csir net, gate, iit-jam, ugc net , tifr, iisc, …examprep.vpmclasses.com/images/uploads/ugc...

C SIR NET, GATE, IIT-JAM, UGC NET , TIFR, IISc , JEST , JNU, BHU , ISM , IBPS, CSAT, SLET, NIMCET, CTET

Phone: 0744-2429714 Mobile: 9001297111, 9829567114, 9001297243 Website: www.vpmclasses.com E-Mail: [email protected] /[email protected] Address: 1-C-8, Sheela Chowdhary Road, SFS, TALWANDI, KOTA, RAJASTHAN, 324005

Page 1

For IIT-JAM, JNU, GATE, NET, NIMCET and Other Entrance Exams

1-C-8, Sheela Chowdhary Road, Talwandi, Kota (Raj.) Tel No. 0744-2429714

Web Site www.vpmclasses.com [email protected]

Fundamental Concepts of Geography

Endogenetic

Exogenetic Forces

Denudation, Weathering and Mass Wasting

UGC NET - GEOGRAPHY

�

SAMPLE THEORY

��

�

�

PAPER - II



Page 2

FUNDAMENTAL CONCEPTS OF GEOGRAPHY

The earth’s crust is dynamic. It has moved and moves vertically and horizontally. Of course,

it moved a bit faster in the past than the rate at w hich it is moving now . The differences in

the internal forces operating from w ithin the earth which built up the crust have been

responsible for the variations in the outer surface of the crust.

The earth’s surface is being continuously subjected to external forces induced basically by

energy (sunlight). Of course, the internal forces are still active though w ith different

intensities. That means, the earth’s surface is being cont inuously subjected to by external

forces originating w ithin the earth’s atmosphere and by internal forcers from within the earth.

The external forces are know n as exogenic forces and the internal forces are known as

endogenic forces. The actions of exogenic forces result in wearing down (degradation) of

relief/elevations and f illing up (aggradation) of basins/depressions, on the earth’s surface.

The phenomenon of wearing down of relief variations of the surface of the earth through

erosion is know n as gradation. The endogenic forces continuously elevate or build up parts

of the earth’s surface and hence the exogenic processes fail to even out the relief variations

of the surface of the earth. So, variations remain as long as the opposing actions of

exogenic and endogenic forces continue. In general terms, the endogenic forces are mainly

land build ing forces and the exogenic processes are mainly land w earing forces. The

surface of the earth is sensitive. Humans depend on it for their sustenance and have been

using it extensively and intensively. So, it is essential to understand its nature in order to use

it ef fectively w ithout disturbing its balance and diminishing its potent ial for the future. Almost

all organis ms contribute to sustain the earth’s environment. How ever, humans have caused

over use of resources. Most of the surface of the earth had and has been shaped over very

long periods of time (hundreds and thousands of years) and because of its use and misuse

by humans, its potent ial is being diminished at a fast rate. If the processes which shaped

and are shaping the surface of the earth into varieties of forms (shapes) and the nature of



Page 3

mater ials of which it is composed of, are understood, precautions can be taken to minimize

the detrimental ef fects of human use and to preserve it for posterity.

GEOMORPHIC PROCESSES

The endogenic and exogenic forces causing physical stresses and chemical actions on

earth materials and bringing about changes in the conf iguration of the surface of the earth

are know n as geomorphic processes. Diastrophism and volcanis m are endogenic

geomorphic processes. Weathering, mass w asting, erosion and deposition are exogenic

geomorphic processes. Any exogenic element of nature (like water, ice, w ind, etc.,) capable

of acquiring and transporting earth materials can be called a geomorphic agent. When these

elements of nature become mobile due to gradients, they remove the materials and

transport them over slopes and deposit them at low er level. Geomorphic processes and

geomorphic agents especially exogenic, unless stated separately, are one and the same.

A process is a force applied on earth materials affecting the same. An agent is a mobile

medium (like running w ater, moving ice masses, w ind, waves and currents etc.) which

removes, transports and deposits earth mater ials. Running water, groundwater, glaciers,

w ind, w aves and currents, etc., can be called geomorphic agents.

Gravity besides being a directional force activating all dow nslope movements of matter also

causes stresses on the earth’s mater ials. Indirect gravitational stresses activate wave and

tide induced currents and w inds. Without gravity and gradients there w ould be no mobility

and hence no erosion, transportation and deposition are possible. So, gravitat ional stresses

are as important as the other geomorphic processes. Gravity is the force that is keeping us

in contact w ith the surface and it is the force that sw itches on the movement of all surface

mater ial on earth. All the movements either w ithin the earth or on the surface of the earth

occur due to gradients — from higher levels to low er levels, f rom high pressure to low

pressure areas etc.

ENDOGENIC PROCESSES

The energy emanating from within the earth, is the main force behind endogenic

geomorphic processes. This energy is mostly generated by radioactivity, rotational and tidal



Page 4

f riction and primordial heat f rom the origin of the earth. This energy due to geothermal

gradients and heat f low from w ithin induces diastrophism and volcanis m in the lithosphere.

Due to variations in geothermal gradients and heat f low from w ithin, crustal thickness and

strength, the action of endogenic forces are not uniform and hence the tectonically

controlled orig inal crustal surface is uneven.

Diastrophism

All processes that move, elevate or build up portions of the earth’s crust come under

diastrophism. They include:

(i) orogenic processes involving mountain building through severe folding and affecting long

and narrow belts of the earth’s crust;

(ii) Epeirogenic processes involving uplif t or w arping of large parts of the earth’s crust;

(iii) earthquakes involving local relat ively minor movements;

(iv) Plate tectonics involving horizontal movements of crustal plates. In the process of

orogeny, the crust is severely deformed into folds. Due to epeirogeny, there may be simple

deformation. Orogeny is a mountain building process w hereas epeirogeny is continental

building process. Through the processes of orogeny, epeirogeny, earthquakes and plate

tectonics, there can be faulting and fracturing of the crust. All these processes cause

pressure, volume and temperature (PVT) changes w hich in turn induce metamorphism of

rocks.

Volcanism

Volcanism includes the movement of molten rock (magma) tow ard the earth’s surface and

also formation of many intrusive and extrusive volcanic forms.

EXOGENIC PROCESSES

The exogenic processes derive their energy from atmosphere determined by the ultimate

energy from the sun and also the gradients created by tectonic factors.

Gravitational force acts upon all earth materials having a sloping surface and tend to

produce movement of matter in dow n slope direction. Force applied per unit area is called

stress. Stress is produced in a solid by pushing or pulling. This induces deformation. Forces

acting along the faces of earth materia ls are shear stresses (separating forces). It is this



Page 5

stress that breaks rocks and other earth materia ls. The shear stresses result in angular

displacement or slippage. Besides the gravitat ional stress earth materia ls become subjected

to molecular stresses that may be caused by a number of factors amongst w hich

temperature changes, crystallization and melting are the most common. Chemical

processes normally lead to loosening of bonds betw een grains, dissolving of soluble

minerals or cementing materia ls. Thus, the basic reason that leads to w eathering, mass

movements, erosion and deposition is development of stresses in the body of the earth

mater ials.

As there are different climatic regions on the earth’s surface the exogenic geomorphic

processes vary from region to region. Temperature and precipitation are the tw o important

climatic elements that control various processes.

All the exogenic geomorphic processes are covered under a general term, denudation. The

word ‘denude’ means to strip off or to uncover. Weathering, mass w asting/movements,

erosion and transportation are included in denudat ion. The f low chart (Figure) gives the

denudation processes and their respective driving forces. It should become clear from this

chart that for each process there exists a distinct driving force or energy.

As there are different climatic regions on the earth’s surface ow ing to thermal gradients

created by latitudinal, seasonal and land and w ater spread variations, the exogenic

geomorphic processes vary from region to region. The density, type and distribution of

vegetation which largely depend upon



Page 6

Figure : Denudational processes and their driving forces.

precipitation and temperature exert inf luence indirectly on exogenic geomorphic processes.

Within dif ferent climat ic regions there may be local variations of the effects of different

climatic elements due to alt itudinal d if ferences, aspect variations and the variation in the

amount of insolation received by north and south facing slopes as compared to east and

west facing slopes. Further, due to dif ferences in w ind velocities and directions, amount and

kind of precipitation, its intensity, the relation betw een precipitation and evaporation, daily

range of temperature, f reezing and thaw ing frequency, depth of f rost penetration, the

geomorphic processes vary w ithin any climat ic region.

Climat ic factors being equal, the intensity of action of exogenic geomorphic processes

depends upon type and structure of rocks. The term structure includes such aspects of

rocks as folds, faults, orientation and inclination of beds, presence or absence of joints,

bedding planes, hardness or softness of constituent minerals, chemical susceptibility of

mineral constituents; the permeability or impermeability etc. Dif ferent types of rocks w ith

dif ferences in their structure offer varying resistances to various geomorphic processes. A

particular rock may be resistant to one process and nonresistant to another. And, under

varying climatic condit ions, particular rocks may exhibit dif ferent degrees of resistance to

geomorphic processes and hence they operate at dif ferential rates and give rise to

dif ferences in topography. The effects of most of the exogenic geomorphic processes are

small and slow and may be imperceptible in a short time span, but w ill in the long run affect

the rocks severely due to continued fatigue.

Finally, it boils down to one fact that the differences on the surface of the earth, though

originally related to the crustal evolut ion continue to exist in some form or the other due to

dif ferences in the type and structure of earth materia ls, dif ferences in geomorphic processes

and in their rates of operation.

Some of the exogenic geomorphic processes are follow ing :

WEATHERING



Page 7

Weathering is an action of elements of weather and climate over earth materia ls. There are

a number of processes w ithin w eathering w hich act either individually or together to affect

the earth materia ls in order to reduce them to fragmental state.

Weathering is def ined as mechanical d isintegration and chemical decomposit ion of rocks

through the actions of various elements of weather and climate.

As very little or no motion of materials takes place in weathering, it is an in-situ or on-site

process.

Weathering processes are conditioned by many complex geological, climatic, topographic

and vegetative factors. Climate is of particular importance. Not only w eathering processes

dif fer from climate to climate, but also the depth of the weathering mantle:-

Figure : Climatic regimes and depth of weathering mantles

Activity

There are three major groups of weathering processes :

(i) chemical



Page 8

(ii) physical or mechanical

(iii) bio logical w eathering processes.

Very rarely does any one of these processes ever operate completely by itself , but quite

often a dominance of one process can be seen.

Chemical Weathering Processes

A group of w eathering processes viz; solution, carbonation, hydration, oxidation and

reduction act on the rocks to decompose, dissolve or reduce them to a f ine clastic state

through chemical reactions by oxygen, surface and/or soil w ater and other acids. Water and

air (oxygen and carbon dioxide) along w ith heat must be present to speed up all chemical

reactions. Over and above the carbon dioxide present in the air, decomposition of plants

and animals increases the quantity of carbon dioxide underground. These chemical

reactions on various minerals are very much similar to the chemical reactions in a

laboratory.

Solution

When something is dissolved in water or acids, the water or acid w ith dissolved contents is

called solution. This process involves removal of solids in solution and depends upon

solubility of a mineral in water or w eak acids. On coming in contact w ith water many solids

disintegrate and mix up as suspension in water. Soluble rock forming minerals like nitrates,

sulphates, and potassium etc. are affected by this process. So, these minerals are easily

leached out w ithout leaving any residue in rainy climates and accumulate in dry regions.

Minerals like calcium carbonate and calcium magnesium bicarbonate present in limestones

are soluble in w ater containing carbonic acid (formed w ith the addition of carbon dioxide in

water), and are carried away in w ater as solution. Carbon dioxide produced by decaying

organic matter along w ith soil water greatly aids in th is reaction. Common salt (sodium

chloride) is also a rock forming mineral and is susceptible to th is process of solution.

Carbonation

Carbonat ion is the reaction of carbonate and bicarbonate w ith minerals and is a common

process helping the breaking dow n of feldspars and carbonate minerals. Carbon dioxide

from the atmosphere and soil air is absorbed by water, to form carbonic acid that acts as a



Page 9

weak acid. Calcium carbonates and magnesium carbonates are dissolved in carbonic acid

and are removed in a solution w ithout leaving any residue resulting in cave formation.

Hydration

Hydration is the chemical addition of water. Minerals take up w ater and expand; th is

expansion causes an increase in the volume of the material itself or rock. Calcium sulphate

takes in w ater and turns to gypsum, w hich is more unstable than calcium sulphate. This

process is reversible and long, continued repetit ion of this process causes fatigue in the

rocks and may lead to their disintegrat ion. Many clay minerals sw ell and contract during

wetting and drying and a repetition of this process results in cracking of overlying materials.

Salts in pore spaces undergo rapid and repeated hydration and help in rock fracturing. The

volume changes in minerals due to hydration w ill also help in physical w eathering through

exfoliation and granular disintegrat ion.

Oxidation and Reduction

In weathering, oxidation means a combinat ion of a mineral w ith oxygen to form oxides or

hydroxides. Oxidation occurs where there is ready access to the atmosphere and

oxygenated w ater. The minerals most commonly involved in this process are iron,

manganese, sulphur etc. In the process of oxidation rock breakdown occurs due to the

disturbance caused by addition of oxygen. Red colour of iron upon oxidation turns to brown

or yellow . When oxidised minerals are placed in an environment w here oxygen is absent,

reduction takes place. Such conditions exist usually below the water table, in areas of

stagnant w ater and w aterlogged ground. Red colour of iron upon reduction turns to greenish

or bluish grey.

These w eathering processes are interrelated. Hydration, carbonation and oxidation go hand

in hand and hasten the w eathering process.

Physical Weathering Processes

Physical or mechanical w eathering processes depend on some applied forces. The applied

forces could be:

(i) gravitational forces such as overburden pressure, load and shearing stress;

(ii) expansion forces due to temperature changes, crystal growth or animal activity;



Page 10

(iii) water pressures controlled by w etting and drying cycles.

Many of these forces are applied both at the surface and within different earth materia ls

leading to rock fracture. Most of the physical weathering processes are caused by thermal

expansion and pressure release. These processes are small and slow but can cause great

damage to the rocks because of continued fatigue the rocks suffer due to repet ition of

contraction and expansion.

Unloading and Expansion

Removal of overlying rock load because of continued erosion causes vertical pressure

release w ith the result that the upper layers of the rock expand producing disintegrat ion of

rock masses. Fractures will develop roughly parallel to the ground surface. In areas of

curved ground surface, arched fractures tend to produce massive sheets or exfoliation slabs

of rock. Exfoliation sheets resulting from expansion due to unloading and pressure release

may measure hundreds or even thousands of meters in horizontal extent. Large, smooth

rounded domes called exfoliat ion domes result due to this process.

Figure : A large exfoliation dome in granite rock near bhongir (Bhuvanagiri) tow n in Andhra

Pradesh



Page 11

Temperature Changes and Expansion

Various minerals in rocks possess their ow n limits of expansion and contraction. With r ise in

temperature, every mineral expands and pushes against its neighbour and as temperature

falls, a corresponding contraction takes place. Because of diurnal changes in the

temperatures, this internal movement among the mineral grains of the superf icial layers of

rocks takes place regularly. This process is most effective in dry climates and high

elevations where diurnal temperature changes are drastic. These movements are very small

they make the rocks w eak due to continued fatigue. The surface layers of the rocks tend to

expand more than the rock at depth and this leads to the formation of stress w ithin the rock

resulting in heaving and fracturing parallel to the surface. Due to dif ferential heating and

resulting expansion and contraction of surface layers and their subsequent exfoliation from

the surface results in smooth rounded surfaces in rocks. In rocks like granites, smooth

surfaced and rounded small to big boulders called tors form due to such exfoliation.

Freezing, Thaw ing and Frost Wedging

Frost w eathering occurs due to growth of ice w ithin pores and cracks of rocks during

repeated cycles of f reezing and melting. This process is most effective at high elevations in

mid-latitudes w here freezing and melting is of ten repeated. Glacial areas are subject to frost

wedging daily. In th is process, the rate of f reezing is important. Rapid freezing of w ater

causes its sudden expansion and high pressure. The resulting expansion affects joints,

cracks and small inter granular fractures to become wider and w ider till the rock breaks

apart.

Salt Weathering

Salts in rocks expand due to thermal action, hydration and crystallisation. Many salts like

calcium, sodium, magnesium, potassium and barium have a tendency to expand. Expansion

of these salts depends on temperature and their thermal properties. High temperature

ranges between 30 and 50o C of surface temperatures in deserts favour such salt

expansion. Salt crystals in near-surface pores cause splitting of individual grains w ithin

rocks, w hich eventually fall of f . This process of falling off of individual grains may result in

granular disintegration or granular foliat ion.



Page 12

Salt crystallisation is most ef fective of all salt-w eathering processes. In areas w ith alternating

wetting and drying condit ions salt crystal grow th is favoured and the neighbour ing grains are

pushed aside. Sodium chloride and gypsum crystals in desert areas heave up overlying

layers of materials and w ith the result polygonal cracks develop all over the heaved surface.

With salt crystal grow th, chalk breaks down most readily, followed by limestone, sandstone,

shale, gneiss and granite etc.

BIOLOGICAL ACTIVITY AND WEATHERING

Biological weathering is contribution to or removal of minerals and ions from the weathering

environment and physical changes due to grow th or movement of organisms. Burrowing and

wedging by organisms like earthw orms, termites, rodents etc., help in exposing the new

surfaces to chemical attack and assists in the penetration of moisture and air. Human

beings by disturbing vegetation, ploughing and cult ivating soils, also help in mixing and

creating new contacts between air, water and minerals in the earth materia ls.

Decaying plant and animal matter help in the production of humic, carbonic and other acids

which enhance decay and solubility of some elements. Algae utilize mineral nutrients for

growth and help in concentration of iron and manganese oxides. Plant roots exert a

tremendous pressure on the earth materials mechanically breaking them apart.

SOME SPECIAL EFFECTS OF WEATHERING

This has already been explained under physical w eathering processes of unloading, thermal

contraction and expansion and salt w eathering. Exfoliation is a result but not a process.

Flaking off of more or less curved sheets of shells from over rocks or bedrock results in

smooth and rounded surfaces. Exfoliation can occur due to expansion and contraction

induced by temperature changes. Exfoliation domes and tors result due to unloading and

thermal expansion respectively.

SIGNIFICANCE OF WEATHERING

Weathering processes are responsible for breaking dow n the rocks into smaller f ragments

and preparing the way for formation of not only regolith and soils, but also erosion and mass

movements. Biomes and biodiversity is basically a result of forests (vegetation) and forests

depend upon the depth of w eathering mantles. Erosion cannot be signif icant if the rocks are



Page 13

not weathered. That means, w eathering aids mass w asting, erosion and reduction of relief

and changes in landforms are a consequence of erosion.

Weathering of rocks and deposits helps in the enrichment and concentrations of certain

valuable ores of iron, manganese, aluminium, copper etc., w hich are of great importance for

the national economy. Weathering is an important process in the formation of soils.

When rocks undergo w eathering, some materials are removed through chemical or physical

leaching by groundw ater and thereby the concentration of remaining (valuable) materia ls

increases. Without such a w eathering taking place, the concentration of the same valuable

mater ial may not be suff icient and economically viable to exploit, process and ref ine. This is

what is called enrichment.

MASS MOVEMENTS

These movements transfer the mass of rock debris dow n the slopes under the direct

inf luence of gravity. That means, air, w ater or ice do not carry debris with them from place to

place but on the other hand the debris may carry w ith it air, w ater or ice. The movements of

mass may range from slow to rapid, affecting shallow to deep columns of materials and

include creep, f low, slide and fall. Gravity exerts its force on all matter, both bedrock and the

products of weathering. So, w eathering is not a pre-requisite for mass movement though it

aids mass movements. Mass movements are very active over w eathered slopes rather than

over unw eathered materials.

Mass movements are aided by gravity and no geomorphic agent like running water, glaciers,

w ind, w aves and currents participate in the process of mass movements. That means mass

movements do not come under erosion though there is a shif t (aided by gravity) of materia ls

from one place to another. Materia ls over the slopes have their ow n resistance to disturbing

forces and w ill yield only when force is greater than the shearing resistance of the materials.

Weak unconsolidated materia ls, thinly bedded rocks, faults, steeply dipping beds, vertical

clif fs or steep slopes, abundant precipitat ion and torrential rains and scarcity of vegetation

etc., favour mass movements.



Page 14

Several activating causes precede mass movements. They are :

(i) removal of support f rom below to materials above through natural or artif icial means;

(ii) increase in gradient and height of slopes;

(iii) overloading through addition of materials naturally or by artif icial f illing;

(iv) overloading due to heavy rainfall, saturation and lubrication of slope materials;

(v) removal of material or load from over the original slope surfaces; (vi) occurrence of

earthquakes, explosions or machinery;

(vii) excessive natural seepage;

(viii) heavy draw down of water f rom lakes, reservoirs and rivers leading to slow outf low of

water f rom under the slopes or river banks;

(ix) indiscriminate removal of natural vegetation.

Heave (heaving up of soils due to frost growth and other causes), f low and slide are the

three forms of movements. Figure shows the relat ionships among dif ferent types of mass

movements, their relative rates of movement and moisture limits.

Figure : Relat ionships among dif ferent types of mass movements, their relative rates of

movement and moisture limits.

Mass movements can be grouped under three major classes:

(i) slow movements; (ii) rapid movements; (iii) landslides.

Slow Movements



Page 15

Creep is one type under this category which can occur on moderately steep, soil covered

slopes. Movement of mater ials is extremely slow and imperceptible except through

extended observation. Materials involved can be soil or rock debris. Fence posts, telephone

poles lean dow nslope from their vertical position and in their linear alignment are due to the

creep effect. Depending upon the type of materia l involved, several types of creep viz., soil

creep, talus creep, rock creep, rock-glacier creep etc., can be identif ied. Also included in th is

group is solif luction w hich involves slow downslope f low ing soil mass or f ine grained rock

debris saturated or lubricated w ith water. This process is quite common in moist temperate

areas w here surface melt ing of deeply frozen ground and long continued rain respectively,

occur frequently. When the upper portions get saturated and when the low er parts are

impervious to w ater percolation, f low ing occurs in the upper parts.

Rapid Movements

These movements are mostly prevalent in humid climatic regions and occur over gentle to

steep slopes. Movement of w ater-saturated clayey or silty earth materials dow n low-angle

terraces or hillsides is known as earthf low. Quite often, the materials slump making steplike

terraces and leaving arcuate scarps at their heads and an accumulation bulge at the toe.

When slopes are steeper, even the bedrock especially of soft sedimentary rocks like shale

or deeply weathered igneous rock may slide downslope.

Another type in th is category is mudf low . In the absence of vegetation cover and w ith heavy

rainfall, thick layers of weathered materia ls get saturated w ith w ater and either slow ly or

rapidly f low down along def inite channels. It looks like a stream of mud w ithin a valley. When

the mudflows emerge out of channels onto the piedmont or plains, they can be very

destructive engulf ing roads, bridges and houses. Mudf lows occur frequently on the slopes of

erupting or recently erupted volcanoes. Volcanic ash, dust and other fragments turn into

mud due to heavy rains and f low down as tongues or streams of mud causing great

destruction to human habitations.

A third type is the debris avalanche, w hich is more characteristic of humid regions w ith or

w ithout vegetation cover and occurs in narrow tracks on steep slopes. This debris avalanche

can be much faster than the mudflow . Debris avalanche is similar to snow avalanche.



Page 16

In Andes mountains of South America and the Rockies mountains of North America, there

are a few volcanoes w hich erupted during the last decade and very devastating mudflows

occurred down their slopes during eruption as well as after eruption.

Landslides

These are know n as relatively rapid and perceptible movements. The materials involved are

relatively dry. The size and shape of the detached mass depends on the nature of

discontinuities in the rock, the degree of w eathering and the steepness of the slope.

Depending upon the type of movement of materia ls several types are identif ied in this

category.

Slump is slipping of one or several units of rock debris w ith a backw ard rotation w ith respect

to the slope over which the movement takes place. Rapid rolling or sliding

Figure : Slumping of debris w ith backward rotation

of earth debris w ithout backward rotation of mass is know n as debris slide. Debris fall is

nearly a free fall of earth debris from a vertical or overhanging face. Sliding of individual rock

masses down bedding, joint or fault surfaces is rockslide. Over steep slopes, rock sliding is

very fast and destructive. Slides occur as planar failures along discontinuities like bedding

planes that dip steeply. Rock fall is f ree falling of rock blocks over any steep slope keeping

itself aw ay from the slope. Rock falls occur from the superf icial layers of the rock

In our country, debris avalanche and landslides occur very frequently in the Himalayas.

There are many reasons for this. One, the Himalayas are tectonically active. They are

mostly made up of sedimentary rocks and unconsolidated and semi-consolidated deposits.

The slopes are very steep. Compared to the Himalayas, the Nilg iris bordering Tamilnadu,

Karnataka, Kerala and the Western Ghats along the west coast are relatively tectonically

stable and are mostly made up of very hard rocks; but, still, debris avalanches and



Page 17

landslides occur though not as frequently as in the Himalayas, in these hills. Many slopes

are steeper w ith almost vertical cliffs and escarpments in the Western Ghats and Nilgiris.

Mechanical w eathering due to temperature changes and ranges is pronounced. They

receive heavy amounts of rainfall over short periods. So, there is almost direct rock fall quite

frequently in these places along w ith landslides and debris avalanches.

EROSION AND DEPOSITION

Erosion involves acquisition and transportation of rock debris. When massive rocks break

into s maller f ragments through weathering and any other process, erosional geomorphic

agents like running w ater, groundw ater, glaciers, w ind and w aves remove and transport it to

other places depending upon the dynamics of each of these agents. Abrasion by rock debris

carried by these geomorphic agents also aids greatly in erosion. By erosion, relief degrades,

i.e., the landscape is w orn dow n. That means, though weathering aids erosion it is not a pre-

condition for erosion to take place. Weathering, mass-wasting and erosion are degradational

processes. It is erosion that is largely responsible for continuous changes that the earth’s

surface is undergoing. Denudational processes like erosion and transportation are controlled

by kinetic energy.

The erosion and transportation of earth materials is brought about by w ind, running w ater,

glaciers, w aves and ground water. Of these the f irst three agents are controlled by climat ic

conditions.

They represent three states of matter — gaseous (wind), liquid (running water) and solid

(glacier) respectively. The erosion can be def ined as “application of the kinetic energy

associated w ith the agent to the surface of the land along w hich it moves”. Kinetic energy is

computed as KE = 1/2 mv2 w here ‘m’ is the mass and ‘v’ is the velocity. Hence the energy

available to perform w ork w ill depend on the mass of the materia l and the velocity w ith

which it is moving. Obviously then you w ill f ind that though the glaciers move at very low

velocities due to tremendous mass are more effective as the agents of erosion and w ind,

being in gaseous state, are less effective.

The w ork of the other tw o agents of erosion waves and ground w ater is not controlled by

climate. In case of w aves it is the location along the interface of litho and hydro sphere —



Page 18

coastal region — that w ill determine the work of waves, w hereas the work of ground water is

determined more by the lithological character of the region. If the rocks are permeable and

soluble and w ater is available only then karst topography develops.

Deposit ion is a consequence of erosion. The erosional agents loose their velocity and hence

energy on gentler slopes and the materials carried by them start to settle themselves. In

other w ords, deposition is not actually the w ork of any agent. The coarser materials get

deposited first and f iner ones later. By deposition depressions get f illed up. The same

erosional agents viz., running water, glaciers, w ind, waves and groundw ater act as

aggradational or depositional agents also.

SOIL FORMATION

Soil and Soil Contents

A pedologist w ho studies soils def ines soil as a collection of natural bodies on the earth’s

surface containing living matter and supporting or capable of supporting plants.

Soil is a dynamic medium in w hich many chemical, physical and biological activit ies go on

constantly. Soil is a result of decay, it is also the medium for grow th. It is a changing and

developing body. It has many characteristics that f luctuate w ith the seasons. It may be

alternatively cold and w arm or dry and moist. Biological activity is slow ed or stopped if the

soil becomes too cold or too dry. Organic matter increases w hen leaves fall or grasses die.

The soil chemistry, the amount of organic matter, the soil f lora and fauna, the temperature

and the moisture, all change w ith the seasons as well as with more extended periods of

time.

That means, soil becomes adjusted to conditions of climate, landform and vegetation and

w ill change internally w hen these controlling condit ions change.

Process of Soil Form ation

Soil formation or pedogenesis depends f irst on weathering. It is this w eathering mantle

(depth of the w eathered material) w hich is the basic input for soil to form. First, the

weathered materia l or transported deposits are colonised by bacteria and other inferior plant

bodies like mosses and lichens. Also, several minor organis ms may take shelter w ithin the

mantle and deposits. The dead remains of organisms and plants help in humus



Page 19

accumulat ion. Minor grasses and ferns may grow; later, bushes and trees w ill start grow ing

through seeds brought in by birds and w ind. Plant roots penetrate down, burrowing animals

bring up particles, mass of material becomes porous and spongelike w ith a capacity to

retain w ater and to permit the passage of air and f inally a mature soil, a complex mixture of

mineral and organic products forms.

Soil-form ing Factors

Five basic factors control the formation of soils:

(i) parent material;

(ii) topography;

(iii) climate;

(iv) biological activity;

(v) time.

In fact soil forming factors act in union and affect the action of one another.

Parent Material

Parent material is a passive control factor in soil formation. Parent materia ls can be any in-

situ or on-site w eathered rock debris (residual soils) or transported deposits (transported

soils). Soil formation depends upon the texture (sizes of debris) and structure (disposition of

individual grains/particles of debris) as well as the mineral and chemical composit ion of the

rock debris/deposits.

Nature and rate of w eathering and depth of w eathering mantle are important consideration

under parent materia ls. There may be differences in soil over similar bedrock and dissimilar

bedrocks may have similar soils above them. But w hen soils are very young and have not

matured these show strong links w ith the type of parent rock. Also, in case of some

limestone areas, where the w eathering processes are specif ic and peculiar, soils w ill show

clear relation w ith the parent rock.

Topography



Page 20

Topography like parent materials is another passive control factor. The inf luence of

topography is felt through the amount of exposure of a surface covered by parent materia ls

to sunlight and the amount of surface and sub-surface drainage over and through the parent

mater ials. Soils w ill be thin on steep slopes and thick over f lat upland areas. Over gentle

slopes w here erosion is slow and percolation of w ater is good, soil formation is very

favourable. Soils over f lat areas may develop a thick layer of clay w ith good accumulation of

organic matter giving the soil dark colour. In middle latitudes, the south facing slopes

exposed to sunlight have dif ferent conditions of vegetation and soils and the north facing

slopes w ith cool, moist condit ions have some other soils and vegetat ion.

Climate

Climate is an important active factor in soil formation. The climat ic elements involved in soil

development are :

(i) moisture in terms of its intensity, f requency and duration of precipitation - evaporation and

humidity;

(ii) temperature in terms of seasonal and diurnal variations.

Precipitation gives soil its moisture content which makes the chemical and biological

activities possible. Excess of w ater helps in the dow nward transportation of soil components

through the soil (eluviation) and deposits the same down below (illuviation). In climates like

wet equatorial rainy areas w ith high rainfall, not only calcium, sodium, magnesium,

potassium etc. but also a major part of silica is removed from the soil. Removal of silica from

the soil is know n as desilication. In dry climates, because of high temperature, evaporation

exceeds precipitation and hence ground w ater is brought up to the surface by capillary

action and in the process the w ater evaporates leaving behind salts in the soil. Such salts

form into a crust in the soil know n as hardpans. In tropical climates and in areas w ith

intermediate precipitat ion condit ions, calcium carbonate nodules (kanker) are formed.

Temperature acts in tw o ways — increasing or reducing chemical and biological activity.

Chemical activity is increased in higher temperatures, reduced in cooler temperatures (w ith

an exception of carbonation) and stops in freezing conditions. That is why, tropical soils w ith



Page 21

higher temperatures show deeper prof iles and in the frozen tundra regions soils contain

largely mechanically broken materia ls.

Biological Activity

The vegetative cover and organisms that occupy the parent materials from the beginning

and also at later stages help in adding organic matter, moisture retent ion, nitrogen etc. Dead

plants provide humus, the f inely divided organic matter of the soil. Some organic acids

which form during humif ication aid in decomposing the minerals of the soil parent materials.

Intensity of bacterial activity shows up differences between soils of cold and warm climates.

Humus accumulates in cold climates as bacterial grow th is slow. With undercomposed

organic matter because of low bacterial activity, layers of peat develop in sub-arctic and

tundra climates. In humid tropical and equatorial climates, bacterial grow th and action is

intense and dead vegetat ion is rapidly oxidised leaving very low humus content in the soil.

Further, bacteria and other soil organisms take gaseous nitrogen from the air and convert it

into a chemical form that can be used by plants. This process is known as nitrogen f ixation.

Rhizobium, a type of bacteria, lives in the root nodules of leguminous plants and f ixes

nitrogen benef icial to the host plant. The inf luence of large animals like ants, termites,

earthw orms, rodents etc., is mechanical, but, it is nevertheless important in soil formation as

they rew ork the soil up and dow n. In case of earthworms, as they feed on soil, the texture

and chemistry of the soil that comes out of their body changes.

Time

Time is the third important controlling factor in soil formation. The length of time the soil

forming processes operate, determines maturation of soils and prof ile development. A soil

becomes mature when all soil-forming processes act for a suff iciently long time developing a

prof ile. Soils developing from recently deposited alluvium or glacial till are considered young

and they exhibit no horizons or only poorly developed horizons. No specif ic length of time in

absolute terms can be f ixed for soils to develop and mature.

DENUDATION, WEATHERING AND MASS WASTING

The collective processes of denudation appear as just tw o facilitating links in the

sedimentary loop of the rock cycle, between the formation of continental crust and the post-



Page 22

depositional fate of derived sediments. In practice they form the principal element in any

review of landsurface development and the core of the science of geomorphology.

This approach permits the linkage of geomorphic processes with the time scales and global

patterns of morphotectonic activity, the tectonic machinery providing init ial uplif t and a global

tectonic framew ork for the location of orogeny, cratons and basins w here geomorphic

landsystems develop. Tectonic forcing also provides a major source of general

environmental and geomorphic change.

General processes of denudation, w eathering and mass wasting are introduced here as the

prelude to later chapters where they are show n to operate in more specif ic w ays in particular

geomorphic environments. An outline history of the signif icance of denudation rates and

earlier attempts to def ine denudation chronologies precedes an introduction to opposing

forces in the geomorphic environment – the static force of gravity and the dynamic force of

moving bodies of water, ice and air versus the strength of earth materials.

These are def ined by Mohr–Coulomb failure criteria, w hich are used w idely in applied

geomorphology and geotechnical investigations to summarize the source of principal

strength components and mobilized eroding forces. Weathering is review ed in its own right

w ith classic physical and chemical processes. It is also treated here as a means to an end –

a source of in situ reduction of rock strength facilitating subsequent mass w asting or erosion

– and as a natural extension of geological f ractionation processes.

Mohr–Coulomb criteria are revisited to demonstrate sliding resistance before a review of

principal styles of slope instability and failure, w ith distinctions betw een rock and debris slopes.

The chapter includes a review of debris f low hazard, which appears to be on the increase in

temperate climates through changes in land use and climate.



Page 1

For IIT-JAM, JNU, GATE, NET, NIMCET and Other Entrance Exams

1-C-8, Sheela Chowdhary Road, Talwandi, Kota (Raj.) Tel No. 0744-2429714

Web Site www.vpmclasses.com [email protected]

INTRODUCTION

VARIOUS STABILITIES AND INSTABILITIES

AIR MASSESS (AM)

TYPES OF AIR MASSES

WEATHER EFFECTIVITIES INDUCED BY

TEST YOUR KNOWLEDGE

UGC NET - GEOGRAPHY

�

SAMPLE THEORY

ATMOSPHERIC STABILITY AND

INSTABILITY

��

�

�

�

�

THE AIR MASSES

PAPER - III



Page 2

STABILITY AND INSTABILITY

Let's use a balloon to demonstrate stability and instability. In f igure 42 w e have, for three

situations, filled a balloon at sea level w ith air at 31° C-the same as the ambient

temperature. We have carried the balloon to 5,000 feet. In each situation, the air in the

balloon expanded and cooled at the dry adiabat ic rate of 3° C for each 1,000 feet to a

temperature of 16° C at 5,000 feet.

Stability related to temperatures alof t and adiabatic cooling. In each situation, the balloon is

f illed at sea level w ith air at 31° C, carried manually to 5,000 feet, and released. In each

case, air in the balloon expands and cools to 16° C (at the dry adiabatic rate of 3° C per

1,000 feet). But, the temperature of the surrounding air alof t in each situation is different.

The balloon on the lef t w ill rise. Even though it cooled adiabat ically, the balloon remains

warmer and lighter than the surrounding cold air; w hen released, it w ill continue upw ard

spontaneously. The air is unstable; it favors vertical motion. In the center, the surrounding



Page 3

air is w armer. The cold balloon w ill sink. It resists our forced lif ting and cannot rue

spontaneously. The air is stable-it resists upw ard motion. On the right, surrounding air and

the balloon are at the same temperature. The balloon remains at rest since no density

dif ference exists to displace it vertically. The air is neutrally stable, i.e., it neither favors nor

resists vertical motion. A mass of air in which the temperature decreases rapidly w ith height

favors instability; but, air tends to be stable if the temperature changes little or not at a ll w ith

altitude.

Stable Or Unstable Process

Stability runs the gamut from absolutely stable to absolutely unstable, and the atmosphere

usually is in a delicate balance somew here in betw een. A change in ambient temperature

lapse rate of an air mass can tip this balance. For example, surface heating or cooling alof t

can make the air more unstable; on the other hand, surface cooling or w arming alof t of ten

tips the balance toward greater stability.

Air may be stable or unstable in layers. A stable layer may overlie and cap unstable air; or,

conversely, air near the surface may be stable w ith unstable layers above.

Stratiform Clouds

Since stable air resists convection, clouds in stable air form in horizontal, sheet-like layers or

"strata." Thus, w ithin a stable layer, clouds are stratiform. Adiabatic cooling may be by

upslope f low ; by lif ting over cold, more dense air; or by converging w inds. Cooling by an

underlying cold surface is a stabilizing process and may produce fog. If clouds are to remain

stratiform, the layer must remain stable after condensation occurs.

Cumuliform Clouds

Unstable air favors convection. A "cumulus" cloud, meaning "heap," forms in a convective

updraft and builds upw ard. Thus, w ithin an unstable layer, clouds are cumuliform; and the

vertical extent of the cloud depends on the depth of the unstable layer.



Page 4

FIGURE

When stable air (lef t) is forced upward, the air tends to retain horizontal f low , and any

cloudiness is f lat and stratif ied. When unstable air is forced upw ard, the disturbance grows,

and any resulting cloudiness shows extensive vertical development.

We can estimate height of cumuliform cloud bases using surface temperature-dew point

spread. Unsaturated air in a convective current cools at about 5.4° F (3.0° C) per 1,000 feet;

dew point decreases at about 1° F (5/9° C). Thus, in a convective current, temperature and

dew point converge at about 4.4° F (2.5° C) per 1,000 feet as illustrated in f igure 44. We can

get a quick estimate of a convective cloud base in thousands of feet by rounding these

values and dividing into the spread or by mult iplying the spread by their reciprocals. When

using Fahrenheit, divide by 4 or multip ly by .25; when using Celsius, divide by 2.2 or mult ip ly

by .45. This method of estimat ing is reliable only w ith instability clouds and during the

warmer part of the day.



Page 5

FIGURE

Cloud base determination. Temperature and dew point in upward moving air converge at a

rate of about 4° F or 2.2° C per 1,000 feet.

Merging Stratiform and Cumuliform

A layer of stratiform clouds may sometimes form in a mildly stable layer w hile a few

ambitious convective clouds penetrate the layer thus merging stratiform w ith cumuliform.

Convective clouds may be almost or entirely embedded in a massive stratiform layer and

pose an unseen threat to instrument f light.



Page 6

Temperature distribution of vertically moving air

The term "adiabatic process" simply means w arming by compression, or cooling by

expansion, w ithout a transfer of heat or mass into a system. As air moves up or down w ithin

the atmosphere, it is af fected by this process. This temperature dif ference w ill be 5-1/2

degree decrease per 1,000 feet increase in altitude. This is also termed the dry adiabat ic

lapse rate. The atmosphere may or may not have a temperature distribution that f its the dry

adiabat ic lapse rate. Usually it does not.



Page 7

Unstable air encourages vertical movement of air and decreases f ire activity.

The actual lapse rate may be greater or less than the dry adiabatic lapse rate and may

change by levels in the atmosphere. This variation from the dry adiabatic lapse rate is what

determines w hether the air is stable or unstable. If the air is unstable, the vertical movement

of air is encouraged, and this tends to increase f ire activity. If the air is stable, vertical

movement of air is discouraged, and this usually decreases or holds dow n fire activity. The

importance of this atmospheric property w ill become evident by the time you have

completed this unit.

Dry Lapse Rates

The actual temperature lapse rate in a given portion of the atmosphere could range from a

plus 15° per 1,000 feet to a minus 15° per 1,000 feet. These would represent the extremes

of very stable air to very unstable air.

AIR MASSES AND FRONTS

The purpose of this module is to introduce air masses, w here they originate from and how

they are modif ied. Clashing air masses in the middle lat itudes spark interesting weather

events and the boundaries separating these air masses are know n as fronts. This module

examines fronts, with detailed explanations about cold fronts and w arm fronts. Finally,



Page 8

dif ferent types of advection are introduced; temperature, moisture and voriticity advection.

The Air Masses and Fronts module has been organized into the follow ing sections:

Air Masses

Air masses that commonly inf luence w eather in the United States.

Fronts

Boundaries separating air masses. Includes w arm fronts, cold fronts, occluded and

stationary fronts and dry lines.

Continental Polar Air Masses

cold tem peratures and little moisture

Those w ho live in northern portions of the United States expect cold w eather during the

w inter months. These conditions usually result f rom the invasion of cold arctic air masses

that orig inate from the snow covered regions of northern Canada. Because of the long

w inter nights and strong radiational cooling found in these regions, the overlying air

becomes very cold and very stable. The longer th is process continues, the colder the

developing air mass becomes, until changing weather patterns transport the arctic air mass

southw ard.



Page 9

Below is a map of surface observations and the leading edge of a large arctic air mass

blanket ing much of the United States has been highlighted by the blue line. The center of

this air mass is a high pressure center located in northern Montana (indicated by the blue

"H").

From these reports, w e see that most stations in the arctic air mass generally exhibit

relatively colder temperatures, w ith low er dew point temperatures, and w inds generally out

of the north. Notice that on the other side of the blue boundary, outside of this air mass,

surface conditions are much dif ferent, which indicates the presence of an entirely dif ferent

air mass.

Maritime Tropical Air Masses

w arm temperatures and rich in m oisture

Maritime tropical air masses originate over the warm w aters of the tropics and Gulf of

Mexico, w here heat and moisture are transferred to the overlying air from the w aters below.

The northward movement of tropical air masses transports warm moist air into the United

States, increasing the potent ial for precipitation.



Page 10

Below is a map of surface observations and the leading edge of a tropical air mass surging

northw ard into the Ohio Valley has been highlighted in red. Southerly w inds behind the

boundary signify the continued northw ard transport of warm moist air.

From these reports, w e see that most stations in the tropical a ir mass generally exhibit

relatively w armer temperatures, w ith higher dew point temperatures, and w inds generally out

of the south. Notice that on the other side of the red boundary, outside of this air mass,

surface conditions are much dif ferent, which indicates the presence of an entirely dif ferent

air mass.

Air masses and their sources

Fahrenheit w hile a short distance behind the front, the temperature decreased to 38

degrees. An abrupt temperature change over a short distance is a good indicator that a front

is located somew here in betw een.

THE HYDROLOGICAL CYCLE

(also know n as the w ater cycle) is the journey water takes as it circulates from the land to

the sky and back again.

The sun's heat provides energy to evaporate water from the earth's surface (oceans, lakes,

etc.). Plants also lose water to the air - this is called transpiration. The w ater vapour

eventually condenses, forming tiny droplets in clouds.

When the clouds meet cool air over land, precipitation (rain, sleet, or snow) is triggered, and

water returns to the land (or sea). Some of the precipitat ion soaks into the ground. Some of

the underground water is trapped betw een rock or clay layers - this is called groundw ater.

But most of the w ater f lows downhill as runoff (above ground or underground), eventually

returning to the seas as slightly salty w ater.

Earth's w ater

Water is the most w idespread substance to be found in the natural environment and it is the

source of all life on earth. Water covers 70% of the earth's surface but it is diff icult to



Page 11

comprehend the total amount of water when we only see a small portion of it. The

distribution of w ater throughout the earth is not uniform. Some places have far more rainfall

than others.

The Water Cycle

To assess the total w ater storage on the earth reliably is a complicated problem because

water is so very dynamic. It is in permanent motion, constantly changing from liquid to solid

or gaseous phase, and back again. The quantity of water found in the hydrosphere is the

usual w ay of estimating the earth's w ater. This is all the free water existing in liquid, solid or

gaseous state in the atmosphere, on the Earth's surface and in the crust down to a depth of

2000 metres. Current estimates are that the earth's hydrosphere contains a huge amount of

water - about 1386 million cubic kilometres. However, 97.5% of this amount exists as saline

waters and only 2.5% as fresh water.

Hydrological Cycle work

The stages of the cycle are:

• Evaporation

• Transport

• Condensation

• Precipitation

• Groundw ater

• Run-off

Evaporation

Water is transferred from the surface to the atmosphere through evaporation, the process by

which water changes from a liquid to a gas. The sun's heat provides energy to evaporate

water from the earth's surface. Land, lakes, rivers and oceans send up a steady stream of

water vapour and plants also lose w ater to the air (transpiration).

Approximately 80% of all evaporation is f rom the oceans, w ith the remaining 20% coming

from inland water and vegetation.



Page 12

Transport

The movement of water through the atmosphere, specif ically f rom over the oceans to over

land, is called transport. Some of the earth's moisture transport is visible as clouds, w hich

themselves consist of ice crystals and/or tiny w ater droplets.

Clouds are propelled from one place to another by either the jet stream, surface-based

circulations like land and sea breezes or other mechanisms. How ever, a typical cloud 1 km

thick contains only enough w ater for a millimetre of rainfall, w hereas the amount of moisture

in the atmosphere is usually 10-50 times greater than this.

Most w ater is transported in the form of water vapour, which is actually the third most

abundant gas in the atmosphere. Water vapour may be invisible to us, but not to satellites

which are capable of collecting data about moisture patterns in the atmosphere.

Condensation

The transported w ater vapour eventually condenses, forming tiny droplets in clouds.

Precipitation

The primary mechanis m for transporting w ater from the atmosphere to the surface of the

earth is precipitation.

When the clouds meet cool air over land, precipitation, in the form of rain, sleet or snow, is

triggered and w ater returns to the land (or sea). A proportion of atmospheric precipitation

evaporates.

Groundwater

Some of the precipitat ion soaks into the ground and this is the main source of the formation

of the w aters found on land - rivers, lakes, groundw ater and glaciers.

Some of the underground water is trapped between rock or clay layers - this is called

groundw ater. Water that inf iltrates the soil f lows dow nward until it encounters impermeable



Page 13

rock and then travels laterally. The locat ions w here water moves laterally are called

'aquifers'. Groundwater returns to the surface through these aquifers, which empty into

lakes, rivers and the oceans.

Under special circumstances, groundw ater can even f low upw ard in artesian wells. The f low

of groundw ater is much slow er than run-off w ith speeds usually measured in centimetres per

day, metres per year or even centimeters per year.

Run-off

Most of the water which returns to land flows downhill as run-off . Some of it penetrates and

charges groundw ater while the rest, as river f low, returns to the oceans w here it evaporates.

As the amount of groundw ater increases or decreases, the water table rises or falls

accordingly. When the entire area below the ground is saturated, f looding occurs because

all subsequent precipitation is forced to remain on the surface.

Dif ferent surfaces hold different amounts of water and absorb w ater at different rates. As a

surface becomes less permeable, an increasing amount of water remains on the surface,

creating a greater potential for f looding. Flooding is very common during w inter and early

spring because frozen ground has no permeability, causing most rainwater and meltwater to

become run-off .

GLOBAL WARMING

Global w arming has become familiar to many people as one of the most important

environmental issues of our day. This review w ill describe the basic science of global

warming, its likely impacts both on human communit ies and on natural ecosystems and the

actions that can be taken to mitigate or to adapt to it. As commonly understood, global

warming refers to the effect on the climate of human activities, in particular the burning of

fossil fuels (coal, oil and gas) and large-scale deforestation—activities that have grown

enormously since the industrial revolution, and are currently leading to the release of about

7 billion tonnes of carbon as carbon dioxide into the atmosphere each year together w ith

substantial quantit ies of methane, nitrous oxide and chlorofluorocarbons (CFCs). These

gases are know n as greenhouse gases.



Page 14

The basic principle of global w arming can be understood by considering the radiation energy

from the sun that warms the Earth’s surface and the thermal radiat ion from the Earth and

the atmosphere that is radiated out to space. On average, these tw o radiation streams must

balance. The greenhouse effect arises because of the presence of greenhouse gases in the

atmosphere that absorb thermal radiat ion emitted by the Earth’s surface and, therefore, act

as a blanket over the surface . It is known as the greenhouse effect because the glass in a

greenhouse possesses similar properties to the greenhouse gases in that it absorbs infrared

radiation while being transparent to radiation in the visib le part of the spectrum. If the

amounts of greenhouse gases increase due to human activit ies, the basic radiat ion balance

is altered.

Because of the long life time in the atmosphere of many greenhouse gases such as carbon

dioxide, their ef fects impact on everyone in the world. Global pollution can only be countered

by global solut ions.

The follow ing sections w ill address the basic science of the greenhouse effect

• climate variability evidenced by past records

• sources and sinks of greenhouse gases

• the concept of radiative forcing and how it is used

• climate models and how well they simulate past and current climate

• projections of climate change over the 21st century

• impacts of climate change especially those on human communit ies

• internat ional policy and action regarding

Figure

A greenhouse has a similar ef fect to the atmosphere on the incoming solar radiat ion and



Page 15

the emitted thermal radiation.

climate change, including the w ork of the IPCC

• stabilization of climate

• mitigat ion of climate change and implicat ions for technology and

• the future challenge

The enhanced greenhouse effect

After our excursion to Mars andVenus, let us return to Earth! To w hat extent are the

greenhouse gases in the Earth’s atmosphere influenced by human activity? The amount of

water vapour depends mostly on the temperature of the surface of the oceans; most of it

originates through evaporation from the ocean surface and is not inf luenced directly by

human activity. Carbon dioxide is dif ferent. Its amount has increased substantially—by over

30 per cent—since the Industrial Revolution, due to human industry and also because of the

removal of forests . Future projections are that, in the absence of controlling factors, its rate

of increase w ill accelerate and its atmospheric concentration w ill double from its pre-

industrial value w ithin the next hundred years .

This increased CO2 is leading to global warming of the Earth’s surface through its enhanced

greenhouse effect. Let us imagine, for instance, that the amount of CO2 in the atmosphere

suddenly doubled, everything else remaining the same . The solar radiat ion budget w ould

not be affected. But the thermal radiat ion emitted from CO2 in the atmosphere w ill originate

on average from a higher and colder level than before . The thermal radiation budget w ill,

therefore, be reduced, the amount of reduction being about 4Wm-2.To restore the radiat ion

balance the surface and lower atmosphere w ill warm. If nothing changes apart f rom their

temperature—in other w ords, clouds, w ater vapour, ice and snow cover and so on, are all

the same as before—a radiative transfer calculation indicates that the temperature change

would be about 1.2°C.

In reality, of course, many of these other factors w ill change, some of them in w ays that add

to the w arming (positive feedbacks), others in w ays that reduce the w arming (negative

feedbacks). The situation is, therefore, much more complicated than this simple calculat ion;



Page 16

it w ill be considered in more detail in section 6. Suff ice it to say here, that the best estimate,

at the present time, of the increased average temperature of the Earth’s surface if CO2

levels w ere to be doubled is about tw ice that of the simple calculation: 2.5°C. As the next

section w ill illustrate, for the global average temperature this is a large change.

Sea-level rise resulting from global w arming w ill, therefore, lag behind temperature change

at the surface. During the follow ing centuries, as the rest of the oceans gradually w arm, sea

level w ill continue to rise at about the same rate, even if the average temperature at the

surface were to be stabilized.

What about the major ice sheets; w ill their contribution continue to be small in the future?

For both ice-sheets there are tw o competing effects. In a w armer w orld, there is more w ater

vapour in the atmosphere that leads to more snowfall. But there is also more ablation

(erosion by melting) of the ice around the boundaries of the ice-sheets and calving of

icebergs during summer months. For Antarctica, the present estimates are that

accumulat ion is greater than ablation, leading to a small net grow th. How ever, it is possible

that larger changes in the ice sheets may begin to occur. The Greenland ice sheet is

probably the more vulnerable; its complete melting w ill cause a sea-level rise of about 7m.

Model studies of the ice sheet show that, w ith a rise in summer temperature in the region of

Greenland of more than 3°C—likely to be realized w ithin a few decades—ablation w ill

signif icantly overtake accumulation and melt down of the ice cap w ill begin. Such melt dow n

is likely to be irreversible. If the temperature continued to rise to say 8°C or more, much of

the melt down would occur during the next 1000 years. Turning to the Antarctic ice-sheet,

the part that is of most concern is in the w est of Antarctica (around 90°W longitude); its

disintegration w ould result in about 6m of sea-level rise. Because a large portion of it is

grounded well below sea level it has been suggested that rapid ice discharge could occur if

the surrounding ice shelves are weakened.

In the absence of such rapid change, about w hich studies at present are inconclusive [89],

the contribution of the West Antarctic Ice Sheet to sea-level rise over the next millennium w ill

be less than 3m.



Page 17

A rise in average sea level of 10 cm by 2030 and about half a metre by the end of the 21st

century may not seem a great deal. Many people live suff iciently above the level of high

water not to be directly affected. How ever, half of humanity inhabits the coastal zones

around the w orld. Within these, the lowest lying are some of the most fertile and densely

populated. To people living in these areas, even a fraction of a metre increase in sea level

can add enormously to their problems. Some of the areas that are especially vulnerable are

f irst, large river delta areas, for instance Bangladesh, second, areas very close to sea level

where sea defences are already in place, for instance the Netherlands and third, small low-

lying islands in the Pacific and other oceans. Here, w e just consider the example of

Bangladesh.

Bangladesh is a densely populated country of about 120 million people located in the

complex delta region of the Ganges, Brahmaputra and Meghna Rivers About 10% of the

country’s habitable land (w ith about 6 million populat ion) w ould be lost w ith half a metre of

sea-level rise and about 20% (w ith about 15 million populat ion) w ould be lost w ith a 1m rise.

Estimates of the sea-level rise are of about 1m by 2050 (compounded by 70 cm due to

subsidence because of land movements and removal of groundw ater and 30 cm from the

effects of global w arming) and nearly 2m by 2100 (1.2m due to subsidence and 70 cm from

global w arming)—although there is a large uncertainty in these estimates.

Further exacerbation of the impact w ill arise through the combinat ion of sea-level rise w ith

likely increases in the intensity of storm surges in that region. Further, increased salt w ater

intrusion into ground w ater w ill occur in many low lying regions. Similar situat ions to that in

Bangladesh exist in other parts of south-east Asia, the Nile delta region of Egypt and delta

regions in other parts of Africa and the Americas.

It is not only in places w here there is dense population that there w ill be adverse effects.

Theworld’s wetlands and mangrove swamps currently occupy an area of about a million

square kilometres, equal approximately to tw ice the area of France. They contain much

biodiversity and their biological productivity equals or exceeds that of any other natural or

agricultural system. Over tw o-thirds of the f ish caught for human consumption, as w ell as

many birds and animals, depend on coastal marshes and swamps for



Page 18

Land affected in Bangladesh by various amounts of sea-level rise.

part of their life cycles, so they are vital to the total world ecology. These areas could not

adjust to the rapid rate of sea-level rise that is likely and in many cases would be unable to

extend inland. Net loss of wetland area w ill therefore occur.

Fresh w ater resources

The global water cycle is a fundamental component of the climate system. Water is cycled

between the oceans, the atmosphere and the land surface. Water is also an essential



Page 19

resource for humans and for ecosytems. During the last 50 years w ater use has grown over

threefold it now amounts to about 10% of the estimated global total of river and groundw ater

f low from land to sea. Increasingly,w ater is used for irrigation. In India about 75% of

availablew ater is so used. Water from major rivers is of ten shared betw een nations; its

growing scarcity is a potentia l source of conf lict. In many areas, groundwater extraction

greatly exceeds its replenishment—a situation that cannot continue indef initely.

With global w arming, there w ill be substantial changes in w ater availability, quality and f low.

On average, some areas w ill become w etter and others drier. Substantial changes in

variations of f low during the year w ill also occur as glaciers and snow cover diminishes

leading to less spring melt. Much of these changes w ill exacerbate the current vulnerability

regarding water availability and use. Especially vulnerable w ill be continental areas w here

decreased summer rainfall and increased temperature result in a substantia l loss in soil

moisture and increased likelihood of drought.

Even greater impact is likely to occur because of increased frequency and intensity of

extremes, especially f loods and droughts. Such disasters are the most damaging disasters

the world experiences; on average they cause more deaths, misery and economic loss than

other disasters. They are especially damaging to developing countries w here, in general,

they are more likely to occur and where there is inadequate infrastructure to cope w ith them.

Impacts of climate change on fresh w ater resources are likely to be exacerbated by other

pressures, e.g. population grow th, land-use change, pollution and economic grow th.

Agriculture and food supply

Climate change w ould affect agriculture and food supply through its impact on crops, soils,

insects, weeds, diseases and livestock. Three factors are particularly important; changes in

water availability changes in temperature and the effect of increased CO2 on plant grow th.

Higher CO2 concentrations stimulate photosynthesis, enabling some plants (e.g. wheat, rice

and soya bean) to fix carbon at a higher rate. This is w hy in glasshouses additional CO2 may

be introduced artif icially to increase productivity. Under ideal conditions it can be a large

effect (for doubled CO2 up to an average of +30% [100]). However, under real conditions on



Page 20

the large scale, w here water and nutrient availability are also important factors, increases

tend to be substantially less than what is potentially possible. For instance, for cereal crops

in mid-latitudes, potent ial yields are projected to increase for small increases in temperature

(2–3°C) but decrease for larger temperature increases.

In a w orld inf luenced by global warming, crop patterns w ill change. But the changes w ill be

complex; economic and other factors w ill take their p lace alongside climate change in the

decision-making process. To estimate the effect of climate change on w orld food supply,

elaborate modelling studies have been carried out. These start w ith climate change

scenarios for different locations and times that are inserted into crop models that then

produce projected changes in crop yields. Included also are farm level adaptations (e.g.

planting date shif ts, more climatically adapted variet ies, irrigation and fertilizer application).

These yield changes are then employed as inputs to a world food trade model that includes

assumptions about global parameters, such as population grow th and economic factors.

The outputs from the total process provide information projected up to the 2080s on food

production, food prices and the number of people at risk of hunger.

Ecosystems

About 10% of the w orld’s land area is under cult ivation. The rest is to a greater or lesser

extent unmanaged by humans. Of this about 30% is natural forest. Within this area climate

is the dominant factor determining the distribution of biomes. Large changes in th is

distribution have occurred during the relatively slow climate changes in the past. It is the

very rapid rate of change of climate that w ill cause excessive stress on many systems. How

much it matters depends on the species and the degree of climate change (e.g. temperature

increase or water availability). Two particularly vulnerable types of species are trees and

coral. The viability of some large areas of tropical forests under climate change is of especial

concern. Many corals are already suffering from bleaching caused by increased ocean

temperatures. Further, as large quant ities of extra carbon dioxide are dissolved in the

oceans, their acidity increases posing a serious threat to living systems in the oceans

especially to corals.



Page 21

Human health

Human health w ill be affected by many of the impacts described in previous paragraphs

such as deteriorating w ater availability, food shortages and more intense and more frequent

f loods and droughts. Increased spread of insect-borne diseases, such as malaria, is also

likely in a warmer world. The main direct ef fect of climate change on humans themselves w ill

be that of heat stress in the extreme high temperatures that w ill become more frequent and

more w idespread especially in urban populat ions. Studies using data from large cities w here

heat

Distribut ion of average summer temperatures (June, July, August) in Sw itzerland from 1864

to 2003 show ing a f itted Gaussian probability distribut ion—standard deviation 0.94°C . The

2003 value is 5.4 standard deviations from the mean show ing it to be an extremely rare

event. Also shown are return periods calculated from conventional statistics assuming no

warming trend.

waves commonly occur show death rates that can be doubled or trip led during days of

unusually high temperatures. On the positive side, mortality due to per iods of severe cold in

w inter w ill be reduced.

An example of record extreme high temperatures is the heat w ave in Europe during June,

July and August, 2003. At many locations temperatures rose above 40°C. In France, Italy,

the Netherlands, Portugal and Spain over 20 000 additional deaths w ere attributed to the

unrelenting heat. The f igure illustrates the extreme rarity of this event. Studies show that it is

very likely that a large part of its cause is due to the increase in greenhouse gases, that by



Page 22

2050 such a summer w ould be likely to be the norm and by 2100 would likely be a cool

summer.

Adaptation and mitigation

There are two kinds of action that can be taken—adaptation to reduce the impacts of climate

change as it occurs and mit igation to reduce emissions of greenhouse gases that in turn w ill

reduce the amount of climate change. Some of the impacts of anthropogenic climate change

are already becoming apparent and a degree of adaptation is already a necessity. Many

adaptation options have already been identif ied that can reduce the adverse impacts of

climate change and can also produce ancillary benefits, but they cannot prevent all

damages. Of particular importance is the requirement for adaptation to extreme events and

disasters such as floods, droughts and severe storms . Vulnerability to such events can be

substantially reduced by more adequate preparat ion.

It is associated w ith both the science and the impacts of climate change are considerable

uncertainties—. Politicians and others making decisions are, therefore, faced w ith the need

to w eigh all aspects of uncertainty against the desirability and the cost of the various actions

that can be taken in response to the threat of climate change.



Page 23

Climate change—an integrat ing framew ork . A complete cycle of cause and effect is show n

beginning w ith economic activity (low er right-hand corner) that results in emissions of

greenhouse gases (of which CO2 is the most important) and aerosols. These emissions lead

to changes in atmospheric composition and hence to changes in climate that impact both

humans and natural ecosystems and affect human livelihood, health and development. An

anticlockw ise arrow represents other effects of development on human communities and

natural systems, for instance changes in land use that lead to deforestation and loss of

biodiversity.

Costing the im pacts

Probably the largest impact of climate change w ill be that of the increased number and

intensity of extreme events. We noted in section 3 the recent increase in extreme events

and the interest of insurance companies w ho have tracked increasing damage from them in

recent decades. Not that insured losses are a good guide to total loss. For instance, the

insured losses for Hurricane Mitch that hit Central A merica in 1998 were small. How ever,



Page 24

9000 people died and the losses in Honduras and Nicaragua, respectively, amounted to

about 70% and 45% of their annual gross national product (GNP) . China is a country

particularly prone to natural d isasters; f rom 1989 to 1996 they resulted in an average annual

loss equivalent to near ly 4% of GDP.

International policy and action

As observational and modelling tools for studying the climate advanced during the 1970s

and 1980s, the attent ion of scientists became increasingly directed tow ards the effects on

the climate of human activit ies. A scientif ic conference in 1985 organized by the Scientif ic

Committee on Problems of the Environment (SCOPE) a committee of the International

Council of Scientific Unions led to an important publication that described the adverse

effects that could result from continued and increased anthropogenic emissions of CO2.

That in turn led to increasing awareness amongst polit icians of the scale of the potential

problem. Two important international bodies were created, one in 1988 concerned w ith

science (the IPCC) and one in 1992 w ith policy (the Framew ork Convention on Climate

Change (FCCC)). These w ill be introduced brief ly in turn.

The Intergovernmental Panel on Clim ate Change (IPC C)

The IPCC w as formed jointly by tw o United Nat ions bodies, the World Meteorological

Organization (WMO) and the United Nations Environment Programme (UNEP) w ith a remit

to prepare thorough assessments of climate change, its causes and effects. The Panel

established three working groups, one to deal w ith the science of climate change, one w ith

impacts and a third w ith policy responses. The IPCC has produced three main

comprehensive reports, in 1990, 1995 and 2001 together w ith a number of special reports

covering particular issues.

csir net, gate, iit-jam, ugc net , tifr, iisc, …examprep.vpmclasses.com/images/uploads/ugc...

Documents