7.3What are the causes of global warming?

© Oxford University Press 2009

Part 7 Global warming─Are humans responsible?

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7.37.3 What are theWhat are the causes of globalcauses of global warming?warming?



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What will happen to the insolation that reaches the earth’s surface?

9 unitsSolar radiation reflected by the earth’s surface

49 unitsSolar radiation absorbed by the earth’s surface

Only ____ units of solar radiation

reaches the earth’s surface

58

As the insolation reaches the earth’s surface, 9 units of it is reflected to space

The remaining 49 units of it is absorbed by the earth’s surface

The absorbed energy heats up the earth’s

surface



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The heat at the earth’s surface is transferred into the atmosphere through conduction, convection

and radiation and eventually leaves the earth

Some heat is trapped by the greenhouse gases in the atmosphere and is re-

radiated back to the earth’s surface

The earth’s average temperature remains relative stable since solar

radiation will eventually leave the earth in the same amount as it comes in.



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The atmosphere

Some radiation is released back into space

DustWater vapour

Greenhouse

gases

A lot of solar radiation enters and passes through the atmosphere

Heat from the earth’s surface is

released in the form of long wave

radiationSome heat is absorbed

and then re-radiated back to the earth’s

surface

As heat is retained, the atmosphere is kept warm


The insolation reaches the earth’s surface The earth’s

surface heats up

Greenhouse gases act as a blanket and keep the lower atmosphere warm. This is know as ________________.greenhouse effect



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The atmosphere

Some radiation is released back into space

DustWater vapour

Greenhouse

gases

A lot of solar radiation enters and passes through the atmosphere

Heat from the earth’s surface is

released in the form of long wave

radiationSome heat is absorbed

and then re-radiated back to the earth’s

surface

As heat is retained, the atmosphere is kept warm


The insolation reaches the earth’s surface The earth’s

surface heats up

The average temperature of the earth’s surface has gone up since the late 19th century.

Why?



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How is the greenhouse effect being intensified?

Greenhouse gases block the ( short / long ) wave radiation from leaving the earth.

Some greenhouse gases are found naturally in the atmosphere

Carbon dioxide (CO2)

Methane (CH4)

Nitrous oxide (N2O)



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What are greenhouse gases?1 Carbon dioxide

Carbon dioxide is the principal greenhouse gas in terms of the greenhouse effect.

It circulates around the earth in the carbon cycle.

Combustion

Respiration

Respiration

Photosynthesis

Photosynthesis

Carbon dioxide

When carbon dioxide reaches the atmosphere, it is

captured, transformed and stored mainly in the oceans

and through photosynthesis.

How does deforestation affect the carbon cycle?



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What are greenhouse gases?2 Methane

Methane is produced when trees or leaves decompose in an environment where there is ( insufficient / sufficient ) oxygen.It is naturally produced in wetlands and oceans.

Wetlands account for about 76% of global methane emissions from natural source(Credit: Biosphoto/Gunther Michel/Peter Arnold Inc.)



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What are greenhouse gases?3 Nitrous oxide

Oceans are a source of nitrous oxide

Nitrous oxide is released naturally from oceans and by bacteria in soils.



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What are the human activities that intensify the greenhouse effect?

The graphs show the changes in the

concentration of the greenhouse gases in the

atmosphere.

Carbon dioxide Methane

Nitrous oxide Since 1800, the concentration of the three greenhouse gases in the atmosphere has been ( decreasing / increasing ).

Why?



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What are the human activities that intensify the greenhouse effect?1 Burning of fossil fuels

Respiration

PhotosynthesisCombustion of fossil fuels

RespirationPhotosynthesis

Human activity has

disturbed the carbon cycle.

Factories, power plants and vehicles emit large amounts of carbon dioxide.



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What are the human activities that intensify the greenhouse effect?2 Deforestation

A cleared site after deforestation

Deforestation through the burning of trees releases carbon dioxide.Fewer trees are available for absorbing the extra amount of carbon dioxide from the atmosphere.

The increased carbon dioxide in the atmosphere will disturb the carbon cycle.



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What are the human activities that intensify the greenhouse effect?3 Agriculture

How does agriculture intensify the

greenhouse effect?



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What are the human activities that intensify the greenhouse effect?3 Agriculture

Burning organic waste and cattle breathing release carbon dioxide.Paddy growing and the digestive process of cattle release methane.The use of nitrogen fertilizers releases nitrous oxide.

How does rapid population growth

intensify global warming?

Population Fooddemand

Agriculturalactivities

Release ofgreenhousegases



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What are the human activities that intensify the greenhouse effect?

People produce enormous amount of garbage every day, while most of the garbage goes into landfills and incinerators.

Methane is released when garbage decomposes in the landfills

A landfill in Phnom Penh, Cambodia (Credit: Ng Kim Hung)

4 Garbage treatment

An incinerator

Carbon dioxide and nitrous oxide are emitted when garbage is burnt in the incinerators

(Credit: Panorama Stock Photo Co. Ltd.)



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What are the human activities that intensify the greenhouse effect?5 Industrial activities

Manufacturing has produced man-made greenhouse gases in the atmosphere.

Chlorofluo-rocarbons

(CFCs)

Hydrofluo-rocarbons

(HFCs)

Perfluoroc-arbons (PFCs)

Sulphur hexafluori-

de (SF6)

Nitrous oxides (N2O)

Production of foams, refrigerants and solventsAluminium smelting and magnesium productionProduction of nylon

Foam products



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Why should we care about the amount of greenhouse gases in the atmosphere?

Greenhouse gases Lifetime (years)

Carbon dioxide 5-200Methane 12Nitrous oxide 114CFCs 10-over 100HFCs 260

PFCs 10,000-over 50,000

SF6 3,200

Estimates of the lifetime of greenhouse gases in the atmosphere

As greenhouse gases are released, they will stay in the atmosphere for a ( short / long ) time.That means greenhouse gases will continue to trap heat for a long time even if people stop emitting them.



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Are there factors other than human activities leading to temperature change?

Temperature change

Changes in the shape of the earth’s orbit

Changes in the tilt of the

earth’s axis

Amount of aerosols in the

atmosphere



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1 Changes in the shape of the earth’s orbit

The earth’s orbit goes from elliptical to nearly circular over a 100,000-year cycle

The shape of the earth’s orbit changes during a cycle that spans about 100,000 years.

Earth Earth

Nearly circular orbit

Elliptical orbit

Sun Sun

The shortest distance

The longest distance

When the orbit is most elliptical, the difference in the amount of solar radiation received by the earth between the nearest and the farthest approach of the earth to the sun will be the ( least / greatest ).



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2 Changes in the tilt of the earth’s axis

The obliquity of the earth’s axis changes over time, ranging from about 22.8° to 24.4° and back over a cycle of about 41,000 years.

Changes in the angle of the earth’s axis

At present, the earth is tilted at an angle of ______ to the vertical.23.5°

(Credit: NASA)



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2 Changes in the tilt of the earth’s axisChanges in obliquity influence the intensity of insolation received in different seasons.

This situation is more obvious at high-latitude regions.

High obliquity (24.4°)

Larger angle of the sun

Low obliquity (10°)

The ( lower / higher ) the obliquity, the stronger the summer sun.

Note: The difference in the degree of obliquity of the earth’s axis has been exaggerated in order to show the variation clearly. The earth does not tilt at 10° in reality.Summer



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2 Changes in the tilt of the earth’s axisChanges in obliquity influence the intensity of insolation received in different seasons.

High obliquity (24.4°)Low obliquity (10°)

Note: The difference in the degree of obliquity of the earth’s axis has been exaggerated in order to show the variation clearly. The earth does not tilt at 10° in reality.

The higher the obliquity, the ( weaker / stronger ) the winter sun.Therefore, high obliquity results in ( smaller / larger ) temperature difference between winter and summer.

Winter

Smaller angle of the sun



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2 Changes in the tilt of the earth’s axisHow do changes in

obliquity affect polar regions?

Obliquity

High Low

( Less / More ) insolation received in

summer in polar regions

( Cooler / Warmer ) summers in polar

regions

Melting of ice sheets

( Less / More ) insolation received in

summer in polar regions

( Cooler / Warmer ) summers in polar

regions

Glaciation



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3 Amount of aerosols in the atmosphereAerosols are tiny particles suspended in the air.Most of the aerosols can reflect radiation back into space, therefore reducing the amount of energy reaching the earth’s surface.

The 1980 eruption of Mount St Helens, the USA

For example, during volcanic eruptions, a large amount of ash, dust and gases are ejected into the air

(Credit: Gary Braasch/Corbis)

7.3What are the causes of global warming?

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