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

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Expected observation: When the concentration of greenhouse gases (CO₂, methane, water vapour) is increased, more of the infrared (heat) radiation given out by the Earth's surface is absorbed and sent back towards the ground, so the surface temperature rises. When the concentration is reduced (or set to "no greenhouse gases"), more heat escapes to space and the surface becomes much colder. This shows that greenhouse gases keep the Earth warm, but too much of them causes global warming.

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• At night the cool, dense air flowing down the slopes lowers the temperature in the valley, reducing heat stress on crops.
• Cooler nights reduce the loss of water by evaporation from soil and by transpiration from plants, so the soil stays moist.
• Cooling of air can lead to dew formation, adding moisture to crops and soil.
• Lower night temperatures slow down respiration in plants, so less stored food is used up – helping better yields (e.g. tea, apples and fruit orchards in hill valleys).

(Very cold air collecting in valleys can sometimes cause frost, so farmers choose crops accordingly.)

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As warm water moves towards the poles as an ocean current, it gradually gives out its heat to the air above and becomes cooler; it also becomes saltier and denser due to evaporation, and finally sinks and returns towards the equator as a deep cold current.

Impact: the heat released warms the air and coastal lands of higher latitudes, giving them milder winters than expected and bringing moisture and rainfall. For example, the warm Gulf Stream (North Atlantic Drift) keeps north-western Europe much warmer than other places at the same latitude. Ocean currents thus redistribute heat from the equator to the poles.

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Warmer water holds less dissolved gas, so the oceans absorb less CO₂ and also less oxygen. At the same time, the extra CO₂ that dissolves makes sea water more acidic (ocean acidification). As a result:

• Corals, shellfish, oysters and tiny plankton find it hard to build their calcium carbonate shells and skeletons; coral reefs bleach and die.
• Less dissolved oxygen stresses fish and other animals; "dead zones" may form.
• Since plankton are the base of marine food chains, fish populations and the whole marine ecosystem (and fishermen's livelihoods) are affected.

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Elements like carbon, nitrogen, oxygen, phosphorus and water would get locked up in one place and would not be available to living things again.

• Carbon cycle stops: CO₂ would not be returned to air by respiration and decomposition, so plants would run out of CO₂ for photosynthesis; food production would stop and all food chains would collapse.
• Nitrogen cycle stops: no conversion of atmospheric nitrogen to nitrates, so plants could not make proteins and DNA; animals depending on plants would also suffer.
• Water cycle stops: no rainfall; land would dry up, rivers would vanish and organisms would die of thirst.
• Oxygen cycle stops: oxygen used in respiration and burning would not be replaced, so animals could not breathe.
• Dead bodies and wastes would pile up without decomposition.

Life on Earth would gradually come to an end.

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Human activities: burning of coal, petrol, diesel and natural gas in power plants, vehicles and industries (CO₂); deforestation (fewer trees to absorb CO₂, and burning releases it); paddy fields, cattle rearing and landfills (methane); excess use of chemical fertilisers (nitrous oxide); refrigerants and ACs (fluorinated gases).

What I can do: walk, cycle or use public transport; switch off lights, fans and appliances when not in use; use LED bulbs and energy-efficient appliances; support solar energy; reduce, reuse and recycle; avoid wasting food; plant and protect trees; avoid burning waste and crackers; spread awareness in my family and school.

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

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

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A warmer climate speeds up the water cycle:

• More evaporation from oceans and soils, and warmer air holds more water vapour – leading to heavier, more intense rainfall and floods in some places (e.g. extreme rain events and cloudbursts in the Himalayas, intense monsoon spells).
• Faster drying of soils elsewhere – more frequent and severe droughts and heatwaves.
• Melting glaciers and less snowfall – rivers fed by glaciers (like the Ganga) may first swell and later shrink, affecting water supply.
• Changing rainfall timing – monsoon onset and distribution become irregular, affecting farming.
• Rising sea level due to melting ice and warming of water – coastal flooding and salt water entering groundwater.

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Albedo is the fraction of incoming sunlight that a surface reflects. Light, shiny surfaces (snow, ice, clouds) have high albedo – they reflect most sunlight and stay cool. Dark surfaces (oceans, forests, asphalt, bare soil) have low albedo – they absorb more sunlight and warm up.

Effect on climate: if ice and snow melt, darker land or sea is exposed, which absorbs more heat, causing more melting – a feedback that speeds up warming (especially in the Arctic). Cutting forests or building cities changes local albedo and temperature (urban heat islands). Changes in cloud cover also change how much sunlight is reflected.

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Valley breeze (day): mountain slopes heat up faster than the valley floor. The air over the slopes becomes warm and light and rises, and air from the valley flows up the slope to take its place.

Mountain breeze (night): after sunset the slopes lose heat faster and become cooler than the valley. The air over the slopes becomes cool and dense and flows down into the valley.

Yes, the temperatures would differ. Barren rock heats up quickly in the day and also loses heat quickly at night by radiation, so the air above it becomes very cold – its mountain breeze would be cooler (and the day-time valley breeze stronger). A grass-covered mountain has moisture in soil and plants and a vegetation cover that holds heat and releases it slowly; temperature changes are smaller, so its night breeze would be milder (less cold).

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The troposphere – the lowest layer of the atmosphere (up to about 8–18 km). It contains almost all the water vapour and most of the air mass. The uneven heating of the Earth's surface by the Sun heats this layer from below, causing convection currents and pressure differences – producing winds, clouds, storms and rainfall.

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1. Nitrogen fixation: atmospheric N₂ is converted into usable forms (ammonia/nitrates) by nitrogen-fixing bacteria (e.g. Rhizobium in root nodules of legumes), by lightning, and industrially (fertilisers).
2. Assimilation: plants absorb nitrates from soil to make proteins and nucleic acids; animals get nitrogen by eating plants.
3. Ammonification: decomposers convert nitrogen in dead organisms and wastes into ammonia.
4. Nitrification: bacteria convert ammonia into nitrites and then nitrates.
5. Denitrification: other bacteria convert nitrates back into nitrogen gas, returning it to the air.

If nitrogen were not cycled, plants would not get nitrates, so they could not make proteins, DNA or chlorophyll. Plant growth would stop, food chains would collapse, and life on Earth could not continue.

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Oxygen and carbon cycles: fewer trees means less photosynthesis, so less CO₂ is absorbed and less O₂ is released. Burning or decay of cut trees releases the stored carbon as CO₂. So atmospheric CO₂ increases, adding to global warming.

Other consequences: soil erosion and loss of fertility, floods and landslides, reduced rainfall and groundwater recharge, loss of habitat and biodiversity, desertification, increased human–wildlife conflict and loss of livelihood for forest-dependent communities.

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Atmospheric CO₂
⇒ photosynthesis in plants (carbon stored in glucose/plant body)
⇒ plants are eaten by animals (carbon passes along food chains)
⇒

• Respiration by plants and animals releases CO₂ back into the air;
• Decomposition of dead plants, animals and wastes by microbes releases CO₂;
• Some remains get buried for millions of years and form fossil fuels; their combustion releases CO₂;
• Burning of wood/forest fires releases CO₂; oceans exchange CO₂ with the air.

Diagram: draw "CO₂ in atmosphere" at the top; arrows down to "Plants (photosynthesis)"
⇒ "Animals (feeding)"
⇒ "Dead matter
⇒ decomposers" and "Fossil fuels"; arrows back up to the atmosphere labelled "respiration", "decomposition" and "combustion".

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Plants need only a limited amount of CO₂, and natural sinks (forests, oceans) can absorb only so much. Excess CO₂ is a greenhouse gas that traps more heat, causing global warming – melting glaciers, rising sea levels, extreme weather, changing monsoons and threats to agriculture. It also dissolves in oceans and makes them acidic, harming corals and marine life. So the balance matters more than the amount.

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The Earth's surface, warmed by sunlight, loses heat by:

• Radiation – it emits infrared (heat) radiation; part escapes to space and part is absorbed by greenhouse gases and re-radiated.
• Conduction – to the air in contact with the ground.
• Convection – warm air rises and carries heat upward.
• Evaporation – heat is used to evaporate water (latent heat), which is released higher up when clouds form.

Significance: this heat loss balances the energy received from the Sun, so the Earth does not keep getting hotter. It keeps the average temperature stable and drives winds, clouds and rainfall.

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On a sphere, sunlight falls directly near the equator and at a slant near the poles, so the equator is hot and the poles are cold. If the Earth were a flat disc facing the Sun, sunlight would fall at almost the same angle everywhere, so all places would receive nearly equal radiation and have nearly the same temperature. There would be very small temperature differences between regions, so winds and ocean currents (which depend on uneven heating) would be very weak, and there would be no tropical, temperate and polar zones. (The side facing away from the Sun would be dark and cold.)

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Cryosphere (ice and snow): glaciers, ice caps, sea ice and permafrost melt; less snowfall; lower albedo causes more warming.

Hydrosphere (water): sea level rises (melting ice + expansion of warm water); more evaporation, heavier rain in some areas and droughts in others; oceans become warmer and more acidic; changes in ocean currents.

Biosphere (living things): species shift to cooler areas or higher altitudes; coral bleaching; loss of biodiversity and extinction of sensitive species; changes in flowering and breeding times; crop yields affected; spread of diseases and pests.

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• Greenhouse effect: gases like CO₂ and water vapour trap part of the heat radiated by the Earth, keeping its average temperature at about 15 °C instead of about −18 °C.
• Ozone layer in the stratosphere absorbs harmful ultraviolet radiation.
• Clouds reflect part of the sunlight during the day and act like a blanket at night, preventing extreme heating and cooling.
• Winds carry heat from hot regions to cold regions, reducing temperature differences.
• Without the atmosphere (like on the Moon), days would be extremely hot and nights extremely cold.

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The Earth has four main spheres – the atmosphere (air), hydrosphere (water, including the cryosphere), lithosphere/geosphere (rocks and soil) and biosphere (living things). Energy and matter constantly flow between them; a change in one affects the others.

Example – a forest during the monsoon: water evaporates from the ocean (hydrosphere) into the air (atmosphere), forms clouds and falls as rain on land (lithosphere). Trees (biosphere) absorb water and minerals from the soil, take CO₂ from the air for photosynthesis and release O₂ and water vapour back into the atmosphere. Roots hold soil and prevent erosion; fallen leaves decay to enrich the soil. If the forest is cut down, rainfall reduces, soil erodes into rivers, CO₂ increases and animals lose their homes – showing how delicately the spheres are balanced.

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(a) forest (b) ocean (c) fresh snow (d) asphalt road

Answer: (c) fresh snow

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Insolation is the solar radiation received by the Earth's surface. It varies due to the Earth's spherical shape, latitude (angle of the Sun's rays), tilt of the Earth's axis (seasons) and cloud cover.

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Atmosphere, hydrosphere (including cryosphere), lithosphere (geosphere) and biosphere.

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Mainly through photosynthesis by green plants, algae and phytoplankton.