Realms of Earth

Astro-physicists and scientists believe that there could be quite a few planets in the Universe similar to our planet Earth. There is also a probable chance that some form of life prevails and sustains in one of such planet. However, so far as we presently know, the Earth is the only planet which supports life which makes it a very unique planet. It is therefore imperative (very important and necessary) for us to understand the spheres of the Earth as the spheres play a crucial role to enable life forms to evolve, prevail and sustain. The three main components of the Earth are water, air and soil. They are not only in contact with each other but are also interactive and interdependent. Life exists only where these three components interact. The Earth’s system consists of four major subsystems. They are known as domains, spheres or realms. They are the Lithosphere (land), Hydrosphere (water), Atmosphere (air) and Biosphere (the parts of the Earth where life is found, including land, water and the lower part of the atmosphere). LITHOSPHERE: the realm of land & MAJOR LANDFORMS

The Lithosphere is the solid surface layer of the Earth. It is generally referred to as the crust. It consists of all the land masses on the surface of the Earth. The surface of the Earth is very uneven. There are: High mountains, e.g., the Himalayas, the Rockies, the Andes, the Alps. Vast plains, e.g., the great plains of India, USA and Brazil. Deep valleys.

The two main features on the Earth’s surface are the continents and oceanic basins. Continents are large, distinct land masses usually separated by vast water-bodies (oceans and seas). There are seven major continents in the world. They are Asia, Africa, Europe, North America, South Ame rica, Australia and Antarctica. Most of the land masses lie in the Northern Hemisphere. The Southern Hemisphere has relatively fewer land masses.

Collectively, the land masses are known as the lithosphere. The lithosphere consists of different types of landforms. Landforms are features that make the Earth’s surface. The different types of landforms include mountains, plateaus, hills, valleys, plains, Ocean bed, sea bed, etc. How are the landforms formed? How are they useful to humans? The surface of the Earth constantly undergoes changes due to the action of various forces. It is essential to understand these forces and how they affect the landforms?

Internal forces
Internal forces are also known as endo-genetic forces or tectonic forces. Internal forces act from within the Earth that lead to sudden changes on the face of the Earth. The crust of the Earth is actually divided into several pieces which we call plates. These plates float over liquid molten rock called magma which lies below the solid crust. The core of the Earth heats the molten magma. The molten magma layer is called the mantle, which is in between the core and crust. Due to the intense activity within the core and the mantle the magma often rises upwards when heated. It spreads, cools and then sinks back again, to get heated and rise once more. This constant rising and sinking of the magma keeps pushing the plates that rest on it. Thus, the plates are also constantly moving. This movement of the plates is termed as plate tectonics. Sometimes the plates move towards each other, or converge. Sometimes the plates move away from each other, or diverge. Sometimes, the plates slide by each other in a transverse movement.

Plate tectonics is responsible for the formation of mountains and valleys o n the surface of the Earth. These features can take millions of years to form. However, sometimes, sudden and strong movements take place within the Earth, which bring about massive changes on the surface of the Earth. Examples are the changes brought about on earth’s surface by the earthquakes, volcanoes, landslides, etc. External forces

External forces or gradational forces are forces acting from above the surface of the Earth. They lead to slow and steady changes on the face of the Earth and include all the agents of erosion like wind, water, glaciers, etc. The external forces lead to wearing away and rebuilding of the Earth’s surface. The agents of erosion physically change the Earth’s surface by eroding (wearing away) land surfaces, transporting the eroded material and depositing the weathered soil, sand and debris at other places. For example, rain, wind and rivers erode the mountains and highlands, the eroded particles are carried and deposited further down to form plains.

Types of landforms
MOUNTAINS
A mountain is a natural elevation of the Earth’s surface rising high above its surrounding. Mountains have narrow summits and broad bases. Mountains are found both on land and on the ocean floor. They are of different types. Some of them are very high and some relatively short. Some have pointed peaks, while some have rounded tops. Some have steep slopes and some have gentle slopes. The higher reaches of the mountains can be very cold. The higher the altitude, the lower will be the temperature.

Fold Mountains
Fold Mountains are formed by the upward movement and folding of land masses due to lateral compression caused by tectonic or internal forces. When two plates move towards each other, the place of convergence of the two plates gets compressed. The crust along the edge gets crumpled or folded. Massive layers of the Earth’s crust get uplifted as a result of these converging forces, resulting in the formation of Fold Mountains. The Himalayas and the Alps were formed in the same way in the recent geological past and are therefore known as young fold mountains. They have rugged relief and high, conical peaks. The Aravalli range in Rajasthan is one of the oldest fold mountain ranges in the world. The range has been lowered greatly due to continuous erosion over millions of years. The Urals in Russia and the Appalachians in North America are also examples of very old fold mountains. They have been worn down considerably and have rounded features. Such mountains can be classified into three forms: Young mountains (recently formed mountains, with sharp peaks, like the Himalayas); Mature Mountains (do not have a sharp peaks which have been worn off due to the agents of erosion like air, water and sunlight) and Old Mountains (the most ancient mountains affected severely by the agents of erosion, over a great period of time like millions of years). Block Mountains

When two plates on the Earth’s crust diverge or are pulled apart, fractures appear on the Earth surface. These fractures are called faults. When two faults are formed alongside each other, large areas get displaced vertically along the two fault lines.

The land between the fault lines either subsides (falls) or rises (gets lifted). The uplifted part of the land is called horst and the subsided part is called graben or rift valleys.
Sometimes, rivers may flow through the rift valleys. The Rhine Valley is a rift valley and the Voges Mountains in Europe are block mountains. River Narmada in the subcontinent of India flows through a rift valley. The Narmada Valley is a graben, a layered block of the Earth’s crust that dropped down relative to the blocks on either side due to ancient spreading of the Earth’s crust. Two normal faults, known as the Narmada North fault and Narmada South

fault, parallel to the river’s course, and mark the boundary between the Narmada block and the Vindhya and Satpura blocks or Horsts which rose relative to the Narmada Graben.
Volcanic Mountains
The conical or dome-shaped structures formed as a result of magma escaping through Earth’s surface are called Volcanic mountains. The opening in Earth’s surface through which the magma escapes is called a vent. Magma that reaches the surface of the earth is called lava. The hot lava flows over the surface of the Earth, cools and then hardens to forms lava sheets. Over thousands of years, several layers of such lava sheets are deposited on top of each other to form volcanic mountains. If the magma is thin and flows easily, gently sloping mountains are formed, such as Mauna Loa in Hawaii.

However, if the lava is thick and viscous, the mountains formed are cone shaped with steeply sloping sides. For example, Mt. Vesuvius in Italy, Mt. Kilimanjaro in Africa and Mt. Fuji in Japan. Pavagadh, near Vadodara, Gujarat is a good example of the Volcanic Mountains.

The Importance of Mountains
Mountains influence the climate of a land. They help cause rain by forcing clouds carrying moisture to rise, condense and come down as rain. Several perennial rivers (rivers that carry water throughout th e year) originate from mountain glaciers. The Indus, Brahmaputra, Ganga, Yamuna and the other rivers are fed by mountain glaciers. Many fresh water lakes and springs are also found in mountains. They act as reservoirs of water. Mountains are rich in flora and fauna. The forests provide various products like fuel, timber, lac, nuts, fruits, medicinal herbs and fodder. Many mountains are a rich source of minerals. Dams built across river valleys provide irrigation and hydro-electricity (e.g. Bhakra-Nangal Dam built on River Sutlej.)

Mountains and their picturesque valleys encourage tourism. They also support sporting activities such as paragliding, rock climbing, hand gliding, river rafting and skiing. PLATEAUS A plateau is a broad, level, elevated area of land. Since a plateau has a flat top with sharply falling sides, it also called a tableland. Often a plateau may have one gently sloping side and one steep side. The steep side of a plateau is called an escarpment. A large part of the surface of the Earth is made up of plateaus. Most of the continents have large plateaus. The largest and highest plateau in the world is the Chang Tang of Tibet, called the roof of the World. India has Malwa plateau in the Central part, ChhotaNagpur plateau in the Eastern side of India. The Deccan Plateau in the peninsular Southern India is one of the oldest plateaus of the world. Most of the African continent is made of plateaus. East Africa, Kenya, Tanzania and Uganda are all located on a plateau. Plateaus are of different types:

Inter-montane plateaus are those that are surrounded completely or partially by mountains, for example, the plateau of Tibet.
Continental plateaus are vast areas of tableland that rise abruptly from the coast or surrounding plains, e.g., the plateaus of Africa.
Lava plateaus are formed when lava flows out of fissures and spreads thickly over extensive areas. The Deccan plateau is a lava plateau.
Plateaus are also very useful to humans. Lava plateaus, like the Deccan plateau, are largely com posed of black soil, which is very fertile. It is useful for the cultivation of crops. Most plateaus have rich deposits of minerals. The Chhota Nagpur plateau in India has rich deposits of iron ore, manganese and coal. The African plateau is rich in gold and diamond deposits.

The rivers flowing along plateaus fall from a great height when they flow across the excarpments. They form waterfalls. The water falls with great force and help rotate turbines to generate electricity. Hundru falls on the river Subarnarekha and Jog falls on the river Sharavati in the Deccan plateau are some examples.

PLAINS
A low-lying vast flat land is called a plain. Plains are formed in different ways. While most of them are levelled alluvial tracts, some are rounded and some undulating (gentle rise and fall). Alluvial tracts are formed when rivers and their tributaries bring down eroded material from the mountains. The eroded material is called debris or alluvium. It contains sand, silt, clay and animal and plant matter. T hey provide the rich nutrients required for the soil. Plains are usually fertile and are extensively used for the cultivation of crops. Plains are the most densely populated regions on Earth. People settle in plains for many reasons. Usually water is abundant.

The land being flat and fertile, it enables cultivation of crops. It is possible to build an excellent network of roads and railways. The ports and
harbours in coastal plains promote international trade. The plains of river Indus, Ganga and Yamuna in India, the plains of the river Nile in Egypt and the plains of the river Hwang Ho in China are densely populated.

VALLEYS
Valleys are the low-lying land between hills or mountains. They may or may not have a river flowing through them. River valleys are usually V-shaped. The Rhine Valley in Europe and the Damodar Valley in India are river valleys formed by glaciers. Large masses of ice that move down a mountain slope are U-shaped. Glacial valleys are found in the Alps and the Himalayas.

BASINS
A basin is a depressed section of the Earth’s crust with surrounding land (E.g. Ocean basins). Many basins are found alongside plateau edges and form areas of inland drainage, i.e., the rivers flowing in the basin do not reach the sea. The Tarim Basin of Asia and the Chad Basin of North Central Africa are basins of inland drainage. Apart from these major landforms, there are some distinctive minor landscapes as well Islands: An island is a piece of land which is surrounded on all sides by water. India has two grou ps of islands – the Lakshadweep Islands in the Arabian Sea and the Andaman -Nicobar islands in the Bay of Bengal. Sri Lanka is an island just south of India, in the Indian Ocean. Malagasy (Madagascar) is another big island in the same ocean. Try to identify other islands on a world map. Many islands together are called Archipelago. (e.g. Lakshadweep and Andaman islands). A small sized island is called an islet. Peninsulas: A peninsula is a piece of land that is surrounded by water on three sides and joined to a larger land mass. The peninsular plateau of India and the Malay Peninsula are two examples of peninsulas attached to the continent of Asia.

Isthmuses: An isthmus is an elongated narrow piece of land connecting two larger land areas, usually with major water-bodies on either side. E.g.: the Isthmus of Panama that joins North America and South America and separates the Pacific Ocean from the Atlantic Ocean. The Isthmus of Suez joins Africa to Asia and separated the Mediterranean Sea and the Red Sea.

Bay: A bay is an open, curving indentation made by the sea into a coastline. E.g. Bay of Bengal Gulf: A gulf is an inlet of the sea of large proportion. Gulfs are more enclosed / narrower than bay. E.g. : Gulf of Kutch and Gulf of Khambhat.

Straits: A strait is a narrow stretch of water joining two large water bodies and separating two large land masses. E.g.: Palk Strait.
Ocean Bed: the earth’s crust found under the water of the ocean or the sea is the ocean bed or sea bed. These are the deepest part of the Earth’s crust. The deep valleys form the abyss and the peaks of high rise mountains often form Islands or islets.

Sea Bed: the earth’s crust found under the sea, forming the sea bed . These are usually shallower than the Ocean bed.

HYDROSPHERE: the realm of water & MAJOR WATER-BODIES
Hydrosphere (hydro= water) refers to the part of Earth that is covered with water. It includes water in all forms, i.e. solid (ice), liquid (water) and gas (water vapour). Water is found as ice sheets in glaciers. It is found as flowing water in oceans, rivers, lakes, ponds and underground streams. It is also found as water vapour in the atmosphere. All these forms together make the Hydrosphere.

Of the Earth’s surface, 71% is covered by water and only 29% by land. This gives t he Earth another name – the Blue Planet. 97% of the Earth’s water is found in the oceans and is salty. 2% of the water is found in the form of glaciers and ice sheets. Only 1% of the water is found as fresh water on the surface and as underground streams. Fresh water is, therefore, a critical resource. Fresh water comes down as precipitation from the atmosphere to the Earth’s surface, flows as rivers and streams along it, and is found as groundwater beneath it. The water cycle

Water cycle refers to the interchange of the forms of water on this planet. It involves three different processes – evaporation, condensation and precipitation.
Water is transferred from the Earth’s surface to the atmosphere through evaporation. It is the process by which water when heated, changes into a gas. Approximately 80% of all evaporation is from the oceans, and the remaining 20% is from inland water and vegetation.

Condensation is the process by which the water vapour in the atmosphere when cooled, changes to its original liquid state. In the atmosphere, condensed water may appear as clouds, mist, fog, dew, etc. depending on the physical conditions of the atmosphere.

When water droplets get too heavy to remain suspended in the air, they fall to the Earth as precipitation. Precipitation occurs in a variety of forms – hail, rain, freezing rain, sleet or snow.

Types of Water-Bodies
OCEANS
Oceans are vast masses of restless water-bodies. The ocean waters are always moving. The three major movements of the ocean waters are waves, tides and currents. Tides are the periodic rising and falling of the water caused by the gravitational force of the Moon and Sun acting upon the rotating Earth. Waves are the rising and falling movements of surface sea water caused by the force of the winds. Currents are the horizontal movements of sea water caused by many factors including wind and the Earth’s movement.

How the Oceans formed: The semi molten surface of the prehistoric Earth was covered by volcanoes. Hot gases and water vapour given off by the volcanoes formed the Earth’s early atmosphere. The water vapour in this early atmosphere condensed as rain. Rainstorms poured down on the planet and filled the vast hollows on the Earth’s surface. These huge pools of water formed the oceans. The water was hot and acidic. Later, plant life evolved and changed the composition of the atmosphere and ocean.

Oceans form the major part of the hydrosphere. All the oceans of the world are interconnected. Oceans help international trade by forming natural marine rou ts. They also support marine life. Oceanic currents – the movement of the mass of oceanic water in a definite direction – help maintain the specific weather patterns and climatic conditions.

Pacific Ocean
It is the deepest and the largest ocean. It covers nearly one -third of the Earth’s surface. In fact, it is so big that all the continents could easily fit into it. The explorer Ferdinand Magellan named the ocean ‘Pacific’ meaning calm or tranquil. The International Date Line passes through the Pacific Ocean. It is located between the Southern Ocean, Asia, Australia, and North and South America. The length of coastline is 135,663 kms. Its lowest point is Challenger Deep in the Mariana trench at 10,924 m below mean sea level. This is the deepest point on the surface of the Earth. The average depth of the Pacific Ocean is around 4,030 m. The Atlantic Ocean

It is the second largest of the world’s oceans. It lies between Africa, Europe, the Southern Ocean, and the Western Hemisphere. The length of its coastline is 111,866 kms. The lowest point in this ocean is Mi lwaukee Deep in the Puerto Rico Trench at 8605 m below the mean sea level. The shape of the Atlantic is like the letter ‘S’ and its average depth is approximately 3330m. Though this ocean is smaller than the Pacific, it has a large number of ports and harbours. The Indian Ocean

It is the third largest of the world’s ocean. It is enclosed by land in the north and lies between Africa and the Southern Ocean, Asia and Australia. The extent of coastline of this ocean is 66,526 kms. The Java Trench at 7258 m deep is its deepest point. Its shores consist of some of the ancient plateaus and the remains of the Gondwanaland.

The Arctic Ocean
It is the smallest of the world’s oceans. It lies well within the Arctic Circle around th e North Pole. The coastline is around 45,389 km. Fram Basin at 4665 m forms the deepest point of the Arctic Ocean. The Arctic is connected to the Pacific by a narrow stretch of shallow water body called the Bering Strait. In the northern coast it is bound by North America and Eurasia. Most of the time, Arctic Ocean remains frozen. Apart from the oceans, there are some smaller water -bodies too: Seas: A sea is a part of an ocean that is smaller and shallower. It is usually located close to the edge of a lan d mass or continent. E.g.: Arabian Sea, Mediterranean Sea.

Lakes: Lakes are small bodies of water which are surrounded by land on all sides. Large lakes are called inland seas, for example, the Aral Sea, the Dead Sea, the Black Sea, the Baikal Lake and the Caspian Sea of Eurasia. Rivers: A river is a stream of water which flows in a channel from high ground to low ground and finally to a lake or sea. The place where a river originates is called its source, which normally lies in a hill or a mountain. The place where a river ends its journey is called its mouth. The mouth is normally the place where the river enters the sea.

The route or the course of a river has three different stages – upper, middle and lower. The upper course generally lies in mountains and, therefore, the land is steep. The river flows very swiftly at this stage. When the river descends to the plains from the mountains, it is in its middle course. The speed of its flow decreases considerably as it flows through areas that have a gentle r slope. The river is in its lower course near its mouth, where the slope is negligible and, therefore, its flow has nearly halted.

ATMOSPHERE: the realm of vapours
The layer of air that surrounds our Earth is called the atmosphere. The word atmosphere is derived from ‘atmos’ which means ‘vapour’ in Greek. This is the most dynamic part of the major domains of our planet. The atmosphere is a mixture of gases, dust and vapour which encompasses the earth as far as 10000 km above the earth surface.

The atmosphere protects us from solar radiation as well as meteors, meteorites and smaller debris which fall towards earth every day. The atmosphere also helps to maintain the Earths’ temperature as well as assists in the climatic and weather patterns. The lower layer of atmosphere contains numerous life sustaining gases like Oxygen, Carbon-di-oxide, Nitrogen, etc as well as water vapour.

The atmosphere is divided into six main layers on the basis of its composition, temperature, pressure, etc.
The troposphere is the layer that is closest to the earth and the place where all weather changes occur. Ninety percent of all air is found in this layer and it contains most of the water vapour and dust particles of the atmosphere. The troposphere extends up to 18 km at the Equator and gradually declines up to 8 km at the poles. This the most important layer for life on Earth. The most upper region of troposphere is known as tropo-pause.

The stratosphere extends beyond the troposphere. It is devoid of water vapour and dust particles and there is no turbulence and clouds in the air. This is where aeroplanes fly. The stratosphere extends from the troposph ere to altitudes ranging from 20 to 50 km above sea level.

The stratosphere contains the ozone layer or ozonosphere, the part of the Earth’s atmosphere which has high concentrations of ozone, a form of oxygen. This layer results when oxygen molecules split due to ultra -violet radiation coming from the sun. The ozone layer absorbs and blocks the harmful ultraviolet rays of the sun.

The mesosphere extends to a height of 80 km above the stratosphere. This sphere has the coldest temperature in the atmosphere. The temperature can be as low as -90 °C.
The ionosphere or thermosphere extends from 80 km to 480 km. The temperatures here are as high as 1480°C. It is electrically charged due to a process called ionization: atoms change into ions through addition or removal of electrons. Ionization happens due to x-rays and gamma rays from the sun. This high temperature of this sphere protects earth from space debris like meteors as well as obsolete satellites. Tele-communications involving radio waves are also possible due to the electrically charged molecules of this sphere. The exosphere is somewhere between 480 km to 960 km. It gradually merges with space. The temperature here ranges from 300°C to 1600°C and air pressure is extremely low.

Composition of the atmosphere
The Earth’s atmosphere is a mixture of twelve gases. The atmosphere contains 78% nitrogen, 21% oxygen and 1% other gases like carbon dioxide, argon, helium etc. Thus nitrogen and oxygen the life-giving gas, make up nearly 99% of clean and dry air. The composition of the atmosphere influences the climate of a place.

Though only a minute amount of carbon dioxide is present in the atmosphere, it is responsible for keeping the planet warm. It traps the heat of the Sun and prevents it from going back into space. It thus acts like a blanket for the Earth.

This warmth is essential for the germination of pla nts and the survival of life. This process of heat getting trapped in the atmosphere to keep the Earth warm is known as the greenhouse effect. Atmospheric pressure and wind: Air has weight. The total weight of the air is called air pressure or atmospheric pressure. Air pressure varies with location and time, because the amount (and weigh) of air above the Earth varies from place to place and time to time.

Atmospheric pressure decreases with increase in altitude. Temperature also decreases with increase in altitude especially in the troposphere. Air always moves from a region of high pressure to region of low pressure. Moving air is called wind.

BIOSPHERE: the realm of the living and BIOMES
Life on Earth exists due to the presence as well as interaction of the three spheres discussed earlier: the Lithosphere, the Hydrosphere and the Atmosphere. The part of the Earth where life exists is called the Biosphere. ‘Bios’ is a Greek word meaning ‘life’.

The Biosphere is the narrow contact zone between the lithosphere, the hydrosphere and the atmosphere, where life forms exist. (Refer the first image on Page 1 of this material). Life on Earth is possible because of

Its optimum distance from the Sun (neither is it too far nor is it too close to the Sun). The presence of a protective atmosphere.
The availability of adequate water required for life forms.
The biosphere consists of the interaction of plants, animals and other living things which constantly interact with their environment. This constant interaction helps maintain a balance between organisms and their environment.

Living organisms range in size from microscopic bacteria to huge mammals. All of them, including humans are interdependent. In the biosphere, living things form communities based on their physical surroundings. These communities are referred to as Biomes. Deserts, grasslands and forests are some important types of biomes. Biomes are defined as “the world’s major communities, classified according to the predominant vegetation and characterized by adaptations of organisms to that particular environment” (Campbell 1996). The importance of biomes cannot be under-estimated. Biomes have changed and moved many times during the history of life on Earth. More recently, human activities have drastically altered these communities. Thus, conservation and preservation of biomes should be a major concern to all. Types of Biomes

FRESH-WATER BIOME
Freshwater is defined as having a low salt concentration — usually less than 1%. Plants and animals in freshwater regions are adjusted to the low salt content and would not be able to survive in areas of high salt concentration (i.e., ocean). There are different types of freshwater regions: Ponds and lakes

Streams and rivers
Wetlands
Ponds and lakes: These regions range in size from just a few square meters to thousands of squa re kilometres. Scattered throughout the earth, several are remnants from the Pleistocene glaciations period. Many ponds are seasonal, lasting just a couple of months (such as sessile pools) while lakes may exist for hundreds of years or more. Ponds and lakes may have limited species diversity since they are often isolated from one another and from other water sources like rivers and oceans.

Streams and rivers: These are bodies of flowing water moving in one direction. Streams and rivers can be found everywhere — they get their starts at headwaters, which may be springs, snowmelt or even lakes, and then travel all the way to their mouths, usually another water channel or the ocean. The characteristics of a river or stream change during the journey from the source to the mouth. The temperature is cooler at the source than it is at the mouth. The water is also clearer, has higher oxygen levels, and freshwater fish such as trout s and hetero-trophs can be found at the mouth.

Wetlands: Wetlands are areas of standing water that support aquatic plants. Marshes, swamps, and bogs are all considered wetlands. Plant species adapted to the very moist and humid conditions are called hydrophytes. These include pond lilies, cattails, sedges, tamarack, and black spruce. Marsh flora also includes such species as cypress and gum. Wetlands have the highest species diversity of all ecosystems. MARINE BIOME

Marine regions cover about three-fourths of the Earth’s surface and include oceans, coral reefs, and estuaries. Marine algae supply much of the world’s oxygen supply and take in a huge amount of atmospheric carbon dioxide. The evaporation of the seawater provides rainwater for the land. The largest of all the ecosystems, oceans are very large bodies of water that dominate the Earth’s surface. DESERT BIOME

Deserts cover about one-fifth of the Earth’s surface. Usually, rainfall is less than 50 cm/year in deserts. Desert biomes can be classified according to several characteristics. There are three major types of deserts:

Hot and dry
Semi-arid
Cold
Hot and dry deserts: The four major North American deserts of this type are the Chihuahuan, Sonoran, Mojave and Great Basin. Others outside the U.S. include the Southern Asian realm, Neo -tropical (South and Central America), Ethiopian (Africa) and Australian wastelands. The seasons are generally warm throughout the year and very hot in the summer. The winters usually bring little rainfall. Semi-arid deserts: The major deserts of this type include the Sagebrush of Utah, Montana and Great Basin. They also include the Ne-arctic realm (North America, Newfoundland, Greenland, Russia, Europe and northern Asia). The summers are moderately long and dry, and like hot deserts, the winters normally bring low concentrations of rainfall. Summer temperatures usually average between 21-27° C. Cold deserts: These deserts are characterized by cold winters with snowfall occasionally over the summer. They occur in the Antarctic, Greenland and the Ne-arctic realm. They have short, moist, and moderately warm summers with fairly long, cold winters.

FOREST BIOME
Forest biomes are biological communities that are dominated by trees and other woody vegetation (Spurr and Barnes 1980). Forest biomes can be classified according to numerous characteristics, with seasonality being the most widely used. Distinct forest types also occur within each of these broad groups. There are three major types of forests, classed according to latitude: Tropical

Temperate
Boreal forests (taiga)
Tropical Forest
Tropical forests are characterized by the greatest diversity of species. They occur near the equator, within the area bounded by latitudes 23.5 degrees N and 23.5 degrees S. One of the major characteristics of tropical forests is their distinct seasonality: winter is absent, and only two seasons – rainy and dry prevail. The length of daylight is 12 hours and varies little.

Temperate Forest
Temperate forests are found in eastern North America, north-eastern Asia, and western and central Europe. Well-defined seasons with a distinct winter characterize this forest biome. Moderate climate and a growing season of 140-200 days during 4-6 frost-free months distinguish temperate forests. Taiga Forests (Boreal)

Boreal forests, or taiga, represent the largest terrest rial biome. Occurring between 50 and 60 degrees north latitudes, boreal forests can be found in the broad belt of Eurasia and North America: two -thirds in Siberia with the rest in Scandinavia, Alaska, and Canada. Seasons are divided into short, moist, and moderately warm summers and long, cold, and dry winters. The length of the growing season in boreal forests is 130 days. TUNDRA BIOME

Tundra is the coldest of all the biomes. Tundra comes from the Finnish word tunturi, meaning treeless plain. It is noted for its frost-moulded landscapes, extremely low temperatures, little precipitation, poor nutrients, and short growing seasons. Dead organic material functions as a nutrient pool. The two major nutrients are nitrogen and phosphorus. Nitrogen is created by biological fixation, and phosp horus is created by precipitation. GRASSLAND BIOME

Grasslands are characterized as lands dominated by grasses rather than large shrubs or trees. In the Miocene and Pliocene Epochs, which spanned a period of about 25 million years, mountains rose in wester n North America and created a continental climate favourable to grasslands. Ancient forests declined and grasslands became widespread. Following the Pleistocene Ice Ages, grasslands expanded in range as hotter and drier climates prevailed worldwide. There are two main divisions of grasslands: Tropical grasslands or savannah

Temperate grasslands
The grasslands of the world

Savannah or Tropical grassland
Savannah is grassland with scattered individual trees. Savannah cover almost half the surface of Africa (about five million square miles, generally central Africa) and large areas of Australia, South America and India. Climate is the most important factor in a savannah. Savannah are always found in warm or hot climates where the annual rainfall is from about 50.8 to 127 cm (20-50 inches) per year.

Temperate grassland
Temperate grasslands have grasses as the dominant vegetation. Trees and large shrubs are absent. Temperatures vary more from summer to winter, and the amount of rainfall is less in temperate grasslands than in savannah. The major manifestations are the veldts of South Africa, the puszta of Hungary, the pampas of Argentina and Uruguay, the steppes of the former Soviet Union, and the plains and prairies of central North America.

Human Impact on Biomes
Human and natural activity affects Biomes in different ways. When humans exploit forest for wood or vegetation.
When humans clear forests for agriculture, industrialization or for exploring minerals. When natural calamities like earthquakes, volcanic eruptions, floods or tsunamis occur. When excessive grazing takes place.

When there is a forest fire, etc.
It is important to protect the realms of the Earth from degradation. These realms of the Earth interact with each other and help sustain the various life forms, including the humans. Thus the realms affect the lives of human beings.

Human activity in terms of industrialization has several negative impacts. Industrial emissions pollute the air. Industrial effluents pollute water bodies. Carbon dioxide is an important gas. However an increase in the level of this gas has caused the temperature of the Earth to rise. This is known as global warming. Human activities are causing the depletion of the ozone layer. The ozone layer protects t he Earth from the harmful ultra-violet rays of the Sun.

The most urgent need today is that all of us learn to limit the use of resources and conserve the same for the future generations. We should save the planet from further pollution and keep it safe – not only for ourselves but also for the diverse life forms that are found on planet Earth.