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Chapter 12: Composition and Structure of the Atmosphere

Every year, as winter closes over Antarctica, the air above the continent grows colder than the air anywhere else on earth. Clouds form high in the stratosphere — not clouds of water droplets, but clouds of ice, more than twenty kilometres above the ground. Nothing seems to happen while the long polar night lasts. Then the sun returns in the Antarctic spring, and the thin shield of ozone that has been sitting quietly overhead begins to disappear. Nobody standing on the ice could watch it happen. It was found from far above. Satellite imagery gathered by NASA in the United States showed an area over the southern polar region where the ozone had thinned away, and scientific work on it has been going on ever since. In September 2000, the largest such ozone-depleted area yet noticed appeared near Antarctica. What makes this strange is the distance involved. The chemicals blamed for it escaped from refrigerators, air conditioners and aircraft in places thousands of kilometres away, where nobody has ever seen an ice cloud. They rose, they drifted, and they did their damage over a continent almost nobody lives on. The atmosphere, it turns out, does not respect the lines we draw on maps.

Diagram

Fig. 12.A1 Caption: Where the ozone layer thins, more of the sun’s ultraviolet radiation reaches the ground below. Brief: Curved section of the earth seen from space with a satellite in orbit above the southern polar region; show a complete ozone band at 20 to 50 km with downward ultraviolet arrows stopping at it, and beside it a thinned patch of the same band with the arrows passing straight through to the surface; label the band Ozone layer and both arrow sets Ultraviolet radiation.

NoteCuriosity Corner

Q1. Why does the air get colder and harder to breathe as you climb higher, when you are moving closer to the sun? Q2. Ozone sits far above the clouds and the weather. Which layer holds it, and what is it doing up there? Q3. Why does an ozone hole form over Antarctica and not over the Arctic, when both are bitterly cold? Q4. How can a gas that leaks from a refrigerator change the temperature of the whole earth? This chapter follows the air upwards, layer by layer, and answers all four.

12.1 Blanket Of Air

The earth is not a single thing. It is divided into three domains — the lithosphere, which is the solid rock and soil of the earth; the hydrosphere, which is all the water on it; and the air that lies above both. Together these three domains make up the biosphere, the zone in which life exists.

Like matter itself, these three domains exist in three states. The lithosphere exists in the solid state, the atmosphere in the gaseous state and the hydrosphere in the liquid state. Each of them occupies space and possesses inertia, which means each resists being moved and, once moving, tends to keep moving until something acts on it.

Each domain has a purpose in supporting life on earth. In its gaseous state, the atmosphere is characterised by momentum and pressure. Momentum is what a moving mass of air carries with it, which is why wind can push a boat or bend a tree. Pressure is the push the air exerts on every surface it touches, and it is the reason the atmosphere regulates the heat around us rather than letting it escape freely.

Flowchart

Fig. 12.A2 Caption: The three domains of the earth exist in three different states, and life occupies the zone where they meet. Nodes: biosphere → lithosphere (solid state — rock and soil) / hydrosphere (liquid state — oceans, rivers and ice) / atmosphere (gaseous state — the air above) → each occupies space and possesses inertia → the three remain in contact → back to biosphere, the zone of life

Q. If air is only gas, how can it press down on us?

A. Because gas still has weight. Gravity pulls every molecule downwards, so the whole column of air above you rests on you and pushes in from every side.

Air That Thins With Height

The blanket of air surrounding the earth is known as the atmosphere. It exists in several layers around the earth, and these layers become thinner at high altitudes. One therefore finds it difficult to breathe as one goes up, because less air is pressing down from above, so each breath draws in fewer gas molecules than the same breath would at sea level.

The thinning is not gradual and even. Half of the total mass of air is found below 5 km, touching the surface of the earth. Above that height, the remaining half is spread out over hundreds of kilometres. What holds all of it close to the earth is gravity, the same pull that keeps you on the ground.

Diagram

Fig. 12.A3 Caption: Half the mass of the air lies below 5 km, so the blanket is thickest exactly where we live. Brief: Vertical column of air from sea level upwards with a height scale marked in km; dots representing air molecules crowded at the base and thinning steadily with height; mark the 5 km line and label it Half the total mass of air lies below this line; a downward arrow along the column labelled Gravity.

An Earth Without Gravity Remove: the gravity that holds the air close to the earth. Then: the molecules, already thinning upwards, drift away into space with nothing pulling them back; the pressure at the surface falls to nothing, and no layers can form. Which tells us: gravity is what packs half the air below 5 km and gives the atmosphere its layered, bottom-heavy shape.

What The Blanket Does For Life

The atmosphere is not merely present above us; it works. Four of its jobs matter most: (a) It protects the earth from the harmful ultraviolet and infrared rays of the sun, absorbing them high above the surface before they can reach living things. (b) It holds oxygen and nitrogen, the life sustaining gases, in the layer where breathing organisms actually live. (c) It helps in retaining the necessary warmth on the earth, so that heat gained by day is not entirely lost by night. (d) It helps in the circulation of water vapour, which is the source of rainfall, carrying moisture from where it evaporates to where it falls.

Each of these depends on what the air is actually made of, and in what proportions.

12.2 Composition Of The Atmosphere

The atmosphere is not one gas but a mixture of many gases and tiny solid particles. A clean dry sample of air is made up of 78% of nitrogen and 21% of oxygen. Clean and dry matters here, because the sample has had its dust and its water vapour taken out before being measured.

The proportions of a clean dry sample are shown in Fig. 12.1. Two gases take up almost the whole of it, and everything else shares what is left.

Graph

Fig. 12.1 Caption: Composition of the atmosphere in a clean dry sample of air. Brief: Pie chart with three segments — nitrogen 78%, oxygen 21%, all others 1%; label each segment with its name and percentage; keep the 1% segment visible and distinctly coloured so it can be pointed to in the text.

Fig. 12.1 The whole circle is one sample of air, and each slice is that gas’s share of it, in per cent. Nitrogen and oxygen together take 99%. It does not tell you how much air there is — only how it is divided. And because the sample is clean and dry, the water vapour and dust of real air are missing from it.

The Two Main Gases

Oxygen gas helps in burning and breathing. Without breathing oxygen, most living things cannot survive, which is why the depth of air we live in matters so much.

Nitrogen is a colourless inert gas and has no taste or smell. Inert means it does not readily take part in chemical reactions. Its job in the air is therefore a quiet one: it dilutes the oxygen and slows down the process of oxidation, which is the reaction in which a substance combines with oxygen, as happens when iron rusts or when fuel burns. Pure oxygen would let things burn far too fiercely.

Nitrogen is essential for all living things, but they cannot use it directly from the atmosphere. Plants obtain it from the soil, and animals obtain nitrogen by consuming plants, directly or indirectly. These two gases are responsible for life on the earth. They are, however, insignificant with respect to weather phenomena — the rain, cloud and storm you actually notice are made by gases present in far smaller amounts.

Q. If nitrogen is 78% of the air, why must plants take it from the soil?

A. Because it is inert. Nitrogen in the air will not react into a form a root or a leaf can use, so plants draw it from the soil instead.

The Remaining One Per Cent

The remaining 1% of air is made up of a number of gases. These include carbon dioxide, water vapour, helium, hydrogen and argon.

Carbon dioxide (CO₂) is needed by plants for their survival. Water vapour is responsible for all forms of precipitation, and hence for the circulation of fresh water — the movement of water from sea to sky to land and back again, which you met in earlier classes as the water cycle.

Water vapour is added to the atmosphere in three ways: (a) Evaporation from oceans, lakes and rivers turns liquid water into vapour and lifts it into the air. (b) Transpiration from vegetation releases vapour through the pores of leaves, so a forest quietly humidifies the air above it. (c) Respiration of animals returns vapour to the air with every breath out.

Flowchart

Fig. 12.A4 Caption: Water vapour enters the air by three routes and returns to the surface as precipitation. Nodes: evaporation from oceans, lakes and rivers / transpiration from vegetation / respiration of animals → water vapour in the atmosphere → precipitation in all its forms → fresh water on land and in the sea → back to evaporation

Solid Particles In The Air

Air also carries solid matter. Pollen from plants, dust from dust storms, ash from meteorites and volcanic eruptions, smoke from fire and salt particles from seas are all present in the lower layer of the atmosphere.

These particles do two things that matter. They act as nuclei for water vapour, which means each particle offers a tiny surface for vapour to gather on and form raindrops around. They also scatter the sun’s radiation during sunrise and sunset, breaking the light up and giving those hours their colour. The amount of dust varies from place to place, so no two skies are quite alike.

Air With No Dust In It Remove: the solid particles that float in the lower atmosphere. Then: water vapour has no surface to gather on, so raindrops cannot form around anything; sunlight passes without being scattered at sunrise and sunset. Which tells us: the particles we think of as dirt are doing real work — rain and the colours of the evening sky both need them.

12.2.1 Functions Of The Atmosphere

The atmosphere earns its place by what it does, and its work can be set out in six points: (a) Air has weight and exerts pressure. Being pressed down, it always remains in contact with land and water. (b) Due to their physical contact, exchange of gases between air, land and water takes place on a continuous basis. (c) Soil absorbs oxygen and nitrogen because of this contact, and water vapours in air rise from oceans on account of evaporation. (d) Carbon dioxide in air helps in absorption of heat and keeps the earth warm at night, when there is no sunshine arriving to replace what is lost. (e) Because of its composition, the atmosphere is the most dynamic entity, with large masses of air being moved up and down and across the surface of the earth. (f) The energy of the sun is responsible for keeping the atmosphere in this dynamic state, since it is solar heating that sets the air moving in the first place.

12.3 Structure Of The Atmosphere

In the atmosphere there are concentric layers of air, one wrapped around the next like the skins of an onion. Each layer has a different density. The upper layers press down on the lower ones, so the air of the lower layer is always heavier or denser, while the upper layers are thinner or less dense.

This is why the air is not spread evenly through its great depth. About 90 per cent of the mass of air lies within a height of about 20 km from the surface of the earth, and everything above that is the thin remainder.

On the basis of the characteristics of temperature and air pressure there exist four thermal layers of the atmosphere. They are the Troposphere, the Stratosphere, the Mesosphere and the Thermosphere. The Thermosphere is itself divided into the Ionosphere and the Exosphere. Their arrangement, one above the other, is shown in Fig. 12.2.

Diagram

Fig. 12.2 Caption: Structure of the atmosphere, showing each thermal layer and the height it reaches. Brief: Curved cross-section of the earth with the layers drawn as bands above the surface — Troposphere 8 to 18 km, Stratosphere to 50 km with the Ozone layer marked at 20 to 50 km, Mesosphere to 80 km, Thermosphere with Ionosphere at 400 km, and Exosphere above; label each band with its name and height, and show aircraft, weather cloud and satellite at their correct levels.

Fig. 12.2 The bands run upward from the ground, and the number on each is the height its top reaches in km. Notice that the layers get thicker as you go up, while the air in them gets thinner. The drawing shows where each layer ends. It cannot show temperature, which does not simply fall all the way up.

Flowchart

Fig. 12.A5 Caption: The four thermal layers, their boundaries, and the two divisions of the thermosphere. Nodes: atmosphere → troposphere (8 to 18 km) → tropopause → stratosphere (to 50 km) → stratopause → mesosphere (to 80 km) → mesopause → thermosphere → splits into ionosphere (to about 400 km) / exosphere (400 to 1500 km) → merges with interplanetary space

12.3.1 Troposphere

The troposphere is the lowermost and densest layer, the one we live in and breathe. Its height varies from 18 km at the Equator to 8 km at the Poles, because warm equatorial air expands upward while cold polar air does not.

Its characteristics follow from that density and that heating: (a) Seventy-five per cent of the atmosphere is found in the troposphere, and its concentration decreases as we go higher from ground level. (b) The troposphere is characterised by a regular decrease in temperature with altitude. This decrease in temperature is known as the normal lapse rate, and the average decrease is 1°C for every 166 metre altitude gained. (c) This temperature variation within the troposphere is responsible for many turbulences, since air of different temperatures does not sit still beside itself. (d) As a result of these turbulences, all weather phenomena take place in this layer — every cloud, every storm and every shower you have ever seen. (e) The troposphere is heated more from below than from above, because the ground absorbs the sun’s energy first and then warms the air resting on it. (f) The upper limit of the troposphere is known as the tropopause, the ceiling at which this cooling with height stops.

WC 12.A1 — How Much Colder At The Top Of The Troposphere The situation: the normal lapse rate given above — an average fall of 1°C for every 166 metre of altitude gained — applied over the depth of the troposphere at the Poles, which this section gives as 8 km. The working: the rule can be written as \[\text{Temperature fall in }°\text{C} = \frac{\text{Altitude gained in metres}}{166}\] where 166 metres is the climb needed for a fall of one degree Celsius. Step 1, put the height into metres: \(8 \text{ km} = 8 \times 1000 = 8000 \text{ m}\). Step 2, divide by 166: \(\frac{8000}{166} = 48.19\). Step 3, read the answer with its unit: the air at the tropopause over the Poles is about 48.2°C colder than the air at the surface below it.

Almost fifty degrees separate the bottom of this layer from its top, and the two are only eight kilometres apart. Air that cold sits directly above air that warm, and the difference will not hold still — the lighter warm air rises, the denser cold air sinks, and the whole layer churns. That churning is the turbulence of point (c), and it is why weather happens here and nowhere else.

Q1. Why does the air get colder and harder to breathe as you climb higher, when you are moving closer to the sun?

A. The troposphere is heated from below, by the ground, not from above. So temperature falls by about 1°C every 166 m of climb. The air also thins upwards, so each breath draws in fewer molecules.

12.3.2 Stratosphere

This layer lies above the tropopause and extends to a height of 50 km from sea level. Its air is thin, cold and dry, and the temperature is about −55°C.

Its characteristics are these: (a) The ozone layer, a thin concentration of the gas ozone, lies within the stratosphere. Ozone is generally found between the altitudes of 20 and 50 km. (b) Ozone absorbs the ultraviolet solar radiation coming from above. The insolation received from the sun is equal to that lost by it, and that is why the temperature of the layer is constant. In the higher levels, however, temperature increases with height. (c) The lower layer of the stratosphere has a virtual absence of water vapour and has constant temperature conditions. With no vapour to build cloud and no temperature difference to stir the air, it is ideal for flying jet aircraft. (d) Temperature rises from −60°C at the base of the stratosphere to 0°C at the stratopause, the reverse of what happens in the layer below. (e) The upper limit of the stratosphere is known as the stratopause.

Q2. Ozone sits far above the clouds and the weather. Which layer holds it, and what is it doing up there?

A. The stratosphere holds it, between 20 and 50 km. There the ozone absorbs the sun’s ultraviolet radiation, losing as much heat as it takes in, which is why the temperature of the layer stays constant.

12.3.3 Mesosphere

This layer lies above the stratosphere and extends to a height of 80 km from ground level. The temperature in this layer decreases with height and reaches a minimum of −110°C at the altitude of about 80 km, near the mesopause, which is the upper limit of this layer.

The mesosphere is also where the highest clouds of all are seen. Meteoric dust particles — the fine debris left by meteors burning up — hang at this height, and sunlight reflected from them shows up as wispy clouds in an otherwise empty layer.

12.3.4 Thermosphere

The thermosphere extends from the mesopause upward, and it consists of two layers: the Ionosphere and the Exosphere.

12.3.4.1 Ionosphere

The ionosphere extends up to a height of about 400 km. It contains electrically charged particles called ions, which are atoms that have gained or lost an electric charge, and these create a sheet like display of light known as the Aurora Borealis in the Northern Hemisphere and the Aurora Australis in the Southern Hemisphere.

The ions do something else that matters daily. They reflect radio waves back to the earth’s surface, so a signal sent up from one place bounces down again far away, and this enables us to have wireless communication.

12.3.4.2 Exosphere

The exosphere is the uppermost layer in the structure of the atmosphere. It lies between 400 and 1500 km above the earth. Its characteristics are these: (a) It merges gradually with interplanetary space, so the atmosphere has no sharp outer edge. (b) In this layer temperature increases with height, though we know very little about the atmosphere extending beyond 640 km height from the sea level. (c) In the exosphere, density becomes extremely low, with enormous distances between the few particles left. (d) The main gases here are the lighter ones, hydrogen and helium, which are the last to be held down by gravity.

Where The Blanket Ends

From the above it is clear that the atmosphere extends more than a thousand kilometres above the earth’s surface. It exercises enormous pressure on the surface of the earth and remains in direct contact with the surface of land as well as the surface of water on the earth.

The contact zone between land, air and water sustains life. The survival of life on earth depends on this contact, and the atmosphere acts and influences life through its various elements.

12.4 Ozone And Its Depletion

Ozone is a form of oxygen. It is present in rare amounts in the earth’s atmosphere — approximately one molecule out of every two million present in the atmosphere is ozone. Counted out, that is one molecule of ozone for two million molecules of everything else.

Ozone is not a pleasant gas in itself. It is highly reactive and toxic, and it forms carcinogenic by-products, which means substances capable of causing cancer. Everything that makes ozone valuable depends on where it sits, not on what it is.

Where Ozone Is Made And Lost

In the troposphere, ozone is generated only in small amounts. But in the stratosphere, at 20 to 50 km above sea level, it is present in significant amounts. There it is produced by the action of high energy radiation from the sun striking some molecules of oxygen and converting them into ozone.

The same layer loses ozone as well as making it. Ozone decreases by sunlight acting on a number of pollutants in air. These pollutants are emitted by jet aircraft and by the hydrocarbons from automobile exhausts — hydrocarbons being compounds of hydrogen and carbon, the unburnt remains of fuel.

Flowchart

Fig. 12.A6 Caption: Ozone is made and destroyed in the same layer, and the shield depends on the balance between the two. Nodes: molecules of oxygen in the stratosphere + high energy radiation from the sun → ozone formed at 20 to 50 km → absorbs ultraviolet rays of the sun → earth and biosphere shielded; separately, pollutants emitted by jet aircraft and hydrocarbons from automobile exhausts + sunlight → ozone decreases → shield thins

Q. If ozone is toxic, why do we want more of it?

A. Height decides. Down here it is a reactive, harmful gas. Twenty kilometres up it is a shield, absorbing ultraviolet rays we never have to meet.

The Shield Above

The concentration of ozone in the upper atmosphere is protective. It absorbs the heat of the ultraviolet rays of the sun, taking that energy in high above the ground rather than letting it reach living things.

The ozone layer in the stratosphere therefore protects the earth and its biosphere from the extra heat as well as from the rays themselves. But there is the danger of its depletion, brought about through harmful gases and through the increase in the level of carbon dioxide in the lower atmosphere.

12.5 Depletion Of Ozone Layer

12.5.1 Ozone Hole

An ozone hole is an ozone depleted area — a part of the layer where much of the ozone that should be there has gone. It was detected through satellite imagery conducted by NASA in the USA, and subsequently much scientific research has been going on.

An annual data is being maintained, so the size of the depleted area in one year can be set against the last. Efforts by almost every country are being made to reduce the breaking down of ozone molecules, and many chemicals, namely chlorine and bromine, are banned, since these are thought to cause the depletion of the ozone layer.

An Ozone Hole Seen From Orbit Evidence: satellite imagery conducted by NASA in the USA, with annual data maintained since. Reasoning: instruments in orbit measure ozone across the whole polar region, so a year-on-year fall over one area shows up as a depleted patch. Limit: the image shows where the ozone is thin and by how much. It cannot show which country or industry released the chemicals that thinned it.

The Chemicals Responsible

Chlorofluorocarbons (CFCs) are regarded as the reason for the ozone hole near Antarctica. CFC gases are used in refrigerators, aircraft and air conditioners, which is why they are made and released in enormous quantities far from the poles.

CFC compounds escape into the atmosphere and finally break down in the stratosphere. There they produce chlorine atoms, and it is these atoms which destroy the ozone layer.

Flowchart

Fig. 12.A7 Caption: A gas released in a city ends up destroying ozone twenty kilometres above the ground. Nodes: CFCs used in refrigerators, aircraft and air conditioners → CFC compounds escape into the atmosphere → rise and reach the stratosphere → break down there → chlorine atoms released → ozone layer destroyed

Natural Causes

Volcanoes play a role in ozone depletion. Large eruptions give out sulphur dioxide, which speeds up destructive chemical reactions. Tropospheric ozone increases especially in the Northern Hemisphere, but this increase is smaller and localised at the eruption site rather than spread across the globe.

Another cause of destruction of the ozone layer, discovered recently, are solar storms. These solar storms consist of coronal mass ejections and solar flares, both of them associated with explosions on the surface of the sun that fling matter and radiation outward.

Why Antarctica

The satellite record has one place at its centre. Fig. 12.3 shows the depleted area over Antarctica, and the largest ozone hole was noticed in September 2000 near Antarctica. The formation of the ozone hole has been attributed largely to the activities of man.

Diagram

Fig. 12.3 Caption: Ozone hole near Antarctica, mapped from satellite data. Brief: Source a satellite ozone map of the south polar region in false colour, with the depleted area shown as a distinct central patch over Antarctica; include the colour key relating each shade to ozone amount, and the date of the observation.

Ozone depletion resulting in an ozone hole is more common to Antarctica, where temperatures are the lowest on earth. Due to the bitterly cold Antarctic winter, stratospheric ice clouds promote the production of chemically active chlorine and bromine. This, in turn, leads to ozone destruction when sunlight returns in the Antarctic spring, because the reactions need light to run.

On the other hand, the Arctic atmosphere, though cold, does not get cold enough for the creation of a fully developed ozone hole. The difference is not one of chemicals but of temperature, and it decides which pole loses its shield.

Flowchart

Fig. 12.A8 Caption: The Antarctic ozone hole is made by cold in winter and completed by sunlight in spring. Nodes: bitterly cold Antarctic winter → stratospheric ice clouds form → chemically active chlorine and bromine produced → sunlight returns in the Antarctic spring → ozone destroyed → ozone hole appears; Arctic winter cold but not cold enough → no fully developed ozone hole

Q3. Why does an ozone hole form over Antarctica and not over the Arctic, when both are bitterly cold?

A. Antarctica gets colder. Its stratospheric ice clouds make chlorine and bromine chemically active, and returning spring sunlight then destroys the ozone. The Arctic never cools enough for a fully developed hole.

12.6 Global Warming

Recent climate studies have shown that the global mean temperature on the earth is rising. This phenomenon, known as Global Warming, is the result of an enhanced concentration of greenhouse gases in the atmosphere, caused mainly by human activities.

The word global matters in that sentence. It is not the temperature of one city or one season that is being measured, but an average taken across the whole earth, which is why a rise of even a fraction of a degree is treated seriously.

12.6.1 Causes Of Global Warming

Some of the chief causes of global warming are the following, and all three are set in motion by what people do.

Destruction Of Ozone Layer

We have studied earlier in this chapter that the ozone layer in the stratosphere protects the earth from the ultraviolet rays of the sun. Its depletion lets more of that radiation through, and so contributes to global warming.

Greenhouse Effect In The Atmosphere

Excess accumulation of some gases — carbon dioxide, methane, chlorofluorocarbons, ozone and water vapour — is responsible for disturbing the energy balance and thus increasing heat on the earth. Fossil fuel based industries and human lifestyle have resulted in an unprecedented rise of these gases, which are known as greenhouse gases. They are called that because they act in the lower levels of the atmosphere like the glass of a greenhouse.

We have all seen a greenhouse, which is covered with glass. The glass allows the sun’s rays to penetrate into the greenhouse, but it blocks the radiation of heat from the greenhouse back into the atmosphere. Heat gets in easily and finds it hard to get out.

The gases do the same work as that glass. Greenhouse gases in the atmosphere allow the near infrared rays from the sun to penetrate into the earth. But these gases do not allow the heat radiated by the heated earth to easily dissipate into space. Thus, by preventing heat rays from escaping into outer space, the greenhouse gases add to the heat already available on the earth’s surface.

The increase in temperature due to the concentration of the greenhouse gases is known as the greenhouse effect. This process is shown in Fig. 12.4, where the two sets of rays can be followed separately.

Diagram

Fig. 12.4 Caption: The greenhouse effect — sunlight passes in, and much of the earth’s own heat is held back. Brief: Sun at upper left with yellow arrows of incoming solar radiation; show some arrows reflected by the earth and the atmosphere and about half absorbed at the surface; red arrows of infrared radiation rising from the warmed surface, some escaping to space but most absorbed and re-emitted downward by clouds and greenhouse gas molecules; label Sun, Atmosphere, Earth, and both arrow sets.

Fig. 12.4 The yellow arrows are the sun’s rays coming in; the red arrows are heat radiated back by the warmed earth. Follow one red arrow upward: a little escapes, and most is absorbed and sent down again. The diagram shows the direction energy travels. It cannot tell you how much warming any one gas causes.

The greenhouse effect is not confined to a particular region but has a global impact. If unchecked, accumulation of these gases from industry and agriculture could change the temperature, the rainfall pattern and the sea level of the earth. By realising the potential danger of these gases to the earth, the United Nations Environment Programme (UNEP) used the expression ‘Global Warming’ to alert people about this danger.

Q4. How can a gas that leaks from a refrigerator change the temperature of the whole earth?

A. CFCs are greenhouse gases and they also destroy stratospheric ozone. Both effects add heat, and neither stays where the gas escaped — the greenhouse effect is not confined to a region but has a global impact.

Deforestation

Clearing away of forests and forest ecosystems on a large scale has posed several dangers to life on earth. Trees are essential for absorbing excess carbon dioxide in air, so their removal leaves more of that gas in the atmosphere than before.

Trees are also responsible for maintaining the fertility of the soil. In their absence, winds blow away the top soil as dust into the atmosphere. That dust plays a key role in reflecting terrestrial radiation — the heat radiated outward by the earth’s own surface — back to the earth.

Bare ground behaves differently from covered ground in another way as well. It may also cause extra cooling at night, because bare soils get cooler and warmer more quickly than soils covered with vegetation.

Clearing Forest On A Large Scale forests cleared → excess carbon dioxide no longer absorbed, and the soil is left bare → winds blow the top soil away as dust into the atmosphere → dust reflects terrestrial radiation back to the earth, while bare soil heats and cools faster than covered soil → extra cooling at night and a disturbed heat balance overall

12.7 Impact Of Global Warming

The atmosphere is being destroyed as a result of global warming, and this can result in a change in the climate of the earth. Already the surface temperature of the earth is on the rise.

The measurements are small in themselves and large in what they imply. The global mean temperature increased by approximately 0.6°C in the 20th century. The average temperature of the earth may increase by 1.4 to 5.8°C by the year 2100 from the 1990 level.

WC 12.A2 — Comparing The Rate Of Warming The situation: the two temperature figures given above — a rise of approximately 0.6°C across the 20th century, and a projected rise of 1.4 to 5.8°C by the year 2100 measured from the 1990 level. The working: rate of warming is found as \[\text{Rate in }°\text{C per year} = \frac{\text{Total rise in }°\text{C}}{\text{Number of years}}\] Step 1, the 20th century covered 100 years, so its rate was \(\frac{0.6}{100} = 0.006\) °C per year. Step 2, the projection runs from 1990 to 2100, which is \(2100 - 1990 = 110\) years. Step 3, the lower projection gives \(\frac{1.4}{110} = 0.0127\) °C per year, and the higher gives \(\frac{5.8}{110} = 0.0527\) °C per year. Step 4, compare with the past rate: \(\frac{0.0127}{0.006} = 2.1\) and \(\frac{0.0527}{0.006} = 8.8\).

Even the gentlest projection has the earth warming about twice as fast as it did through the whole of the last century, and the steepest has it warming close to nine times as fast. That is the reason these figures are treated as urgent rather than merely interesting: it is not only the size of the change but the speed of it. If the atmosphere is not protected, it can have some of the following major impacts.

Rise In Sea Level

Over the last 100 years, because of global warming, the ice reserves have melted and the sea level has risen by about 10 to 25 cm. It is predicted that by the year 2100, the oceans are expected to rise between 2.5 and 6.5 feet.

This can have disastrous effects on human habitations on sea coasts, because one-third of the human population lives within 60 km of a coastline. Many towns and cities could even be submerged under water.

Precipitation Change

Warming of the atmosphere will considerably increase its moisture carrying capacity, since warm air can hold more water vapour than cold air. While the troposphere warms up, the stratosphere will cool down. This would cause widespread changes in rainfall patterns, due to the changed pattern of air-mass movements.

The change will not fall evenly over the earth: (a) Rainfall is expected to increase at higher latitudes, in both summer and winter. (b) Rainfall is expected to increase in southern and eastern Asia in summer. (c) Winter rainfall may decrease at lower latitudes.

Q. Why would a warmer atmosphere bring floods to some places and drought to others?

A. Because the air masses that carry rain shift. Where they arrive more often there is more rain than the land can drain; where they no longer arrive, the rain simply stops coming.

These changes in rainfall patterns are expected to lead to flooding in some areas and drought in others. Monsoon flooding is expected to cause great loss of life in India, Bangladesh and South-East Asian countries, where large numbers of people live on low, densely settled river plains.

Radiation Imbalance

Radiation imbalance is a change in the balance between radiation coming into the atmosphere and radiation going out. Any change in this balance can have a rapid impact on lifeforms on earth, because it affects the sensitivity, adaptability and vulnerability of the whole biospheric system.

With increasing global warming, many species are expected to shift slowly poleward, or towards high elevations in mountain areas, following the temperatures they can live in. Since trees are sensitive to temperature changes, an increase in temperature may cause large-scale death of trees and their replacement by scrub vegetation.

The difficulty is one of speed. Many species may not be able to migrate fast enough to track temperature changes, and may disappear.

Warming Faster Than Species Can Move temperature rises → the belt each species can live in shifts poleward and upslope → species that can move follow it, and trees die back and give way to scrub → species that cannot move fast enough are left behind in conditions they cannot survive → those species disappear

Agriculture

Climate change will bring about substantial changes in cereal production, especially in the Tropics. In the Tropics, cereals dominate the food sources and are the mainstay of agriculture. There will be more shortages of cereals.

Water Resources

There is a great deal of controversy over the likely impact on water resources, and the chapter does not settle it. However, all agree that global freshwater conditions may worsen more due to population pressure than due to climatic factors.

What Is Draining The Fresh Water? The population argument: more people drawing on the same rivers and wells is the pressure that counts. The climate argument: shifting rainfall decides how much water there is to draw on in the first place. Both agree: freshwater conditions are worsening, and the two pressures act on the same supply. What would you need to know to decide? Water use per person and rainfall received, measured in the same place over the same years.

Diseases

Temperature and precipitation are two important causative factors of diseases. Malaria is one such disease, and it may reappear in different forms as a result of change in the climate, as the conditions that allow it to spread move into new areas.

What The Past Tells Us

The earth in the past has had several phases of warm climates followed by Ice Ages, so warming and cooling are not new in themselves. In the past, these changes took place over several thousand years.

What is new is the pace. Such changes may now take place over a few decades or a century. During such changes in the past, life forms, landforms and waterbodies have changed, and we can now expect such changes taking place within a few centuries if we do not protect our atmosphere.

Ozone Layer — a layer in the stratosphere of the atmosphere which keeps temperature constant in the lower levels and absorbs ultraviolet rays of the sun Global Warming — the rise in average mean temperature of the earth on account of enhanced concentration of greenhouse gases in the atmosphere CFCs — chlorofluorocarbons

12.8 Summary

  1. The earth is divided into three domains — the lithosphere, the hydrosphere and the atmosphere — which exist in the solid, liquid and gaseous states and together make up the biosphere.
  2. The blanket of air surrounding the earth is known as the atmosphere. It exists in layers that become thinner at high altitudes, half of its total mass lies below 5 km, and gravity holds it close to the earth.
  3. The atmosphere protects the earth from harmful ultraviolet and infrared rays, holds the life sustaining gases, retains warmth and circulates water vapour, which is the source of rainfall.
  4. A clean dry sample of air is made up of 78% nitrogen and 21% oxygen, with the remaining 1% shared among other gases.
    1. Oxygen helps in burning and breathing, and most living things cannot survive without it.
    2. Nitrogen is a colourless inert gas that dilutes the oxygen and slows down oxidation; plants take it from the soil and animals get it by eating plants.
    3. The remaining 1% includes carbon dioxide, water vapour, helium, hydrogen and argon; water vapour enters the air by evaporation, transpiration and respiration.
    4. Solid particles such as pollen, dust, ash, smoke and sea salt act as nuclei for raindrops and scatter the sun’s radiation at sunrise and sunset.
  5. The functions of the atmosphere follow from its weight and its contact with land and water: it exerts pressure, allows a continuous exchange of gases, keeps the earth warm at night through carbon dioxide, and stays the most dynamic entity on earth because the sun’s energy keeps it moving.
  6. The atmosphere is made of concentric layers of decreasing density, and about 90 per cent of the mass of air lies within about 20 km of the surface. On the basis of temperature and air pressure there are four thermal layers.
    1. The troposphere is the lowermost and densest layer, from 18 km at the Equator to 8 km at the Poles; it holds 75 per cent of the atmosphere, cools by 1°C for every 166 metre of climb, and contains all weather phenomena. Its upper limit is the tropopause.
    2. The stratosphere extends to 50 km, holds the ozone layer between 20 and 50 km, has a constant temperature because ozone loses as much heat as it absorbs, and is ideal for flying jet aircraft. Its upper limit is the stratopause.
    3. The mesosphere extends to 80 km, falls to −110°C near the mesopause, and holds wispy clouds lit by reflected sunlight from meteoric dust.
    4. The thermosphere is divided into the ionosphere, up to about 400 km, where ions cause the auroras and reflect radio waves for wireless communication, and the exosphere, from 400 to 1500 km, which merges gradually with interplanetary space.
  7. Ozone is a form of oxygen, present in rare amounts — about one molecule in every two million — and it is produced in the stratosphere when high energy radiation from the sun strikes molecules of oxygen.
  8. Ozone in the upper atmosphere absorbs the ultraviolet rays of the sun and so protects the earth and its biosphere, but it decreases when sunlight acts on pollutants from jet aircraft and automobile exhausts.
  9. An ozone hole is an ozone depleted area, detected through satellite imagery conducted by NASA, and chlorine and bromine chemicals have been banned because of it.
    1. CFCs, used in refrigerators, aircraft and air conditioners, escape upward and break down in the stratosphere to release chlorine atoms that destroy ozone.
    2. Large volcanic eruptions give out sulphur dioxide, which speeds up destructive reactions, and solar storms are a recently discovered cause.
    3. The largest ozone hole was noticed in September 2000 near Antarctica, where the bitterly cold winter forms stratospheric ice clouds; the Arctic never gets cold enough for a fully developed hole.
  10. Global warming is the rise in the global mean temperature caused by an enhanced concentration of greenhouse gases, mainly through human activities.
    1. Destruction of the ozone layer lets more solar radiation through.
    2. Greenhouse gases such as carbon dioxide, methane, CFCs, ozone and water vapour let the sun’s rays in but stop the earth’s heat escaping, which is the greenhouse effect.
    3. Deforestation removes the trees that absorb carbon dioxide and hold the soil, and the loose dust that follows alters how radiation is reflected.
  11. The global mean temperature increased by approximately 0.6°C in the 20th century and may increase by 1.4 to 5.8°C by the year 2100 from the 1990 level.
  12. The major impacts expected from global warming are these:
    1. Sea level has risen by about 10 to 25 cm in the last 100 years and may rise between 2.5 and 6.5 feet by 2100, threatening the one-third of people who live within 60 km of a coastline.
    2. Rainfall patterns will change, bringing flooding to some regions and drought to others, with monsoon flooding a serious danger in India, Bangladesh and South-East Asian countries.
    3. A radiation imbalance will push species poleward and upslope, and those that cannot move fast enough may disappear.
    4. Cereal production will change substantially in the Tropics, bringing more shortages.
    5. Freshwater conditions may worsen, though more from population pressure than from climatic factors.
    6. Diseases such as malaria may reappear in different forms as temperature and precipitation change.
  13. The earth has passed through warm phases and Ice Ages before, but those changes took several thousand years, while changes of the same kind may now take only a few decades or centuries.

Flowchart

Fig. 12.A9 Caption: One chain runs through this chapter — from the gases in the air to the layers they form, and from what we release into them to what follows. Nodes: energy from the sun → atmosphere of gases, water vapour and dust held to the earth by gravity → troposphere holds 75 per cent of the air and all weather → stratosphere holds the ozone layer, which absorbs ultraviolet rays → human activity releases CFCs and greenhouse gases → ozone layer thins and outgoing heat is trapped → global mean temperature rises → rise in sea level / precipitation change / radiation imbalance / cereal shortage / water stress / spread of disease

What This Chapter Cannot Tell You

The composition figures describe a clean dry sample of air, so they leave out the water vapour and dust that real air always carries, and those are exactly the components that make weather. The layer heights are averages too: the troposphere alone runs from 8 km to 18 km depending on where you stand, and a single number for its top would hide that.

The temperature projections are given as a wide range, from 1.4 to 5.8°C, because they are estimates of what may happen rather than measurements of what has happened. A global mean also hides everything local — it cannot tell you what will happen to the rainfall of one district, or in one season, which is the scale at which a farmer or a city actually feels the change.

12.9 Check Your Concepts

  1. Look at Fig. 12.1. State the share taken by each part of the circle, and then explain why the water vapour and the dust of ordinary air do not appear anywhere on it.

  2. The normal lapse rate is an average fall of 1°C for every 166 metre of altitude gained. Over the Equator the troposphere is 18 km deep. Calculate the fall in temperature between the surface and the tropopause there, showing every step of your working and carrying the units through.

  3. A CFC gas escapes from an air conditioner in a city. Set out, in order, the steps by which it comes to destroy ozone, and name the layer in which that destruction takes place.

  4. Antarctica and the Arctic are both bitterly cold, yet a fully developed ozone hole forms over only one of them. Explain why, and state which condition is doing the deciding.

  5. Pratap was travelling by air from Kolkata to Chennai. There was a heavy storm with rain over Bhubaneswar. Pratap’s flight passed over Bhubaneswar during that storm, and he felt no turbulence because of it. As a student of geography, how would you explain this?

  6. Could ozone in the lower layer of the atmosphere make up for the loss of ozone in the ozone layer? Give reasons to support your answer.

  7. Describe the structure of the atmosphere, using Fig. 12.2.

  8. Give a reason for each of the following:

    1. We find it difficult to breathe when we climb mountains.
    2. The atmosphere is the most dynamic entity.
    3. All the weather phenomena take place in the troposphere.

  9. Draw a well labelled diagram to explain the greenhouse effect.

  10. Given below are some activities. How does each one cause global warming?

  1. Conversion of forest land into agricultural land.
  2. Construction of high rise buildings.
  3. Fossil fuel production.
  4. Transportation industry.