NCERT Solutions for Class 9th Science Chapter 13 End-of-chapter questions — Revise, Reflect, Refine

Book page 267 – 268 Updated on2026-09-08

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.

Answer

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.

Answer

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

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

Why the others fail:

(i) CO₂ is largely transparent to incoming visible sunlight. It absorbs the outgoing infrared, not the incoming radiation.
(ii) Particles do absorb and scatter a little, but this is a small effect; the atmosphere is heated mainly from below, by the surface. That is why temperature falls with height in the troposphere.
(iv) Radiation reflected by clouds is energy lost to space — it cannot heat the Earth. The word "only" makes this option plainly wrong.

Q3.

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

Answer

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.

Step of the water cycle	How climate change alters it	Example from the chapter
Evaporation	Warmer seas and land evaporate faster; warmer air holds more moisture	Warmer Arabian Sea feeds the southwest monsoon
Precipitation	More moisture is released in short, intense bursts; other regions get none	"Heavier rains in some areas (like intensified monsoons) and droughts elsewhere"
Melting (cryosphere)	Glaciers melt faster, adding water to rivers now and raising sea level later	"Melting glaciers add more water to rivers, raising sea levels in the long run and threatening coastal cities, such as Mumbai and Chennai"
Run off	Intense rain runs off the surface instead of soaking in, and erodes soil	"Sudden bursts of intense rainfall result in more run off that erodes soil"
Infiltration and groundwater	Less water soaks in, so the groundwater is not recharged	"less infiltration reduces the recharge of 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.

Answer

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.

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.

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?

Answer

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 first	Sunlit slope heats faster than the valley floor	Slope loses heat faster; valley floor stays warmer
Air on the slope	Warms, expands, becomes less dense, rises	Cools, becomes denser, sinks
Pressure	A low pressure region forms over the slope	Denser cold air presses down the slope
Direction of the wind	Cooler valley air flows up the slope	Cold slope air flows down into the valley

Part 2 — grassy slope versus barren rock

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?

Answer

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.

Surface absorbs sunlight → surface becomes warm
Air in contact with it is heated → expands, becomes less dense
Less dense air rises → convection currents form
Rising air cools → water vapour condenses → clouds → rainfall
Air rushing in to replace the rising air = wind; if the rise is violent = storm

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. Warm air lying on top of cooler air is a stable arrangement — nothing wants to rise through it. The chapter says this "calms the layer due to the lack of vertical mixing of air, keeping weather confined to the troposphere." That stability is also why aircraft cruise near the top of the troposphere and in the lower stratosphere, above the weather.

Note: the troposphere is also where almost all the atmosphere's water vapour lives — and without water vapour there is no cloud, no rain and no latent heat to power a storm. Its height is greatest above the equator (where heating is strongest) and least above the poles.

Q7.

Explain the processes involved in the nitrogen cycle. How would life on Earth be affected if nitrogen were not cycled?

Answer

Nitrogen makes up the largest reservoir in the atmosphere, but N₂ gas is unreactive, so it must be converted into soluble compounds before life can use it. Five processes do this and undo it in a closed loop.

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 fixation	Atmospheric N₂ → ammonia (NH₃)	Rhizobium in root nodules of legumes, 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 proteins and nucleic acids; animals get nitrogen by eating	Plants, then animals
Ammonification	Dead bodies and waste are broken down, returning ammonia to the soil	Decomposer bacteria and fungi
Denitrification	Some nitrate → N₂ gas, back to the atmosphere — closing the cycle	Pseudomonas

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?

Answer

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.

Cycle	Effect of clearing forest
Oxygen cycle	Photosynthesis is the only large-scale process that restores atmospheric O₂. Fewer trees means less O₂ produced, while respiration and combustion keep consuming it — so the balance of the cycle tilts towards consumption.
Carbon cycle	Two blows at once. (a) A carbon sink is removed — trees no longer absorb CO₂ for photosynthesis. (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.

Answer

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.

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

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

The fast cycle runs "over days to years" — photosynthesis, respiration and decomposition.
The slow cycle runs "over millions of years" — burial of dead organisms and their conversion into fossil fuels.

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

Q10.

Why is an excess of CO2 in the atmosphere considered undesirable even though it is required by plants?

Answer

Because CO₂'s usefulness to plants and its effect on the Earth's temperature are two different jobs, and only one of them saturates. Plants can use only so much; the greenhouse effect keeps growing.

Why "required" and "excess" are not in conflict:

A plant's demand has a ceiling. Photosynthesis is limited by water, light, temperature and mineral supply as well as CO₂. Once another factor becomes limiting, extra CO₂ adds nothing.
The greenhouse effect has no such ceiling. Every extra CO₂ molecule intercepts more outgoing infrared. The chapter's words: "While some amounts of carbon dioxide are necessary to keep the Earth warm enough to sustain life, the balance is critical."

The consequences the chapter lists for excess CO₂:
Intensified greenhouse effect → global warming
→ melting of glaciers and Arctic sea ice
→ rising sea level → threat to coastal cities
→ more extreme weather conditions
→ in India, more intense monsoons and changing rainfall patterns → threat to agriculture
→ more CO₂ dissolving in the sea → ocean acidification → harm to plankton and coral

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?

Answer

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 is lost	How it works	Where the energy goes
Radiation	The warm surface emits infrared radiation (it is far cooler than the Sun, so it radiates in the IR, not in visible light)	Partly absorbed by greenhouse gases; the rest escapes to space
Conduction	Heat passes to the thin layer of air in direct contact with the ground	Into the lowest air layer
Convection	That warmed air expands, becomes less dense and rises; cooler air takes its place	Carried upward through the troposphere
Evaporation	Water takes in latent heat to change from liquid to vapour, cooling the surface it left	Stored in the vapour; released high up when the vapour condenses into cloud

Why this loss matters — four reasons:

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.
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.
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.
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?

Answer

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.

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

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?

Answer

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.

Sphere	What happens	Why
Cryosphere	Glaciers, snow cover and polar ice melt faster; the ice sheets shrink	More heat is supplied to ice already at 0 °C, so more of it changes state to liquid. Losing bright ice also lowers albedo, so still more radiation is absorbed — the melting accelerates itself
Hydrosphere	Rivers first swell with melt water; sea level rises in the long run; evaporation increases; the sea warms and dissolves more CO₂	Water stored on land as ice returns to the ocean, adding volume. Warmer surfaces evaporate faster, so the water cycle runs harder — "heavier rains in some areas … and droughts elsewhere"
Biosphere	Habitats are lost (mangroves, coral reefs, polar species), crop yields and rainfall patterns change, ocean acidification threatens plankton and reefs, species shift or die out	Every organism is adapted to a particular range of temperature, water supply and season. Change those faster than the species can move or adapt, and it is lost — "causing a habitat loss"

Rise in atmospheric temperature
→ cryosphere: accelerated melting of glaciers and polar ice
→ hydrosphere: flooding of low-lying regions now, rising sea level later, faster water cycle
→ biosphere: habitat loss, disrupted ecosystems, threatened agriculture and fisheries

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 and Chennai face rising seas. And a warmer Arabian Sea makes the monsoon more intense and less reliable — which is a direct threat to agriculture.

Q14.

Explain how the Earth's atmosphere helps in maintaining a suitable temperature for life to survive on the Earth.

Answer

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.

Role	What the atmosphere does	Effect on temperature
1. Absorbs part of the incoming radiation	The ozone layer in the stratosphere absorbs harmful short-wavelength UV; clouds, dust and gases reflect and absorb some sunlight before it reaches the ground	Prevents overheating and protects living tissue from UV damage
2. Traps the outgoing heat	The surface absorbs sunlight and re-radiates it as infrared; greenhouse gases (CO₂, CH₄, water vapour) absorb that infrared and send part of it back down	Keeps the surface warm enough — "without the atmosphere, the Earth would be too cold for life to survive"
3. Distributes heat	Winds and convection carry warmth from hot regions to cold ones; water vapour carries latent heat and releases it elsewhere	Evens out the day–night and equator–pole extremes
4. Acts as a blanket at night	Slows the escape of infrared once the Sun has set	Stops the surface from freezing 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.

Q15.

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

Answer

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 interior	The Deccan plateau, the Thar desert
Hydrosphere	Liquid water — oceans, rivers, lakes, groundwater	The Ganga–Brahmaputra river system
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:

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."

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