Composition and structure of the atmosphere | ICSE Class 9 Geography Notes

This note covers the atmosphere and its constituents, the roles of gases, water vapour and dust, atmospheric layers, the ozone layer and its depletion, the greenhouse effect, global warming and its impacts.

What is the atmosphere, and why does it matter?

Definition: The atmosphere is the envelope of air surrounding the Earth. Air is a mixture of gases and also contains water vapour and small solid particles.

The atmosphere provides the air that living beings depend on. Oxygen is the gas humans and animals take in when breathing. Carbon dioxide is a gas that green plants use to make food. These different functions make the mixture important for life.

The atmosphere also protects living things from harmful solar rays and helps maintain liveable temperatures. Its importance therefore goes beyond breathing. The way its gases interact with energy from the Sun and energy leaving the Earth affects conditions near the surface.

How do composition and structure differ?

Composition means what the atmosphere contains. It includes the gases present and their proportions, together with water vapour, which is water in its gaseous form, and dust particles. A description of composition answers the question, “What is air made of?”

Structure means the arrangement of the atmosphere into layers. These layers differ in temperature and density. Temperature describes how hot or cold the air is. Density means mass per unit volume, so denser air contains more mass within the same volume.

Atmospheric density is greatest near the Earth's surface and decreases with altitude, meaning height above the Earth's surface in this discussion. Of the atmosphere's total mass, 99 per cent is confined within 32 kilometres of the Earth's surface. The abbreviation km means kilometres.

Do not confuse a percentage of atmospheric mass with a percentage of a gas in air. The first tells us how the atmosphere's mass is distributed vertically. The second tells us the relative amount of a constituent in the mixture.

Which gases make up air, and what are their roles?

Nitrogen, the most plentiful gas in air, and oxygen together make up the bulk of the atmosphere. In a simplified composition, nitrogen accounts for 78 per cent and oxygen for 21 per cent. The symbol % means per cent, or parts in a hundred.

Other gases occur in much smaller quantities. They include carbon dioxide, argon, helium, hydrogen and ozone. Ozone is a form of oxygen whose molecules contain three oxygen atoms. A molecule is a group of bonded atoms; an atom is a basic unit of an element. Its small quantity does not prevent it from performing an important protective role high in the atmosphere.

How do gases support living organisms?

Plants need nitrogen, but they cannot take it directly from the air. Bacteria living in soil and in the roots of some plants change atmospheric nitrogen into a form that plants can use. Thus, abundance in the air does not mean direct availability to plants.

Humans and animals take oxygen from the air as they breathe. Green plants produce oxygen during photosynthesis, the process by which they use sunlight to make food. They use carbon dioxide in food production, while humans and animals release carbon dioxide.

Burning fuels such as coal and oil adds carbon dioxide to the atmosphere. The amount of this gas can change, with consequences for weather and climate. Weather is the hour-to-hour and day-to-day condition of the atmosphere; climate is the average weather condition of a place over a longer period.

Ultraviolet radiation, abbreviated UV, is a form of energy from the Sun that can damage living organisms. The term “radiation” means energy travelling as waves. Ozone's absorption of UV and carbon dioxide's role in retaining heat are different functions and should be explained separately.

Why are water vapour and dust important atmospheric constituents?

Water vapour varies in amount from place to place and with height. Its amount decreases with altitude and also decreases from the equator towards the poles. The equator is the imaginary circle midway between the poles, the northern and southern ends of the Earth's axis.

In the warm and wet tropics, the belt between the Tropics of Cancer and Capricorn, water vapour may account for four per cent of air by volume. In dry and cold desert and polar areas, it may be less than one per cent.

On an outline world map, mark the equator and polar regions. Draw arrows from the equator towards both poles and label them “decreasing water vapour”.

Annotate the warm and wet tropics with “may account for 4 per cent of air by volume”, and dry and cold desert and polar regions with “may be less than 1 per cent”.

Water vapour absorbs some incoming solar energy and helps retain heat radiated by the Earth. Solar radiation means energy received from the Sun; terrestrial radiation means energy radiated by the Earth. Water vapour therefore helps moderate temperature as well as supplying moisture.

How do tiny particles help clouds form?

Atmospheric particles include sea salts, fine soil, smoke-soot, ash, pollen, dust and broken particles of meteors. Meteors are the luminous phenomena associated with material entering the atmosphere from space. These particles have different origins, so atmospheric dust is not simply soil lifted from the ground.

Dust is generally concentrated in the lower atmosphere. However, convectional air currents may carry particles to great heights. Convection is the transfer of heat through the movement of air, including the upward movement of heated air.

Dust and salt particles act as hygroscopic nuclei, tiny centres around which water vapour condenses. Condensation is the change of water vapour into liquid water. The particles provide centres for droplets to form, helping produce clouds.

1. Water evaporates from land and water bodies; evaporation is the change of liquid water into water vapour.
2. Water vapour rises with the air and starts cooling.
3. Water vapour condenses around tiny dust and salt particles.
4. Droplets gather as clouds, connecting atmospheric moisture with suspended particles.

What makes the troposphere the most important layer for life?

The troposphere is the lowest atmospheric layer, next to the Earth's surface. It contains the air we breathe, water vapour and dust particles. Almost all weather phenomena, including rainfall, fog and hailstorms, occur here. This makes it especially important for biological activity, meaning the activities of living organisms.

Rainfall is precipitation in liquid form, while precipitation is water falling from the atmosphere. Fog consists of tiny water droplets near the ground, and hail consists of balls or pieces of ice falling from clouds. These examples connect atmospheric moisture with familiar weather.

How does the height of this layer vary?

The troposphere's average height is 13 km. It extends roughly to 8 km near the poles and about 18 km at the equator. These are different descriptions of its extent: an average height and approximate heights at two different geographical positions.

Its thickness is greatest at the equator because strong convectional currents transport heat to great heights. Heated air rises, carrying heat upwards. The resulting greater vertical extent explains why a single height cannot describe the troposphere everywhere.

Within the troposphere, temperature decreases with height at a rate of 1 degree Celsius for every 165 metres. °C means degrees Celsius, a unit of temperature; m means metres, a unit of length. This temperature pattern belongs to this layer, not to the entire atmosphere.

What is the tropopause?

The tropopause is the zone separating the troposphere from the stratosphere, the layer above it. Temperature at the tropopause is nearly constant. It is about minus 80°C over the equator and about minus 45°C over the poles.

Note: Preserve the words “average”, “roughly”, “about” and “nearly” when describing these values. They distinguish an average or approximate atmospheric condition from an exact, uniform boundary.

Three ideas explain this layer's importance together: its position next to the Earth's surface, its supply of air for life, and its concentration of weather activity. Naming the layer alone does not explain why it is central to everyday environmental conditions.

How does the stratosphere protect life on Earth?

The stratosphere lies above the tropopause and extends up to a height of 50 km. It contains the ozone layer, the region of the atmosphere in which ozone provides protection by absorbing harmful ultraviolet radiation from the Sun.

The stratosphere is almost free from clouds and associated weather phenomena. These conditions make it favourable for flying aeroplanes. The word “almost” matters: the description is not a claim that clouds and weather-related features are absolutely absent.

What is special about ozone?

An ordinary oxygen molecule contains two oxygen atoms and is represented by O₂. An ozone molecule contains three oxygen atoms and is represented by O₃.

High in the atmosphere, UV radiation acts on oxygen molecules. It splits some of them into separate oxygen atoms. These atoms then combine with oxygen molecules to form ozone. This process connects oxygen and ozone without making them identical substances.

Ozone absorbs ultraviolet radiation and shields life from this harmful energy. The protection depends on ozone's position high in the atmosphere. Ozone is poisonous, so the usefulness of the ozone layer should not be taken to mean that breathing ozone is beneficial.

Both layers matter for life, but in different ways. The troposphere supplies the immediate atmospheric setting for living organisms. The stratosphere contains a shield against harmful solar radiation. Keeping those roles distinct helps explain why damage higher in the atmosphere can affect life at the surface.

What are the ionosphere and exosphere?

Above the stratosphere lies the mesosphere, a layer extending up to 80 km. Its upper boundary is the mesopause. These names help locate the higher regions even when the main focus is the troposphere, stratosphere, ionosphere and exosphere.

Above the mesosphere is the thermosphere, where temperature rises rapidly with increasing height. The ionosphere is a region containing electrically charged particles called ions. It is described here as part of the thermosphere, extending between 80 and 400 km.

Why is the ionosphere important for communication?

The ionosphere helps in radio transmission. Radio waves are waves of electromagnetic energy used to carry signals. Waves transmitted from the Earth are reflected back towards the Earth by this region. This communication role differs from the ozone layer's absorption of harmful solar radiation.

In describing the ionosphere, connect its name with its charged particles, its position with the upper atmosphere, and its importance with radio transmission. Do not identify it as the ozone layer: the two terms describe different atmospheric features with different functions.

How does the atmosphere merge with space?

The exosphere is the uppermost atmospheric layer, above the thermosphere. Its contents are extremely rarefied, meaning very thinly spread. It gradually merges with outer space, rather than ending at a sharply defined upper surface.

Light gases such as helium and hydrogen pass into space from this layer. The exosphere's very thin air contrasts with the denser air near the Earth's surface. Remember this contrast when interpreting a layer diagram: equally sized drawn bands do not represent equal amounts of air.

The two higher regions illustrate different aspects of atmospheric structure. The ionosphere is identified by charged particles and its role in radio transmission. The exosphere is identified by its outermost position, extremely thin contents and gradual transition to space.

How can the layers and their heights be compared?

A vertical atmospheric profile places the layers in order above the Earth's surface. In a temperature-based division, the sequence is troposphere, stratosphere, mesosphere, thermosphere and exosphere. The ionosphere can be located within the upper atmosphere without replacing the entire sequence with four adjoining bands.

How should a temperature profile be read?

The stratopause is the boundary between the stratosphere and mesosphere. A temperature profile is a line showing how temperature changes with height. Follow its direction within each layer separately. A line moving towards lower temperatures indicates cooling with height; a line moving towards higher temperatures indicates warming with height.

The vertical axis shows altitude in km and the horizontal axis shows temperature in °C. A profile passes through labelled troposphere, stratosphere, mesosphere and thermosphere regions, with exosphere above. Dashed boundaries label the tropopause, stratopause and mesopause.

See Fig. 7.1 in your NCERT textbook

In the diagram, the profile falls in temperature through the troposphere, rises through the stratosphere, falls through the mesosphere and rises again through the thermosphere. Temperature therefore does not decrease continuously throughout the atmosphere.

Distinguish an upper height from a thickness. Saying that the stratosphere extends up to 50 km locates its upper extent above the surface; it does not say that it begins at the surface. The troposphere and tropopause are below it.

What causes ozone depletion, and why is it harmful?

Ozone depletion means a reduction in the amount or concentration of ozone. Concentration describes how much of a substance is present in a mixture. Depletion in the stratosphere weakens the protective shield that absorbs harmful ultraviolet radiation.

The drop in atmospheric ozone has been linked to synthetic chemicals such as chlorofluorocarbons, abbreviated CFCs. Synthetic means made through human chemical processes. CFCs have been used as refrigerants, substances used for cooling in equipment such as refrigerators.

What is the sequence from release to harm?

1. CFCs are released through human activities and enter the atmosphere.
2. These chemicals drift into the stratosphere, where they destroy ozone.
3. Ozone concentration is depleted, weakening the protection against ultraviolet radiation.
4. More harmful UV radiation can pass through towards the Earth's surface, where it can damage living organisms.

UV radiation is known to cause skin cancer in human beings. This gives a direct example of why ozone depletion matters to people living far below the stratosphere. The environmental problem is damage to a protective atmospheric component, with consequences for life at the surface.

What does the ozone hole mean?

Large depletion of ozone occurs over Antarctica, the continent around the South Pole. The term ozone hole refers to depletion of ozone concentration in the stratosphere. It does not mean that all the air above Antarctica has disappeared or that a physical opening exists in the atmosphere.

On an outline world map, locate and label Antarctica. Add the annotation “large depletion of stratospheric ozone”. Label this depletion “ozone hole”, and note that reduced ozone protection allows ultraviolet rays to pass through the troposphere.

The geographical example and the process should be connected: Antarctica is associated with large ozone depletion, while CFCs provide an explanation for destruction of stratospheric ozone. The name of the place alone does not explain how the protective layer is weakened.

Draw the Earth's surface below a band labelled “stratospheric ozone”. Show incoming ultraviolet radiation and label ozone's absorption of it. Alongside, show reduced ozone protection, with more ultraviolet radiation passing towards the surface. Label CFCs as ozone-destroying chemicals.

Reducing the use and release of ozone-destroying chemicals addresses the cause of depletion. CFC-free refrigeration is an example of avoiding these chemicals in a familiar application. Protecting the ozone layer and limiting global warming require attention to different atmospheric processes, even though some gases are involved in both.

How does the greenhouse effect lead to global warming?

Greenhouse gases are gases that absorb the long-wave radiation emitted by the Earth's surface. Long-wave radiation has longer wavelengths than incoming short-wave solar radiation; wavelength is the distance between successive wave crests. Carbon dioxide and water vapour both help retain the Earth's radiated heat.

The processes that warm the atmosphere are often collectively referred to as the greenhouse effect. The atmosphere transmits incoming solar radiation but absorbs much of the outgoing terrestrial radiation. This helps maintain temperatures suitable for life.

What happens when heat retention increases?

1. Solar radiation reaches the Earth and provides energy that warms its surface.
2. The Earth emits energy upwards as long-wave terrestrial radiation.
3. Greenhouse gases absorb outgoing radiation and help retain heat in the atmosphere.
4. An increase in greenhouse gases can increase heat retention, raising the Earth's temperature.

Global warming is the rise in the Earth's average temperature associated with increased heat retention. The natural greenhouse effect and its strengthening are related but distinct ideas. The existence of heat-retaining gases supports liveable conditions; an increase in their concentration can disturb those conditions.

Carbon dioxide, represented by the chemical formula CO₂, is released mainly by fossil fuel combustion. Combustion means burning, and fossil fuels are fuels such as coal, oil and natural gas. Burning these fuels adds carbon dioxide to the atmosphere.

Other greenhouse gases include methane, nitrous oxide, ozone and CFCs. Their effects depend on factors including changes in concentration, their lifetime in the atmosphere and the wavelengths they absorb. The mere presence of a gas in a small quantity does not make its atmospheric effect unimportant.

Why does clearing forests matter?

Forests and oceans are carbon dioxide sinks, places that take up carbon dioxide. Forests use the gas in growth. Deforestation, meaning the removal of forests, can increase carbon dioxide concentration by reducing this uptake as land use changes.

The increasing concentration of greenhouse gases may, in the long run, warm the Earth. This statement describes a trend and its possible long-term consequence; it is not a claim that every place becomes warmer by the same amount at the same time.

What impacts can global warming have, and how is it different from ozone depletion?

The effect of global warming may not be uniform everywhere. Its consequences can affect the systems supporting life. Understanding these effects requires a chain of causes and consequences rather than treating “warming” as simply a description of one hot day.

How can warming affect coastal areas?

Warming can melt glaciers, large masses of moving ice, and ice caps, extensive coverings of ice. It also causes thermal expansion of seawater, meaning an increase in the water's volume as it warms. These processes contribute to a rise in sea level.

A rise in sea level may inundate, or flood, large parts of coastal areas and islands, leading to social problems. Coastal flooding is therefore a possible consequence connected to warming through changes in ice and ocean volume.

There may be drastic changes in the climate of a place, leading to the extinction of some plants and animals in the long run. Extinction means the disappearance of a species. The words “may”, “some” and “in the long run” are essential to this statement.

How do the two environmental problems compare?

CFCs appear in both discussions because they destroy ozone and also act as greenhouse gases. This overlap does not make the two problems identical. State which process is involved: loss of UV protection or increased retention of heat radiated by the Earth.

Efforts to limit greenhouse gas emissions address the warming problem. Restricting fossil fuel use and saving energy reduce activities associated with emissions, while protecting forests preserves carbon uptake. These responses connect human choices to atmospheric composition and its effects on living conditions.

Glossary

• Atmosphere — The envelope of air surrounding the Earth, containing gases, water vapour and particles.
• Altitude — Height above the Earth's surface when describing the atmospheric layers in this note.
• Density — Mass per unit volume, greatest near the Earth's surface and decreasing upwards.
• Water vapour — Water in its gaseous form, present in varying amounts in the atmosphere.
• Hygroscopic nuclei — Tiny particles such as dust and salt around which water vapour condenses.
• Troposphere — The lowest atmospheric layer, containing the air we breathe and almost all weather phenomena.
• Tropopause — The zone separating the troposphere from the stratosphere, where temperature is nearly constant.
• Stratosphere — The layer above the tropopause that contains the protective ozone layer.
• Ionosphere — An upper atmospheric region containing electrically charged particles and helping radio transmission.
• Exosphere — The uppermost atmospheric layer, with extremely rarefied contents, gradually merging with outer space.
• Ozone depletion — A reduction in ozone concentration that weakens protection against harmful ultraviolet radiation.
• Greenhouse gases — Gases that absorb outgoing long-wave terrestrial radiation and help retain atmospheric heat.
• Global warming — A rise in the Earth's average temperature associated with increased heat retention.
• Carbon dioxide sink — A place, such as a forest or ocean, that takes up carbon dioxide.
• Thermal expansion — An increase in volume as a substance warms, contributing to sea-level rise in seawater.

Common errors and misconceptions

• Misconception: Oxygen is the largest constituent of air. Correct: Nitrogen is the most plentiful gas; oxygen is second.
• Misconception: Every place has a troposphere exactly 13 km high. Correct: This is its average height; it extends roughly to 8 km near the poles and about 18 km at the equator.
• Misconception: The stratosphere has absolutely no clouds. Correct: It is almost free from clouds and associated weather phenomena.
• Misconception: Temperature falls continuously through every atmospheric layer. Correct: The temperature profile changes direction between different layers.
• Misconception: The ionosphere and ozone layer perform the same function. Correct: The ionosphere helps radio transmission; stratospheric ozone absorbs harmful UV radiation.
• Misconception: The ozone hole is an empty gap in all atmospheric gases. Correct: It means depletion of ozone concentration in the stratosphere.
• Misconception: Ozone depletion and global warming are identical. Correct: One weakens UV protection; the other concerns increased temperature through heat retention.
• Misconception: Global warming has exactly the same effects everywhere. Correct: Its effects may not be uniform everywhere.

Exam-style questions with model answers

Q1. A simplified air composition gives nitrogen as 78 per cent and oxygen as 21 per cent. Identify the most abundant gas and the gas that is second in abundance, giving the supplied proportion for each. [2 marks]

1. Nitrogen is the most abundant gas in the supplied composition, accounting for 78 per cent of air.
2. Oxygen is second in abundance in the supplied composition, accounting for 21 per cent of air.

Q2. Water vapour may form four per cent of air in warm, wet tropics and less than one per cent in dry, cold desert and polar areas. It decreases with altitude. State three conclusions from these data, preserving their qualifications. [3 marks]

1. In warm and wet tropical areas, water vapour may account for four per cent of air; this is not a fixed proportion everywhere.
2. In dry and cold desert and polar areas, its proportion may be less than one per cent of the air.
3. Water vapour decreases with altitude, so its proportion is variable vertically as well as between the geographical settings given.

Q3. The troposphere averages 13 km in height, extends roughly to 8 km near the poles and about 18 km at the equator, and is thickest at the equator because strong convection carries heat upwards. Use this information to describe its average, polar and equatorial heights and explain the equatorial thickness. [4 marks]

1. The troposphere has an average height of 13 km, which should not be treated as its exact height everywhere.
2. Near the poles, it extends roughly to a height of 8 km, retaining the qualification attached to the figure.
3. At the equator, it extends to about 18 km, making it thicker there than near the poles.
4. Strong convectional currents at the equator transport heat to great heights, explaining the troposphere's greater thickness there.

Q4. Use these facts to explain five atmospheric features: the troposphere is lowest and contains almost all weather; the stratosphere extends up to 50 km and contains ozone; ozone absorbs harmful UV radiation; the ionosphere contains ions and reflects radio waves towards Earth; the exosphere has extremely thin air and merges with space. Give one point for each feature. [5 marks]

1. The troposphere is the lowest atmospheric layer. Almost all weather occurs within it, connecting its position near the surface with familiar atmospheric conditions.
2. The stratosphere extends up to 50 km above the Earth's surface and contains ozone, placing this protective atmospheric component above the troposphere.
3. The ozone layer protects life by absorbing harmful ultraviolet radiation. Its role concerns protection from solar radiation, rather than the occurrence of everyday weather.
4. The ionosphere contains electrically charged particles called ions. It helps radio transmission by reflecting radio waves transmitted from the Earth back towards it.
5. The exosphere contains extremely thin air and gradually merges with outer space. Its outer edge is therefore described as a transition rather than a sharp surface.

Q5. CFCs released by human activities drift into the stratosphere and destroy ozone. Ozone absorbs harmful UV radiation, which is known to cause skin cancer. Explain the sequence from CFC release to harm in four points. [4 marks]

1. Human activities release CFCs into the atmosphere, introducing chemicals that can affect the ozone present higher above the Earth's surface.
2. The CFCs drift into the stratosphere, bringing them into the atmospheric region that contains the protective ozone layer.
3. They destroy ozone there, reducing the protection provided by ozone's absorption of harmful ultraviolet radiation from the Sun.
4. More harmful UV can therefore reach the surface, threatening living organisms; the given example of harm is skin cancer in humans.

Q6. Solar energy warms the Earth's surface; the Earth emits long-wave radiation; greenhouse gases absorb this outgoing radiation; increasing their concentration increases heat retention. Explain this sequence in four points. [4 marks]

1. Incoming solar energy reaches and warms the Earth's surface, providing the starting energy for the sequence described in the question.
2. The warmed Earth emits long-wave terrestrial radiation upwards, transferring energy away from its surface towards the atmosphere.
3. Greenhouse gases absorb outgoing terrestrial radiation, helping retain heat instead of allowing all that energy to leave directly.
4. Increasing the concentration of these gases increases heat retention, which can raise the Earth's temperature and contribute to global warming.

Q7. Warming can melt glaciers and ice caps and expand seawater. Sea-level rise may flood coastal areas and islands, causing social problems. Climate changes may lead to extinction of some plants and animals in the long run. Effects may not be uniform everywhere. Explain five impacts or qualifications using only this information. [5 marks]

1. Warming can melt glaciers and ice caps. Loss of this ice is one of the processes connected with rising sea levels in the information given.
2. Warming also expands seawater. This thermal expansion increases water volume and contributes to sea-level rise alongside the effects of melting ice.
3. Rising sea levels may flood coastal areas and islands. Such flooding can create social problems, linking physical environmental change with consequences for people.
4. Changes in climate may lead to extinction of some plants and animals in the long run. The possibility, limited scope and timescale must be retained.
5. The effects may not be uniform everywhere. The information does not justify claiming that every place experiences identical changes or the same degree of harm.

Q8. Ozone depletion reduces stratospheric ozone and weakens absorption of harmful UV. Global warming involves increased heat retention by greenhouse gases and rising average temperature. CFCs destroy ozone and are also greenhouse gases. Distinguish the two problems and explain the role of CFCs in three points. [3 marks]

1. Ozone depletion is the reduction of stratospheric ozone, which weakens the atmosphere's protection against harmful ultraviolet radiation from the Sun.
2. Global warming concerns increased heat retention by greenhouse gases and a rise in average temperature, rather than simply loss of UV protection.
3. CFCs are involved in both because they destroy ozone and act as greenhouse gases, but these are two different atmospheric effects.

Key takeaways

• The atmosphere contains gases, water vapour and particles; nitrogen and oxygen together make up its bulk.
• Water vapour varies with place and altitude, while dust and salt provide centres for condensation.
• The troposphere contains the air we breathe and almost all weather, and its thickness varies geographically.
• The stratosphere contains ozone, which absorbs harmful ultraviolet radiation and protects life on the Earth's surface.
• The ionosphere helps radio transmission, while the extremely rarefied exosphere gradually merges with outer space.
• Ozone depletion weakens protection against UV; the ozone hole is depletion, not an empty opening in air.
• Greenhouse gases retain terrestrial radiation, and increasing their concentration can increase heat retention and temperature.
• Global warming may have uneven effects, including sea-level rise that may flood coastal areas and islands.

Test yourself

How does composition differ from structure?

Composition describes atmospheric constituents; structure describes the arrangement of the atmosphere into different layers.

How do bacteria make atmospheric nitrogen available to plants?

Plants need nitrogen in a usable form. Bacteria in soil and some plant roots change atmospheric nitrogen into such forms.

What do dust and salt particles do in cloud formation?

They act as hygroscopic nuclei around which water vapour condenses to form droplets and clouds.

Why is the troposphere thicker at the equator?

Strong convectional currents transport heat to great heights, giving the equatorial troposphere a greater thickness.

What is the difference between O₂ and O₃?

O₂ represents ordinary oxygen with two atoms per molecule; O₃ represents ozone with three oxygen atoms per molecule.

What does the term ozone hole describe?

It describes depletion of stratospheric ozone concentration, with large depletion occurring over Antarctica.

Why can deforestation increase atmospheric carbon dioxide?

Forests take up carbon dioxide for growth. Removing them reduces this uptake and can increase atmospheric carbon dioxide concentration.

How can warmer seawater contribute to rising sea level?

Seawater expands as it warms. This thermal expansion increases its volume and contributes to sea-level rise.