The air we breathe is a mixture of gases that has changed dramatically over billions of years. Understanding this journey helps us understand our planet’s climate today.

The Atmosphere Today 

For the last 200 million years, the composition of our atmosphere has been stable. It is a mixture made up of:

• Nitrogen (N₂): Approximately 78% (about four-fifths).

• Oxygen (O₂): Approximately 21% (about one-fifth).

• Argon (Ar): Approximately 0.9%.

• Carbon Dioxide (CO₂), water vapour, and other noble gases: Tiny amounts making up the rest.

Exam Tip: You must remember the approximate proportions of nitrogen and oxygen. A common question asks you to state that the atmosphere is roughly 80% nitrogen and 20% oxygen.

The Earth’s Early Atmosphere 

About 4.6 billion years ago, the Earth’s atmosphere was very different. Theories suggest it was formed by gases released from intense volcanic activity.

This early atmosphere likely consisted of:

• Mainly carbon dioxide (CO₂), similar to the atmospheres of Mars and Venus today.

• Nitrogen (N₂), which gradually built up over time.

• Water vapour (H₂O).

• Small amounts of methane (CH₄) and ammonia (NH₃).

• Crucially, there was little or no oxygen (O₂) a.

Exam Tip: Questions about the early atmosphere often test your understanding that evidence is limited because it was so long ago. Answers should be based on theories, such as the one involving volcanic activity.

How Did the Atmosphere Change?

Two major processes transformed the early, CO₂-rich atmosphere into the oxygen-rich one we have today.

1. How Carbon Dioxide Decreased 

The huge amount of carbon dioxide in the early atmosphere was reduced in several ways:

• Dissolving in Oceans: As the Earth cooled, water vapour condensed to form oceans. A large amount of CO₂ dissolved in this seawater.

• Formation of Sedimentary Rocks: Dissolved CO₂ reacted to form carbonate precipitates. Over millions of years, these built up on the seabed as sediments, forming rocks like limestone (calcium carbonate). This locked away vast amounts of carbon.

• Photosynthesis: Algae and then plants evolved, absorbing CO₂ from the atmosphere for photosynthesis.

• Formation of Fossil Fuels: When these plants and the animals that ate them died, their carbon became trapped in rocks, forming fossil fuels like coal, oil, and natural gas.

2. How Oxygen Increased 

The oxygen in our atmosphere was produced by early life.

• Photosynthesis: Algae began producing oxygen through photosynthesis around 2.7 billion years ago. Later, as plants evolved, they also released oxygen.

• The word equation is: Carbon dioxide + Water ⟶ Glucose + Oxygen

• The balanced symbol equation is: 6CO₂ + 6H₂O ⟶ C₆H₁₂O₆ + 6O₂

This gradual increase in oxygen levels over billions of years allowed for the evolution of more complex animal life that relies on oxygen for respiration.

Exam Tip: You must be able to recall the word equation for photosynthesis. Linking the increase in oxygen to the evolution of animals is a key concept that is often tested.

The air around us today is vastly different from what it was when the Earth was young. Understanding the planet’s early atmosphere is like piecing together a puzzle from billions of years ago.

Theories and Limited Evidence

The Earth formed around 4.6 billion years ago. Because of this enormous timescale, the direct evidence for what the early atmosphere was like is very limited. As a result, scientists have developed theories based on the evidence they can find, and these theories have changed and developed over time.

Exam Tip: If a question asks about the early atmosphere, it’s a good idea to mention that our knowledge is based on theories and that evidence is limited due to the 4.6-billion-year timescale. This shows a deeper understanding.

Formation and Composition

One leading theory suggests that for the first billion years, intense volcanic activity shaped the planet and its atmosphere. These volcanoes released huge quantities of gases.

This early atmosphere was likely very similar to the atmospheres of Mars and Venus today. Its composition was thought to be:

• Mainly carbon dioxide (CO₂).

• Little or no oxygen (O₂) a.

• Nitrogen (N₂), which was also released by volcanoes and gradually built up.

• Small amounts of methane (CH₄) and ammonia (NH₃).

• Lots of water vapour (H₂O).

Exam Tip: You must remember the main gas was carbon dioxide and that there was little or no oxygen. This lack of oxygen is a crucial difference compared to today’s atmosphere.

The First Big Change: Losing Carbon Dioxide

As the Earth began to cool, the water vapour released by volcanoes condensed and formed the oceans and seas. This was the first major step in changing the atmosphere.

The new oceans played a vital role in reducing the huge amount of CO₂ in the air:

1. Dissolving: A large amount of the atmospheric carbon dioxide dissolved into the ocean water.

2. Rock Formation: The dissolved CO₂ reacted to form carbonate compounds. These solid compounds then precipitated (sank to the bottom of the ocean) as sediments, eventually forming sedimentary rocks like limestone.

This process locked away enormous quantities of carbon dioxide from the atmosphere, paving the way for the next stage of its evolution.

The Earth’s early atmosphere had almost no oxygen, which meant complex animal life couldn’t exist. The arrival of oxygen was a revolutionary event, driven entirely by the evolution of early life.

The Power of Photosynthesis

The single process responsible for putting oxygen into the atmosphere was photosynthesis. Early life forms, like algae and later plants, developed the ability to use sunlight to create their own food (glucose). Oxygen is released as a waste product of this vital reaction.

You need to know the equation for photosynthesis:

• Word Equation: Carbon dioxide + Water ⟶ Glucose + Oxygen

• Balanced Symbol Equation: 6CO₂ + 6H₂O ⟶ C₆H₁₂O₆ + 6O₂

Exam Tip: You must be able to recall at least the word equation for photosynthesis. It is a fundamental concept that links biology and chemistry and is frequently tested.

A Timeline for Oxygen’s Rise

The increase in oxygen was a very slow and gradual process that took billions of years.

1. First Appearance: The first organisms to produce oxygen were primitive algae, which started photosynthesising around 2.7 billion years ago. Soon after, oxygen began to appear in the atmosphere for the first time.

2. Plant Evolution: Over the next billion years, more complex plants evolved, and the amount of photosynthesis happening on the planet increased dramatically. This caused the percentage of oxygen in the atmosphere to gradually rise.

3. Enabling Animal Life: Eventually, oxygen levels became high enough to support the evolution of more complex animal life, which uses oxygen for respiration.

4. Reaching Stability: This process continued until about 200 million years ago, when the atmosphere reached its current stable composition of approximately 21% oxygen.

Exam Tip: A common exam question asks you to explain the significance of increasing oxygen levels. A key point to make is that the rise in atmospheric oxygen allowed for the evolution of complex animals that rely on aerobic respiration.

The Earth’s early atmosphere was dominated by carbon dioxide, creating a very different world from today’s. The significant reduction of this gas was a crucial process involving the formation of oceans, the evolution of life, and the creation of rocks and fuels.

Oceans and Rock Formation 

The first major step in reducing atmospheric CO₂ began as the planet cooled.

1. Ocean Formation: Water vapour released from volcanoes condensed to form the first seas and oceans.

2. Dissolving CO₂: Carbon dioxide is soluble in water, so a vast amount of the atmospheric CO₂ dissolved into these new oceans.

3. Precipitation and Sedimentation: The dissolved CO₂ reacted in the water to form solid carbonate compounds. These insoluble solids precipitated (sank to the bottom), forming layers of sediment. Over millions of years, this process created sedimentary rocks like limestone (calcium carbonate), locking away huge quantities of carbon from the atmosphere.

The Impact of Life and Photosynthesis 

The evolution of early life, particularly algae and plants, dramatically accelerated the removal of CO₂.

• Photosynthesis: These organisms absorbed carbon dioxide from the atmosphere and oceans for photosynthesis, converting it into glucose for energy and growth. This biological process was a major factor in decreasing the percentage of CO₂ in the air.

Exam Tip: Photosynthesis is a critical process to understand. It connects two major atmospheric changes: it decreased carbon dioxide and increased oxygen. Be ready to explain both effects in an exam.

Formation of Fossil Fuels 

The carbon taken in by early life was eventually trapped underground.

• When plants and animals died, their remains were buried under layers of mud and sediment.

• Over millions of years, immense heat and pressure transformed this trapped carbon into fossil fuels—coal, crude oil, and natural gas.

Both the formation of sedimentary rocks like limestone and the creation of fossil fuels locked away carbon that was once in the atmosphere, permanently reducing its concentration to the low levels we see today.

Below are the key molecules and ions involved in locking away carbon dioxide.

Exam Tip: Remember the two main “sinks” that locked away atmospheric carbon: limestone (sedimentary rock) and fossil fuels. Questions often ask you to describe how atmospheric composition has changed and to explain the processes responsible.

Greenhouse gases in our atmosphere play a crucial role in keeping the Earth warm enough for life to exist. However, human activities are increasing their concentration, leading to a potentially dangerous enhancement of this natural process.

What are Greenhouse Gases? 

Greenhouse gases are gases in the atmosphere that trap heat energy. This natural process is called the greenhouse effect, and it maintains the Earth’s average temperature at a level that can support life.

The main greenhouse gases you need to know are:

• Water vapour (H2​O)

• Carbon dioxide (CO2​)

• Methane (CH4​)

The greenhouse effect works like this:

1. 1. The Sun emits energy as short-wavelength radiation, which passes through the atmosphere and warms the Earth.

   2. The Earth’s surface cools down by emitting this energy as long-wavelength radiation (infrared/heat).

   3. Greenhouse gases in the atmosphere absorb some of this outgoing long-wavelength radiation, trapping the heat and keeping the planet warm.

Exam Tip: You must be able to describe the two types of radiation involved. Remember: short-wavelength radiation comes IN from the sun, and long-wavelength radiation goes OUT from the Earth, which is then absorbed by greenhouse gases.

Human Activities and Increasing Greenhouse Gases

Many scientists agree that human activities are increasing the amounts of greenhouse gases in the atmosphere.

• Carbon Dioxide (CO2​) Sources:

  • Burning fossil fuels in cars and power stations is the biggest source.

  • Deforestation means there are fewer trees to remove CO₂ from the atmosphere through photosynthesis.

• Methane (CH4​) Sources:

  • Farming cattle and other grazing animals, which release methane.

  • Growing rice in flooded paddy fields.

  • The decomposition of waste in landfill sites.

Methane is a much more powerful greenhouse gas than carbon dioxide, though it is present in smaller concentrations.

Exam Tip: A common exam question will ask you to state two human activities that increase either carbon dioxide or methane levels. Memorise the main sources for each gas.

The Consequences: Global Climate Change 

The increase in greenhouse gases is enhancing the greenhouse effect, leading to a rise in the Earth’s average temperature—a phenomenon known as global warming. This warming is causing global climate change, which has several potential effects:

• Rising Sea Levels: Polar ice caps and glaciers melt, causing sea levels to rise. This can lead to coastal flooding and habitat loss.

• Extreme Weather: Warmer oceans provide more energy for storms, leading to more frequent and intense hurricanes, heatwaves, and heavy rainfall.

• Changing Rainfall Patterns: This can cause severe droughts in some regions, leading to crop failure and food shortages.

There is a strong correlation in scientific data between rising CO₂ concentrations and rising global temperatures.

Reducing our Carbon Footprint 

The carbon footprint of a product, service, or event is the total amount of carbon dioxide and other greenhouse gases emitted over its full life cycle. Reducing our carbon footprint is essential to combat climate change.

Actions we can take include:

• Reducing CO₂ emissions by using less electricity (insulating homes, switching off appliances), using public transport, and switching to renewable energy sources.

• Reducing CH₄ emissions by eating less beef and dairy, and by capturing methane from landfill sites to use as a fuel.

However, taking these actions can be difficult, as many solutions are expensive or seen as inconvenient by people.

While the greenhouse effect is a natural process, scientists widely agree that various human activities are releasing extra greenhouse gases into the atmosphere. This enhancement of the greenhouse effect is leading to global warming and climate change. The two main gases of concern are carbon dioxide and methane.

Sources of Carbon Dioxide (CO2​) 

The increase in atmospheric carbon dioxide is primarily linked to our energy needs and how we use land.

1. Burning Fossil Fuels

This is the largest human source of CO₂.

• Activity: We burn fossil fuels like coal, oil, and natural gas in power stations to generate electricity and in cars for transport.

• Reason: This combustion reaction releases huge amounts of stored carbon into the atmosphere as carbon dioxide.

2. Deforestation

This is the large-scale cutting down of forests.

• Activity: Clearing land, often by burning or felling trees, for farming or building.

• Reason: Trees absorb carbon dioxide from the atmosphere for photosynthesis. Having fewer trees means that less CO₂ is removed, causing its concentration in the atmosphere to rise.

Exam Tip: A common question asks you to name two human activities that increase atmospheric CO₂. The easiest two to remember are burning fossil fuels and deforestation.

Sources of Methane (CH4​) 

Methane is a more powerful greenhouse gas than carbon dioxide. Its levels are rising mainly due to agriculture and waste.

1. Farming and Agriculture

• Activity: Farming cattle and other grazing animals releases enormous amounts of methane when the animals digest their food and pass wind. Growing rice in flooded paddy fields also releases methane.

2. Waste in Landfill Sites

• Activity: As the global population increases, more waste is sent to landfill sites.

• Reason: When this waste decomposes without oxygen, it releases methane gas.

Exam Tip: For sources of methane, think Farms and Landfills. Specifically, remember cattle farmingand waste decomposition as key examples.

Global climate change refers to the long-term shift in weather patterns across the world. A major part of this is global warming, which is the steady increase in the Earth’s average temperature observed over the last century.

The Cause: An Enhanced Greenhouse Effect

The Earth is kept warm by a natural process called the greenhouse effect. While this is essential for life, human activities are making it stronger.

1. The Sun heats the Earth with short-wavelength radiation (like UV and visible light).

2. The Earth’s surface cools down by giving off long-wavelength radiation (infrared/heat).

3. Greenhouse gases in the atmosphere (like CO₂, CH₄, and water vapour) absorb this outgoing long-wavelength radiation, trapping the heat.

Human activities, especially since the Industrial Revolution, have released more greenhouse gases into the atmosphere. This means more heat is trapped, causing the Earth’s average temperature to rise.

Exam Tip: You must be able to describe the two types of radiation involved. A simple way to remember is: Short waves from the Sun, long waves from the ground.

Scientific Evidence and Uncertainty

The link between human activity and climate change is supported by strong evidence, but it’s also a complex topic.

• Evidence: Data collected over many years shows a strong correlation between the rising concentration of atmospheric carbon dioxide and the rise in global average temperatures. The overwhelming majority of climate scientists (around 97%) agree that this is caused by human activity.

• Uncertainty: Predicting future climate is very difficult because it’s such a complex system. This can lead to simplified models and speculation in the media. It’s important to rely on peer-reviewed scientific evidence, which has been checked by other experts to ensure it is valid.

Exam Tip: If a question asks you to evaluate evidence about climate change, a good answer will mention the strong correlation in the data but also acknowledge that modelling complex systems is difficult, which leads to uncertainty.

Potential Effects of Global Climate Change

A warmer world has serious consequences. You should be able to describe at least four potential effects:

• Rising Sea Levels: As polar ice caps and glaciers melt, sea levels rise. This leads to coastal flooding, the destruction of habitats, and the loss of usable land.

• More Extreme Weather Events: Warmer oceans provide more energy for storms, making events like hurricanes more frequent and powerful. Extreme heatwaves are also becoming more common.

• Changes in Rainfall Patterns: Some areas may experience more frequent and severe droughts, leading to crop failures and water shortages. Other areas might experience more intense rainfall and flooding.

• Threats to Ecosystems: Changes in temperature and weather patterns can destroy habitats and threaten the survival of many species.

Everything we do, make, and use has an impact on the environment. The “carbon footprint” is a way of measuring this impact in terms of greenhouse gas emissions.

What is a Carbon Footprint?

The carbon footprint is the total amount of carbon dioxide (CO2​) and other greenhouse gases (like methane, CH4​) emitted over the full life cycle of a product, service, or event.

This includes everything from the raw materials used to make a product, to its manufacturing, use, and final disposal. An individual’s carbon footprint is the sum of all their activities throughout the year, such as:

• Heating a home.

• Using electricity.

• Travelling by car, bus, or plane.

• The food they eat (e.g., beef and rice farming produce a lot of methane).

How to Reduce the Carbon Footprint

Reducing our carbon footprint means cutting emissions of both carbon dioxide and methane.

Reducing Carbon Dioxide (CO2​) Emissions

Most CO₂ emissions come from burning fossil fuels for energy. We can reduce this by:

• Improving Home Energy Efficiency: Insulating homes and turning down the heating reduces the amount of fuel burned.

• Switching to Renewables: Using renewable energy sources like wind or solar power instead of fossil fuel power stations.

• Changing Transport Habits: Using public transport, cycling, or walking instead of driving a car reduces emissions.

Reducing Methane (CH4​) Emissions

Methane is a powerful greenhouse gas, mainly from agriculture and waste. We can reduce it by:

• Changing Diets: Eating less beef and dairy can reduce the amount of methane produced by cattle farming.

• Managing Waste: Capturing methane from landfill sites and burning it to generate electricity prevents it from escaping into the atmosphere.

Exam Tip: When asked how to reduce the carbon footprint, give a specific action and link it to a reduction in either CO₂ (from fossil fuels) or CH₄ (from farming/waste). For example, “Insulate your home to use less energy from burning fossil fuels, which reduces CO₂ emissions.”

Why is it Difficult to Reduce Our Carbon Footprint?

Although there are many ways to reduce emissions, putting them into practice can be difficult for several reasons:

• Cost: Many solutions, like installing solar panels or buying an electric car, are very expensive.

• Inconvenience: People may find it inconvenient to use public transport instead of their own car.

• Lifestyle: Some people may be unwilling to make changes to their lifestyle, such as reducing the amount of meat they eat.

Exam Tip: If a question asks why it might be difficult for a government to implement policies to reduce carbon emissions, think about the practical limitations. A good answer would mention that people may be resistant to changes that are expensive or inconvenient.

The combustion of fuels, especially fossil fuels like coal, oil, and gas, is a major source of atmospheric pollutants. Most fuels contain carbon and hydrogen, but they can also have impurities like sulfur. The gases released when these fuels burn can have harmful effects on our health and the environment.

Pollutants from Incomplete Combustion

When a fuel burns in a limited supply of oxygen, it undergoes incomplete combustion. This produces harmful substances instead of just carbon dioxide and water.

Carbon Monoxide (CO)

• Formation: Carbon monoxide is produced when there isn’t enough oxygen for the fuel to burn completely. The carbon is only partially oxidised.

• Properties & Effects: CO is a toxic gas that is colorless and odorless, making it very difficult to detect. It is dangerous because it binds to hemoglobin in red blood cells, stopping them from carrying oxygen around the body. This can lead to fainting, a coma, or even death.

Particulates (Soot)

• Formation: During incomplete combustion, tiny solid particles of carbon (soot) can also be released. This is common in diesel engines.

• Properties & Effects: These particulates can cause health problems for humans, including worsening asthma and other respiratory issues. They also cause global dimming by blocking sunlight from reaching the Earth’s surface.

Exam Tip: Remember the difference between complete and incomplete combustion. Complete combustion (plenty of oxygen) produces CO₂ and water. Incomplete combustion (limited oxygen) produces harmful CO and/or soot.

Pollutants from Fuel Impurities and High Temperatures

Sulfur Dioxide (SO₂) 

• Formation: Most fossil fuels contain some sulfur impurities. When the fuel is burned, this sulfur reacts with oxygen in the air to produce sulfur dioxide gas.

  • S + O₂ ⟶ SO₂

• Properties & Effects: Sulfur dioxide is a major cause of acid rain. It dissolves in water droplets in the atmosphere to form an acidic solution. Acid rain damages buildings, corrodes metal structures, and harms ecosystems by killing trees and aquatic life. It can also cause respiratory problems in humans.

Oxides of Nitrogen (NOₓ) 

• Formation: Oxides of nitrogen are not formed from the fuel itself, but from the air. The extremely high temperature and pressure inside car engines and power stations cause nitrogen and oxygen from the air to react together.

• Properties & Effects: Like sulfur dioxide, oxides of nitrogen also contribute to the formation of acid rain. They can also cause photochemical smog in cities and lead to breathing difficulties, especially for people with asthma.

Exam Tip: A common point of confusion is the source of the different pollutants. Remember:

CO and Soot come from the carbon in the fuel during incomplete combustion.

SO₂ comes from sulfur impurities in the fuel.

NOₓ comes from the nitrogen in the air at high temperatures.

Carbon Monoxide (CO)

Carbon Monoxide is a dangerous gas produced during the incomplete combustion of fuels.

• Properties: It is a toxic gas that is also colorless and odorless, which makes it impossible for humans to detect. This is why it’s often called the “silent killer”.

• Effects: When inhaled, CO binds to the hemoglobin in red blood cells much more strongly than oxygen does. This stops the blood from being able to carry oxygen around the body. A lack of oxygen can quickly lead to fainting, a coma, or death.

Exam Tip: You must remember the specific health risk of carbon monoxide. Stating that it “prevents red blood cells from carrying oxygen” is the key biological effect you need to describe.

Sulfur Dioxide (SO₂) 

Sulfur dioxide is formed when sulfur impurities, present in most fossil fuels, burn along with the fuel.

• Properties: It is a colorless gas with a pungent, sharp smell.

• Effects: SO₂ is the main cause of acid rain. It dissolves in rainwater to form an acidic solution. Acid rain has widespread environmental consequences:

  • It corrodes metal structures and damages buildings and statues made from carbonate rocks (like limestone and marble).

  • It damages forests and kills plants and animals in rivers and lakes.

  • It causes respiratory problems in humans by irritating the lungs and throat.

Exam Tip: When discussing sulfur dioxide, always link it to its primary environmental impact: acid rain. Be prepared to give an example of the damage acid rain causes, such as corroding limestone buildings.

Oxides of Nitrogen (NOₓ)

Oxides of nitrogen are formed under the high temperature and pressure conditions found inside car engines and power stations. This causes nitrogen and oxygen from the air to react together.

• Effects:

  • They contribute to the formation of acid rain, along with sulfur dioxide.

  • They cause photochemical smog, a type of air pollution seen as a brown haze over cities, which is formed when they react with sunlight.

  • They cause respiratory problems in humans and can make conditions like asthma worse.

Exam Tip: A common mistake is to think that nitrogen oxides come from the fuel. Remember, they are formed from the air (which is ~78% nitrogen) under the extreme conditions inside an engine.

Particulates (Soot) 

Particulates are tiny solid carbon particles that are released during the incomplete combustion of fuels, especially diesel.

• Effects:

  • Human Health: When inhaled, they can damage the lungs and cause serious respiratory problems.

  • Global Dimming: Particulates in the atmosphere block incoming sunlight, which can lead to a cooling effect on the Earth’s surface and potentially reduce rainfall.

  • Environmental Damage: They build up as soot, which makes buildings look dirty and can accelerate corrosion.