Environment of Earth

September 2, 2008

HYDROSPHERE OF EARTH

Filed under: Environment — gargpk @ 12:07 pm
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All the water in its various forms, present in all the components of Earth’s environment together constitutes the hydro­sphere. Most of the water is present as liquid water on the Earth’s surface and some liquid water is present underground. Apart of total water is present as snow or ice on the Earth’s surface while a substantial part of water in Earth’s environment is also present as water vapor in the atmosphere. General fea­tures of various parts of the Earth’s hydrosphere are as given below:

1. Oceans: Major part of water is present in the oceans of the Earth. Average depth of oceans is about 3.7 kilometers and about 1300 million cubic kilometers water is present in oceans.

2. Ice sheets: Substantial quantity of water, about 24 million cubic kilometers, is present as solid in the ice sheets of Earth. About 90% of the volume of such water is found in Antarctica.

3. Groundwater: About 24 million cubic kilometer water is present under the ground surface at depths of upto two kilometers.

4. Lakes and rivers: On the land surface, approximately 0.18 million cubic kilometer water is present in lakes while about 0.002 million cubic kilometer water is found in rivers.

5. Atmospheric moisture: The amount of water present as water vapor in the atmosphere is about 0.013 million cubic kilometer.

6. Biological water: In addition to above categories, about 0.001 million cubic kilometer water is contained in the bodies of living organisms.

The Earth did not have any hydrosphere in the beginning. It is thought that hydrosphere emerged as the result of processes taking place in the lithosphere. These processes released a substantial quantity of water vapor and juvenile waters during the geological history of Earth. Further, the amounts of water present in oceans, as ice and in atmosphere have fluctuated with appearance and disappearance of major periods of glaciations on the Earth. Palaeogeographic data indicates that the level of oceans on Earth had declined by more than 100 meters during the age of greatest glaciations during Quaternary period. It is estimated that if ice sheets present on Earth today were to melt completely, the level of oceans of Earth would rise by about 66 meters.

Most important feature of global environment of Earth is the hydrological cycle which determines the distribution of water on Earth’s surface and in the atmosphere. The water is evaporated from surface of water bodies like oceans, lakes, rivers etc. as water vapor into atmosphere. It is also absorbed by plants from soil and lost as vapor to atmosphere through transpiration. The water vapor condenses in the atmosphere to form precipitation and thus water is returned from the atmosphere to the surface of Earth. Cyclic movement of water in different components of the global environment is termed hydrological cycle.

OCEAN-WATER
Major portion of earth’s surface is covered with oceans. The water body in the oceans absorbs a large amount of solar radia­tion and has far reaching impact on the heat balance of earth. The water in oceans moves up and down as well as from one place to other. These movements of oceanic water result in transfer of heat from one place to other and have fundamental influence on various components of water and energy balance of land and oceans. The distribution of water balance components has major role in creating and maintaining the climatic and weather conditions in different regions of earth. Therefore, a brief discussion of various aspects of ocean water, ocean currents and waves has been given below.

COMPOSITION OF SEA WATER

Sea water may be described as a brine i.e. the solution of dissolved salts which have accumulated over past periods of geological time from the inflow of runoff water from the land masses. On the land masses, the salts have been formed by the process of weathering of rocks in which weak acids corrode and dissolve the rocks forming various minerals. Due to evaporation of water from oceans, the concentration of salts in the sea water rises resulting in rise of salinity

The composition of sea water results in important properties which are important in understanding its role in Earth’s environment. One way to describe the composition of sea water is to state the principal ingredients that would be required to make an artificial brine approximately like sea water. These ingredi­ents are listed below:

Table : Ingradients in ocean water.

Salt

gm salt per 1000 gm water

Sodium chloride (NaCl)

23

Magnesium chloride (MgCl2)

5

Sodium sulfate (Na2SO4)

4

Calcium chloride (CaCl2)

1

Potassium chloride (KCl)

0.7

With other minor ingredients to total

34.5

Of the various elements combined in these salts, Chlorine alone makes up 55% by weight of all the dissolved matter and Sodium 31%. In addition to elements of above listed five salts, less abundant but important are Bromine, Carbon, Strontium, Boron, Silicon and Fluorine. At least some traces of half of the known elements can be found in sea water. Small amounts of all the gases of the atmosphere are also present in dissolved form in the sea water. Chief among these are Nitrogen, Oxygen, Carbon dioxide, hydrogen and argon.

SALINITY OF SEA WATER

Salinity is described as the proportion of dissolved salts to pure water. It is usually stated in units of parts per thousand by weight and is designated by a special symbol 0/00. The total figure 34.5 0/00 in the above table represents 3.45 percent. Salinity of sea water varies slightly from one place to other in the oceans. Where diluted by abundant rainfall, as in the equatorial oceans, the salinity may be between 34.5 and 35.0 0/00, whereas in the subtropical high-pressure belts, where extreme dryness prevails, the evaporation may increase the salin­ity of surface sea water upto 35.5 0/00.

DISSOLVED GASES

Most important gases dissolved in the oceanic water are oxygen and carbon dioxide. The quantity of oxygen dissolved in the ocean­ic water changes within wide boundaries depending on the tempera­ture, living activities and certain other factors. The concentra­tion of carbon dioxide dissolved in sea water also changes but such change is has negligible importance since the overall quan­tity of carbon dioxide dissolved in sea water is about sixty times more than its amount in atmosphere. Carbon dioxide in sea water is assimilated by autotrophic organisms during photosynthe­sis and enters the organic matter cycle. Part of such assimilated carbon dioxide returns back to ocean water by respiration and after death and decomposition of living organisms but a substan­tial part of such assimilated gas is deposited as carbonate sediments at the bottoms of oceans.

DENSITY OF WATER

The density of water is given in grams per cubic centimeter. The density of pure water is greatest at 4 degrees C. At this tempera­ture, one cubic centimeter of water weighs exactly one gram i.e. its density is 1.000. However, the density of sea water ranges from 1.027 to 1.028. Two factors determine the density of sea water: salinity and temperature. Density increases with salinity and with low temperature upto -2 degrees C.

Density of sea water is of prime importance in circulation of ocean waters because slight density differences causes water to move. Where density of sea water increases by lowering of temperature of evaporation at the surface, the water tends to sink displacing less dense water below it. Just like convection wind systems, such vertical movements of sea water are also described as convectional currents.

OCEAN CURRENTS

Surface winds and density differences are two most important factors in generating and controlling ocean currents. Another factor affecting ocean current is the configuration of ocean basins and coasts.

Virtually all of the important surface currents of the oceans are set in motion by prevailing winds. Energy of winds is transferred to sea water by the frictional drag of the air blowing over the water surface. The Coriolis force impels the water drift toward the right of its path of motion in Northern hemisphere and, therefore, the current at the water surface is in the direction approximately 45 degrees to the right of the wind direc­tion. Under the influence of winds, currents may tend to bank up the water close to the coast of a continent, in which case the force of gravity, tending to equalize the water level, will cause other currents to be set up.

Density differences in oceans arise from greater heating by insolation or greater cooling by radiation, in one place than another. Thus the surface water chilled in the arctic and polar seas sinks to the ocean floor and spreads equatorward displacing upward the warmer, less dense water. Density differences can also be set up due to salinity differences. A currents tends to flow from the area of low salinity to the area of higher salinity, but this flow is also deflected by Coriolis force through a right angle in Northern hemisphere so that the flow is actually paral­lel with the slope of density gradient between the two places.

Configuration of ocean basins and coasts also affects the ocean currents. Currents initially set up by winds impinge upon a coast and are locally deflected to a different path or are confined in straits or gulfs.

The combined action of wind and density differences sets up the global oceanic circulation system including not only horizontal motions but vertical upswelling and down-sinking motions also. Oceanic currents of a shallow surface water zone have strong climatic influence upon overlaying layer of atmosphere and, hence, have been briefly discussed below.

GENERALIZED SCHEME OF OCEAN CURRENTS

There are a number of defined and permanent oceanic currents involving different oceans at the global scale. They have important impact on the climatic conditions of different regions of earth. It is now well recognized that oceanic circulation involves the complex motions of water masses of different temperatures and salinity characteristics. Important such movements have been briefly described below. However, this account does not take into account the movements of water masses at different depths.

1. Most striking features of generalized oceanic currents are the gyrals. These are circular movements of sea water around the subtropical highs, centered about 25 to 30 degrees S.

2. Two equatorial currents marks the belt of trade winds. Whereas the trade winds blow to the southwest and northwest obliquely across the parallels of latitudes, the water movement of this current flows the parallels. Thus the currents are turnd at an angle of about 45 degrees with the prevailing surface winds, because of the deflective force of the earth’s rotation.

3. The equatorial countercurrent separates the north and south equatorial currents and flows in opposite direction to them. It is well developed in the Pacific, Atlantic and Indian oceans.

4. Along the west sides of the oceans in low latitudes, the equatorial currents turn poleward forming warm currents parallel­ing the coasts. Examples of such oceanic currents are Gulf Stream (Florida and Caribbean stream), Japan current (Kuroshio) and the Brazil current. These currents bring higher than average tempera­tures along the respective coasts.

5. The west-wind drift is the slow eastward movement of oceanic water over the zone of westerlies. It covers a broad belt between 35degrees and 45degrees in the Northern hemisphere and between 30-35degrees to 70-75 degrees in Southern hemisphere where open ocean exists in the higher latitudes.

6. The west-wind drift, upon approaching the east side of the ocean is deflected both south and north along the coast. This results in equatorward flow of cool current produced by upwelling of colder water from greater depths. The Humboldt current (Peru current) off the coast of Chile and Peru, Benguela current off the southwest African coast, California current off the west coast of U.S.A. and Canaries current off the Spanish and North African coast are such currents. These currents bring colder than average temperatures along the respective coasts.

7. The North Atlantic current is a relatively warm current formed in the northern eastern Atlantic Ocean due to poleward deflection of west-wind drift. The current spreads around the British Isles, into the North Sea and along the Norwegian coast bringing about warming effect in summers alongwith it. This effect is more pronounced in winters.

8. In the Northern hemisphere, where the polar sea is largely landlocked, cold water flows equatorward along the west side of the large straits connecting the Arctic oceans with the Atlantic basin. Three such cold currents are Kamchatka current flowing southward along the Kamchatka Peninsula and Kurile Islands, Labrador current moving south from the Baffin Bay area through Davis Strait to reach the coast of Newfoundland, Nova Scotia and New England.

9. In both north Atlantic and Pacific oceans, the Icelandic and Aleutian lows coincide in a very rough manner with two centers of counterclockwise circulation involving the cold arctic currents and the west-wind drifts.

10. The Antarctic region has a relatively simple current scheme. It consists of a single Antarctic circumpolar current moving clockwise around the Antarctic continent in latitudes 50 to 60 degrees S where the expanse of open ocean exists. well.

OCEAN WAVES

Almost all the waves in the oceans that can be seen and felt, are produced by wind. The energy of moving air is transferred to water wave motion and this can, in turn, be expended upon the coasts of the lands causing the landforms of erosion and deposition. Thus ocean waves have important role in global energetics and coastal environments. A brief description of the waves in deep water, their growth and decay is given below.

The ocean waves generated by wind belong to a type known as progressive oscillatory waves since wave form travels through the water and causes an oscillatory water motion. Following terms are used in description of waves:

  1. Wave height: It is the vertical distance between the trough and the crest of the wave and is usually measured in feet or meters.

  2. Wave length: It is the horizontal distance from trough to trough or crest to crest and is also stated in feet or meters.

  3. Wave velocity: It is the speed at which wave advances through water and is given in feet or meter per second or in knots (nautical miles per hour).

  4. Period: It is the time elapsed between successive passages of wave crests past a fixed point and is given in seconds.

In the progressive oscillatory wave a tiny particle, such as a drop of water or small floating object, completes one vertical circle, or orbit, with passage of each wave length. Particles move forward on the wave crest, backward in the wave trough. At the sea surface the orbit is of same diameter as the wave height, but dies out rapidly with depth.

In the long waves the water particles return to the same starting point at the completion of each orbit. Hence there is no net motion in the direction of the wind. Only the energy of the wave and its form are transmitted through the water. However, in case of steep, high waves the orbits are not perfect circles. The particle moves just a bit faster forward when on the crest than when it returns in the trough, so that at the end of each circuit the particle makes a slight advance. This produces a very slow surface drift in the direction in which waves are traveling. The rate of this drift is called mass transport velocity. Under favorable conditions, the flow may reach a velocity as high as two knots and will tend to raise the water level along a coast against which the waves are breaking. This motion is not the same as set up by wind friction.

Ocean waves are usually not simple parallel crests and troughs. Instead, they appear highly irregular in height and form because of the interference among several wave trains that are normally present. These trains are not only of different periods, but travel in slightly different directions, so as to intersect at many points. Where two wave crests intersect, the wave height is increased, forming a peak. Where two troughs intersect, the depression is accentuated.

Two forms are waves are usually recognized: wind waves and swell.

(a) Wind waves: These are waves that are being formed and active­ly maintained by the wind. These grow through two mechanisms:

(1) The direct push of wind upon the windward slope of wave drives it forward, just as with floating object.

(2) The skin drag of air flowing over the water surface exerts a pull in the direction of wave motion. Over the wave crest, where drag is strongest, the orbital movement is supplemented adding energy to the wave. In the trough, which is protected, drag is weaker, hence does not counteract the reverse orbital movement as strongly as it is assisted on the crests. This results in a steady increase in the wave height and wave length to some maximum point possible under given wind strength. The wind waves commonly reach speeds much faster than the winds that produce and sustain them. This condition is possible only through the mechanism of skin drag.

The maximum height to which wind waves can grow is controlled by three factors.

(i) Wind velocity: It is obviously a major factor since this determines the amount of energy that can be supplied to the wave.

(ii) Duration of wind: This determines whether or not the waves the opportunity to grow to maximum size.

(iii) Available expanse of water (fetch): This is important because the waves travel as they grow. If waves are developed in a very large body of water over a period of many hours, so that neither duration nor fetch are limiting factors, the maximum wave height varies as the square of the wind velocity i.e.

Wave height (ft.) = 0.026 x Wind velocity2 (knots)s

This would represent the greatest waves to be expected.

Wind duration is important in the early stages of wave growth. Under strong winds, say 30 knots, waves will continue to grow for more than 32 hours although most rapid growth will be in the first 15 hours. Fetch may be an important limiting factor in small bays and straits but has no appreciable effect for water expanses greater than 1000 kilometers across.

(b) Swell: These consist of wind waves that have left the region where they were formed and are gradually dying out in a region of calm or lesser winds. As waves continue to grow, they not only increase their speed but also become longer i.e. their wave length increases. When they have passed beyond the region of strong winds that formed them, waves are transformed into a swell, consisting of very long, low waves of simple form and parallel, even crests. For each time that the swell has traveled a distance in nautical miles equivalent to its length in feet, the swell looses one-third of its height. The energy is lost by friction from air resistance.

SEISMIC SEA WAVES

When sudden displacements of large earth masses occur on the ocean floor, a series of waves is sent out across the ocean. The cause may be slippage along a fault, a volcanic eruption or a large submarine landslide. The waves thus produced are called the seismic sea waves or tsunami (Japanese). These waves are enormous in length (100 to 200 km) and the height of the waves upon reach­ing the shore is observed to be as great as 50 meters in many cases or may even be upto 100 meters in rare instances. In the deep ocean, wave height is only a foot or two and because their length is much greater than the height, such waves may pass unnoticed by observes in a ship at sea. The period of such waves may be 10 to 30 minutes and the velocity of travel of the wave form may be 450 to 800 km per hour. Upon reaching the shallow water of a coastline, a seismic sea wave has the effect of caus­ing an unusual rise of water level. The low areas are inundated and the wind waves which are superimposed upon them are able to break upon much higher grounds than normal.

SEA ICE ICEBERGS AND ICE ISLANDS

Large area in high latitudes of Arctic and Antarctic regions is characterized by presence of ice in various forms over the oceans. Sea ice, pack ice, icebergs and ice islands are important such forms of ice in these regions.

Sea ice is formed by direct freezing of ocean water. It begins to form when the surface water is cooled to temperatures of about -20C and is limited in thickness to about 5 meters be­cause once the insulating layer of floating ice has been formed over the water, heat is supplied from the underlying water as rapidly as it is lost from upper surface. Surface zone of sea ice is composed of fresh water, the salt being excluded in the proc­ess of freezing.

Pack ice is the name given to the ice that completely covers the sea surface. Under the force of wind and currents, pack ice breaks up into individual patches which are termed ice floes. Narrow strips of open water between such floes are called leads. Where ice floes are forcibly brought together by winds, the ice margins buckle and turn upward into pressure ridges resembling walls or irregular hummocks.

The North Polar Sea, which is surrounded by land masses, is normally covered by pack ice throughout the year, although open leads are numerous in the summer. The relatively warmer North Atlantic drift maintains an ice-free zone off the northern coast of Norway. In Antarctica, a vast ocean bounds the sea ice zone on the equatorward margin. Because the ice floes can drift freely north into warmer waters, the Antarctic ice pack does not spread beyond about 600S latitude in the cold season. In March, close to the end of the warm season, the ice margin shrinks to a narrow zone bordering the Antarctic continent.

Icebergs and ice islands differ from sea ice in origin and thickness. Icebergs are formed by breaking off or calving of the blocks from a valley glacier or tongue of an ice cap. These may be several hundred meters in thickness. Icebergs are only slight­ly less dense than the sea water and so these float very low in the sea water, about 5/6th of the bulk lying below the water level. The ice in icebergs is fresh since it is formed of com­pacted and re-crystallized snow. In the Northern hemisphere, icebergs are derived mostly from glacier tongues of the Greenland icecap. They drift slowly south with Labrador and Greenland currents and may find their way into North Atlantic sea in the vicinity of Grand Banks of Newfoundland. Icebergs of Antarctic region are distinctly different from those of arctic region. Whereas those of arctic region are irregular in shape and , therefore, present rather peaked outlines above sea water, the Antarctic icebergs are commonly tabular in form with flat tops and steep cliff-like sides. This is because the tabular icebergs are parts of ice shelves, the great, floating plate-like exten­sions of the continental icecap. In dimensions, a large tabular iceberg of the Antarctic may be tens of kilometers broad and over 700 meters thick, with an ice wall rising 70-100 meters above sea level.

Ice islands of North Polar Sea are somewhat related to tabular icebergs of Antarctic in origin. These are huge plates of floating ice which may be 25 kilometers across and have an area of 300 to 400 square kilometers. The bordering ice cliff, 7 to 10 meters above sea level indicates an ice thickness of 70 meters or more. The few ice islands known are probably derived from a shelf of land-fast glacial ice attached to Ellesmere Island about 83degrees N latitude. The ice islands move slowly with the water drift of Polar Sea and a charting of their tracks reveals much about the circulation in that ocean.

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