CBSE Class 9 Science Natural Resources Notes

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NATURAL RESOURCES

NATURAL RESOURCES

It indicates the potential wealth of a country. The variety of substance that man gets from earth and nature to meet his basic needs are called natural resources. The word resource means a source of supplying a material generally held in reserve. Natural resources are both living and non-living. Some of these resources are found in abundance, while others are found in limited quantities and that too in some restricted parts of our land. For this reason, the natural resource have to be wisely used. However, in reality it is not so. They are being used indiscriminately.

(a)Types of Natural Resources : 

 Depending upon the abundance and availability, the natural resources are categorized into two types, i.e.

 (i) inexhaustible natural resources (ii) exhaustible natural resources.

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AIR OR ATMOSPHERE

The multilayered, transparent and protective envelope of gases surrounding the planet earth is called atmosphere. In other words atmosphere is the layer or air above the earth’s surface and air is a mixture of several gases. About 95% of total air is present up to the height of 20 km above earth’s surface.  Remaining 5% is up to the height of 280 km. 

(a)Composition of Air : 

Gas                                  relative percentage / volume

Nitrogen                           78.08%

Oxygen                             20.94%

Argon                                 0.9%

Carbon dioxide                    0.03%

He, Ne, Kr, Xe,                     in trace amounts

Ch4.O3

Besides these gaseous components air also possesses water vapour, industrial gases, dust, smoke particles, microorganisms, pollen grains, fungal spores etc.

(b)The different zones of Atmosphere or Air :

  1. Troposphere: It is the basal part that extends about 20 km above the earth’s surface. (Upto 8 km on poles), in this layer important climatic events occur like cloud formation, lightening, thundering etc. in this region air temperature gradually decreases with height.
  2. Stratosphere: It lies next to troposphere and is 30 km high. in this layer temperature rises. there is a formation of ozone layer in this region which can absorb the harmful ultra violet rays coming from sun.
  3. Mesosphere: It lies next to stratosphere and is 40 km in height. Temperature decreases in this region.
  4. Lonosphere: It lays upto the height of 300 km above earth’s surface. In this layer gaseous components become ionized to sun’s energy and remain there as ions.

(c)Role of Air or Atmosphere :

  1. It acts as medium for movement of insects, birds etc.
  2. It protects the life on earth from harmful ultra violet rays.
  3. It is a source of oxygen, carbon dioxide and nitrogen required for various metabolic activities of living beings.
  4. It helps in dispersal of spores, pollen, seeds etc.
  5. It maintains temperature on earth required for life.
  6. It transmits sound for communication.
  7. Ionosphere reflects the radio waves back to earth for long distance communication due to presence of ions and free electrons.
  8. Burning (combustion) takes place in presence of oxygen and produces carbon dioxide.
  9. Specific climatic conditions and water cycle is maintained due to circulation of air.
  10. Eukaryotic cells and many prokaryotic cells require O2 for break down of glucose to get energy through respiration, they release CO2.

(d)The Role of Atmosphere in Climate Control :

Climate is an average weather of an area. Temperature, light and rainfall are important factors that determine climate of an area. Atmosphere plays a crucial role in its control :

  1. It acts like a blanket covering the whole earth.
  2. It keeps the temperature of earth steady. It acts as bad conductor of heat thus prevents the sudden increase in temperature during the day as well as slows down the escape of heat into the outer space during night.
  3. The role of atmosphere on earth, when compared with moon (with no atmosphere), temperature range varies at moon from – 1900 C to 1100 Although both lie at same distance from sun.

(e)Wind :

Air  in  called wind. Speed of wind can be determined by :

a Heating of air    b Formation of water vapour

Atmosphere can be heated from below by radiations, such radiations are reflected back. Convection currents appear in air on being heated. When the air gets hot due to radiations emitted from land or water, it rises. Air over land get heated speedily as compared to air above water bodies because, land gets heat faster. Wind occurs because of horizontal and vertical differences in atmospheric pressure. During daytime in coastal regions, air on lands rises due to heating at faster rate. Air over sea moves into the space of low pressure created due to rising of air above land. This creates wind. During day time, thus air moves from to land. Sea and land both become cool during night hours. Cooling of water is slower than land, thus air above water becomes warmer. Thus air will move from land sea. 

(i)Factors controlling movement of air

(A)rotation of earth in the path of wind, mountain ranges may come across. The general pattern of winds over earth is known as general circulation and specific winds are named for the direction from which they originate (e.g. wind blowing from west to east is westerly). Wind speeds are often classified according to Beaufort scale.

(f)Rain : The warm, moist and rising air cools and forms clouds in the sky. This happens due to heating of water bodies during day time which get mixed with atmosphere. The air rises, it expands and cools. cool air in the atmosphere sinks towards the ground. Due to cooling water vapours present in air get facilitated. These tiny droplets become bigger and bigger due to condensation. When they become heavy, they fall down in the from of rain. Four main types of precipitation are as follows: 

(i)rain. Precipitation in the form of liquid

(ii)sleet. Rain which freezes.

(iii)snow. Small ice crystals that form around dust or salt particles.

(iv)hail. Frozen rain that is circulated up and down in a cloud until, it is hard frozen ball of ice.

Cloudy, wet, changeable weather is common in low pressure zones with rising unstable areas. Such conditions are found at temperate latitudes, where warm air along polar fronts. Here spiraling low pressure cells known as depressions (mid-latitude cyclones) are formed. In India mostly rains are brought by usually southwest or northeast monsoon. Depressions in Bay of Bengal also cause rain at some places of India.

POLLUTION    

Any undesirable change in physical, chemical or biological characteristics in the air, water and land which is harmful to the men directly or indirectly though animals, plants,industrial units or raw materials is called as pollution. pollution is mostly man made. But it can also be natural.

(a)Pollutants :

Any material or act of man, or nature which leads to pollution is called as pollutants. The pollution is usually brought about by the addition to the environment of waste products of human activity. When the waste products are not efficiently assimilated, decomposed or other wish removed by natural, biological and physical processes (recycling) and the system is unable to utilize them properly, so that the balance of the system breaks down. Therefore such type of pollutants can stimulate or inhibit the biological reactions or change in their capacity. Therefore changes also take place in the ecosystem. The amount, numbers and types of pollutants are increasing with the growth of the population.

(b)Air Pollution :

Air pollution is caused due to the addition of the unwanted substances or gases. The atmospheric pollution is mainly caused by the activities of man and concentrated to the inhabited and the industrial complexes in cities. There are two main categories of air pollutants.

(i)Gaseous: The gaseous materials include various gases and vapours of volatile substances or the compound with a boiling point below 2000C.

(ii)Particulate: Dust particles, carbon particles, particles of other matels etc.

(c)Major air Pollutants and Their Effects :

(i)carbon monoxide (CO) : this is the main air pollutant. Carbon monoxide is a highly toxic as which is colourless and odourless in nature. It combines with hemoglobin of the blood and blocks the transportation of oxygen. Thus, it impairs respiration and it causes death.

(ii)Unburnt hydrocarbons : Out of them 3, 4 – benzpyrene is the main pollutant. This causes cancer in lungs.

(iii)Ethylene: The falling of leaves without particular reason, falling buds et. Effects are seen in plants are due to ethylene.

(iv)Oxides of nitrogen: these oxides form photochemical smog in the atmosphere and release ozone. Ozone causes harm to mucilaginous membrane. The oxide pollutants of nitrogen are nitric oxide (NO), and nitrogen di oxide (NO2). These oxides and ozone are very harmful for the plants. The entry of these pollutants causes various diseases in animals like- respiratory trouble such as emphysema, bronchitis, swelling of lungs and lung cancer etc.

(v)smoke : Many constituents are present in smoke such as sulphur dioxide(SO2), Sulphur trioxide (SO3), Sulphuric acid (H2SO4), Ozone (O3), Carbon dioxide (CO2), PAN (Peroxyacetyle nitrate), Arsenic and Fluoride etc. the distribution area of lichen and mosses are the indicators of SO2 pollution because lichen and mosses cannot grow in the industrial regions or the regions containing SO2 pollutants. The higher concentration of ozone produces harmful effects. But normally, ozone layer absorbs U.V. rays which are harmful for the living things. 

(vi)aerosol : The aerosol like C.F.C. (chloro fluoro carbon)  release into the atmosphere from the refrigerators, air conditioners and jet planes deplete or reduce the ozone layer. This thin layer of ozone is also known as ozone hole results in the increase in temperature of the earth.

(d)Measures to Control Air Pollution :

  1. Barium compounds should be mixed with petrol which reduce the smoke.
  2. It is also very essential to check the quality of gases released from the factories.
  3. Industries should not be established at one place.
  4. The smoke should be released into the atmosphere after filtration and purification (by cyclone collector or electrostatic precipitators).

WATER (HYDROSPHERE)

It is renewable resource which is essential for sustenance of life. It covers 3/4   th of the earth’s surface. Of the total water present in hydrosphere 97% is present in oceans which is not utilizable by living beings. Only 3% water is fresh water. Among this 3%, 72.2% is stored in glaciers and ice caps (frozen), 22.4% is ground water and soil moisture. Remaining 0.36% is found in lakes, rivers, streams and swamps.

(a)Types of Water Resources :

(i)Fresh water resource : It consists of pounds, lakes, large rivers. It can be recycled. It is essential for life on earth as well as for survival. It can be obtained by three different types of natural resources.

  1.  Rain water : India receives 3 trillion m3of water from rainfall or precipitation. Its intensity is different in different zones, on this basis zones are classified as Wet zone : with very high rainfall ­ intermediate zone : with heavy rainfall
  2. Semi arid zone : with moderate rainfall ­ Arid zone ; with low rainfall.
  3. There are 14 major river systems with plenty of lakes, pounds etc.
  4. It is the water which percolate into the ground. There is a certain level below the surface where the rocks are saturated with water and this level is known as the zone of saturation. The upper level of the zone of saturation is called the water table. However, the vertical distance from the surface from the surface of a region of the water table is called the water level.

(ii)Salt water resource :It consists of oceans, seas etc. it cannot be used by living beings for drinking.

(b)Role of Water or Hydrosphere :

  1. Water is the main constituent of protoplasm.
  2. It is the universal solvent. Through which mineral salts are transported from one part of the plant to the other.
  3. Various metabolic reaction take place in the medium containing water.
  4. It acts as a reactant in numerous metabolic reactions.
  5. During photosynthesis, water releases oxygen.
  6. Turgidity of the growing cells is maintained with water.
  7. Various movements of plant organs like movements in sensitive plant (touch-me-not) are controlled by water.
  8. The growth of the cells during elongation phase is mainly depends upon absorption of water.
  9. Metabolic end product of respiration is water.
  10. It acts as a temperature buffer as its specific heat is highest (only exception – liquid ammonia).
  11. It show the properties of cohesion and adhesion which account for the capillary action of water.

(c)Water pollution :

The water pollution is caused by the addition of organic and inorganic chemicals as well as the biological materials which change the physical and chemical properties of water. This harmful process is called as water pollution. The water pollution is caused by many sources such as sewage matter, industrial wastage, agricultural wastage, domestic wastage, not water of thermal plants and nuclear reactors etc. water pollution can be caused by the following man made sources :

(i)Household detergents : The household detergents include the compounds of

phosphate, nitrate, ammonium and alkylbenzene sulphonate etc. harmful substances which are gathered in water. Alkyl benzene sulphonate (ABS) is not degradable, so that its concentration increases which is harmful for aquatic life.

Control measures : For the control of this pollution lime, ferric chloride etc. are used to precipitate the phosphate. Zirconium is considered best for this purpose.

(ii)Sewage : Sewage contains highest amount of carbonic materials and biological materials. These carbonic materials increase the number of decomposers like bacteria and fungus. The rate of reoxygenation reduced as compared to deoxygenaion in a water reservoirs. The acceleration of microbial activity increases the BOD of water. BOD is very less in pure water. The higher BOD is the indication of water pollution and the water of polluted reservoir can not be utilized and produces a very bad smell spreading around the locality. The infection or disease also takes place. Daphnia and some fishes are sensitive to water pollution and show the intensity of water pollution.

Control measures : To control the water pollution of sewage water it should be left into reservoir after the primary and secondary treatment. The big particles are mainly separated in primary treatment through floatation and sedimentation. Micro organisms are used for secondary treatment such as oxidation chamber or activated sludge process. Oxidation chamber is a shallow reservoir in which the sewage is stored. Algae and bacteria grow very well because of the higher amount of carbonic materials in it. Bacteria decomposes the organic materials and produce CO2 which is utilized by the algae in photosynthesis. Oxygen released by photosynthesis protects the water pollution. Therefore oxidation pond is the example of symbiosis in between algae and bacteria. The infectious bacteria are destroyed during the activity (reactions) in the oxidation pond. So that the simple substances are left after decomposition of organic matter.

 (iii)Industrial wastes : the wastes of industries are discharged into the running water, rivers and canals, industrial wastes mainly contain inert suspended particles such as dust, coal, toxins like acid, base, phenols, cyanides, mercury, zinc etc., inorganic materials like-ferrous salts, sulphides, oils and other residues of organic material and hot water. The water polluted by mercury, lead etc. causes disorganization of nervous system. It means it produces insanity. The minamata disease is caused in Japan by eating of mercury polluted fishes. So many people died because of this disease.

  1. Control measures: The industrial wastes and toxic components should be made pure before releasing into rivers, lakes, ponds or sea. So that the water pollution of industrial effluents can be controlled by suitable treatment to remove the pollutants.
  2. Bioaccumulation of pesticides: Pesticides like DDT are poisonous chemicals sprayed on crops to protect them frompests and diseases. This increase in concentration of harmful non-biodegradable chemical substances in the body of living organisms at each trophic level of a food chain is called biological magnification.
  3. Eutrophication: the discharger of sewage water and detergents in water bodies promotes excessive growth of phytoplanktons (minute aquatic algae). This excessive growth causes reduction in oxygen level of water. The excessive growth of phytoplanktons brings about a reduction in dissolved oxygen which affects other aquatic organisms. Consequently potential sources of food are highly reduced.

 LITHOSPHERE

Lithosphere is the main life supporting system. Top layer of earth is called soil. It is the main natural resource essential for survival and development.

(a)Structure and Formation of Soil :

Soil is formed due to interaction between weathering of rocks, rain, wind, temperature (physical components) and plants, animals and microbes (biological components). .it is formed by combined action of climatic factors such as temperature, rainfall, light etc. and biotic factors such as plants and microbes on earth crust.

(b)Constituents of Soil :

Soil contains: (a) inorganic constituents of parent rocks (b) organic products of living organisms; (c) living organisms including microorganisms (d) air in the pores. There are four important components of soil. They are

  1. Mineral matter           50-60%
  2. Organic matter            10%

(A) Living organisms  (B) decomposed matter

  1. Soil water               25-35%
  2. Soil air              15-25%

(c)Types of Soil :

On the basis of its nature and composition, soil is mainly of six types ­­­­­----

  1. Alluvial soil: rich in loam and clay.   (ii)Black soil-which has clay.

(iii) Red soil: which is sandy to loam.  (iv) Mountain soil-which is a stony and sandy soil.

(v) Desert soil- which is sandy.  (vi) Laterite soil- which has porous clay.

Outer most layer of earth is called crust. Many types of minerals are found in crust. They provide many types of nutrients to living beings.

(d)Factors / Processes Responsible for Formation of Soil :

  1. Sun: rocks get expanded due to heat produced by sun during day time. At night, The rocks cool down and contract. Due to this unequal expansion and contraction of rocks, cracks in rocks appear. This leads to formation of smaller pieces of rocks.
  2. Water : due to continuous movement of rain and fast flowing river water, rock Pieces collide and break down in still finer particles due to their abrasive effect.
  3. Wind: wind has abrasive effect on rocks. Finer rock particles are blown away and get deposited at other distant places.
  4. Living organisms : the step of weathering is brought about by plants and animals. Lichens are first to appear on bare rocks. They produce acids which corrode the rocky surface to produce fine particles. Now plants like mosses can appear on it. In such type of soil, certain microbes, algae, insects and worms appear and die. Organic matter gets accumulated. Roots of some plants grow into the cervices of rocks.

(e)Soil pollution :

Soil is also polluted through the polluted water and air. These pollutants are mixed into the soil through the rainy water. Such as H2 SOacid is formed by mixing of SO2 with rainy water in the air. The fertilizers are used to increase yield of the crops. Various types pesticides and weedicides etc. are sprayed over the crops. All these mixed with soil to produce harmful effects. The growth of plants inhibited or reduced due to this type of pollution and sometimes death also takes place. Excluding to these soil pollution is also caused by the disposal of house hold detergents, sewage, flowing oils, radioactive substances and hot water etc. the main substances of pesticides in soil pollutants are D.D.T. and weedicides 2, 4-D (2,4 di-chlorophenoxy acetic acid) 2,4,5-T (2,4,5, tri-chlorophenoxy acetic acid).

(i)Control measures: soil pollution can be controlled through biological degradation of waste materials. The various carbonic materials are of agricultural waste, cattle dung etc. which can be minimized by the use of biogas plants which can produce energy also. Inspite all measures pesticides and weedicides should be used in limited quantity only when they are required. Bhopal Gas Tragedy is the best example of human hazard which took the life of many persons the tank of methyl isocyanate burst during the manufacturing of savin insecticide on 3rd December 1984.

(ii)Soil erosion :

Fertility of soil depends on Presence of organic matter(humus) and nutrients, (ii) capacity of soil to retain water and air. A loamy soil is the best-suited for plant growth. The fertility of soil is threatened due to various activities of humans. The main threat to the fertility of soil is from soil erosion, which is the loss of soil due to wind or water flow.

(f)Methods of Preventing Soil Erosion :

Prevention of soil erosion can be brought about by controlling the factors which cause soil erosion. The methods would thus be follows:

  1. Deforestation should be stopped, rather, trees should be planted (afforestation). Afforestation should be undertaken not only in areas already cut, but additional areas should be brought under plantation.
  2. To reduce the effect of strong wind in the fields, the boundaries of the fields should be planted with trees in two or three rows.
  3. To maintain the soil in its natural condition, it is advisable to grow different crops. Crop rotation helps to maintain the fertility of the soil. The water – holding capacity of the soil is also maintained by this method.
  4. Proper drainage and irrigation arrangements should be made in the fields.
  5. On the sloping areas in hills, strip cropping should be practiced, thereby reducing the steepness of the slopes and checking soil erosion.
  6. Strip – cropping means the planting of crops in rows or strips to check flow of water. 

BIOGEOCHEMIC CYCLES

These are the cyclic pathways through which chemical Elements move from environment to organisms and back to the environment . Such cycling is essential as the earth and its environment, with reference to these elements, are considered As closed system and there is no inflow of such elements from Outside the earth and their amount is limited.

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Two types of biogeochemical cycles are :

i Gaseous cycles   ii Sedimentary cycles

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(a)Water Cycle :

Water is the most abundant (60-90%) component of protoplasm. It acts as a habital for hydrophytes and many aquatic animals, a good ionizer, good solvent, temperature, buffer and perform transportation of materials. It also helps in digestion of organic compounds and in photosynthesis of plants.

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(i) Types of water cycles are:

  1. Global water cycle : does not involve living organisms and involves the interchange of water between the earth’s surface and the atmosphere via the processes of precipitation and evaporation. Ocean is the biggest store house of water. Evaporation involves the conversion of liquid and solid forms of water into vapours and later form the clouds. Precipitation involves the rainfall, hail, snow, etc. energy for global water cycle is provided by sunlight.
  2. Biological water cycle : it is the interchange of water between Abiotic and biotic components of environment e.g. the plants absorb water from water bodies and soil while loose most of the water by the process of transpiration, animals consume water from water bodies or the food ingested, while release water via the processes of respiration and excretion.
  3. Nitrogen Cycle : Nitrogen is an essential component of amino acids, proteins, enzymes and nucleic acids of the protoplasm. Reservoir pool of nitrogen is atmosphere which contains about 78.08% of nitrogen in gaseous state. But it cannot be used directly and is changed into nitrites and nitrates and then utilized.

(i)Steps of nitrogen cycle are:

(A) Nitrogen fixation: it involves the conversion of free diatomic nitrogen (N2) into nitrites and nitrates. It occurs in three ways :

  1. Atmospheric nitrogen fixation in the presence of photochemical and electrochemical reactions induced by thundering and lightening.
  2. Industrial nitrogen fixation in the industries at night temperature and high pressure.
  3. Biological nitrogen fixation in the presence of certain living organisms as
  4. Rhizobium bacterium in the root nodules of legumes.
  5. Azotobacter bacterium in the soil.
  6. Anabaena (blue green algae) in water in the paddy fields.
  7. Azospirillum bacterium in loose association with the roots of maize, sorghum, etc.

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(B)Ammonification: it involves the decomposition of proteins of dead plants and animals to ammonia in the presence of ammonifying bacteria like Bacillus ramosus.

(C)Nitrification: it involves the oxidation of ammonia to nitrites (NO2) and nitrates (NO3) in the presence of nitrifying bacteria like Nitrosomonas (Ammonia to nitrite), Nitrobacter (Nitrite to nitrate), etc. plants absorb the nitrites and nitrates from the soil through their roots and convert them into organic compounds(e.g. proteins) of protoplasm by the process called nitrogen assimilation.

(D)Denitrification: it involves reduction of ammonium compounds, nitrites and nitrates to molecular nitrogen in the presence of denitrifying bacteria like Thiobacillus denitrificans.

(C)Carbon Cycle :

Carbon is the basic component of all the organic compounds like carbohydrates, proteins, lipids, enzymes and nucleic acid of the protoplasm. In atmosphere, it is present as carbon dioxide. It involves two types of processes, one involving COutilization and another involving COproduction. They are expressed as follows: 

(i)COutilization : carbon dioxide is utilized by the Photosynthetic organisms like green plants, Photosynthetic bacteria, diatoms and blue green algae In the presence of Photosynthesis, it occurs in the Presence of chlorophyll and radiant energy of Sunlight. Glucose synthesized in photo synthesis is Used to synthesize other organic compounds.

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(ii)CO2 production :

  1. COis released during respiration of both producers And consumers.
  2. During decomposition of organic compounds of deadn Bodies.
  3. During burning of fossil fuels like wood, coal, Petroleum, etc.
  4. Volcanic eruptions and hot springs.
  5. During weathering of rocks by acids produced by microorganisms and roots of higher plants.

(d)Oxygen Cycle :

Oxygen is present in water and from 20% of air in Atmosphere. All living beings need it for respiration. Oxygen content of atmosphere has remained constant For the last several million years. Most of Olost is Replenished by photosynthesis. During photosynthesis COis used by plants to from food along with release Of oxygen.

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The oxides can be reduced both chemically And biologically to produce oxygen. Microbial oxidation can also occur. Due to burning materials oxygen form carbon dioxide. When oxygen combines with nitrogen, it forms oxides of nitrogen, amino acids, proteins etc. these compounds on breakdown release the oxygen in atmosphere.

GREEN-HOUSE EFFECT

Usually carbon dioxide is not considered as pollutant, but its higher concentration forms the thick layer above the earth surface which checks the radiation of the heat from the earth surface. Because of this the temperature of the earth surface increases. This is called as “Green house effect”. The various green house gases are CO(Warming effect 60%) ,CH4(Warming effect 20%), chlorofluoro carbon or CFCs(14%) and Nitrous oxide(N2O6%) :

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Even 2-30 C rise in Temperature will lead to melting glaciers and Ice caps of polar regions & consequently Causes floods in rivers, rise in sea level and Changes in cycle of rain. Islands may be Emerged in sea water. The present growth rate Is continued then the amount of COwill be double upto 2020.

(a)Global warming :

Global warming is the increase in average global temperature due to increase in amount of GHGs in earth’s atmosphere.

consequences of global warming :

  1. Increase in the sea level : Global warming will melt polar ice caps. If all the ice on the earth will melt, about 200 feet of water would be added to surface of all oceans. Thus low lying coastal cities like shanghai, Kolkata, Bangkok, Dhaka, Venice, etc. will be inundated.
  2. Increase in global temperature : If present input of GHGs will be continued, the earth’s global temperature will rise.
  3. Effect on agriculture : Grain production will be reduced. India’s annual monsoon rains may even cease together. One third of global forest might be swept away. Deserts are likely to increase
  4. Chances of hurricanes, cyclones and floods will be more.
  5. Increased temperature and humidity caused by global warming will lead to spread of diseases like malaria, filariasis etc. due to spread of vectors. Incidences of respiratory and skin diseases are likely to increase.

OZONE DEPLETION

Between 320 And 26 Km Above The Sea Level It Occurs Ozone Layer And The Part Of Atmosphere Containing It Is Called Ozonosphere (Stratosphere). This layer is established due to an equilibrium between phto-dissociaiation of ozone by UV-radiation and regeneration of ozone. The thickness of this ozonosphere averages 5km. the ozone layer acts as a shield and absorbs the harmful UV-radiations of the sunlight so protects the earth’s biota from the harmful effects of strong UV-radiations. So this layer is very important for the survival and existence of life n earth.

(a)Causes of Thinning of Ozone Layer :

The decline in spring-layer thickness is called ozone hole is largest over Antarctica and was just short of 27 million sq.km. during September 2003.main chemicals responsible for destruction of ozone-layer are : chlorofluorocarbons(CFCs), halogens (used in fire extinguishers) methane and nitrous oxide. Out of these, most damaging is the effect of CFCs which are a group of synthetic chemicals and are used as coolants is refrigerators and air conditions; as cleaning solvents, propellants and sterilants etc. these CFCs produce “active chlorine” (CI and CIO radicals) in the presence of UV-radiations. These active chlorine radicals catalytically destroy ozone and convert it into oxygen.

Nitrous oxide : It is produced by industrial processes, forest fires, slid waste disposal, spraying of insecticides and pesticides, etc. methane and nitrous oxide also cause ozone destruction. 

(b)Effects of Ozone Depletion :

The thinning of ozone layer results in an increase in the UV radiation (in the range of 290-320nm) reaching the earth’s surface. It is estimated that a 5 per cent loss of ozone results in a 10 per cent increase in UV-radiations. These UV-radiations:

  1. Increased incidences of cataract and skin cancer.
  2. Decrease the functioning of immune system : due to killing of melanin- producing cells of the skin.
  3. Inhibit photosynthesis in most of phytoplanktons so adversely affecting the food chains of aquatic ecosystems.
  4. Damage nucleic of the living organisms.

Strip-cropping means the planting of crops in rows or strips to check flow of water.

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