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

Earth functions as one interconnected system where energy and matter continuously move between different parts of the planet.

The Sun is the main source of energy for the Earth. This energy drives many natural processes such as:

• Winds
• Ocean currents
• Water cycle
• Climate and weather
• Photosynthesis
• Biogeochemical cycles

Apart from solar energy, the Earth’s hot interior and chemical reactions in air, water and rocks also influence the movement of energy and matter.

The Earth system is made up of five important spheres:

• Geosphere: Solid rocks, soil, landforms and Earth’s interior.
• Hydrosphere: Liquid water in oceans, rivers, lakes and groundwater.
• Cryosphere: Frozen water such as glaciers, snow and polar ice.
• Atmosphere: Layer of air surrounding the Earth.
• Biosphere: All living organisms and their habitats.

These spheres are interconnected. A change in one sphere affects the others.

Example: If snowfall decreases in the mountains, less snow melts in summer. This can reduce water in rivers and lakes, affecting plants, animals and human life.

The Earth does not receive solar energy equally everywhere. This is called uneven heating of the Earth.

The main reasons for uneven heating are:

• Shape of the Earth
• Latitude
• Tilt of Earth’s axis
• Different nature of land and water
• Role of atmosphere
• Albedo of surfaces

Solar radiation reaches the Earth in the form of electromagnetic waves. The important parts of solar radiation are:

• Ultraviolet rays
• Visible light
• Infrared radiation

Visible light helps plants in photosynthesis. Infrared radiation helps in warming the Earth’s surface. Ultraviolet radiation can be harmful, but most of it is absorbed by the ozone layer.

Insolation

The amount of solar radiation received by the Earth’s surface is called insolation.

• The average solar energy received at the top of the atmosphere is called the solar constant.
• Its value is about 1.4 kW m⁻².
• Under clear sky conditions, the maximum insolation reaching the Earth’s surface is about 1 kW m⁻².

India receives abundant sunlight because of its location in tropical and sub-tropical regions. Therefore, India has great potential for solar energy.

Different surfaces absorb and reflect solar radiation differently.

• Dark surfaces absorb more sunlight and become hotter.
• Light-coloured surfaces reflect more sunlight and remain cooler.
• Land heats up faster than water.
• Water heats and cools slowly.

The fraction of solar radiation reflected by a surface is called albedo.

Albedo

• High albedo means more reflection and less heating.
• Low albedo means less reflection and more absorption.

Examples:

• Snow and ice have high albedo, so they reflect more sunlight.
• Black soil and ocean water have low albedo, so they absorb more sunlight.

This is one reason why polar regions remain cold and dark surfaces become hot quickly.

Threads of Curiosity: Urban Heat Island Effect

The urban heat island effect means that cities are often warmer than nearby rural areas.

This happens because cities have:

• Concrete buildings
• Asphalt roads
• Steel and brick structures
• Less vegetation

These materials absorb and retain heat during the day and release it at night.

Rural areas remain cooler because they have:

• More trees
• More open land
• More plant transpiration
• More shade

The urban heat island effect shows how human land use can change local climate.

The Earth is spherical. Therefore, the Sun’s rays do not fall equally everywhere.

• Near the equator, Sun’s rays fall more directly and are concentrated over a smaller area.
• Near the poles, Sun’s rays fall at a slant and spread over a larger area.

So:

• Equatorial regions are warmer.
• Polar regions are colder.

The uneven distribution of solar energy causes:

• Temperature differences
• Pressure differences
• Winds
• Ocean currents
• Climatic variations

The tilt of the Earth’s axis also causes seasons and changes in the length of day and night.

The atmosphere is the layer of air surrounding the Earth. It is held by Earth’s gravity.

The atmosphere mainly contains:

• Nitrogen — 78%
• Oxygen — 21%
• Small amounts of argon, carbon dioxide, water vapour and other gases

The atmosphere has several layers, but two important layers are:

1. Troposphere

• Extends from 0 to about 12 km
• Most weather events occur here
• Temperature decreases with height
• Clouds, rain, storms and winds occur in this layer

2. Stratosphere

• Extends from 12 km to 50 km
• Contains the ozone layer
• Ozone absorbs harmful ultraviolet rays
• Temperature increases with height in this layer

Importance of the Atmosphere

The atmosphere protects life on Earth in two main ways:

• It absorbs harmful solar radiation, especially ultraviolet rays.
• It traps some outgoing heat, keeping Earth warm enough for life.

The Earth’s surface absorbs sunlight and re-radiates heat as infrared radiation. Greenhouse gases such as carbon dioxide, methane and water vapour absorb some of this heat and keep the planet warm.

This is called the greenhouse effect.

However, too much carbon dioxide from human activities increases the greenhouse effect and causes global warming.

Threads of Curiosity: Why is the Ozone Layer So Important?

The ozone layer is important because it absorbs harmful ultraviolet radiation from the Sun.

If too much UV radiation reaches the Earth, it can:

• Damage skin
• Damage eyes
• Increase the risk of cancer
• Harm plants and animals
• Disturb ecosystems

Human-made chemicals called chlorofluorocarbons or CFCs damaged the ozone layer and caused the ozone hole over Antarctica.

The Montreal Protocol helped reduce the use of CFCs, and now the ozone layer is slowly recovering.

Uneven heating of the Earth creates temperature and pressure differences.

Wind is the movement of air from a region of high pressure to a region of low pressure.

Uneven heating causes:

• Local winds
• Planetary winds
• Ocean currents

Local winds are winds that blow over a small area due to local differences in heating.

Examples include:

• Sea breeze
• Land breeze
• Valley breeze
• Mountain breeze

Valley Breeze

During the day:

• Mountain slopes heat up faster.
• Warm air over slopes rises.
• Cool air from the valley moves up the slopes.

This upward movement of air is called valley breeze.

Mountain Breeze

At night:

• Mountain slopes cool faster.
• Cool and dense air flows down into the valley.

This downward movement of air is called mountain breeze.

These breezes are common in hilly regions such as Shimla, Dehradun and Himalayan valleys.

They affect:

• Local weather
• Agriculture
• Soil moisture
• Crop growth

Planetary winds are large-scale winds that blow across the Earth due to pressure belts.

Uneven heating between the equator and the poles creates:

• Equatorial low pressure belt
• Sub-tropical high pressure belts
• Sub-polar low pressure belts
• Polar high pressure belts

Near the equator, air becomes warm and rises. This creates a low pressure area. The air moves towards the poles at higher altitudes and sinks around 30° North and South latitudes, creating high pressure belts.

Some air moves back towards the equator, while some moves towards the poles.

The Earth’s rotation deflects winds:

• To the right in the Northern Hemisphere
• To the left in the Southern Hemisphere

This deflection causes winds to follow curved paths.

Ocean currents are the continuous movement of large masses of ocean water.

They are caused by:

• Planetary winds
• Temperature differences
• Salinity differences
• Rotation of the Earth
• Shape and position of continents

Warm water from the equator moves towards the poles. Cold and dense water from polar regions moves towards the equator at deeper levels.

Ocean currents form large circular patterns called gyres.

• Gyres rotate clockwise in the Northern Hemisphere.
• Gyres rotate anticlockwise in the Southern Hemisphere.

Importance of Ocean Currents

Ocean currents are important because they:

• Transfer heat from equator to poles
• Reduce temperature differences on Earth
• Influence climate
• Support marine life
• Transport nutrients
• Help in trade and navigation

Example: The North Atlantic Drift, an extension of the Gulf Stream, carries warm water towards northwestern Europe. This keeps many ports ice-free even in winter.

Living organisms continuously exchange matter and energy with air, water, soil and rocks.

The cyclic movement of matter and energy between living and non-living components is called a biogeochemical cycle.

These cycles help recycle essential nutrients such as:

• Water
• Carbon
• Nitrogen
• Oxygen

Biogeochemical cycles maintain the balance of ecosystems and make nutrients available to living organisms.

The water cycle is the continuous movement of water between the atmosphere, land, oceans and living organisms.

Important processes in the water cycle are:

• Evaporation
• Transpiration
• Condensation
• Precipitation
• Run off
• Infiltration
• Groundwater recharge

Steps of Water Cycle

• Water evaporates from oceans, rivers and lakes.
• Plants release water vapour through transpiration.
• Water vapour condenses to form clouds.
• Water returns to Earth as rain, snow or hail.
• Some water flows into rivers and oceans as run off.
• Some water seeps into the ground and becomes groundwater.

Water also dissolves minerals from rocks and soil and carries nutrients to rivers and oceans.

Effect of Climate Change on Water Cycle

Climate change affects the water cycle in many ways:

• A warmer atmosphere holds more moisture.
• Some areas may receive heavier rainfall.
• Some areas may face droughts.
• Glaciers melt faster.
• Sea level rises.
• Intense rainfall causes soil erosion.
• Less infiltration reduces groundwater recharge.

This affects agriculture, drinking water, fisheries and coastal cities such as Mumbai and Chennai.

Carbon is very important for life. It is present in:

• Proteins
• Carbohydrates
• Fats
• DNA
• Fossil fuels
• Carbon dioxide
• Carbonate rocks
• Marine shells

The carbon cycle is the movement of carbon between atmosphere, biosphere, geosphere and hydrosphere.

Fast Carbon Cycle

This happens over days to years.

• Plants absorb CO₂ from air during photosynthesis.
• Plants make glucose.
• Animals get carbon by eating plants or other animals.
• Plants and animals release CO₂ during respiration.
• Dead organisms decompose and release carbon back into the environment.

Slow Carbon Cycle

This happens over millions of years.

• Dead plants and animals get buried.
• Over time, they form fossil fuels such as coal, petroleum and natural gas.
• Burning fossil fuels releases carbon dioxide into the atmosphere.

Oceans also absorb carbon dioxide from the atmosphere. Marine organisms use carbon compounds to make shells. Some carbon gets stored on the ocean floor for long periods.

Human Impact on Carbon Cycle

Human activities have increased carbon dioxide in the atmosphere.

Major causes are:

• Burning fossil fuels
• Deforestation
• Industrial activities
• Transport emissions

Excess carbon dioxide causes:

• Enhanced greenhouse effect
• Global warming
• Melting of glaciers
• Rise in sea level
• Extreme weather
• Changing rainfall patterns
• Threats to agriculture

Plants need carbon dioxide for photosynthesis, but excess CO₂ is harmful because it disturbs Earth’s climate balance.

Nitrogen is essential for making:

• Proteins
• Nucleic acids
• DNA
• RNA

The atmosphere contains a large amount of nitrogen gas, but plants and animals cannot use atmospheric nitrogen directly.

Nitrogen must be converted into usable compounds.

The main steps of the nitrogen cycle are:

• Nitrogen fixation
• Nitrification
• Assimilation
• Ammonification
• Denitrification

1. Nitrogen Fixation

Nitrogen-fixing bacteria convert atmospheric nitrogen into ammonia.

Examples:

• Rhizobium in root nodules of leguminous plants
• Azotobacter in soil

Lightning also fixes a small amount of atmospheric nitrogen.

2. Nitrification

Nitrifying bacteria convert ammonia into nitrites and then nitrates.

• Nitrosomonas converts ammonia into nitrite.
• Nitrobacter converts nitrite into nitrate.

Plants absorb nitrates from the soil.

3. Assimilation

Plants take nitrogen compounds from the soil and use them to make proteins and other compounds.

Animals get nitrogen by eating plants or other animals.

4. Ammonification

When plants and animals die or produce waste, decomposers such as bacteria and fungi break down organic matter and release ammonia into the soil.

5. Denitrification

Denitrifying bacteria such as Pseudomonas convert nitrates back into nitrogen gas.

This nitrogen returns to the atmosphere and completes the cycle.

Oxygen is essential for life.

About 21% of the atmosphere consists of oxygen gas.

Oxygen is used in:

• Respiration
• Combustion
• Formation of oxides
• Biological molecules

Oxygen Cycle

• Plants release oxygen during photosynthesis.
• Animals and plants use oxygen for respiration.
• Combustion of fuels uses oxygen and releases carbon dioxide.
• Plants again use carbon dioxide and release oxygen.

Thus, oxygen keeps cycling between atmosphere, land, oceans and living organisms.

The balance between photosynthesis and respiration/combustion helps maintain oxygen levels in the atmosphere.

Human activities disturb Earth’s natural systems and biogeochemical cycles.

Major human activities that affect Earth’s processes include:

• Burning fossil fuels
• Deforestation
• Overuse of fertilisers
• Industrial pollution
• Vehicular emissions
• Unsustainable consumption
• Excessive waste generation

Impact of Excess CO₂

Excess carbon dioxide:

• Increases global warming
• Melts glaciers and polar ice
• Raises sea level
• Causes extreme weather
• Makes oceans more acidic
• Harms plankton and coral reefs
• Disturbs marine ecosystems

Warmer ocean water also reduces the ocean’s ability to absorb CO₂.

Impact of Overuse of Fertilisers

Excess fertilisers add too many nitrates to rivers and lakes.

This causes rapid growth of algae called algal bloom.

The process is called eutrophication.

Eutrophication causes:

• Decrease in oxygen in water
• Death of fish
• Damage to aquatic life
• Pollution of water bodies
• Harm to coastal fisheries

Impact of Deforestation

Deforestation affects many Earth systems.

It causes:

• Less photosynthesis
• More carbon dioxide in atmosphere
• Less oxygen production
• Less transpiration
• Reduced local rainfall
• Increased soil erosion
• Habitat destruction
• Loss of biodiversity
• Disturbance in water cycle
• Change in surface albedo

Forests act as important carbon sinks. Cutting forests reduces the Earth’s ability to absorb carbon dioxide.

Impact of Vehicular Emissions

Vehicular emissions react with sunlight and form smog.

They also form ground-level ozone.

Ground-level ozone is harmful to health, while ozone in the stratosphere is useful because it protects us from UV radiation.

Ways to Reduce Human Impact

We can help restore Earth’s balance by:

• Saving energy
• Using renewable energy such as solar and wind energy
• Planting trees
• Saving water
• Reducing waste
• Reusing and recycling materials
• Practising sustainable farming
• Reducing fossil fuel use
• Using public transport
• Conserving food and natural resources

Threads of Curiosity: Mission LiFE

Mission LiFE means Lifestyle for Environment.

It is an India-led global initiative introduced at the United Nations Climate Change Conference in 2021.

It encourages people to adopt eco-friendly lifestyles.

Mission LiFE promotes habits such as:

• Saving energy
• Saving water
• Reducing waste
• Conserving resources
• Using materials responsibly
• Living in harmony with nature

It teaches that small actions by individuals and communities can help build a sustainable future.

At a Glance — Summary

• Earth is an interconnected system made up of the geosphere, hydrosphere, cryosphere, atmosphere and biosphere.
• The Sun is the primary source of energy for the Earth.
• Solar radiation reaches Earth mainly as ultraviolet, visible and infrared radiation.
• Uneven heating of the Earth occurs due to Earth’s shape, latitude, tilt and different surface properties.
• Albedo is the fraction of solar radiation reflected by a surface.
• High albedo surfaces like snow reflect more sunlight and stay cooler.
• Low albedo surfaces like black soil and ocean water absorb more sunlight and become warmer.
• The atmosphere protects life by absorbing harmful radiation and trapping heat.
• Most weather events occur in the troposphere.
• The ozone layer in the stratosphere absorbs harmful UV radiation.
• Uneven heating creates pressure differences, which cause winds.
• Local winds include mountain breeze and valley breeze.
• Planetary winds are caused by large-scale pressure belts.
• Ocean currents move large masses of ocean water and help regulate Earth’s climate.
• Biogeochemical cycles recycle matter between living and non-living components.
• The main cycles studied are the water cycle, carbon cycle, nitrogen cycle and oxygen cycle.
• The water cycle connects atmosphere, land, oceans, glaciers and living organisms.
• The carbon cycle moves carbon between air, organisms, rocks, fossil fuels and oceans.
• The nitrogen cycle converts atmospheric nitrogen into usable forms for plants and animals.
• The oxygen cycle maintains oxygen balance through photosynthesis, respiration and combustion.
• Human activities such as burning fossil fuels, deforestation and overuse of fertilisers disturb Earth’s natural cycles.
• Excess carbon dioxide causes global warming, climate change and ocean acidification.
• Overuse of fertilisers causes eutrophication in water bodies.
• Sustainable actions like saving energy, planting trees, using renewable energy and reducing waste can help maintain Earth’s balance.

• Earth System: The interconnected system of Earth’s spheres.
• Geosphere: Solid part of Earth including rocks, soil and landforms.
• Hydrosphere: All liquid water on Earth.
• Cryosphere: Frozen water such as glaciers and snow.
• Atmosphere: Air surrounding the Earth.
• Biosphere: All living organisms and their habitats.
• Solar Radiation: Energy received from the Sun.
• Insolation: Solar radiation received by Earth’s surface.
• Albedo: Fraction of sunlight reflected by a surface.
• Greenhouse Effect: Trapping of heat by greenhouse gases.
• Ozone Layer: Protective layer that absorbs UV radiation.
• Wind: Movement of air from high pressure to low pressure.
• Ocean Current: Continuous movement of ocean water.
• Biogeochemical Cycle: Cyclic movement of matter between living and non-living components.
• Eutrophication: Excessive algal growth due to nutrient pollution.
• Deforestation: Large-scale cutting of forests.
• Mission LiFE: Lifestyle for Environment initiative to promote eco-friendly living.

Important Keywords and Their Definitions

1. Earth System

Earth System is the complete system of the Earth in which different spheres such as geosphere, hydrosphere, cryosphere, atmosphere and biosphere interact with one another.

2. Geosphere

Geosphere refers to the solid part of the Earth, including rocks, soil, landforms, mountains, plateaus, deserts and the Earth’s interior.

3. Hydrosphere

Hydrosphere includes all the liquid water present on the Earth, such as oceans, rivers, lakes, ponds and groundwater.

4. Cryosphere

Cryosphere refers to the solid form of water on Earth, such as glaciers, snow, ice caps and polar ice.

5. Atmosphere

Atmosphere is the layer of air surrounding the Earth. It contains gases like nitrogen, oxygen, carbon dioxide, water vapour and other gases.

6. Biosphere

Biosphere includes all living organisms and their habitats on the Earth, such as forests, grasslands, oceans, farms and wetlands.

7. Solar Radiation

Solar radiation is the energy received from the Sun in the form of electromagnetic waves. It is the main source of energy for the Earth.

8. Electromagnetic Waves

Electromagnetic waves are waves that can travel through a vacuum. Solar radiation reaches the Earth as electromagnetic waves.

9. Electromagnetic Spectrum

Electromagnetic spectrum is the complete range of electromagnetic radiation, including gamma rays, X-rays, ultraviolet rays, visible light, infrared rays, microwaves and radio waves.

10. Ultraviolet Radiation

Ultraviolet radiation is a high-energy part of solar radiation. It can harm the skin and eyes, but most harmful UV radiation is absorbed by the ozone layer.

11. Visible Light

Visible light is the part of solar radiation that can be seen by human eyes. It provides energy for photosynthesis in plants.

12. Infrared Radiation

Infrared radiation is the part of solar radiation that mainly helps in warming the Earth’s surface.

13. Insolation

Insolation is the amount of solar radiation received by the Earth’s surface.

14. Solar Constant

Solar constant is the average amount of solar energy received per unit time per unit area at the top of the Earth’s atmosphere. Its value is about 1.4 kW m⁻².

15. Albedo

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

• High albedo surfaces reflect more sunlight and remain cooler.
• Low albedo surfaces absorb more sunlight and become warmer.

16. Uneven Heating of the Earth

Uneven heating of the Earth means that different parts of the Earth receive and absorb different amounts of solar energy.

It occurs due to:

• Latitude
• Shape of the Earth
• Tilt of Earth’s axis
• Different surfaces like land, water, ice and soil

17. Urban Heat Island Effect

Urban heat island effect is the condition in which cities become warmer than surrounding rural areas because buildings, roads and concrete surfaces absorb and retain more heat.

18. Latitude

Latitude is the angular distance of a place north or south of the equator. It affects the amount of solar radiation received by a place.

19. Troposphere

Troposphere is the lowest layer of the atmosphere. Most weather events such as clouds, rain, storms and winds occur in this layer.

20. Stratosphere

Stratosphere is the atmospheric layer above the troposphere. It contains the ozone layer, which absorbs harmful ultraviolet radiation.

21. Ozone Layer

Ozone layer is a layer in the stratosphere that absorbs harmful ultraviolet rays from the Sun and protects life on Earth.

22. Greenhouse Gases

Greenhouse gases are gases that trap heat in the atmosphere. Examples include carbon dioxide, methane and water vapour.

23. Greenhouse Effect

Greenhouse effect is the process by which greenhouse gases trap outgoing heat from the Earth and keep the planet warm enough for life.

24. Global Warming

Global warming is the rise in Earth’s average temperature due to increased greenhouse gases, mainly from human activities.

25. Wind

Wind is the movement of air from a region of high pressure to a region of low pressure.

26. Local Winds

Local winds are winds that blow over a small area due to local differences in heating and pressure.

Examples:

• Sea breeze
• Land breeze
• Valley breeze
• Mountain breeze

27. Valley Breeze

Valley breeze is the wind that blows from the valley up the mountain slopes during the day because the slopes heat up faster.

28. Mountain Breeze

Mountain breeze is the wind that blows down from mountain slopes into the valley at night because the slopes cool faster.

29. Planetary Winds

Planetary winds are large-scale winds that blow across the Earth due to global pressure belts formed by uneven heating of the Earth.

30. Pressure Belt

Pressure belts are large zones of high or low air pressure on the Earth caused by unequal heating.

31. Ocean Currents

Ocean currents are the continuous movement of large masses of ocean water.

They are caused by:

• Planetary winds
• Temperature differences
• Salinity differences
• Earth’s rotation
• Distribution of landmasses

32. Gyres

Gyres are large circular patterns of ocean currents formed due to the Earth’s rotation and movement of ocean water.

33. Salinity

Salinity is the amount of salt dissolved in ocean water. It affects the density and movement of ocean water.

34. North Atlantic Drift

North Atlantic Drift is a warm ocean current that carries warm water towards northwestern Europe and helps keep many ports ice-free in winter.

35. Biotic Components

Biotic components are the living parts of the environment, such as plants, animals, bacteria, fungi and humans.

36. Abiotic Components

Abiotic components are the non-living parts of the environment, such as air, water, soil, rocks, sunlight and minerals.

37. Biogeochemical Cycle

Biogeochemical cycle is the cyclic movement of matter and energy between living organisms and non-living components of the Earth.

38. Water Cycle

Water cycle is the continuous movement of water between the atmosphere, land, oceans and living organisms.

39. Evaporation

Evaporation is the process by which water changes into water vapour due to heat.

40. Transpiration

Transpiration is the process by which plants release water vapour into the atmosphere through their leaves.

41. Condensation

Condensation is the process by which water vapour cools and changes into tiny water droplets to form clouds.

42. Precipitation

Precipitation is the falling of water from clouds to the Earth in the form of rain, snow, hail or sleet.

43. Run off

Run off is the flow of water over the land surface into rivers, lakes and oceans.

44. Infiltration

Infiltration is the process by which water seeps into the soil and rocks.

45. Groundwater

Groundwater is the water stored below the Earth’s surface in soil and rocks.

46. Carbon Cycle

Carbon cycle is the movement of carbon between the atmosphere, living organisms, oceans, rocks and fossil fuels.

47. Photosynthesis

Photosynthesis is the process by which green plants use sunlight, carbon dioxide and water to make food and release oxygen.

48. Respiration

Respiration is the process by which living organisms use oxygen to break down food and release energy, carbon dioxide and water.

49. Decomposition

Decomposition is the breakdown of dead plants and animals by decomposers such as bacteria and fungi.

50. Fossil Fuels

Fossil fuels are fuels formed from the remains of dead plants and animals over millions of years.

Examples:

• Coal
• Petroleum
• Natural gas

51. Carbon Sink

Carbon sink is a natural system that absorbs and stores carbon dioxide from the atmosphere.

Examples:

• Forests
• Oceans

52. Nitrogen Cycle

Nitrogen cycle is the movement of nitrogen between the atmosphere, soil, water and living organisms.

53. Nitrogen Fixation

Nitrogen fixation is the process by which atmospheric nitrogen is converted into usable nitrogen compounds such as ammonia.

54. Rhizobium

Rhizobium is a nitrogen-fixing bacterium found in the root nodules of leguminous plants.

55. Azotobacter

Azotobacter is a nitrogen-fixing bacterium found freely in the soil.

56. Nitrification

Nitrification is the process by which ammonia is converted into nitrites and then into nitrates by nitrifying bacteria.

57. Nitrosomonas

Nitrosomonas is a nitrifying bacterium that converts ammonia into nitrite.

58. Nitrobacter

Nitrobacter is a nitrifying bacterium that converts nitrite into nitrate.

59. Assimilation

Assimilation is the process by which plants absorb nitrogen compounds from the soil and use them to make proteins and other substances.

60. Ammonification

Ammonification is the process by which decomposers convert dead organic matter and waste into ammonia.

61. Denitrification

Denitrification is the process by which nitrates are converted back into nitrogen gas by denitrifying bacteria.

62. Pseudomonas

Pseudomonas is a denitrifying bacterium that converts nitrates into nitrogen gas.

63. Oxygen Cycle

Oxygen cycle is the movement of oxygen between the atmosphere, living organisms, land and oceans.

64. Combustion

Combustion is the burning of a substance in the presence of oxygen, producing heat, light and carbon dioxide.

65. Eutrophication

Eutrophication is the process in which excess nutrients, especially nitrates from fertilisers, enter water bodies and cause rapid algal growth.

66. Algal Bloom

Algal bloom is the rapid growth of algae in water bodies due to excess nutrients. It reduces oxygen in water and can kill fish.

67. Deforestation

Deforestation is the large-scale cutting or clearing of forests.

It causes:

• Increase in carbon dioxide
• Less oxygen production
• Soil erosion
• Habitat loss
• Reduced rainfall
• Loss of biodiversity

68. Biodiversity

Biodiversity means the variety of living organisms present in an area, including plants, animals and microorganisms.

69. Smog

Smog is polluted air formed when smoke, gases and pollutants react with sunlight. It is harmful to health.

70. Ground-level Ozone

Ground-level ozone is ozone formed near the Earth’s surface due to pollution. It is harmful to human health and plants.

71. Montreal Protocol

Montreal Protocol is an international agreement made to reduce the use of ozone-depleting substances such as CFCs.

72. Kyoto Protocol

Kyoto Protocol is an international agreement aimed at reducing greenhouse gas emissions.

73. Paris Agreement

Paris Agreement is a global agreement to reduce greenhouse gas emissions and limit climate change.

74. Renewable Energy

Renewable energy is energy obtained from natural sources that can be replenished.

Examples:

• Solar energy
• Wind energy
• Hydropower

75. Mission LiFE

Mission LiFE stands for Lifestyle for Environment. It is an India-led initiative that encourages people to adopt eco-friendly habits and conserve resources.

76. Sustainable Lifestyle

Sustainable lifestyle means living in a way that reduces harm to the environment and conserves resources for future generations.

77. Climate Change

Climate change refers to long-term changes in temperature, rainfall, wind patterns and other climate conditions of the Earth.

78. Ocean Acidification

Ocean acidification is the increase in acidity of ocean water due to absorption of excess carbon dioxide from the atmosphere.

79. Plankton

Plankton are tiny organisms that float in water. They form the base of many aquatic food chains.

80. Coral Reefs

Coral reefs are marine ecosystems built by tiny organisms called corals. They support a large variety of marine life.