Earth as a System: Energy, Matter, and Life | CBSE Class 9 Science Notes

This note covers Earth’s interacting spheres, solar radiation, uneven heating, atmospheric layers, winds, ocean currents, water, carbon, nitrogen and oxygen cycles, human impacts, and actions that help maintain environmental balance.

How do Earth’s five spheres function as one system?

The Earth system consists of interacting parts through which energy and matter move. The Sun is its main energy source. Earth’s hot interior and chemical reactions in air, water and rocks also help drive these movements.

The five spheres are the major parts of this system. Each includes particular materials or forms of life, but changes in one can affect others.

How can one disturbance spread?

Less winter snowfall may lead to less lake water in summer, leaving less water to support grass growth. This connects the cryosphere, hydrosphere and biosphere. Solar heating, movement of air and water, and cycling of nutrients, substances needed by organisms for life and growth, maintain connections across the spheres.

Warmer Arabian Sea water increases evaporation, the change of liquid water into water vapour. This causes fluctuations in the southwest monsoon, producing variable rainfall, floods in some regions and drought elsewhere.

A rise in atmospheric temperature could eventually accelerate glacier and polar ice melting. This may flood low-lying regions and, in the long run, raise sea levels that may threaten coastal cities. Habitat loss could disturb ecosystems, the interacting living and non-living components of an environment.

Note: A sphere is part of an interconnected system. Ice melting, water movement and changes in living habitats illustrate connections, rather than isolated events.

What is solar radiation, and how is incoming solar energy measured?

Solar radiation is energy received from the Sun as electromagnetic waves, abbreviated EM waves. These travel through a vacuum, or space without a material medium. Unlike sound waves, they do not require a medium for travel.

Their speed in a vacuum is 3 × 10⁸ metres per second. The electromagnetic spectrum is the complete range of electromagnetic radiation. Frequency means the number of wave cycles per second; wavelength is the distance occupied by one complete wave cycle.

Which parts of the spectrum matter here?

About 99 per cent of the Sun’s energy falls within the ultraviolet (UV), visible and infrared (IR) ranges. UV has shorter wavelengths than visible light, while IR has longer wavelengths. These regions influence climate and support life.

Gamma rays and X-rays have high frequencies, short wavelengths and very high energy, and can harm life. They are mostly filtered by the upper atmosphere. Microwaves and radio waves carry very little energy to significantly warm Earth.

Short-wavelength UV is mostly absorbed by the ozone layer, the atmospheric region containing ozone that absorbs harmful UV radiation. Visible light powers photosynthesis, in which plants use sunlight, water and carbon dioxide to make glucose, a sugar, and release oxygen.

Photosynthesis is the primary food source for most organisms. Visible light also partly warms land and water. IR warms the surface, which re-radiates heat towards the atmosphere.

How do insolation and the solar constant differ?

Insolation is the amount of solar radiation reaching Earth’s surface. The solar constant is the average solar energy received per unit time and area, perpendicular to the Sun’s rays, at the top of the atmosphere.

The solar constant is approximately 1.4 kilowatts per square metre. Under clear skies, maximum surface insolation is about 1 kilowatt per square metre, because gases, clouds and dust absorb and scatter some incoming energy.

Worked example 1. Find the energy received by 1 square metre in one hour at an insolation of 1 kilowatt per square metre. One kilowatt equals 1,000 joules per second, and one hour equals 3,600 seconds.

Answer: E = intensity × area × time, where E is received energy in joules. Thus E = 1,000 × 1 × 3,600 = 3,600,000 joules, or 3.6 × 10⁶ joules.

Why does Earth’s surface warm unevenly?

Different surfaces absorb sunlight differently. Land heats faster than water. There may be some variation due to the material forming the land and the colour of the soil. Dark roads heat more quickly, while light-coloured surfaces reflect more sunlight and remain comparatively cooler.

Definition: Albedo is the fraction of incoming solar radiation reflected by a surface. High albedo means more reflection; low albedo means less reflection and more absorption.

Snow and ice reflect much incoming radiation, contributing to cold polar conditions. Black soil and ocean water have lower albedo, absorb more solar radiation and are relatively warmer.

How do location and buildings affect heating?

Latitude describes position north or south of the equator. Earth’s spherical shape makes sunlight strike different latitudes at different angles. Near the equator, radiation is concentrated over a smaller area; near the poles, it spreads across a larger area.

Equatorial regions therefore remain relatively warm throughout the year, while polar regions are much colder. Earth’s spherical shape and tilted rotation axis also produce seasons and changing daytime length during its yearly revolution around the Sun.

The urban heat island effect means cities are warmer than surrounding rural areas, especially in summer and at night. Steel, concrete, brick and asphalt absorb solar radiation, retain heat and re-radiate it. Greater air-conditioning demand further stresses urban ecosystems.

Rural vegetation and forests provide shade and cooling through transpiration, the release of water vapour by plants. Thus human land use can change local climate as well as surface heating.

Yellow arrows show incoming energy from the Sun, partial reflection by atmosphere and clouds, reflection by Earth’s surface, and partial absorption by atmosphere, clouds and the surface.

See Fig. 13.5 in your NCERT textbook

How does the atmosphere regulate temperature and protect life?

Earth’s gravity holds the atmosphere in place. Air consists mainly of nitrogen, 78 per cent, and oxygen, 21 per cent, with small amounts of argon, carbon dioxide, water vapour and other gases.

The troposphere is the lowest atmospheric layer, averaging about 12 kilometres in height. Nearly all weather occurs here. Heating from Earth’s surface causes warm air to rise and helps drive winds and storms.

Temperature in this layer decreases by approximately 6.5 degrees Celsius per kilometre of height. Its height is greatest above the equator and lowest over polar regions. These are approximate descriptions, rather than a uniform height everywhere.

How does the stratosphere differ?

The stratosphere extends approximately from 12 to 50 kilometres. Its ozone absorbs UV, so temperature increases with height. The lack of vertical air mixing keeps this layer calm and weather confined to the troposphere.

The mesosphere, thermosphere and exosphere are successively higher atmospheric layers. They play only a minor role in regulating surface climate. The region called outer space starts at about 100 kilometres above Earth.

What is the greenhouse effect?

The greenhouse effect is warming caused by atmospheric gases absorbing heat re-radiated from Earth’s surface. Carbon dioxide, written CO₂, methane, written CH₄, and water vapour are greenhouse gases because they absorb this outgoing heat.

1. Incoming solar radiation reaches the atmosphere, where some is absorbed by gases and clouds.
2. Earth’s surface absorbs sunlight and becomes warmer.
3. The warmed surface re-radiates energy in the infrared region.
4. Greenhouse gases absorb a portion of this outgoing heat, helping maintain temperatures suitable for life.

Without the atmosphere, Earth would be too cold for life to survive. Excess carbon dioxide enhances this effect and causes global warming, a rise in Earth’s temperature. If left unchecked, this could make Earth uninhabitable.

Human-made chlorofluorocarbons (CFCs), used in refrigerators and aerosols, caused severe ozone loss over Antarctica, called the ozone hole. The Montreal Protocol, a global agreement reducing CFC use, has helped the ozone layer begin slowly recovering.

How does uneven heating produce local and planetary winds?

Wind is air moving from a high-pressure region to a low-pressure region. Air pressure is the force exerted by air per unit area. Uneven solar heating is the main cause of these pressure differences.

How do mountain and valley breezes compare?

Density means mass per unit volume. The two breezes arise because slopes and valley floors heat and cool at different rates.

Cooler, denser air sinking down mountain slopes helps explain the night-time breeze. Such daily wind changes are commonly experienced in Shimla, Dehradun and other Himalayan valleys, influencing temperature, moisture, agriculture and daily life.

The daytime panel has orange arrows pointing uphill and the label “Warm air rising”. The night-time panel has pale arrows pointing downhill and the label “Cold air sinking”.

See Fig. 13.8 in your NCERT textbook

What produces planetary winds?

Planetary winds are large-scale air movements driven by pressure differences between belts around Earth. The equator is at zero degrees latitude, with northern and southern latitudes measured towards their respective poles.

1. Intense equatorial heating makes warm air rise, creating the equatorial low-pressure belt.
2. Air moves poleward at higher altitudes, cools and sinks around 30 degrees north and south, creating subtropical high-pressure belts.
3. Some surface air returns towards the equator. Some moves poleward and rises around 60 degrees, meeting cold polar air and forming subpolar low-pressure belts.
4. Near the poles, around 90 degrees north and south, cold dense air sinks. These polar high-pressure regions send air towards subpolar belts.

Earth’s rotation deflects winds to the right in the Northern Hemisphere, the half north of the equator, and to the left in the Southern Hemisphere. Planetary winds therefore follow curved paths.

What drives ocean currents, and why do they matter?

Ocean currents are continuous movements of large masses of ocean water. Strong planetary winds drag surface water through friction, setting surface currents in motion. Temperature, salinity or salt content, Earth’s rotation and the distribution of land also influence their movement.

Lower-salinity water is less dense and tends to remain near the surface. Higher-salinity, denser water sinks and moves through deeper ocean levels. Salinity and temperature therefore influence vertical differences in ocean circulation.

How is heat redistributed?

Warm equatorial waters travel towards the poles at the surface. Colder, denser water slowly flows back towards the equator through deeper levels. By carrying heat poleward, currents reduce temperature differences across Earth and regulate climate.

Earth’s rotation deflects moving water into large circular patterns called gyres. These rotate clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere. Continents block and redirect currents, further modifying their routes.

The North Atlantic Drift is a warm current extending from the Gulf Stream. The Gulf Stream carries warm water from the southern part of North America’s east coast across the Atlantic. Its extension flows towards northwestern Europe.

This warm current keeps many European ports ice-free during winter, even at high latitudes. Its moderating effect supports trade and commerce. Ocean currents also transport nutrients, helping support a massive ecosystem.

Scientists at the Indian Institute of Tropical Meteorology (IITM), Pune, model interactions between atmosphere, oceans, land and ice to simulate the Indian monsoon. Data from satellites, Indian Ocean buoys and Antarctic stations help improve seasonal forecasts and study possible rainfall changes under global warming.

How does the water cycle connect Earth’s spheres?

Biotic means living, while abiotic means non-living. Organisms exchange matter and energy with air, water, soil and rocks. Biogeochemical cycles connect these components and recycle essential nutrients, including carbon, nitrogen and oxygen, so they remain available to support life.

The water cycle circulates water between water bodies, air, land and living organisms. It also transports minerals dissolved from soil and rocks, supporting organisms on land and carrying nutrients towards the oceans.

1. Evaporation and transpiration: Water evaporates from rivers, lakes and oceans, while plants release water vapour into the air.
2. Condensation: Water vapour changes into liquid water and forms clouds.
3. Precipitation: Water returns from the atmosphere as rain, hail or snow.
4. Run-off and infiltration: Surface water flows towards water bodies, while some seeps through soil and rocks. This seepage is infiltration and replenishes groundwater.

A landscape cross-section shows clouds, mountains, surface water and underground water. Arrows are labelled evaporation, condensation, precipitation, transpiration and run off, with ground water labelled beneath the surface.

See Fig. 13.12 in your NCERT textbook

How does warming disturb the cycle?

A warmer atmosphere holds more moisture, causing heavier rains in some areas, including intensified monsoons, and drought elsewhere. Melting glaciers add water to rivers and raise sea levels in the long run, threatening coastal cities such as Mumbai and Chennai.

Sudden intense rainfall increases run-off and soil erosion, the removal of soil. Less infiltration reduces groundwater recharge. This makes agriculture harder to sustain, especially in dry months, because underground water reserves receive less replenishment.

These changes link glaciers in the cryosphere, rivers and oceans in the hydrosphere, atmospheric moisture, soil in the geosphere, and crops and fisheries in the biosphere. The water cycle is therefore also a connection between climate, land and life.

How does carbon move through fast and slow cycles?

The carbon cycle moves carbon among air, water, rocks and organisms. Carbon occurs in proteins, carbohydrates and fats, which are biological substances, and in genetic material, the material carrying hereditary information. Its stores include atmospheric carbon dioxide, organisms, carbonate rocks, fossil fuels, dissolved carbon dioxide and marine shells.

What happens over days to years?

In the fast carbon cycle, plants absorb atmospheric carbon dioxide during photosynthesis and make glucose. Animals obtain carbon by eating plants or other animals. Respiration, the life process that releases energy from food, returns carbon dioxide to the atmosphere.

When organisms die, decomposition, the breakdown of dead organic matter, returns carbon dioxide to the air. These processes link the food-making activity of plants with feeding, respiration and the breakdown of remains.

What happens over millions of years?

In the slow carbon cycle, buried dead plants and animals are converted into fossil fuels such as coal, oil and gas over millions of years. Burning these fuels releases their stored carbon as carbon dioxide on a very short timescale.

The atmosphere and oceans continuously exchange carbon dioxide. Ocean water absorbs it to form carbonate and bicarbonate ions, meaning electrically charged particles in solution, used by phytoplankton, photosynthetic plankton, for photosynthesis. Some marine organisms also use these forms to build shells.

Dead marine organisms sink to the ocean floor, storing their organic matter as carbon for a long period. A carbon sink is a store such as a forest or ocean that takes up carbon.

What does the atmospheric record show?

Burning fossil fuels and deforestation, meaning forest clearing, have raised atmospheric carbon dioxide by about 33.3 per cent since 1960, from 315 to 420 parts per million (ppm). This unit expresses concentration as parts of a substance per million parts of the mixture.

Excess carbon dioxide intensifies greenhouse warming, glacier and Arctic sea-ice melting, sea-level rise and extreme weather. In India, it may produce more intense monsoons and threaten agriculture through changing rainfall patterns.

The Keeling curve plots year from 1960 to 2025 horizontally and carbon dioxide concentration in ppm vertically. Its rising line has a sawtooth pattern, with seasonal dips associated with yearly Northern Hemisphere plant growth absorbing carbon dioxide.

See Fig. 13.14 in your NCERT textbook

How does the nitrogen cycle make atmospheric nitrogen usable?

Nitrogen is needed to make proteins and nucleic acids, biological molecules associated with genetic material. The atmosphere is its largest reservoir. However, nitrogen gas, written N₂, is rather non-reactive and cannot be used directly by plants and animals.

The nitrogen cycle moves nitrogen between air, soil, water and organisms. Nitrogen must first enter soluble compounds that organisms can absorb. Different bacteria perform different conversions, while feeding and decomposition move nitrogen through living organisms and back to soil.

1. Nitrogen fixation: Rhizobium bacteria in the root nodules of legumes and Azotobacter in soil convert atmospheric nitrogen into ammonia, written NH₃. Legumes are a group of flowering plants; their root nodules are swellings containing nitrogen-fixing bacteria.
2. Nitrification: Nitrosomonas bacteria convert ammonia into nitrite, written NO₂⁻. Nitrobacter then convert nitrite into nitrate, written NO₃⁻. Nitrite and nitrate are different nitrogen-containing ions; the superscript minus sign means each has a negative electrical charge.
3. Assimilation: Plants take up nitrogen compounds from soil. Animals obtain nitrogen by consuming plants or other animals.
4. Ammonification: Decomposers, organisms such as bacteria and fungi that break down dead matter and waste, return nitrogen compounds such as ammonia to soil.
5. Denitrification: Bacteria such as Pseudomonas convert some nitrates into nitrogen gas, returning it to the atmosphere and completing the cycle.

Fixation makes atmospheric nitrogen available in compounds, while denitrification returns some nitrogen to its gaseous form. Confusing these processes reverses their roles.

How else is nitrogen fixed?

Lightning fixes a tiny amount of atmospheric nitrogen into nitrogen oxides, compounds containing nitrogen and oxygen. Artificial fixation through the Haber-Bosch process makes ammonia from atmospheric nitrogen and produces most fertilisers used today.

This process revolutionised agriculture and enabled India’s Green Revolution, a major increase in agricultural production. However, it is energy intensive, using approximately 1 to 2 per cent of global energy. Overuse of fertilisers has degraded soil and water, showing why nitrogen supply must remain balanced.

Arrows connect atmospheric nitrogen, ammonia, nitrite and nitrate. Plants, an animal and decomposers appear alongside arrows labelled nitrogen fixation, nitrification, assimilation, ammonification and denitrification.

See Fig. 13.15 in your NCERT textbook

How is atmospheric oxygen consumed and restored?

The oxygen cycle circulates oxygen between the atmosphere, land, oceans and organisms. About 21 per cent of the atmosphere consists of free oxygen gas, written O₂. Oxygen also occurs within most biological molecules, including carbohydrates, proteins, nucleic acids and fats.

Oxygen is present in combined forms too. In Earth’s crust it occurs in minerals and metal oxides, compounds of metals with oxygen. In the atmosphere it also occurs within carbon dioxide. Free oxygen gas is therefore one form of Earth’s oxygen.

Which processes balance oxygen levels?

Plants and animals use oxygen during respiration and release carbon dioxide. Combustion, the burning of fuels, also consumes oxygen and releases carbon dioxide. Oxide formation is another process that uses atmospheric oxygen.

Photosynthesis restores oxygen: plants use sunlight, water and carbon dioxide to produce glucose and release oxygen. Atmospheric oxygen is restored mainly through this process. Consumption by respiration and combustion and production by photosynthesis together maintain circulation.

The carbon and oxygen cycles are closely linked through these processes. Photosynthesis takes in carbon dioxide and releases oxygen; respiration and combustion consume oxygen and release carbon dioxide. Their balance connects organisms with air, land and oceans.

Forest clearing reduces photosynthesis. It therefore disrupts both the uptake of carbon dioxide and the process that restores atmospheric oxygen. This is one reason why changes in the biosphere affect the composition of the atmosphere.

How do human activities disturb Earth’s natural processes?

Human actions can affect several spheres together. Burning fossil fuels and clearing forests increase atmospheric carbon dioxide and saturate natural carbon sinks. The resulting enhanced greenhouse warming disrupts the carbon cycle and contributes to extreme weather and loss of biodiversity, the variety of living organisms.

When habitats are damaged, many species lose their natural homes. Human impacts on carbon, water and soil therefore also become impacts on living communities.

What happens in oceans and freshwater?

Excess atmospheric carbon dioxide increases ocean absorption, making seawater more acidic. This could threaten tiny plankton and coral reefs and disrupt marine ecosystems. However, warmer ocean water reduces the ocean’s capacity to absorb carbon dioxide as an effective carbon sink.

Eutrophication occurs when excess nutrients promote widespread algal growth in water. Overused fertilisers add excessive nitrogen as nitrates to rivers and lakes. The resulting algal blooms, extensive growths of algae, deplete oxygen and kill fish, threatening water bodies and coastal fisheries.

How do forest clearing and emissions spread harm?

Deforestation decreases photosynthesis and transpiration, which can lead to declining local rainfall. It also alters albedo. Without tree roots holding soil together, erosion could increase. Over time, habitats could be destroyed, leading to biodiversity decline.

Vehicular emissions react with sunlight to produce ground-level smog, polluted air formed through these reactions. Ground-level ozone also forms and is harmful to health. This differs from protective ozone in the stratosphere, where it absorbs UV radiation.

Note: Preserve the distinction between atmospheric locations: stratospheric ozone protects life from UV, while ground-level ozone is harmful. Likewise, some carbon dioxide is necessary for warmth, but excess carbon dioxide disrupts the balance.

Which actions help maintain the balance of the Earth system?

Restoring environmental balance involves local action and global cooperation. Conserving energy and other resources reduces waste, while switching to renewable energy sources such as solar and wind can help reduce carbon released into the atmosphere.

Planting trees, saving water and practising sustainable farming can help restore balance. Sustainable farming means farming practices that conserve resources and limit environmental damage. These actions address connections among air, soil, water and living organisms.

What can individuals and countries contribute?

Individuals can save water, food and energy by reducing waste, reusing materials and recycling. These actions support conservation of resources across Earth’s spheres. Their importance follows from the fact that unsustainable consumption disturbs the interconnected system.

India has expanded solar and other renewable energy sources, planted billions of trees and promoted sustainable farming practices. Fossil fuels still generate a significant part of its electricity, so increasing renewable sources remains relevant to reducing carbon emissions.

Mission LiFE, meaning Lifestyle for Environment, is an India-led global initiative introduced at the United Nations Climate Change Conference in 2021. It encourages mindful, environmentally friendly lifestyles, including saving energy and conserving resources.

The Montreal Protocol has started ozone-layer recovery through international cooperation. The Kyoto Protocol and Paris Agreement, agreements under which countries were supposed to reduce carbon dioxide emissions, have been less successful. Cooperation and individual action both have roles in environmental protection.

Scientific measurements also support action. Atmospheric scientist Anna Mani mapped solar insolation across India in the 1950s. With S. Rangarajan, she published Solar Radiation Over India in 1982, creating India’s first insolation atlas and demonstrating its substantial solar energy potential.

Glossary

• Geosphere — Earth’s solid rocks, soil, landforms and interior, interacting with water, air and living organisms.
• Hydrosphere — Liquid water in oceans, rivers, lakes and underground stores beneath Earth’s surface.
• Cryosphere — Solid water on Earth, including ice, snow, glaciers and polar ice caps.
• Biosphere — All living organisms and their habitats, including forests, farms and marine communities.
• Insolation — The amount of radiation from the Sun that reaches Earth’s surface.
• Albedo — The fraction of incoming solar radiation that a surface reflects rather than absorbs.
• Troposphere — The lowest atmospheric layer, heated from Earth’s surface, where nearly all weather occurs.
• Greenhouse effect — Warming caused when atmospheric gases absorb heat re-radiated from Earth’s surface.
• Gyres — Large circular patterns of ocean-water movement formed as Earth’s rotation deflects moving water.
• Biogeochemical cycles — Cycles connecting living and non-living components and keeping essential nutrients available for life.
• Nitrogen fixation — Conversion of atmospheric nitrogen into compounds that make nitrogen available to living organisms.
• Nitrification — Bacterial conversion of ammonia into nitrite and then conversion of nitrite into nitrate.
• Ammonification — Decomposition of dead organisms and waste that returns nitrogen compounds such as ammonia to soil.
• Denitrification — Conversion of some nitrates back into nitrogen gas by bacteria such as Pseudomonas.
• Eutrophication — Excess nutrient enrichment causing widespread algal growth that depletes oxygen and kills fish.

Common errors and misconceptions

• Misconception: Earth’s spheres act independently. Correct: Energy, water and nutrients connect them, so a disturbance in one can affect others.
• Misconception: The solar constant is the radiation received at ground level. Correct: It is measured at the top of the atmosphere, before atmospheric absorption, scattering or reflection.
• Misconception: High albedo causes strong absorption. Correct: High-albedo surfaces reflect more sunlight and remain cooler.
• Misconception: Temperature decreases upwards through every atmospheric layer. Correct: It decreases in the troposphere but increases in the stratosphere because ozone absorbs UV.
• Misconception: Mountain breezes blow uphill during daytime. Correct: Valley breezes blow uphill during the day; mountain breezes blow downhill after sunset.
• Misconception: Plants directly use atmospheric nitrogen gas. Correct: Nitrogen must first be converted into soluble compounds that organisms can absorb.
• Misconception: Ozone is beneficial wherever it occurs. Correct: Stratospheric ozone protects against UV, whereas ground-level ozone harms health.
• Misconception: All carbon dioxide is undesirable. Correct: Some is necessary for suitable warmth and photosynthesis; excessive amounts intensify greenhouse warming.

Exam-style questions with model answers

Q1. Define albedo and state how a high-albedo surface responds to sunlight. [2 marks]

1. Albedo is the fraction of incoming solar radiation reflected by a surface.
2. A high-albedo surface reflects more sunlight and stays cooler because it absorbs less radiation.

Q2. A surface of area 1 square metre receives constant insolation of 1 kilowatt per square metre for one hour. Given 1 kilowatt = 1,000 joules per second and 1 hour = 3,600 seconds, calculate the received energy in three steps. [3 marks]

1. Use E = intensity × area × time, where E means received solar energy in joules. The receiving area is 1 square metre.
2. Convert the given intensity to 1,000 joules per second per square metre and the exposure time to 3,600 seconds.
3. Substitution gives E = 1,000 × 1 × 3,600 = 3,600,000 joules. Thus the surface receives 3.6 × 10⁶ joules of solar energy.

Q3. Explain the formation of a valley breeze during the day and a mountain breeze after sunset, giving two steps for each. [4 marks]

1. During the day, sun-facing mountain slopes heat more rapidly than the valley floor. Air over the slopes becomes warmer and rises, creating low pressure.
2. Cooler air from the valley moves uphill to replace the rising air. This uphill movement is called a valley breeze.
3. After sunset, mountain slopes lose heat faster than the valley floor, which remains relatively warmer. Air over the slopes becomes cooler and denser.
4. This cooler, denser air flows down into the valley. The resulting downhill movement is called a mountain breeze.

Q4. Explain nitrogen fixation, nitrification, assimilation, ammonification and denitrification, naming the bacterial examples associated with fixation, nitrification and denitrification. [5 marks]

1. Nitrogen fixation converts atmospheric nitrogen into ammonia. Rhizobium in legume root nodules and Azotobacter in soil perform this conversion, making atmospheric nitrogen available in a compound.
2. Nitrification occurs in two conversions: Nitrosomonas changes ammonia into nitrite, and Nitrobacter changes nitrite into nitrate. These are distinct bacterial roles.
3. Assimilation involves plants taking up nitrogen compounds from soil. Animals obtain nitrogen by consuming plants or other animals, transferring it through feeding.
4. Ammonification occurs when decomposers such as bacteria and fungi break down dead organisms and waste, returning nitrogen compounds such as ammonia to soil.
5. Denitrification is the conversion of some nitrates back into nitrogen gas by bacteria such as Pseudomonas. This returns nitrogen to the atmosphere and completes the cycle.

Q5. Explain three ways in which climate change affects the water cycle and its consequences for agriculture or coastal settlements. [3 marks]

1. A warmer atmosphere holds more moisture, producing heavier rain in some areas, including intensified monsoons, while drought occurs elsewhere.
2. Melting glaciers add water to rivers and raise sea levels in the long run, threatening coastal cities such as Mumbai and Chennai.
3. Sudden intense rainfall increases run-off and soil erosion. Reduced infiltration means less groundwater recharge, making agriculture harder to sustain during dry months.

Q6. Describe the fast and slow carbon cycles and ocean carbon exchange in five points, beginning with atmospheric carbon dioxide entering plants. [5 marks]

1. During photosynthesis, plants use sunlight to convert atmospheric carbon dioxide into glucose. This forms part of the fast cycle, operating over days to years.
2. Animals obtain carbon by eating plants or other animals. Respiration releases carbon dioxide, while decomposition after organisms die also returns carbon dioxide to the air.
3. In the slow cycle, buried dead plants and animals become fossil fuels such as coal, oil and gas over millions of years.
4. Burning fossil fuels releases their stored carbon as carbon dioxide on a very short timescale, returning carbon from long-term stores to the atmosphere.
5. Oceans exchange carbon dioxide with the atmosphere. Dissolved carbon supports photosynthesis and shell formation, while dead organisms sinking to the ocean floor store carbon for long periods.

Q7. Distinguish between the roles of stratospheric ozone and ground-level ozone. [2 marks]

1. Stratospheric ozone absorbs harmful ultraviolet radiation from the Sun, providing a protective shield for life.
2. Ground-level ozone forms with pollution associated with vehicular emissions and sunlight, and is harmful to health.

Q8. Explain four effects of deforestation on photosynthesis, local rainfall, surface conditions and biodiversity. [4 marks]

1. Forest clearing reduces photosynthesis, affecting carbon dioxide uptake and the process that restores atmospheric oxygen.
2. Reduced transpiration can lead to a decline in local rainfall, linking vegetation loss with changes in atmospheric moisture.
3. Deforestation alters surface albedo. Without tree roots holding soil together, soil erosion could increase.
4. Over time, habitats could be destroyed, leading to a decline in biodiversity as many species lose the natural homes on which they depend.

Key takeaways

• Earth’s geosphere, hydrosphere, cryosphere, atmosphere and biosphere interact through movements of energy, water and nutrients.
• Solar radiation is the main energy source, while surface properties, latitude and Earth’s tilt produce uneven heating.
• High-albedo surfaces reflect more sunlight; greenhouse gases absorb part of the heat re-radiated by Earth’s surface.
• Uneven heating creates pressure differences that drive winds, while ocean currents distribute heat and transport nutrients.
• The water, carbon, nitrogen and oxygen cycles connect living organisms with air, water, soil and rocks.
• Nitrogen fixation and denitrification have different roles: one forms usable compounds, while the other returns nitrogen gas.
• Fossil-fuel burning, forest clearing and fertiliser overuse disturb connected processes across several Earth spheres.
• Resource conservation, renewable energy, tree planting, sustainable farming and international cooperation can help maintain environmental balance.

Test yourself

Which sphere contains glaciers, and which contains liquid rivers?

Glaciers belong to the cryosphere, while liquid river water belongs to the hydrosphere.

Why is surface insolation lower than the solar constant?

Gases, clouds and dust absorb and scatter some incoming solar radiation before it reaches the surface.

Where does nearly all weather occur, and how is that layer heated?

Nearly all weather occurs in the troposphere, which receives heat from Earth’s surface.

How does Earth’s rotation deflect planetary winds?

It deflects winds towards the right in the Northern Hemisphere and towards the left in the Southern Hemisphere.

What do Nitrosomonas and Nitrobacter do?

Nitrosomonas converts ammonia into nitrite, while Nitrobacter converts nitrite into nitrate during nitrification.

Which processes consume oxygen, and which mainly restores it?

Respiration, combustion and oxide formation consume atmospheric oxygen. Photosynthesis is the main process that restores it.

How can excess fertiliser harm fish?

Excess nitrates entering rivers and lakes encourage algal blooms that deplete oxygen and kill fish.

What does the sawtooth pattern of the Keeling curve represent?

The seasonal dips reflect yearly plant growth in the Northern Hemisphere absorbing atmospheric carbon dioxide.