Correct option: (B) The Sun.

Solar radiation is the principal external energy source driving atmospheric circulation, the water cycle, photosynthesis and many other Earth-system processes.

Correct option: (B) Geosphere.

Correct option: (A).

The cryosphere includes water occurring in solid form such as ice and snow.

Correct option: (A).

Correct option: (B).

Electromagnetic waves can travel through a vacuum, whereas sound is a mechanical wave and requires a medium.

Correct option: (C).

Correct option: (B).

Correct option: (A).

Correct option: (B) Albedo.

Correct option: (A).

Snow reflects a large fraction of incoming solar radiation.

Correct option: (B).

Correct option: (B).

This contributes to the urban heat island effect.

Correct option: (A).

Correct option: (C) 78%.

Correct option: (C).

Correct option: (B).

The chapter gives an approximate decrease of 6.5°C per kilometre.

Correct option: (A).

Correct option: (A).

Correct option: (B).

Correct option: (B).

Correct option: (C).

Correct option: (A).

Correct option: (A).

Correct option: (B).

Correct option: (B).

Correct option: (B).

Correct option: (B).

Correct option: (B) Nitrobacter.

Correct option: (B).

Correct option: (B).

Geosphere: the solid rocks, soil, landforms and interior of the Earth.

Biosphere: all living organisms together with the habitats in which they live.

Insolation is the solar radiation reaching the Earth’s surface.

The solar constant is the average solar energy received per unit time per unit area perpendicular to the Sun’s rays at the top of the atmosphere, before atmospheric absorption, scattering and reflection.

Albedo is the fraction of incoming solar radiation reflected by a surface.

Snow has a high albedo and reflects much of the incoming sunlight, whereas black soil has a lower albedo and absorbs more radiation.

At the equator, solar radiation is concentrated over a relatively smaller surface area.

Near the poles, incoming rays strike at a more oblique angle and their energy is spread over a larger area.

• It absorbs or blocks harmful incoming radiation, especially much of the ultraviolet radiation through ozone.
• Greenhouse gases absorb part of the Earth’s outgoing infrared radiation, keeping the planet sufficiently warm for life.

Valley breeze: occurs during the day when heated mountain slopes cause air to rise and cooler valley air moves upslope.

Mountain breeze: occurs after sunset when mountain slopes cool rapidly and cold dense air flows down into the valley.

Higher salinity generally increases the density of ocean water.

Denser water therefore tends to sink and can contribute to deeper ocean circulation.

A biogeochemical cycle is the cyclic movement of matter between living components and non-living parts of the Earth system.

Examples: water cycle, carbon cycle, nitrogen cycle or oxygen cycle.

Fast carbon cycle: occurs over days to years through processes such as photosynthesis, respiration and decomposition.

Slow carbon cycle: operates over very long periods and includes burial of organic matter and formation of fossil fuels.

Nitrogen fixation converts atmospheric nitrogen into biologically usable nitrogen compounds such as ammonia.

Denitrification converts nitrates back into atmospheric nitrogen gas.

Nitrosomonas: converts ammonia into nitrite.

Nitrobacter: converts nitrite into nitrate.

Both participate in nitrification.

Oxygen is consumed in respiration and combustion.

It is restored mainly through photosynthesis, in which plants use carbon dioxide and water and release oxygen.

Eutrophication is the excessive enrichment of water with nutrients such as nitrates, often resulting from fertiliser runoff.

It promotes algal blooms that can reduce dissolved oxygen and kill fish.

Forests return substantial amounts of water vapour to the atmosphere through transpiration.

Removing forests reduces transpiration and alters interactions between land and atmosphere, which can contribute to a decline in local rainfall.

Suppose snowfall or glacial storage decreases.

• Less snow and ice may reduce meltwater available to lakes or rivers.
• This changes the hydrosphere by lowering water availability.
• Reduced water can decrease vegetation growth and affect grazing animals, agriculture and aquatic ecosystems in the biosphere.

Thus, one disturbance can propagate through several Earth spheres.

• UV: much of the harmful short-wavelength UV is absorbed by ozone in the upper atmosphere.
• Visible light: reaches the surface and provides energy for photosynthesis while also contributing to surface heating.
• Infrared: warms the surface; the heated Earth re-radiates infrared energy, some of which is absorbed by greenhouse gases.

I = 800 W m⁻²
A = 2 m²
t = 30 min = 1800 s

E = 800 × 2 × 1800

E = 2,880,000 J

Answer = 2.88 × 10⁶ J.

Albedo = fraction reflected = 0.85

Reflected energy = 0.85 × 1000 = 850 J

Absorbed energy = 1000 − 850 = 150 J

Reflected = 850 J; absorbed = 150 J.

• Concrete, steel, brick and asphalt absorb large quantities of solar radiation.
• These materials store heat and re-radiate it later, including during nighttime.
• Cities have less vegetation, so they receive less shade and less cooling from plant transpiration.

The effect increases local temperatures and can increase energy demand for cooling.

Temperature fall = 6.5 × 2 = 13°C

Temperature at 2 km = 26 − 13

Temperature = 13°C.

This is an idealised calculation using the average lapse rate quoted in the chapter.

The troposphere is heated from the Earth’s surface.

Warm air can rise and cooler air can sink, creating vertical movement that drives winds, clouds and storms.

In the stratosphere, ozone absorption causes temperature to increase with height, reducing vertical mixing and making that layer more stable.

1. The Earth’s surface absorbs incoming solar energy and becomes warm.
2. The warm surface re-radiates energy mainly in the infrared region.
3. Greenhouse gases such as CO₂, CH₄ and water vapour absorb part of this outgoing infrared energy and reduce the rate at which heat escapes to space.

Without this natural effect, Earth would be too cold for life as we know it.

Stratospheric ozone absorbs harmful ultraviolet radiation and therefore protects organisms and ecosystems.

CFCs caused severe ozone depletion, particularly over Antarctica.

The Montreal Protocol reduced the use of ozone-depleting chemicals, allowing the ozone layer to begin a gradual recovery and demonstrating the value of international scientific cooperation.

1. Mountain slopes facing the Sun heat more rapidly during daytime.
2. Air above the slopes becomes warm, expands and rises, producing relatively lower pressure.
3. Cooler air from the valley moves upward along the slopes to replace it.

This upslope flow is called a valley breeze.

Strong solar heating near the equator warms the air and causes it to rise, creating an equatorial low-pressure belt.

The rising air moves poleward at higher altitudes and cools.

It becomes denser and sinks around approximately 30° N and 30° S, producing subtropical high-pressure belts.

Any three:

• Planetary winds drag surface water through friction.
• Temperature differences produce density differences.
• Salinity differences also affect water density.
• Earth’s rotation deflects moving water masses.
• Continents block and redirect ocean-current paths.

Warm surface water transports heat from equatorial regions towards higher latitudes.

Colder, denser water can return towards lower latitudes at deeper levels.

This movement redistributes heat and reduces temperature differences between regions. For example, the North Atlantic Drift helps keep parts of northwestern Europe comparatively mild and many ports ice-free.

Evaporation: liquid water from oceans, lakes and rivers changes into water vapour.

Condensation: water vapour cools and forms tiny droplets, contributing to clouds.

Precipitation: water returns to Earth’s surface as rain, snow or hail.

• A warmer atmosphere can hold more moisture, contributing to heavier rainfall in some regions.
• Other regions may experience prolonged drought.
• Glacier melting can initially alter river flows and eventually contribute to sea-level rise.
• Intense rainfall can increase runoff and soil erosion while reducing infiltration and groundwater recharge.

• Photosynthesis removes CO₂ from the atmosphere and converts carbon into organic molecules such as glucose.
• Respiration releases part of this carbon back into the atmosphere as CO₂.
• After organisms die, decomposers break down organic material, returning carbon compounds and CO₂ to the environment.

Ocean water continuously exchanges carbon dioxide with the atmosphere.

Dissolved CO₂ forms carbonate and bicarbonate ions; phytoplankton use dissolved carbon in photosynthesis and some marine organisms use carbon compounds to form shells.

When marine organisms die and sink, some carbon can be stored on the ocean floor for long periods.

Nitrogen fixation: bacteria such as Rhizobium and Azotobacter convert atmospheric N₂ into ammonia.

Nitrification: Nitrosomonas converts ammonia to nitrite, and Nitrobacter converts nitrite to nitrate.

Assimilation: plants absorb nitrogen compounds from soil and incorporate them into biological molecules; animals obtain nitrogen through feeding.

The Haber-Bosch process converts atmospheric nitrogen into ammonia and supplies much of the nitrogen used in modern fertilisers.

It greatly increased agricultural productivity and contributed to food security.

However, it is energy intensive, and excessive fertiliser use can degrade soil and water and contribute to eutrophication.

• Atmosphere: reduced photosynthesis can decrease carbon removal and reduced transpiration can affect rainfall.
• Geosphere: removal of roots increases soil erosion.
• Biosphere: habitat destruction can reduce biodiversity.
• Energy balance: changing vegetation cover can alter surface albedo.

• Geosphere: rocks, soil, landforms and Earth’s interior.
• Hydrosphere: liquid water in oceans, rivers, lakes and groundwater.
• Cryosphere: snow and ice.
• Atmosphere: gases surrounding Earth.
• Biosphere: organisms and their habitats.

They are interconnected by continuous transfers of matter and energy. For example, glacier melt moves water from the cryosphere into rivers, soil moisture affects plants, plants exchange gases with the atmosphere, and rainfall influences both soil and ecosystems.

A disturbance in one sphere can therefore produce changes in several others.

Warmer seawater increases evaporation and therefore changes the transfer of water vapour from the hydrosphere to the atmosphere.

This can contribute to fluctuations in the southwest monsoon and variability in rainfall, including heavy rain or floods in some regions and drought conditions in others.

Changed rainfall alters rivers and groundwater in the hydrosphere, soil erosion and infiltration in the geosphere, and agriculture and habitats in the biosphere.

The example demonstrates that the monsoon is an Earth-system process rather than an isolated atmospheric event.

I = 900 W m⁻²
A = 5 m²
t = 4 × 3600 = 14,400 s

Incident energy = 900 × 5 × 14,400

= 64,800,000 J

= 6.48 × 10⁷ J

Electrical energy = 20% of incident energy

= 0.20 × 6.48 × 10⁷

= 1.296 × 10⁷ J.

Incoming solar radiation does not all reach or remain at Earth’s surface.

• A portion is reflected by clouds and the atmosphere.
• A portion is absorbed by atmospheric gases and clouds.
• A portion is reflected by the Earth’s surface according to its albedo.
• The remainder is absorbed by land and water, causing surface warming.

The heated Earth subsequently re-radiates energy, mainly as infrared radiation. Greenhouse gases absorb part of this outgoing radiation and help regulate Earth’s temperature.

Because Earth is spherical, solar rays strike different latitudes at different angles.

Near the equator, energy is concentrated over a smaller area, causing greater heating. At higher latitudes the same incoming radiation is spread over a larger area.

The temperature differences create pressure differences in the atmosphere and density differences in water.

These energy gradients help drive large-scale atmospheric winds and ocean circulation, transporting heat around the planet.

At the equator intense heating causes air to rise, creating low pressure.

The air moves poleward aloft, cools and sinks around 30° N and S, forming subtropical high-pressure belts.

Some surface air returns towards the equator, while some moves poleward and rises again around approximately 60° where it meets colder polar air, contributing to subpolar low pressure.

Cold dense air sinks near 90° N and S, forming polar high-pressure regions. Earth’s rotation then deflects these moving air masses from straight paths.

• Planetary winds drag surface water through friction.
• Warm equatorial water tends to move along surface pathways towards higher latitudes.
• Cold water is denser and may move at deeper levels.
• Higher salinity also increases density and encourages sinking.
• Earth’s rotation deflects the moving water, contributing to large circular patterns or gyres.
• Continents block and redirect the currents.

Thus ocean circulation results from several interacting physical processes rather than one single cause.

A warmer atmosphere can hold more water vapour and alter precipitation patterns.

Higher temperature accelerates melting in the cryosphere.

The additional meltwater and changing rainfall alter rivers, oceans and groundwater in the hydrosphere.

Intense runoff increases soil erosion and reduced infiltration can reduce groundwater recharge, affecting the geosphere.

Agriculture, fisheries and natural habitats are then affected in the biosphere.

Increase = 420 − 315

= 105 ppm

Percentage increase =

≈ 33.3%

Increase ≈ 105 ppm, or about 33% using the two quoted endpoint values.

• Nitrogen fixation: Rhizobium and Azotobacter convert atmospheric nitrogen to ammonia.
• Nitrification: Nitrosomonas changes ammonia to nitrite; Nitrobacter changes nitrite to nitrate.
• Assimilation: plants absorb nitrogen compounds; animals obtain them by feeding.
• Ammonification: decomposers return ammonia-containing compounds to soil from wastes and dead matter.
• Denitrification: bacteria such as Pseudomonas convert nitrates back to nitrogen gas.

This cyclic movement maintains nitrogen availability in ecosystems.

Photosynthesis is a major process that removes atmospheric CO₂ and produces oxygen.

If it stopped, biological removal of CO₂ would fall sharply while respiration and combustion would continue releasing CO₂.

Atmospheric oxygen would gradually decline because respiration and combustion would continue consuming it without equivalent biological replacement.

Food production by primary producers would also stop, causing collapse of food chains and severe disruption of the biosphere.

• Reduced photosynthesis lowers biological uptake of atmospheric CO₂.
• Reduced transpiration can alter local moisture and rainfall patterns.
• Loss of roots increases soil erosion.
• Loss of vegetation changes surface albedo and energy exchange.
• Habitat destruction reduces biodiversity.

Any four suitably explained points earn full credit.

• Reduce fossil-fuel dependence and expand renewable energy such as solar and wind.
• Plant and protect trees to support carbon storage, habitats and water cycling.
• Use water efficiently and protect water bodies.
• Practise sustainable farming and reduce excessive fertiliser use.
• Reduce waste and reuse and recycle materials.
• Conserve food and energy through responsible consumption.

Such measures reduce pressure on several Earth spheres simultaneously.

The statement is incorrect because drought affects several interconnected spheres.

• Hydrosphere: rivers, lakes and groundwater decline.
• Biosphere: plants, animals, agriculture and food availability are affected.
• Geosphere: dry soil may become more prone to erosion.
• Atmosphere: changing evaporation and transpiration can influence humidity and local climate.

Earth-system disturbances rarely remain confined to only one sphere.

Surface A:

Reflected = 0.80 × 1000 = 800 W m⁻²

Absorbed = 1000 − 800 = 200 W m⁻²

Surface B:

Reflected = 0.20 × 1000 = 200 W m⁻²

Absorbed = 1000 − 200 = 800 W m⁻²

Snow and ice have high albedo and reflect large fractions of incoming sunlight.

Darker land or ocean generally has lower albedo and therefore absorbs more solar radiation.

Replacing reflective ice with darker surfaces would therefore increase the amount of absorbed solar energy and tend to promote additional warming.

This is a logical consequence of the chapter’s albedo concept.

On spherical Earth, rays strike different latitudes at different angles, so equatorial radiation is more concentrated while polar radiation is spread over a larger area.

On the hypothetical flat disc described, the same geometric difference in angle would largely disappear if sunlight struck the entire surface equally and normally.

The strong equator-to-pole heating contrast generated by Earth’s spherical geometry would therefore be greatly reduced or absent.

Consequently, the large-scale temperature and pressure gradients driving many planetary winds would be very different.

The dark barren rocky slope is likely to absorb more solar radiation and heat more strongly.

Vegetation can provide shade and cooling through transpiration, while dark rock can store more heat.

Warmer air over the barren slope may rise more strongly, potentially producing a larger local temperature and pressure contrast.

The exact breeze depends on several local conditions, but the different surface properties can clearly alter local atmospheric circulation.

Mercury is closer to the Sun than Venus, yet Venus is hotter.

The chapter attributes this to Venus’s atmosphere and its uncontrolled greenhouse effect.

Therefore, the way a planet’s atmosphere absorbs and traps outgoing heat can be as important as incoming solar energy in determining surface temperature.

In the present Earth system, rotation causes winds and ocean currents to be deflected from straight paths.

Without that deflection, moving air and water would tend to follow more direct paths from regions of high pressure or density contrast towards lower-pressure or different-density regions.

The curved circulation patterns and ocean gyres described in the chapter would therefore be substantially altered.

Lower temperature increases water density, and higher salinity also increases density.

A cold, salty water mass can therefore become denser than surrounding water and sink.

This sinking transfers water from the surface to deeper ocean levels and contributes to large-scale circulation connecting surface and deep ocean waters.

• Surface runoff increases.
• Greater runoff can increase soil erosion.
• Less time is available for water to infiltrate into the ground.
• Reduced infiltration decreases groundwater recharge.
• Lower groundwater availability can make agriculture more difficult during later dry periods.

Thus, heavier rain does not automatically mean improved long-term water availability.

Carbon dioxide is essential for photosynthesis and its natural greenhouse effect helps maintain a habitable temperature.

However, the Earth system depends on balance rather than simply the presence of CO₂.

Excess CO₂ absorbs additional outgoing infrared radiation and strengthens greenhouse warming.

Consequences can include glacier melting, sea-level rise, altered rainfall, extreme weather and disturbance of terrestrial and marine ecosystems.

Fossil fuels form from buried organic matter over millions of years and therefore belong to long-term geological carbon storage.

Combustion releases that stored carbon as CO₂ within minutes or hours.

The released CO₂ immediately becomes part of the active atmosphere-ocean-biosphere exchange system.

Humans are therefore moving carbon from very slow storage into the fast carbon cycle at a rapid rate.

Most organisms cannot directly use atmospheric N₂.

Without biological nitrogen fixation, natural conversion of N₂ into usable nitrogen compounds would greatly decline.

Plants would eventually face nitrogen shortages that would limit protein and nucleic-acid synthesis and reduce growth.

Animals would then receive less nitrogen through food chains, reducing ecosystem productivity and ultimately threatening many populations.

Denitrification does convert nitrates back into atmospheric nitrogen, so it can reduce nitrate concentration locally.

However, it is an essential stage of the nitrogen cycle because it returns nitrogen to the atmosphere and helps prevent nitrogen compounds from accumulating indefinitely.

The ecological requirement is a balanced nitrogen cycle containing fixation, assimilation, decomposition, nitrification and denitrification.

The effect depends strongly on where ozone occurs.

In the stratosphere, ozone absorbs harmful solar UV radiation and protects life.

Near the ground, ozone formed as part of photochemical smog can be harmful to health and contributes to unhealthy urban air.

Therefore, describing ozone simply as “good” or “bad” without specifying its location is scientifically incomplete.

1. Energy balance:

Replacing vegetation with buildings, concrete and roads changes surface albedo, absorption and heat storage. The built environment can produce an urban heat island.

2. Water cycle:

Reduced vegetation lowers transpiration. Impervious surfaces can increase rapid runoff and reduce infiltration.

3. Groundwater:

Reduced infiltration can decrease groundwater recharge.

4. Carbon cycle:

Forest removal reduces photosynthetic uptake and storage of CO₂. Urban transport and energy use may additionally release more CO₂.

5. Oxygen cycle:

Less vegetation means less local oxygen production through photosynthesis, while increased fuel combustion consumes oxygen and releases CO₂.

6. Geosphere:

Loss of roots can increase soil erosion before or during construction, while paving dramatically changes the natural land surface.

7. Biosphere:

Habitat destruction reduces biodiversity and displaces organisms.

8. Atmosphere:

Vehicular and industrial emissions can worsen air quality and contribute to greenhouse-gas concentrations and ground-level smog.

The scenario demonstrates the central principle of the chapter: Earth’s spheres function as an interconnected system, so a major change in one component produces a chain of consequences throughout the system.