1. Composition of the Atmosphere

2. The Carbon Cycle

Introduction

Carbon is one of the most important elements on Earth. It forms the backbone of all living organisms and is found in the air, oceans, rocks, soil, and fossil fuels. Although the total amount of carbon on Earth changes very little, carbon is constantly moving between these different parts of the planet in a process known as the carbon cycle.

The carbon cycle is essential because it recycles carbon, making it continuously available for living organisms. Without this cycle, plants would eventually run out of carbon dioxide for photosynthesis, animals would lack food and oxygen, and Earth's climate would become unstable.

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What Is the Carbon Cycle?

The carbon cycle is the continuous movement of carbon between:

• the atmosphere
• living organisms
• oceans
• soils
• rocks
• fossil fuels

Carbon changes form as it moves through these reservoirs, but the total amount of carbon on Earth remains nearly constant.

Definition:
The carbon cycle is the natural process by which carbon moves between Earth's atmosphere, biosphere, hydrosphere, and geosphere.

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Major Carbon Reservoirs

A carbon reservoir is any place where carbon is stored for a period of time.

The largest reservoirs include:

Some reservoirs store carbon for only days or years, while others, such as sedimentary rocks, can store carbon for millions of years.

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Carbon in the Atmosphere

Carbon in the atmosphere is found mainly as carbon dioxide (CO₂).

Although carbon dioxide makes up only about 0.04% of the atmosphere, it is essential because it:

• supplies carbon for photosynthesis
• helps regulate Earth's temperature through the greenhouse effect
• forms part of the global carbon cycle

Atmospheric carbon dioxide is constantly exchanged with plants, oceans, and living organisms.

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Photosynthesis

Plants, algae, and some bacteria remove carbon dioxide from the atmosphere during photosynthesis.

During this process:

• carbon dioxide enters leaves,
• water is absorbed through the roots,
• sunlight provides energy,
• glucose is produced,
• oxygen is released.

This transfers carbon from the atmosphere into living organisms.

What Happens During Photosynthesis?

• Plants absorb carbon dioxide from the air.
• Carbon atoms become part of glucose molecules.
• Glucose is used to build:
  • cellulose
  • starch
  • proteins (after combining with nitrogen)
  • fats
• Animals obtain this carbon by eating plants or other animals.

Photosynthesis is the main way carbon enters the food chain.

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Respiration

Living organisms release carbon back into the atmosphere through respiration.

During respiration:

• glucose is broken down,
• energy is released,
• carbon dioxide is produced,
• carbon dioxide returns to the atmosphere.

Both plants and animals respire continuously.

This means that:

• photosynthesis removes CO₂,
• respiration returns CO₂.

Together, these processes keep carbon moving through ecosystems.

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Decomposition

When plants and animals die, decomposers such as bacteria and fungi break down their remains.

During decomposition:

• carbon stored in dead organisms is released,
• some returns to the atmosphere as carbon dioxide,
• some becomes part of the soil,
• some may eventually form fossil fuels over millions of years.

Without decomposers, carbon would remain locked in dead material.

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Combustion

When fossil fuels or wood are burned, carbon stored within them combines with oxygen to form carbon dioxide.

Examples include:

• burning coal
• burning petrol
• burning natural gas
• forest fires

Combustion transfers carbon rapidly from long-term storage back into the atmosphere.

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Carbon Exchange with the Oceans

The oceans store enormous amounts of carbon.

Carbon dioxide continuously moves between:

• the atmosphere
• ocean water

Marine organisms also absorb carbon to build:

• shells
• coral skeletons
• marine sediments

Some of this carbon eventually becomes limestone and other sedimentary rocks, storing carbon for millions of years.

The oceans therefore act as one of Earth's largest carbon sinks.

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Human Activities and the Carbon Cycle

Human activities have altered the natural carbon cycle.

Major activities include:

Burning Fossil Fuels

• coal
• oil
• natural gas

These release carbon that has been stored underground for millions of years.

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Deforestation

Trees remove carbon dioxide through photosynthesis.

When forests are cut down:

• fewer trees absorb carbon dioxide,
• stored carbon is released if wood is burned or decomposes,
• atmospheric carbon dioxide increases.

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Agriculture

Certain farming practices increase carbon emissions through:

• soil disturbance
• livestock producing methane
• burning vegetation

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Industry

Manufacturing processes such as cement production also release significant amounts of carbon dioxide.

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Why Is the Carbon Cycle Important?

The carbon cycle supports life by:

• providing carbon for living organisms,
• supplying carbon dioxide for photosynthesis,
• recycling nutrients,
• maintaining food webs,
• regulating Earth's climate.

Without carbon cycling:

• plants would eventually run out of carbon dioxide,
• food chains would collapse,
• Earth's climate would become unstable.

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Worked Example

A student observes the following sequence:

Plant → Rabbit → Fox → Respiration

Question

How does carbon move through this sequence?

Solution

1. The plant absorbs carbon dioxide through photosynthesis.
2. Carbon becomes part of the plant's tissues.
3. The rabbit eats the plant.
4. The fox eats the rabbit.
5. Both animals respire and release carbon dioxide back into the atmosphere.

This demonstrates how carbon cycles through living organisms before returning to the atmosphere.

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Real-World Connection

Forests are often called "carbon sinks" because they absorb large amounts of atmospheric carbon dioxide. Tropical rainforests such as the Amazon help slow climate change by storing billions of tonnes of carbon in trees and soils. Protecting forests and restoring degraded ecosystems are important strategies for maintaining a balanced carbon cycle and reducing the impacts of global warming.

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Did You Know?

The world's oceans contain around 50 times more carbon than the atmosphere. Tiny marine organisms called phytoplankton absorb enormous amounts of carbon dioxide through photosynthesis. When they die, some sink to the ocean floor, where their carbon may be stored in sediments for thousands to millions of years.

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Key Terms

• Carbon cycle — the continuous movement of carbon through Earth's systems.
• Carbon reservoir — a place where carbon is stored.
• Photosynthesis — the process by which plants use sunlight to convert carbon dioxide and water into glucose and oxygen.
• Respiration — the process by which organisms release energy from glucose, producing carbon dioxide.
• Decomposition — the breakdown of dead organisms by decomposers.
• Combustion — the burning of fuels, releasing carbon dioxide.
• Carbon sink — a reservoir that absorbs more carbon than it releases.

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Key Takeaways

• Carbon continuously cycles between the atmosphere, living organisms, oceans, soils, rocks, and fossil fuels.
• Photosynthesis removes carbon dioxide from the atmosphere and transfers carbon into living organisms.
• Respiration, decomposition, and combustion return carbon dioxide to the atmosphere.
• Major carbon reservoirs include the atmosphere, oceans, living organisms, soils, rocks, and fossil fuels.
• Human activities such as burning fossil fuels and deforestation have increased atmospheric carbon dioxide, affecting Earth's climate.
• A balanced carbon cycle is essential for sustaining life and regulating the global climate.

3. Greenhouse Gases

Introduction

Earth receives enormous amounts of energy from the Sun every day. Some of this energy is reflected back into space, while the rest warms Earth's surface. As the warm surface cools, it releases energy in the form of infrared radiation (heat).

If all of this heat escaped directly into space, Earth would be an icy planet with an average temperature of about –18°C. Fortunately, certain gases in the atmosphere absorb and re-radiate some of this heat, keeping Earth's average surface temperature close to 15°C. These gases are called greenhouse gases, and together they produce the natural greenhouse effect, which makes life on Earth possible.

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What Are Greenhouse Gases?

Greenhouse gases (GHGs) are gases in the atmosphere that absorb and re-emit infrared radiation, trapping some heat near Earth's surface.

Unlike nitrogen and oxygen, which make up most of the atmosphere, greenhouse gases interact strongly with infrared radiation.

Definition:
Greenhouse gases are atmospheric gases that absorb and re-radiate infrared radiation, helping to warm Earth's lower atmosphere.

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Major Greenhouse Gases

The main greenhouse gases are:

Although some greenhouse gases exist in very small concentrations, they can have a significant effect on Earth's temperature.

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How Greenhouse Gases Trap Heat

The Sun emits mostly visible light, which passes easily through the atmosphere.

After Earth's surface absorbs this energy, it warms up and emits infrared radiation (heat).

Greenhouse gases:

• absorb some of this outgoing infrared radiation,
• re-radiate it in all directions,
• send some heat back toward Earth's surface,
• reduce the amount of heat escaping into space.

This process keeps Earth much warmer than it would otherwise be.

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The Greenhouse Effect vs Global Warming

These two terms are often confused, but they are not the same.

 

The Natural Greenhouse Effect

The natural greenhouse effect:

• has operated for hundreds of millions of years,
• maintains a suitable climate,
• supports liquid water,
• allows ecosystems to thrive.

Without it, most life on Earth would not exist.

Enhanced Greenhouse Effect

Human activities have increased greenhouse gas concentrations.

More greenhouse gases trap more heat, strengthening the greenhouse effect and contributing to global warming.

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Natural Sources of Greenhouse Gases

Greenhouse gases are produced naturally by many processes.

These natural sources have existed for millions of years and are balanced by natural sinks such as forests and oceans.

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Human Sources of Greenhouse Gases

Human activities have greatly increased greenhouse gas emissions.

Major sources include:

Burning Fossil Fuels

Produces large amounts of:

• carbon dioxide

Examples:

• power stations
• vehicles
• factories
• aircraft

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Deforestation

Removing forests:

• reduces carbon dioxide absorption,
• releases stored carbon when trees are burned or decay.

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Agriculture

Agriculture produces:

• methane from cattle and sheep,
• methane from rice paddies,
• nitrous oxide from fertilisers.

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Industry

Industrial activities produce:

• carbon dioxide,
• fluorinated gases,
• nitrous oxide.

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Why Different Greenhouse Gases Matter

Not all greenhouse gases trap heat equally.

For example:

• Water vapour is the most abundant natural greenhouse gas and responds quickly to changes in temperature.
• Carbon dioxide is less effective molecule-for-molecule than methane but remains in the atmosphere for much longer and is emitted in much larger quantities.
• Methane traps much more heat per molecule than carbon dioxide over a 100-year period, but it has a shorter atmospheric lifetime.
• Nitrous oxide is both a powerful greenhouse gas and contributes to ozone depletion in the upper atmosphere.
• Fluorinated gases are present in very small amounts but can remain in the atmosphere for centuries or even thousands of years.

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Climate Regulation

Greenhouse gases help regulate Earth's climate by maintaining a balance between incoming solar energy and outgoing heat.

A balanced greenhouse effect helps maintain:

• liquid water
• stable ecosystems
• agriculture
• biodiversity
• habitable temperatures

However, if greenhouse gas concentrations rise too much, more heat is retained, increasing the risk of:

• rising global temperatures,
• melting glaciers,
• sea-level rise,
• more frequent extreme weather events,
• changes to ecosystems.

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Worked Example

A student says:

"The greenhouse effect is bad because it causes global warming."

Question

Is this statement completely correct?

Solution

No.

The natural greenhouse effect is essential because it keeps Earth warm enough for life.

Global warming occurs when additional greenhouse gases, mainly from human activities, strengthen this natural effect and trap more heat than usual.

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Real-World Connection

Many countries are working to reduce greenhouse gas emissions by increasing the use of renewable energy sources such as solar, wind, and hydroelectric power. Improvements in energy efficiency, electric vehicles, forest conservation, and carbon capture technologies are all designed to reduce the amount of carbon dioxide entering the atmosphere while maintaining reliable energy supplies.

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Did You Know?

Although water vapour is the most abundant greenhouse gas, scientists focus heavily on carbon dioxide because human activities directly increase its concentration. As carbon dioxide warms the atmosphere, warmer air can hold more water vapour, creating a positive feedback loop that can amplify warming.

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Key Terms

• Greenhouse gas (GHG) — a gas that absorbs and re-radiates infrared radiation.
• Greenhouse effect — the natural warming of Earth caused by greenhouse gases trapping some outgoing heat.
• Global warming — the long-term increase in Earth's average surface temperature, largely driven by enhanced greenhouse gas concentrations.
• Infrared radiation — heat energy emitted by Earth's surface.
• Climate regulation — the processes that help maintain Earth's long-term temperature and weather patterns.
• Enhanced greenhouse effect — the strengthening of the natural greenhouse effect due to increased greenhouse gas emissions from human activities.

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Key Takeaways

• Greenhouse gases include water vapour, carbon dioxide, methane, nitrous oxide, ozone, and fluorinated gases.
• These gases absorb and re-radiate infrared radiation, helping to keep Earth warm enough for life.
• The natural greenhouse effect is essential for maintaining a habitable planet.
• Global warming refers to the recent increase in Earth's average temperature caused primarily by higher concentrations of greenhouse gases from human activities.
• Burning fossil fuels, deforestation, agriculture, and industrial processes are major human sources of greenhouse gases.
• Maintaining a balanced greenhouse effect is essential for regulating Earth's climate and supporting life.

4. Air Pollution

Introduction

Every breath we take contains a mixture of gases, tiny particles, and water vapour. Under normal conditions, Earth's atmosphere is clean enough to support healthy ecosystems and human life. However, human activities and some natural events can introduce harmful substances into the air. These substances are known as air pollutants.

Air pollution is one of the world's most significant environmental challenges. It affects millions of people every year, damages ecosystems, contributes to climate change, and reduces the quality of life in many cities. Understanding the causes and effects of air pollution is an important step toward protecting both human health and the environment.

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What Is Air Pollution?

Air pollution occurs when harmful substances are released into the atmosphere in concentrations that can damage living organisms, ecosystems, or materials.

Air pollutants may be:

• gases
• tiny solid particles
• liquid droplets
• biological materials such as pollen or spores

Some pollutants occur naturally, while many are produced by human activities.

Definition:
Air pollution is the contamination of the atmosphere by substances that can harm people, other living organisms, or the environment.

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Major Sources of Air Pollution

Air pollution comes from both natural and human sources.

Human Sources

The largest contributors include:

• burning fossil fuels
• vehicle exhaust
• factories and power stations
• agriculture
• construction activities
• waste burning
• household heating and cooking

Natural Sources

Natural events also release pollutants into the atmosphere.

Examples include:

• volcanic eruptions
• wildfires
• dust storms
• pollen
• sea spray

Although natural pollution has always existed, human activities have greatly increased pollution levels in many parts of the world.

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Common Air Pollutants

Several pollutants are responsible for most air quality problems.

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Carbon Monoxide (CO)

Carbon monoxide is produced when fuels burn incompletely.

Common sources include:

• petrol and diesel engines
• faulty gas heaters
• wood-burning stoves

Carbon monoxide is especially dangerous because it is:

• colourless
• odourless
• tasteless

It binds strongly to haemoglobin in red blood cells, reducing the blood's ability to carry oxygen.

High concentrations can cause:

• headaches
• dizziness
• unconsciousness
• death

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Sulfur Dioxide (SO₂)

Sulfur dioxide is mainly released when fuels containing sulfur are burned.

Major sources include:

• coal-fired power stations
• metal smelting
• volcanic eruptions

Sulfur dioxide contributes to:

• acid rain
• breathing difficulties
• damage to forests and lakes

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Nitrogen Oxides (NOₓ)

Nitrogen oxides form when nitrogen and oxygen react at the high temperatures found in engines and power stations.

They contribute to:

• photochemical smog
• acid rain
• ground-level ozone formation
• respiratory illnesses

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Particulate Matter (PM)

Particulate matter (PM) consists of tiny solid particles and liquid droplets suspended in the air.

Two common categories are:

• PM₁₀ — particles smaller than 10 micrometres
• PM₂.₅ — particles smaller than 2.5 micrometres

Because PM₂.₅ particles are extremely small, they can travel deep into the lungs and even enter the bloodstream.

Sources include:

• diesel engines
• construction sites
• wildfires
• dust
• industrial processes

Health effects include:

• asthma
• bronchitis
• heart disease
• reduced lung function

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Ground-Level Ozone (O₃)

Ozone has two very different roles depending on where it is found.

Ground-level ozone forms when sunlight causes nitrogen oxides and VOCs to react.

It is a major component of photochemical smog.

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Effects on Human Health

Air pollution affects nearly every organ system.

Short-term effects include:

• coughing
• sore throat
• eye irritation
• headaches
• shortness of breath

Long-term exposure may increase the risk of:

• asthma
• chronic lung disease
• heart disease
• stroke
• lung cancer

Children, older adults, and people with existing respiratory illnesses are particularly vulnerable.

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Effects on Ecosystems

Air pollution also damages the natural environment.

Plants

Pollutants can:

• reduce photosynthesis
• damage leaves
• slow plant growth
• reduce crop yields

Animals

Animals may suffer from:

• poor air quality
• contaminated food and water
• habitat degradation

Water Bodies

Acid rain lowers the pH of lakes and rivers, harming fish and other aquatic organisms.

Buildings

Certain pollutants react with stone and metals, causing buildings and monuments to deteriorate over time.

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Factors That Influence Air Quality

Air quality varies from day to day depending on several factors.

Weather

Wind can disperse pollutants, while still air allows them to build up.

Rain helps remove many pollutants from the atmosphere.

Temperature Inversions

Normally, warm air rises and carries pollutants upward.

During a temperature inversion, a layer of warm air traps cooler air near the ground, preventing pollutants from dispersing.

This can lead to severe smog episodes.

Geography

Cities surrounded by mountains often experience poorer air quality because pollutants become trapped.

Population and Industry

Areas with:

• heavy traffic
• large industries
• high population density

usually experience higher pollution levels.

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Reducing Air Pollution

Many strategies can reduce air pollution.

Cleaner Energy

Replacing fossil fuels with:

• solar power
• wind power
• hydroelectric power

reduces emissions.

Cleaner Transportation

Examples include:

• electric vehicles
• public transportation
• cycling
• walking
• carpooling

Industrial Controls

Factories can reduce emissions using:

• filters
• scrubbers
• catalytic converters
• cleaner production methods

Government Regulations

Many countries limit emissions through:

• vehicle emission standards
• industrial pollution limits
• air quality monitoring
• clean-air legislation

Individual Actions

Individuals can help by:

• conserving energy
• using public transport
• avoiding unnecessary burning
• maintaining vehicles properly
• planting trees

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Worked Example

A city experiences a week of hot, sunny weather with very little wind.

Question

How is air quality likely to change?

Solution

Hot, sunny conditions encourage the formation of ground-level ozone.

Little wind means pollutants remain concentrated instead of being dispersed.

Therefore, air quality is likely to decrease, with increased smog and higher concentrations of pollutants.

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Real-World Connection

Many cities now publish a daily Air Quality Index (AQI) that reports pollution levels and provides health advice. On days when pollution levels are high, people with asthma or heart disease may be advised to limit outdoor exercise. Governments also use AQI data to evaluate pollution-control measures and identify areas where air quality improvements are needed.

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Did You Know?

According to the World Health Organization (WHO), air pollution contributes to millions of premature deaths each year. Fine particulate matter (PM₂.₅) is considered one of the most harmful pollutants because its tiny particles can travel deep into the lungs and even enter the bloodstream, affecting both the respiratory and cardiovascular systems.

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Key Terms

• Air pollution — contamination of the atmosphere by harmful substances.
• Air pollutant — any substance that negatively affects air quality.
• Carbon monoxide (CO) — a poisonous gas produced by incomplete combustion.
• Sulfur dioxide (SO₂) — a gas that contributes to acid rain and respiratory problems.
• Nitrogen oxides (NOₓ) — gases that contribute to smog and acid rain.
• Particulate matter (PM) — tiny solid or liquid particles suspended in the air.
• Ground-level ozone (O₃) — a harmful pollutant formed by reactions involving sunlight, NOₓ, and VOCs.
• Temperature inversion — a weather condition that traps pollutants close to Earth's surface.
• Air Quality Index (AQI) — a measure used to describe how clean or polluted the air is.

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Key Takeaways

• Air pollution results from both natural processes and human activities.
• Major pollutants include carbon monoxide, sulfur dioxide, nitrogen oxides, particulate matter, ground-level ozone, and volatile organic compounds.
• Air pollution can damage human health, ecosystems, crops, water bodies, and buildings.
• Weather, geography, traffic, and industrial activity all influence local air quality.
• Cleaner energy, improved transportation, pollution-control technologies, and environmental regulations can significantly reduce air pollution.
• Protecting air quality benefits both human health and the long-term sustainability of Earth's ecosystems.

5. Acid Rain