 kW m–2 under a clear Section 13.1
the Earth's surface sky

Albedo Fraction of the solar radiation a surface Snow 0.80 – 0.90; ice 0.50 – Table 13.1
reflects (Latin albedo = whiteness) 0.70; crushed rock 0.25 – 0.30

Troposphere Lowest layer, heated from below by the 0 – 12 km; temperature falls Table 13.2
Earth's surface; carries almost all at about 6.5 °C km–1
weather

Stratosphere Holds the ozone layer; UV absorption 12 – 50 km Table 13.2
warms it, so temperature rises with
height and the layer stays calm

Greenhouse gases Gases that absorb the infrared radiated CO2, CH4, water vapour Section 13.1.3
back by the warm Earth and stop it
escaping to space

Valley breeze Daytime wind — sunlit slope heats, air Shimla, Dehradun and other Fig. 13.8(a)
rises, cool valley air flows up the slope Himalayan valleys

Mountain breeze Night wind — slope cools fast, dense Shimla, Dehradun and other Fig. 13.8(b)
cold air slides down into the valley Himalayan valleys

Pressure belts Bands of high and low pressure set up Equatorial low, sub-tropical Fig. 13.9(b)
by uneven heating between equator and high (30°), sub-polar low
poles (60°), polar high (90°)

Gyre Large circular pattern of ocean currents Clockwise in the Northern Fig. 13.10(a)
made by the Earth's rotation deflecting Hemisphere, anticlockwise in
moving water the Southern

Biogeochemical Cyclic movement of matter and energy Water, carbon, nitrogen and Section 13.3
cycle between the abiotic and biotic oxygen cycles
components of the Earth

Page 55 of 56

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

TERM WHAT IT MEANS VALUE / EXAMPLE FROM WHERE IT
THE CHAPTER APPEARS

Nitrogen fixation Converting unreactive N2 into ammonia Rhizobium in root nodules, Section 13.3.3
that living things can use Azotobacter in soil, lightning,
Haber–Bosch process

Nitrification / NH3 → NO2– → NO3–, and nitrate back Nitrosomonas, Nitrobacter; Fig. 13.15
Denitrification to N2 gas Pseudomonas

Eutrophication Excess nitrate causes an algal bloom, the Fig. 13.17 Section 13.4
algae use up dissolved oxygen and fish
die

Keeling curve Graph of atmospheric CO2 since 1960; 315 ppm (1960) → about 420 Fig. 13.14
the saw-teeth are the yearly plant growth ppm (2025)
in the Northern Hemisphere

Urban heat island Cities stay warmer than the countryside Higher air-conditioning Threads of
because concrete, brick and asphalt store demand in cities Curiosity, p.
and re-radiate heat 257

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