Global warming and Green House effect | ICSE Class 9 Physics Notes

This note covers global warming, the greenhouse effect, the passage of solar energy through the atmosphere, greenhouse gases, human causes of increased warming, impacts on life, ways to reduce the problem, and the meaning and examples of energy degradation.

What are global warming and the greenhouse effect?

Definition: Global warming is the long-term rise in the Earth's average temperature. Temperature indicates how hot or cold something is. The greenhouse effect is the warming associated with gases in the atmosphere absorbing outgoing heat radiation from the Earth's surface.

The atmosphere is the air surrounding the Earth. Radiation is the transfer of energy by electromagnetic waves, which can travel through empty space. Heat is energy transferred because of a temperature difference. The Sun supplies the main source of energy for the Earth, while the atmosphere influences how that energy enters and leaves.

The surface absorbs some incoming sunlight and becomes warmer. It then gives out energy as infrared radiation, radiation with a longer wavelength than visible light. Wavelength means the distance between successive corresponding points of a wave, such as neighbouring crests.

Some atmospheric gases absorb a portion of this outgoing infrared radiation. They are called greenhouse gases. This natural warming helps keep the Earth warm enough to support life. The existence of the greenhouse effect is therefore essential to understanding the Earth's suitable temperature.

The name comes from a greenhouse, a glass structure used to preserve warmth in cold areas. Glass transmits incoming short-wave solar radiation but absorbs outgoing long-wave radiation. The comparison highlights the different treatment of incoming and outgoing radiation.

How does natural warming differ from increased warming?

The natural greenhouse effect and an increase in that effect must be distinguished. Carbon dioxide, written as CO₂, is one greenhouse gas. Excess carbon dioxide from human activities strengthens the greenhouse effect and causes global warming.

Global refers to the Earth as a whole. A change in its average temperature does not require an identical change in every place. The effects of global warming may not be uniform everywhere, even though the problem concerns the planet's connected systems.

Note: The aim is to reduce excessive human enhancement of the greenhouse effect. Removing the natural warming effect would leave the Earth too cold for life.

How does radiation produce the greenhouse effect?

The starting point is solar radiation, meaning energy received from the Sun. Incoming visible light passes relatively easily through the atmosphere. Some incoming solar radiation is reflected, and some is absorbed by the atmosphere, clouds and the Earth's surface.

Absorption means taking in radiation's energy; reflection means sending radiation back from a surface. These processes have different results. Energy reflected away is not the energy absorbed at that surface to produce warming.

What happens to energy absorbed by the surface?

1. The Sun sends radiation towards the Earth. The atmosphere transmits part of it towards the surface.
2. Land and water absorb some of the incoming radiation. This absorbed energy warms the Earth's surface.
3. The warmed surface emits infrared radiation upwards. Emission means giving out radiation; the surface's outgoing radiation has a longer wavelength than incoming visible light.
4. Greenhouse gases absorb a portion of this outgoing infrared radiation. This absorption contributes to maintaining warmth near the Earth's surface.

The phrase trapping heat is a short description of this interaction between gases and outgoing radiation. It should not be taken to mean that every ray is retained. A portion of the outgoing heat is trapped, while energy also escapes to space.

The distinction between incoming and outgoing radiation explains why sunlight can reach the surface while gases affect the escape of heat. The surface does not simply send all the absorbed visible light back unchanged. It absorbs energy and re-radiates energy in the infrared region.

Draw the Sun, the Earth's surface and an atmospheric region above it. Show incoming sunlight reaching the surface and outgoing infrared radiation rising from the warmed surface. Label absorption of part of the outgoing infrared radiation by greenhouse gases, and show some radiation escaping to space.

Why does the kind of radiation matter?

A gas's contribution depends partly on the wavelengths it absorbs. Identifying the outgoing infrared radiation is therefore central to the explanation. Saying merely that gases “stop sunlight” misses the link between the warmed ground and atmospheric absorption.

The same sequence connects the natural and enhanced effects. When excess carbon dioxide increases atmospheric absorption of outgoing heat, it strengthens an existing process. The energy still originates mainly from the Sun, while the atmosphere alters the Earth's loss of that energy.

Which gases contribute to the greenhouse effect?

Carbon dioxide, methane and water vapour absorb outgoing heat radiation. Methane is a gas written as CH₄; water vapour is water in its gaseous state. Their greenhouse role is linked to radiation absorption, rather than to being visible in the air.

Other greenhouse gases include nitrous oxide, written as N₂O, and ozone, written as O₃. Chlorofluorocarbons, abbreviated to CFCs, are human-made compounds containing chlorine, fluorine and carbon. They also contribute to the greenhouse effect.

What determines a greenhouse gas's effect?

A gas's concentration means how much of it is present in a given amount of air. Its effect depends on the increase in concentration, how long it remains in the atmosphere, and which wavelengths of radiation it absorbs.

These factors explain why listing gases is only part of the explanation. An increase in concentration changes atmospheric absorption, while a longer atmospheric lifetime means a longer time for recovery from the change. Different gases need not have identical effects.

Is ozone depletion the same as global warming?

Ozone depletion is the reduction of ozone in the protective ozone layer. That layer absorbs harmful ultraviolet radiation, radiation with shorter wavelengths than visible light. Depletion allows more of this harmful radiation to reach the surface.

Global warming concerns increased temperature associated with enhanced retention of outgoing infrared radiation. Ozone depletion concerns weakened protection against incoming ultraviolet radiation. CFCs are connected with both problems, but that connection does not make the two processes identical.

How do human activities increase greenhouse warming?

Two important causes are burning fossil fuels and deforestation. Fossil fuels are fuels formed from the buried remains of ancient organisms over very long periods; examples are coal, oil and gas. Deforestation means clearing forests.

Fossil fuels supply energy for heating, cooking, transport and industry. Combustion means burning. When these fuels burn, stored carbon is released as carbon dioxide on a very short timescale compared with the long timescale on which the fuels formed.

Why does clearing forests affect carbon dioxide?

Plants remove carbon dioxide from the atmosphere through photosynthesis, the process in which they use sunlight to make food from carbon dioxide and water. Clearing forests decreases photosynthesis and reduces the removal of carbon dioxide from the air.

A carbon sink is a store that takes up carbon from the atmosphere. Forests and oceans act as carbon sinks. Human activities affect both the addition of carbon dioxide and the natural processes that take it out of the atmosphere.

1. Fossil fuel combustion transfers long-stored carbon into the atmosphere as carbon dioxide.
2. Forest clearance reduces the photosynthesis that removes atmospheric carbon dioxide.
3. These activities increase atmospheric carbon dioxide and disturb the movement of carbon between its stores.
4. Excess carbon dioxide intensifies the greenhouse effect, contributing to global warming and its consequences.

How does the carbon cycle connect these processes?

The carbon cycle is the movement of carbon between air, living organisms, water and other stores. Plants take in carbon dioxide, while respiration, the energy-releasing process in organisms, returns carbon dioxide to the air. Decomposition, the breakdown of dead organisms, also returns carbon dioxide.

The drawing shows carbon dioxide in the atmosphere, trees, animals, water with plankton, and underground coal, oil and gas. Arrows are labelled with processes including photosynthesis, respiration, decomposition, absorption and combustion of fossil fuels. Plankton are organisms that drift in water.

See Fig. 13.13 in your NCERT textbook

The atmosphere and ocean water continuously exchange carbon dioxide. Thus, the amount in the atmosphere is connected with movement into and out of other stores. A source that releases carbon and a sink that takes it up have different roles in this balance.

Deforestation and combustion reinforce the same problem through different routes: one reduces a removal process, while the other adds carbon dioxide. A complete explanation should identify these routes instead of treating every human activity as merely “producing heat”.

How can global warming affect ice, sea level and coastal life?

The Earth's air, water, ice and living organisms interact. The cryosphere is the Earth's frozen water, including ice and snow. The hydrosphere is its liquid water, including oceans, rivers and lakes. Changes in temperature can connect changes in both.

A glacier is a large mass of ice on land that moves slowly. Rising atmospheric temperature could eventually accelerate the melting of glaciers and polar ice. This may lead to flooding in low-lying regions and can raise sea levels in the long run.

Why can sea level rise?

Melting glaciers and ice caps can contribute to rising sea level. Another contribution is thermal expansion, meaning an increase in volume as a material warms. Sea water expands as it warms, so rising sea level is not explained by melting ice alone.

Sea-level rise may inundate large parts of coastal areas and islands. Inundation means flooding land with water. Such flooding may threaten coastal cities and affect the places where people and other organisms live.

Why are these changes important for living organisms?

A habitat is the natural place where an organism lives. Flooding may cause habitat loss. An ecosystem consists of organisms interacting with one another and their physical surroundings, so a change to a habitat can disturb more than one kind of organism.

These connections show why global warming is a concern for life as well as for temperature. A change in air temperature can alter ice and water, which can then affect habitats and human settlements. The physical change and its biological or social consequence belong in the same explanation.

Note: The effects of global warming may not be uniform everywhere. A possible coastal consequence should be described as a possibility, without claiming that every coast will experience an identical change.

How can warming affect rainfall, agriculture and ecosystems?

Weather means the atmospheric conditions at a place over a short period. Climate concerns the longer-term pattern of those conditions. Global warming can affect the connected processes that shape rainfall, water availability and conditions for living organisms.

More extreme weather conditions are associated with excessive greenhouse warming. In India, this may lead to more intense monsoons and threats to agriculture from changing rainfall patterns. A monsoon is a seasonal wind system associated with seasonal rainfall; agriculture means growing crops and rearing animals.

How does warmer sea water connect with rainfall?

Evaporation is the change of liquid water into water vapour. Warmer Arabian Sea water leads to greater evaporation. Changes in the southwest monsoon can then produce variability in rainfall, bringing floods to some regions of India while leaving others in drought.

Drought is a prolonged shortage of rainfall and water. The same large-scale disturbance can therefore have different regional consequences. Describing global warming simply as “less rain everywhere” or “more rain everywhere” misses this variation.

Warmer air holds more moisture, which helps explain the possibility of more intense monsoons. Changing rainfall patterns may threaten agriculture because crop growth depends on water. The sequence connects an atmospheric change with a practical effect on food production.

How can changes extend to marine life?

Marine means related to the sea. Excess atmospheric carbon dioxide increases ocean absorption of the gas and makes sea water more acidic. This could threaten tiny plankton and coral reefs, disrupting marine ecosystems. Coral reefs are underwater structures built by colonies of coral animals.

This is a related consequence of excess carbon dioxide, while warmer ocean water also reduces the ocean's capacity to absorb carbon dioxide as an effective carbon sink. The chemical effect of added carbon dioxide and the effect of warming on uptake should be kept distinct.

Biodiversity means the variety of living organisms. Rising carbon dioxide levels from fossil fuel use are linked with extreme weather and biodiversity loss. Habitat changes help explain why the effects are not restricted to air temperature.

Forests, oceans, rainfall and living organisms form connected systems. A useful explanation follows those connections: identify the physical change, describe what it alters, and then show how life may be affected. Preserve the difference between a stated effect and a possible consequence.

How can people reduce excessive greenhouse warming?

Reducing the problem requires attention to both greenhouse gas additions and natural carbon removal. Conserving energy resources means using them carefully and avoiding waste. Renewable energy resources are resources replenished by natural processes; solar and wind energy are examples.

How do energy choices help?

Burning fossil fuels releases carbon dioxide. Conserving energy and switching to renewable resources can help restore environmental balance. Solar and wind energy provide alternatives to continued dependence on fossil fuels for energy supplies.

The connection is between the source of energy and the carbon released in obtaining it. Measures addressing energy use should therefore be linked to fossil fuel combustion. Simply listing “solar energy” without explaining that connection leaves out the reason for the measure.

Why are trees and resource conservation important?

Planting trees supports the removal of atmospheric carbon dioxide by photosynthesis. Forests also form habitats. Protecting their role in the carbon cycle connects action on warming with the protection of living systems.

Reuse means using an item again; recycling means processing discarded material so that it can be used again. Individuals can contribute by saving water, food and energy and by reducing waste. These actions form part of conserving the Earth's resources.

Local actions and global cooperation can work together. The atmosphere connects regions, while many decisions about resource use occur in homes and communities. Careful energy use and protection of carbon sinks address different parts of the same environmental problem.

The underlying aim is to limit excess greenhouse warming while maintaining the natural systems that support life. Preventing waste, changing energy sources and supporting vegetation are connected measures, rather than separate slogans to memorise without their causes.

What does energy degradation mean?

Definition: Energy degradation is the change of energy into a form that is less available for useful work, commonly when energy spreads into the surroundings as heat.

A force is a push or pull, and displacement is a change in position. Work is energy transferred when a force has a component parallel to an object’s displacement. Heat transfer instead results from a temperature difference.

Internal energy is the energy associated with the microscopic motion and interactions of the particles making up a body. Energy that was available as organised motion can become spread among those particles. It is then less readily available for the original task of producing motion.

Energy degradation concerns usefulness, rather than the disappearance of energy. When motion is reduced by friction, energy is transferred into internal energy of the object and its surroundings. Friction is a force opposing relative motion between surfaces in contact.

How can energy be conserved but become less useful?

Conservation of energy means that total energy is neither created nor destroyed, but transferred or transformed. Here, transformation means a change from one form of energy to another. The energy account must include the surroundings as well as the moving object.

Mechanical energy is energy associated with motion and position. Its conversion into internal energy can reduce the energy available for useful motion even though total energy remains conserved. This distinction is essential when discussing practical energy losses.

A body and its surroundings do not spontaneously collect dispersed heat and convert it entirely back into the body's original motion. Spontaneously means happening on its own. The direction of the process matters as well as the total amount of energy involved.

Note: “Lost energy” in an everyday description often means energy no longer useful for the intended task. It does not mean energy has ceased to exist.

Which examples show energy degradation in everyday processes?

Examples of degradation are clearest when the initial useful energy and the final energy transfer are both identified. The important question is what happens to the energy when a moving body slows or when heat spreads through a body and its surroundings.

What happens when a moving body stops through friction?

A moving body has kinetic energy, meaning energy due to motion. When friction brings it to rest, its mechanical energy is transferred as heat to the body and floor. The body's motion disappears, but its energy has been transferred rather than destroyed.

This is energy degradation because energy formerly available as organised motion becomes less available for doing useful work. Merely saying “friction wastes energy” is incomplete; the explanation should state that the body and floor gain internal energy.

Draw a moving body on a rough floor, with an arrow showing its motion. Beside it, draw the body at rest. Connect the stages with the label “friction”, and label the final transfer “increase in internal energy of body and floor”.

What happens to a rotating blade in a liquid?

Viscosity is a liquid's resistance to relative motion within it. A rotating blade can slow down and stop because of this resistance. Its loss of mechanical energy is accompanied by a gain in the liquid's internal energy.

The comparison with sliding friction is useful: the resisting interaction differs, but both examples transfer mechanical energy into internal energy. Dissipation describes this spreading of energy into forms less available for the original useful motion.

What happens when a hot vessel cools?

The base of a vessel on an oven is hotter than its other parts. After removal, heat spreads from the base to the other parts until the vessel reaches a uniform temperature. In due course, it cools to the temperature of its surroundings.

The reverse does not happen spontaneously: one part does not cool itself to make the base hot again. This shows that heat spreading and energy conservation are compatible. The total energy account remains valid even when the original temperature difference is lost.

Frictional stopping, resistance in a liquid and cooling all illustrate why careful energy use matters. Degradation describes reduced availability of energy; global warming describes a rise in the Earth's average temperature. They are distinct ideas connected with energy use and its consequences.

Glossary

• Global warming — The long-term rise in the average temperature of the Earth as a whole.
• Greenhouse effect — Warming associated with atmospheric gases absorbing outgoing heat radiation from the Earth's surface.
• Greenhouse gas — A gas that absorbs outgoing infrared radiation and contributes to atmospheric warming.
• Infrared radiation — Radiation with wavelengths longer than visible light, emitted by the warmed Earth's surface.
• Atmosphere — The air surrounding the Earth, affecting incoming radiation and the escape of heat.
• Fossil fuels — Fuels such as coal, oil and gas formed from ancient buried organic remains.
• Deforestation — The clearing of forests, reducing vegetation and its removal of carbon dioxide through photosynthesis.
• Carbon sink — A store, such as a forest or ocean, that takes up atmospheric carbon.
• Thermal expansion — An increase in the volume of a material as its temperature rises.
• Habitat — The natural place in which an organism lives and obtains conditions needed for life.
• Energy degradation — A change making energy less available for useful work, commonly through dispersal as heat.
• Internal energy — Energy associated with microscopic particle motion and interactions within a body or material.

Common errors and misconceptions

• Misconception: The greenhouse effect is entirely harmful. Correct: Natural greenhouse warming helps support life; excessive enhancement of it causes global warming and associated problems.
• Misconception: Greenhouse gases keep all radiation from escaping. Correct: A portion of outgoing heat is absorbed; energy also escapes to space.
• Misconception: Carbon dioxide is the only greenhouse gas. Correct: Methane, water vapour, nitrous oxide, ozone and CFCs also contribute.
• Misconception: Deforestation has no connection with atmospheric warming. Correct: Forest clearance decreases photosynthesis and reduces atmospheric carbon dioxide removal.
• Misconception: Ozone depletion and global warming describe the same process. Correct: Ozone depletion weakens protection against ultraviolet radiation; enhanced greenhouse warming involves outgoing infrared radiation.
• Misconception: Global warming changes every place in the same way. Correct: Its effects may not be uniform everywhere, including changes in rainfall and coastal conditions.
• Misconception: Energy degradation means energy is destroyed. Correct: Energy remains conserved while becoming less available for the intended useful work.

Exam-style questions with model answers

Q1. Define global warming and state one reason why the natural greenhouse effect is important for life. [2 marks]

1. Global warming is the long-term rise in the Earth's average temperature.
2. The natural greenhouse effect retains part of the outgoing heat and keeps the Earth warm enough to support life.

Q2. Explain the greenhouse effect in four stages, beginning with incoming sunlight and ending with atmospheric absorption of outgoing radiation. [4 marks]

1. Incoming visible sunlight passes relatively easily through the atmosphere and reaches the Earth's surface.
2. The surface absorbs some of this incoming energy, causing land and water to become warmer.
3. The warmed surface emits energy upwards as infrared radiation, which has a longer wavelength than visible light.
4. Greenhouse gases absorb a portion of this outgoing infrared radiation, contributing to warming near the Earth's surface.

Q3. Burning coal releases carbon dioxide. Forest plants remove carbon dioxide through photosynthesis. Use these facts to explain, in three linked points, how coal burning and forest clearance can increase greenhouse warming. [3 marks]

1. Burning coal transfers stored carbon into the atmosphere as carbon dioxide, increasing the addition of this greenhouse gas.
2. Clearing forests reduces the photosynthesis carried out by forest plants, so less carbon dioxide is removed through that process.
3. The combined increase in additions and reduction in removal raises atmospheric carbon dioxide, strengthening absorption of outgoing heat and enhancing greenhouse warming.

Q4. Rising temperature could accelerate glacier melting, while warming sea water expands. Rainfall patterns may also change. Explain five possible consequences for sea level, coastal land, habitats and agriculture, using these processes. [5 marks]

1. Accelerated melting of glaciers can contribute to rising sea level by transferring water from ice on land into the sea.
2. Thermal expansion of warming sea water increases its volume, providing a further contribution to sea-level rise alongside melting ice.
3. Higher sea level may inundate coastal areas and islands, threatening settlements and creating problems for the people living there.
4. Flooding and other changes to natural living places may cause habitat loss, disturbing organisms and the ecosystems of which they form part.
5. Changing rainfall patterns may threaten agriculture by altering the water conditions on which crops depend; effects may not be uniform everywhere.

Q5. Suggest three measures that can help reduce excessive greenhouse warming, and explain the link between each measure and fossil fuel use or carbon dioxide removal. [3 marks]

1. Conserve energy resources and avoid waste, reducing unnecessary use of the resources that supply energy, including fossil fuels.
2. Switch towards renewable resources such as solar and wind energy, providing alternatives to energy obtained through burning fossil fuels.
3. Plant trees to support photosynthesis, which removes carbon dioxide from the atmosphere and helps vegetation act as a carbon sink.

Q6. A body moves over a rough floor and stops. Its lost mechanical energy increases the internal energy of the body and floor. Explain energy degradation in this example and why it does not contradict conservation of energy. Give four separate points. [4 marks]

1. The moving body initially has kinetic energy, which is energy associated with its motion over the floor.
2. Friction opposes the motion and brings the body to rest, so its mechanical energy decreases.
3. That energy increases the internal energy of the body and floor, becoming less available for the original useful motion; this is degradation.
4. Total energy is conserved when the body and floor are both included, because the energy is transferred rather than destroyed.

Q7. Distinguish ozone depletion from enhanced greenhouse warming by identifying the radiation involved and the main effect in each case. [2 marks]

1. Ozone depletion reduces absorption of incoming ultraviolet radiation by the protective ozone layer, allowing more harmful ultraviolet radiation to reach the surface.
2. Enhanced greenhouse warming increases retention of outgoing infrared radiation by greenhouse gases, contributing to a rise in the Earth's average temperature.

Q8. A rotating blade slows in a liquid, whose internal energy increases. Separately, a hot vessel cools to the temperature of its surroundings. Explain these two energy changes and energy degradation in five points. Do not use numerical calculations. [5 marks]

1. The rotating blade initially has mechanical energy associated with its motion, while the liquid resists relative movement within it through viscosity.
2. As the blade slows, its mechanical energy decreases and the liquid gains internal energy, accounting for the energy transferred during the process.
3. The hot vessel transfers heat to its cooler surroundings, eventually losing the temperature difference that existed between the vessel and those surroundings.
4. These processes illustrate degradation because energy becomes more spread out and less available for the original useful motion or work.
5. The energy has not been destroyed: conservation requires including the liquid or surroundings that receive it, as well as the blade or vessel.

Key takeaways

• The natural greenhouse effect helps maintain warmth needed for life, while excessive enhancement contributes to global warming.
• The warmed Earth's surface emits infrared radiation, and greenhouse gases absorb a portion of this outgoing energy.
• Carbon dioxide, methane, water vapour, nitrous oxide, ozone and chlorofluorocarbons all contribute to the greenhouse effect.
• Fossil fuel combustion adds carbon dioxide, while deforestation reduces its removal through photosynthesis in forest plants.
• Melting glaciers and thermal expansion of sea water contribute to sea-level rise, which may threaten coastal habitats and settlements.
• Global warming may affect rainfall and agriculture differently across regions; its effects may not be uniform everywhere.
• Energy conservation, renewable resources, tree planting and reducing waste contribute to protecting the Earth's environmental balance.
• Energy degradation reduces availability for useful work without destroying energy; frictional stopping and cooling illustrate this distinction.

Test yourself

Which radiation emitted by the Earth's warmed surface is central to the greenhouse effect?

Outgoing infrared radiation is absorbed by greenhouse gases, contributing to atmospheric warming.

Why is some greenhouse warming necessary?

It keeps the Earth warm enough to support life by retaining a portion of the outgoing heat.

How does removing forest trees affect carbon dioxide uptake?

It decreases photosynthesis, reducing the removal of atmospheric carbon dioxide by forest vegetation.

What process besides melting ice contributes to sea-level rise?

Thermal expansion increases the volume of sea water as its temperature rises.

Why should regional effects of global warming be described carefully?

The effects may not be uniform everywhere, and rainfall changes can differ between regions.

What is the difference between ultraviolet protection and greenhouse warming?

The ozone layer absorbs harmful incoming ultraviolet radiation. Greenhouse gases absorb outgoing infrared radiation from the Earth's warmed surface.

Where does a body's mechanical energy go when friction stops it?

It increases the internal energy of the body and the floor or other surroundings.

Does energy degradation contradict conservation of energy?

No. Total energy remains conserved, but becomes less available for the intended useful work.