Environment of Earth

March 9, 2008

MECHANISMS OF CLIMATIC CHANGE

Filed under: Climate,Environment — gargpk @ 2:57 pm
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There are distinctly different and quite complex patterns of energy transfers at different latitudes of Earth. The global climate and energy budgets at latitudes are correlated so that if climate of one large area or globe as a whole changes then con­siderable changes in energy budgets of different latitudes may occur. Similarly, if energy budget of one large area or globe as a whole changes then considerable changes in the global climate may also occur. Three major types of mechanisms of climatic change can be recognized viz. deterministic extrinsic, stochastic extrinsic and stochastic intrinsic mechanisms.

1. DETERMINISTIC EXTRINSIC MECHANISMS
These mechanisms of climatic change include those processes that are external to Earth’s climate system, are repetitive events i.e. cyclic events of definite periodicity and have forcing effect on global climate. Such mechanisms may be categorized as follows:

a) Mechanisms related to geometry of solar system

These include those processes that are due to cyclic varia­tions in the geometry of Sun-Earth-Moon system.

i) Lunar cycle: It is the 14-day cycle of lunar tidal influences due to orbiting of Moon around Earth. The effect of tidal influ­ences on the climate are observed as variations in cloudiness and rainfall.

ii) Milankovitch cycles: Three cycles of different periodicity have been recognized due to changes in Sun-Earth geometry.

100,000-year cycle: It is cyclic change in the shape of Earth’s orbit around Sun from nearly circular to markedly elliptical and back. At present the orbit is nearly circular and the difference between perihelion and aphelion is about 3.5%. In most elliptical condition of the orbit, this difference may be as large as 30%.

21,000-year cycle: It is the cycle due to wobbling of Earth’s axis of rotation. This wobble in 10,000 years time causes a shift to more extreme difference between summer and winter climates and hemispherical differences in solar constant.

40,000-year cycle: It is the cyclic change in the angle of incli­nation of Earth’s rotation within a range of 21.8o to 24.4o (presently being about 23.45o). Increase in angle of incli­nation results in increases contrast between summer and winter climates.

Most important aspect of these cycles of variations in Earth-Sun geometries is that the distribution of solar irradiance of Earth, particularly between the two hemispheres, varies regu­larly during the period of each of these cycles. This results in changes in the radiation balance and consequently the energy balance which affects the global climate. The above three cycles occur simultaneously but not synchronously (due to different periodicity) and, therefore, lead to a long and complex time series of climatic variation.

Considerable evidence has now accumulated in favour of the Milankovitch model in which he proposes that these three cycles lead to major ice ages on Earth. At the time when during these cycles, the summer insolation in northern hemisphere decreases by 2%, the result is spread of the ice cover of Earth particularly between latitudes 50o to 70o N. As a result of the spread of ice cover, the albedo of Earth’s surface increases leading to de­creased absorbed absorption of solar radiation and consequent decline in temperature. The spread of ice cover is also associat­ed with decrease in atmospheric concentration of carbon dioxide. This decrease in carbon dioxide in atmosphere results in reduced atmospheric absorption of long-wave re-radiation from Earth’s surface which results in further decrease in temperature. Thus the initial spread of ice cover acts as trigger for increasing decline in temperature and consequently more and more spread of ice cover leading to major ice age.

b) Mechanisms related to solar radiant emittance
These mechanisms also bring about climatic changes due to changes in solar irradiance of Earth but the cause of change in solar irradiance is actual cyclic variation in the output of radiation from Sun i.e. variation in solar radiant emittance.

Most important such mechanisms are Sunspot cycles of different periodicity. Sunspots are areas on the Sun’s surface which have temperatures lower than average surface temperature of Sun but have intense magnetic and solar activity. The number and area of sunspots is not constant but shows regular cyclic variation between some minima and maxima. Such sunspot cycles having peri­ods of 11-years (occurrence of successive sunspot minima) and 22 to 23 years (double sunspot cycle associated with reversal of solar magnetic field) are quite important. It has been shown that solar constant varies by about 2% during such sunspot cycles. Other cycles having periodicity of 45, 80, 150, 200, 500, 1000 years have also been recognized. The impact of these cycles depends on their effectiveness in perturbing the global energy balance. Their effect may be realized as changes in pattern of atmospher­ic circulation.

2. STOCHASTIC EXTRINSIC MECHANISMS
These include processes that are external to Earth’s climat­ic system, have forcing effect on global climate but are not regularly repetitive but occur at irregular intervals i.e. are stochastic in nature. Most important such mechanism is volcanic eruptions. A very large quantity of volcanic ash and sulphuric acid droplets is injected into the Earth’s atmosphere, particu­larly into stratosphere during volcanic eruption. This addition of volcanic dust in the atmosphere reduces the amount of solar radiation reaching the Earth’s surface. This reduction in solar irradiance of Earth’s surface may result in reduced temperature, particularly during summers. Lamb (1971) has shown that for British Isles many of the coldest and wettest summers (e.g. during 1695, 1725, 1816, 1879, 1903, 1912 A.D.) occurred at times when volcanic dust content of stratosphere and atmosphere was high. Bray (1974) has proposed that volcanic activity may also have triggered the relatively recent glacial advances such as in periods of 5400-4700 B.C., 2850-2150 B.C. and 470-50 B.C. Possibly the injection of volcanic dust into atmosphere during a sensitive stage of Milankovitch cycles may hasten the approach of an ice age. Important considerations related to impact of volcan­ic eruption on global climate are:

a) Type and quantity of volcanic aerosols injected into atmos­phere: Higher the amount and proportion of volcanic ash injected into the atmosphere, greater is the reduction in solar radiation reaching Earth’s surface.

b) Residence time of aerosols in stratosphere: If aerosols injected by volcanic eruption stay in stratosphere for sufficiently long time only then the decrease in solar irradiance will be realized as effect on the climate. The residence times of aero­sols are inversely proportional to their size i.e. smaller the size, longer the residence time. Acid droplets in particular and ash to lesser extent may have residence times in stratosphere and atmosphere in the order of a few years.

c) Global air circulation: The particles injected into the atmosphere during volcanic eruption are spread over a range of latitudes by stratospheric winds depending on the pattern of global air circulation. According to Lockwood (1979), volcanic dust injected into equatorial zones of stratosphere is spread over whole of globe and perhaps is accumulated over polar ice caps. The dust injected in high latitudes does not spread to latitudes lower than about 300 N. Maximum effect of volcanic dust on global climate is due to this apparent tendency particles to accumulate and persist over high latitudes. This is because the effect of stratospheric particles on incoming solar radiation increases with latitude due to greater reflectance of radiation with increasing angle of incidence of solar beam.

3. STOCHASTIC INTRINSIC MECHANISMS
These mechanisms related to climatic change include those processes that originate within the climatic system of Earth i.e. are internal (intrinsic) processes. These mechanisms have forcing effect on cli­mate and are not regular repetitive events but are probabilistic or stochastic in nature. Important such mechanisms of climate change are discussed below.

a) High-pressure and low-pressure atmospheric systems: Most familiar stochastic events are changes in synoptic weather patterns typi­fied by movements of regions of high and low atmospheric pres­sure. Such pressure areas occur only for few days at most and their spatial and temporal distribution is controlled by the great flows of circumpolar vortex.

b) Anticyclonic and cyclonic systems: In each hemisphere of globe, there is considerable wind flow from west to east through most of the atmosphere. Majority of this flow is in middle lati­tudes. This circumpolar flow plays important role in creating and maintaining characteristic pattern of global climate. Variations in atmospheric pressure cause disturbances in this circumpolar flow and lead to climatic change. Such disturbances arise due to differences in degree of radiant heating of Earth’s surface. Low-pressure areas develop over colder and high-pressure areas over warmer regions. These high- and low-pressure areas in atmosphere in turn affect the velocity of circumpolar vortex. Local high-pressure areas accelerate while low-pressure areas deaccelerate the circumpolar flow. These processes thus result in development of high- and low-pressure atmospheric systems which in turn are moved around the globe by the circumpolar vortex. In general, high-pressure systems or anticyclones are maintained along the warm side of the upper wind and low pressure or cyclonic systems occur near the cold side of the main flow.

c) Meridonial flow system: The above described smooth zonal pattern of circumpolar vortex changes stochastically to a system of meanders from the middle latitudes to the pole and back again. This pattern is called meridonial flow system and may be long lasting. It exerts a blocking effect on the more typical pattern of climate or weather. The meridonial flow leads to periods of abnormally extreme weather with drought in one location and floods in another. The extremes of temperatures also occur in the same manner. Striking example of the effect of meridonial flow on climate was in summer of 1976 A.D. when climate of British Isles was particularly hot and dry while Greece and Turkey experienced wetter than average climate. It has also been suggested that increases in meridonial flow and resultant blocking of atmospher­ic flow might have caused or amplified the course of Little Ice Age during late 16th to early 18th centuries. The climate of this period varied considerably over whole of globe, observed as low precipitation during the Indian monsoons, expansion of polar ice caps, severe winters in Europe and yet a warmer climate in Sibe­ria during the 17th century.

d) El Nino effect: The ocean currents also have very important relationship with characteristic global climatic pattern and any change in pattern of ocean currents results in considerable change in climate. Best known periodic variation of ocean cur­rents is El Nino effect. It is the irregular fluctuation of ocean currents off the coast of Peru and Equador. Its recurrence time is stochastic between 3 to 8 years. Due to dominant, cold Peruvi­an current the coastline of Peru and Ecuador has very little precipitation and the adjacent sea is colder than the land. This climatic pattern is generally emphasized by trade winds from the southeast. During an El Nino event, these trade winds are re­placed by northerly winds, which cause southward flow of warm equatorial water. This leads to heavy rainfall over the arid regions of Peru. The time scales of variations in the circumpolar vortex and of the El Nino event are quite short being in the order of few years only. It has been emphasized by several workers that intrinsic proc­esses operating on time scales of millennia with interactions between the atmosphere, ocean currents and global ice may be important in understanding the regular occurrence of Ice Ages.

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HISTORY AND CAUSES OF CLIMATE CHANGES THROUGH AGES

Filed under: Palaeoenvironment — gargpk @ 2:53 pm
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The climate of Earth has not remained same throughout the geological history of the planet. A broad history of climate changes has been constructed based on palaeogeomorphic studies for the geological past and on observations through network of meteorological stations for the present time. Palaeobotanical studies of the microfossils like pollens, spores etc. as well as megafossils have contributed significantly in elucidating the palaeoclimates and palaeoenvironments of different geological ages in different areas of the earth. An outline of this history of climate changes has been given below.

Precambrian: Very little data is available to deduce the climatic conditions during Precambrian times.

Palaeozoic era (570 to 235 million years B.C.): Somewhat more information is available for this era. It has been deduced that during major part of Palaeozoic era the climate was very warm in all parts of the globe and moisture conditions on the continents fluctuated within wide boundaries.

Towards the end of Palaeozoic, at the boundary between Carboniferous and Permian periods, a glaciation developed. It extended over a large part of the land area that is presently located at tropical latitudes. It is difficult to estimate the geological location of that glaciation during its development since substantial shifts may have subsequently occurred in the location of Earth’s continents and poles. During Permo-carboniferous glaciation, climatic conditions in other regions of Earth were relatively warm.

During Permian period, a thermal zonality appeared and areas of dry climate were greatly enlarged on continents.

Mesozoic era (235 to 66 million B.C.): The climate of this era was relatively uniform. Climatic conditions similar to today’s tropical climate prevailed over a large part of Earth. At higher latitudes, climate was cooler, though it remained quite warm with negligible seasonal changes in temperature. Moisture conditions on the continents appear to have been homogeneous during this era by comparison with the present, even though zones of insufficient and excess humidity did exist. Towards the end of Cretaceous period, the zone of hot climate became less extensive while the zone of dry climatic conditions spread.

Cenozoic era: In passing from Mesozoic to Cenozoic era, no perceptible changes in climate occurred.

During the second half of Tertiary period (towards the middle of Oligocene epoch) a process of progressive cooling began that was most pronounced at middle and especially at high latitudes. Since then, a new climatic zone developed at high latitudes and gradually widened. Its meteorological regime was similar to that of current climatic conditions at middle latitudes. The winter air temperature in that zone fell below zero and this made possible the seasonal formation of snow covers. At the same time the specific properties of continental climates became more pronounced in continental regions distant from the ocean.

During Miocene epoch the cooling process was not uniform and there were periods of temperature rises as well. But these did not alter the general tendency towards an intensification of thermal zonality attributable to declining temperatures at high latitudes.

During the Pliocene epoch the above process was further intensified when the continental glaciation that started during Oligocene in Antarctic (and which continues to exist today) began to spread. Towards the end of Pliocene the climate became warmer than it is today, it was more similar to contemporary climatic conditions than to those of Mesozoic era and of the first half of the Tertiary period.

During Pleistocene epoch the climate differed sharply from preceding conditions in Mesozoic era and the Tertiary period when the thermal zonality was not very pronounced.

Pleistocene epoch began about 1.5 to 2.0 million years ago following an intensification of the cooling that occurred towards the end of Tertiary period at middle and high latitudes. This contributed to development of large continental glaciations. The number of such glaciations and their time of occurrence are known only approximately. Studies in Alps have identified four principal European ice ages termed Gunz, Mindel, Riss and Wurm. Each of these glacial ages could be further subdivided into several stages and glaciations receded during the intervals that divided them. Periods of spread and receding of glaciations occupied only a smaller part of Pleistocene and relatively warm intervening periods were more prolonged. During these warmer periods, ice cover on continents disappeared and existed only in mountainous regions and at high latitudes.

It has been established that the advance and receding of glaciations in Europe, Asia and North America occurred more or less at the same time and a similar correspondence occurred in ice ages in Northern and Southern hemispheres. The spread of ice covers on continents was greatest in regions of more humid maritime climates. In relatively dry climates of Northern Asia, glaciations occupied a relatively small area. During periods of particularly vas glaciations the continental ice cover in Northern Hemisphere reached on the average, 57o North Latitude and in individual regions 40o N latitude. The thickness of continental ice cover over a major part was several hundred meters and in some regions it reached to several kilometers.

With continental glaciation, the boundary of sea polar ice also moved to lower latitudes. This greatly increased the overall area of our globe’s permanent ice cover. During each glaciation period the snow level in mountainous regions not subjected to glaciation moved down by hundreds of meters and sometimes more than a kilometer. During these ice ages the zones of permafrost also increased greatly; their boundary moving to lower latitudes over distances that sometimes reached several thousand kilometers.

During ice ages, along with extensive glaciations, the level of World Ocean declined by 100 to 150 meters below its present level. During warm periods between ice ages, glaciations receded and level of oceans rose by several tens of meters over the present level. The climate of ice ages was characterized by perceptible decline in air temperature in all the regions of world. This reduction in temperature on the average amounted to several degrees below the current temperature and was more pronounced at higher latitudes. In warm periods between ice ages, air temperature was higher than it is today. The influence of ice ages on precipitation is less clear. Some data suggest that moisture conditions in different parts of the world changed in different ways during periods of glaciation. This points to changes in the system of atmospheric circulation produced by glaciations and to corresponding changes in temperature differences between Equator and poles.

Holocene epoch represented a relatively short time period in the history of climate changes that followed the end of Quaternary glaciations. There were several fluctuations in the climatic conditions during this epoch.

Maximal development of Wurmian glaciation occurred approximately 20,000 B.C. and after few thousand years this glaciation was destroyed. After this an epoch followed in which climate was relatively cold and humid at middle latitudes of Northern Hemisphere.

About 12,000 B.C. temperature increased substantially (the Allered) and soon after this a cold period followed. As a result of these climatic fluctuations, Europe’s summer air temperature changed by several degrees.

Between 5,000 to 7,000 B.C. the temperature increased once again and the last large-scale glaciations in Europe and North America disappeared. Post-glaciation temperature increases reached a maximum at that time. Between 5,000 to 6,000 B.C. air temperature at middle latitudes of Northern Hemisphere was about 1 to 3 degrees C higher than today. During this time, changes in atmospheric circulation also occurred. While polar ice boundaries shifted to the north, the sub-tropical high-pressure belt moved to higher latitudes leading to an expansion of arid zones in a number of regions of Europe, Asia and North America. The volume of precipitation in contemporary desert areas at low latitudes also increased. During this period, Sahara’s climate was relatively humid.

Subsequently a trend towards lower temperatures prevailed that was especially pronounced during the fist half of the first millennium B.C. Together with these changes in thermal regime the precipitation regime also changed and gradually approached its present state.

An appreciable rise in temperature occurred at the end of first and beginning of second millennium A.D. At that time polar ice receded to high latitudes. The process of cooling that began in 13th century and reached a maximum at the beginning of 17th century was accompanied by an extension of mountain glaciers and is sometimes referred to as a small ice age. Subsequently temperature increased again and ice has receded. The climatic conditions of 18th and 19th century differed little from those that exist today.

Towards the end of 19th century there has been a gradual increase in air temperature at all latitudes of Northern Hemisphere and in all seasons, especially at high latitudes during cold seasons. Increase in temperature reached a maximum in 1930s when average temperature in Northern Hemisphere increased by about 0.6o C by comparison with the end of 19th century. In 1940s a cooling started that continued until recently. This cooling, however, was relatively slow in comparison with preceding cycle of increased temperatures.

In Northern Hemisphere, increased air temperature has been accompanied by reduction in polar ice area, receding of the boundary of permafrost to high latitudes and a northward movement of forest and tundra boundaries as well other changes in natural conditions. The amount of precipitation in a number of regions of insufficient moisture was reduced as the air temperature increased, particularly during cold seasons of the year. It is also believed that that during the first half of 20th century, temperature increased in Southern Hemisphere as well.

Causes Of Changes In Climate

Until recently studies of the causes underlying changes in climate were made difficult by the absence of a corresponding physical theory. However, in recent years, numerical models of climate theory including the semi-empirical theory of atmospheric thermal regimes have significantly contributed to the studies of causes underlying climatic changes.

Quantitative data concerning climatic conditions in the distant past are not very reliable. Therefore, study of the causes of climatic changes in Prequaternary period is quite difficult while that of Quaternary period is relatively more reliable.

Prequaternary period

The study of the causes of climatic change during Prequaternary period can only be limited to an examination of the secular trend of air temperature during second half of the Mesozoic era and the Tertiary period. Palaeotemperatures determined through the method of isotopic analysis can be reliably used in such studies. In has been established from the studies of palaeotemperatures that during the last 130 million years there has been a tendency for average air temperatures at the Earth’s surface to decline. This tendency was interrupted several times by perceptible increases in air temperature but these were always followed by periods of cooling and did not alter the overall course of climate evolution. In order to explain the changes in climatic conditions, consideration of the influence of fluctuations in the composition of atmosphere and in the structure of the Earth’s surface on the thermal regime is necessary.

1. Effect of atmospheric carbon dioxide on thermal regime of atmosphere

Since carbon dioxide influences the absorption of long-wave radiation, thus sustaining the atmospheric green-house effect, a reduction in its mass produces a decline in air temperature at Earth’s surface. It has been established that only for relatively low concentrations (<0.1%) the dependence of temperature on the concentration of carbon dioxide becomes perceptible. At higher concentrations, changes in the volume of carbon dioxide in the atmosphere influence air temperature insignificantly. As a result, only during Pliocene and Pleistocene epochs the reductions in the atmospheric carbon dioxide content have produced considerable changes in air temperature. Calculations based on semi-empirical theory of thermal regime have shown that during that period the average air temperature at the Earth’s surface has declined by several degrees.

2. Effect of structure of Earth’s surface on thermal regime of atmosphere

During the second half of Mesozoic epoch of Tertiary period, the average elevation of continents was relatively low. As a result, a large part of continental plateform was covered by shallow seas. During that time a large number of uplifts and sinkings of different continental areas occurred but this did not alter the overall pattern in which straights and seas of various sizes separated various continents.

During Neogene epoch of Tertiary period, intensive tectonic movements produced the elevation of continents and caused a gradual vanishing of many intercontinental seas. Particularly, the sea on the present-day tertiary of Western Siberia that linked tropical oceans with the polar basin ceased to exist. This resulted in transformation of Arctic Ocean into its today’s condition of a relatively isolated body of water that is linked only with Atlantic Ocean and poorly connected to Pacific Ocean through the very narrow Bering Strait.

Possibly the slow drift of Antarctic was completed during the Tertiary period. It is presumed that such changes in the structure of Earth’s surface produced substantial decline in meridonial heat exchanges in the oceans. Such reductions in meridonial flows of heat in oceans resulting from increases in the levels of continents have contributed to the reduction in temperatures that have taken place at middle and high latitudes during the last 100-150 million years.

The reasons why the temperatures fluctuate over intervals extending to tens of millions of years is an interesting problem. It may be assumed that cause was local changes in circulation processes in the atmosphere and oceans.

Quaternary period

The Quaternary period was preceded by a prolonged evolution of climate in the direction of a more pronounced thermal zonality resulting from changes in Earth’s surface structure. This was largely expressed in a continual decline in air temperature at middle and high latitude. Peculiar climatic conditions during Quaternary period appear to have emerged due to further decline in concentration of carbon dioxide in the atmosphere and also due to drifts and elevation of continents.

1. Effect of decline in atmospheric Carbon dioxide

During Pliocene epoch, climatic conditions began to be influenced by a reduction in the atmospheric carbon dioxide concentration. Such reduction led to a decline in average global air temperature by 2o to 3o C (3o to 5o C at high latitudes). This resulted in development and extension of polar ice caps that in turn, caused further decline in average global temperature, particularly at higher latitudes.

2. Effect of shifts and elevation of continents

Development of polar ice caps sharply increased the sensitivity of thermal regime to even very small changes in climate-forming factors. This made possible the very large oscillations in the boundaries of snow and ice covers on land and in oceans as a result of changes in the location of land in relation to the Sun; earlier this factor had not influenced the climate substantially.

Continuous reduction in atmospheric CO2 contributed to the advance of glaciations, even though the major influence on their scope was a combination of astronomical factors determining the location of Earth’s surface in relation to the Sun. These factors include the eccentricity of Earth’s orbit, the inclination of Earth’s axis in relation to the plane of its orbit and the time of equinoxes. These astronomical changes occur in periods of tens of thousands of years. In comparison to changes in astronomic factors, all other factors appear to have exerted a lesser influence on climatic fluctuations during Quaternary period.

The above conclusions have been verified by using a numerical model that makes possible the calculation of individual ice covers as influenced by external factors. Budyko, M.I. & Vasishcheva, M.A. (1971, The Influence of Astronomical Factors on Quaternary Glaciations. Meteorologiya I gidrologiya. No. 6) studied the climatic conditions during glaciation periods using a model describing the distribution of average latitudinal temperature in different seasons. The study showed that fluctuation in the radiation regime caused by changes in the position of Earth’s surface in relation to Sun may lead to substantial changes in climate. Though average global temperature does not fluctuate much at such times, such modest fluctuations are accompanied by perceptible shifts in the boundaries of ice covers.

Climatic changes in 20th century

Meteorological observations for first half of present century show a trend towards increased temperatures that reached a maximum in 1930s. It appears from the studies of reasons explaining climatic change during this period that the increase in temperature has been caused by an increase in transparency of stratosphere. This caused increased inflow of solar radiation (i.e. increased meteorological Solar constant) into the troposphere and has led to an increase in average global air temperature at Earth’s surface.

The changes in air temperature at different latitudes and in different seasons have depended on:

the optical thickness of stratospheric aerosol and

shifts in the boundaries of sea polar ice.

The shrinking Arctic sea ice due to rise in air temperature has further contributed to increase in air temperature during cold seasons at high latitudes in Northern Hemisphere.

It seems probable that changes in the transparency of stratosphere during first half of this century have been associated with the regime of volcanic activities, particularly with the changes in inflow of the products of volcanic eruptions (including sulphur dioxide) into stratosphere. It is now increasingly being recognized that during last 3 to 4 decades, changes in climate have also begun to depend, atleast in some measure, on human activities. Increasing pollution with rapid industrialization is adding various substances into the atmosphere. This is changing various climatic and meteorological factors causing changes in the climate that are still not well understood.

CLIMATIC INDICES AND BIOGEOGRAPHICAL ZONES

Filed under: Environment — gargpk @ 2:43 pm
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Study of the parameters of radiation and water balance of Earth clearly shows that these parameters differ with latitudes. The interactions of these parameters of radiation and water balance are responsible for particular meteorological conditions in different regions of Earth which, in turn, result in climatic zones. Further, the biosphere of Earth, particularly the plant cover over land surface is characteristically dependent on the climatic conditions of a particular region. Thus, a number of distinct biogeographical zones can be recognized over Earth’s surface based on the characteristic climate, soil and plant cover. Dokuchavev for the first time observed that the boundaries of biogeographical zones are largely determined by climatic factors and are particularly dependent on moisture conditions. His classical studies included estimates of the ratios of precipitation to potential evaporation for major biogeographical zones of Earth. Various studies after him examined the relationship of soil and forest types with ratios of precipitation to potential evaporation. Grigoriyev and Budyko in comprehensive studies of global environment identified the most important parameters of radiation and water balance which determine the biogeographical zonality and gave the Law of Geographical Zonality.

LAW OF GEOGRAPHICAL ZONALITY

Budyko showed that equations for mean yearly heat and water balances of land may be written in the following form:

R/Lr = E/r + P/Lr and 1 = E/r + f/r

(the members of heat balance equation are divided by Lr and those of water balance equation by r).

The linkage equation of water and heat balance may then be added to these relations:

E/r = @ (R/Lr)

These equations link four relative values of components of heat and water balances. Therefore, it is sufficient to know any one of them for determining the others. Owing to special form of the linkage equation, ratio R/Lr or P/Lr may be selected as the parameter that determines all relative values of the components of heat and water balances. The ratios E/r and f/r can not be decisive factor for the first two variables in case of dry climates when small changes in E/r or f/r produce large changes in R/Lr or P/Lr. Further, it seems more appropriate to select R/Lr as the principal parameter determining the relative values of the components of heat and water balances. This parameter may also be viewed as the ratio of potential evaporation R/L to precipitation r, or else as a ratio of the radiation balance of a moistened surface to heat expended on the evaporation of the total yearly precipitation

While the relative values of components of heat and water balances are determined by one parameter R/Lr, the determination of absolute values of these components requires two parameters namely R/Lr and R.

It has been shown by Budyko that it is possible to calculate potential evaporation from the radiation balance at the Earth’s surface. The method for calculating potential evaporation from air humidity deficit as various other empirical methods yield less accurate results.

A world map of the radiation index of dryness (R/Lr) has been constructed by Budyko (1955). A comparison of this map with geobotanical and soil maps confirms that the positions of isoclines of the index of dryness confirm well to the distribution of major climatic and biogeographical zones. It has been shown that the radiation index of dryness accords well with the boundaries of major natural climatic and biogeographical zones. In general, values of the radiation index of dryness (< 1/3) correspond to tundras, values between 1/3 and 1 to forest zone, from 1 to 2 to the steppe zone, more than 2 to semi-desert zone and more than 3 to the desert zone. Further, within particular zones at various latitudes substantial differences in conditions determining the development of natural processes can be observed. These differences derive from the fact that the energy base of natural processes may be characterized through the magnitude of the radiation balance R and it differs at various latitudes. Accordingly, while it may be possible to make use of single parameter R/L in characterizing general zonal conditions of natural processes, the characterization of absolute values of the intensity of natural processes requires the use of two parameters i.e. R/Lr and R which determine the absolute magnitudes of the elements of heat and water balances.

The relation of biogeographical conditions with the parameters R/r and R plotted along its axes and on which major biogeographical zones are divided by straight lines. A schematic form of such a graph is given in Fig.

While the use of more accurate precipitation normals found with help of currently available measuring instruments introduce certain changes in the characteristic values of moisture indices, this does not alter the general patterns of the relation of moisture indices to biogeographical zones. The close relationship of parameters R/Lr and R with biogeographical zones has been examined and named the Law of Geographical Zonality (Grigoriyev and Budyko; 1956, 1962). It was specifically noted that within each latitudinal belt there exists a definite correspondence between the boundaries of natural zones and the isoclines for particular values of R/Lr. Table- represents the overall structure. In the compilation of the table, use was made of the values of R relating to the conditions of moistened surface and of more detailed data on precipitation and plant cover in various regions. To each section in the table, characterizing gradations of moisture conditions correspond to specific values of the coefficients of river run-offs. At the same time, yearly run-off normal in each section increases as radiation balance increases. However, this does not apply to desert areas where the run-off at low latitudes is close to zero. Important conclusions of the Law of Geographical Zonality are discussed below.

1. R/Lr and botanical zones: Similar types of plant cover correspond to each section in the table. In conditions of excessive moisture, forests are prevalent at all latitudes except for areas with substantial excess of moisture i.e. R/r < 2/5. In such cases, forests are replaced by tundras at high latitudes and by swamps at lower latitudes. Since R/r < 2/5 over vast territories is found only at relatively high latitudes, swampy areas at low latitudes can not be viewed as autonomous biogeographical zones. It should be noted that the ordinate represents values of the radiation balance for actual state of Earth’s surface, which differ from the values of the radiation balance for moistened surfaces. The continuous line on the graph bounds the region of actually occurring values of R/r and R (except for mountainous regions) within whose ranges specific values of R/Lr (shown as vertical lines) delimit the major botanical zones. Despite common features of plant covers in the forest zone, large changes in the values of radiation balance correspond to perceptible geobotanical changes within these zones.

Each gradation in moistening is characterized not only by a definite type of plant cover but also by a specific value of its productivity. In the studies of Grigoriyev and Budyko (1956, 1962), it was assumed that the productivity of natural vegetation increases as the moisture conditions approach optimal ones for a given radiation balance. Similarly, productivity increases with increase in radiation balance for given moisture conditions.

2. R and R/Lr and soil zones: It has long been established that there exists a close relationship between the zonality of plant covers and the zonality of soils. Therefore, the conclusions concerning the relation of botanical zones to specific values of parameters R and R/Lr can be fully applied to soil zones as well. It can be established that as the values of parameter R/Lr increase, soil types change in the following sequence:

(i) Tundra soils;

(ii) Podzols, brown forest soils, yellow soils, red soils and laterite soils (the diversity of soil types within that group corresponds to changes in the parameter R within the broad range);

(iii) Chernozems and black soils of savannah regions;

(iv) Chestnut brown soils;

(v) Grey soils.

General relation of soil zonality to climatic indices R/Lr and R may be represented as a graph similar to the graph for botanical zones. Considering the Table-, to each section of the table, there corresponds a specific sequence of changes in soil types which is largely similar within each section. For example, the third section is characterized by following sequence of soil types: tundras, podzols and brown forest soils, subtropical red soils and yellow soils, tropical podzol red soils and laterites. For the fourth section, the sequence is chernozems and dark chestnut soils, black soils and brown soils, weakly leached subtropical soils, red brown tropical soils etc. To each such sequence there correspond specific values of quantitative characteristics of the process of soil formation. The region of eternal snow occupies a special place within the table. It is described by negative value of R and R/Lr due to practical absence of plant and soil covers. Similarly, the subzone of Arctic deserts has negligible values of yearly radiation balance and a high humidity.

3. R/Lr and R and zonality of hydrological regime of land: The relation of the zonality of hydrological regime of land to parameters R/Lr and R may be established on both quantitative and qualitative terms. From the linkage equation it follows that to each gradation in value of parameter R/Lr there corresponds specific gradation in the value of run-off coefficient. A consideration of the influence of energy factors makes it possible to explain the zonal changes in run-off coefficients in quantitative terms. The absolute values of total run-off are determined by two parameters, namely R/Lr and R. Graph in Fig. , which is similar to that in Fig. represents the distribution of yearly total run-off and characterizes the absolute values of run-off in different biogeographical zones.

CAUSES OF PATTERNS OF GEOGRAPHICAL ZONALITY
From the above discussion, it is evident that the patterns in the Table-6 of Geographical Zones may be explained in terms of the following factors:

1. Latitudinal difference in Earth’s surface radiation balance: Owing to the spherical shape of Earth, its surface is divided into several latitudinal belts that differ in terms of radiation energy inflow to the surface of Earth.

2. Differences in moisture conditions: Within each of these belts (except for the region of eternal snow) there exist different moisture conditions ranging from excessive to highly insufficient moisture. These differences in moisture conditions within each latitudinal belt are responsible for the zonality within the belt..

Within different latitudinal belts, geographical conditions with similar moisture indices have a certain number of common characteristics, which are combined with differences resulting from different inflows of radiation energy. These common characteristics are periodically repeated as regions of two latitudinal belts in which humidity (or dryness) increases) are compared with each other.

SEASONAL CHANGES IN CLIMATIC FACTORS OF BIOGEOGRAPHICAL ZONALITY
The influence of climatic factors on biogeographical zones may be represented more clearly by considering the specific features of the climatic regimes in different seasons. In specifying the climatic conditions of particular seasons, following major types of climatic regimes may be identified:

1. Arctic climatic regime: It is characterized by the sow cover, negative air temperatures and negative values of radiation balance or else values close to zero.

2. Tundra climatic regimes: Such climatic regimes have average monthly temperatures ranging from zero to 10o C and a small positive radiation-balance.

3. Climatic regimes of forest zones: These have average monthly temperatures of more than 10o C and a positive radiation balance as well as sufficient moisture, when evaporation exceeds one half of the potential evaporation.

4. Climatic regimes of arid zones (Steppes and savannahs): These have a positive radiation balance and actual evaporation ranging from 1/10 to ½ of the potential evaporation.

5. Climatic regimes of deserts: These have positive radiation balance and an evaporation less than 1/10 of potential evaporation.

Important features of seasonal changes in climatic factors determining biogeographical zonality are:

(i) It has been shown that the type of climatic conditions in each month corresponds with the type of natural zone throughout the year. However, in most biogeographical regions, several types of climatic regimes replace each other during the year.

(ii) There are substantial differences in seasonal changes of climatic regimes at various longitudes. These are especially felt at middle and low latitudes where types of climatic regimes depend on moisture conditions.

(iii) In Africa and Europe, a regime of low humidity exists within a wide latitudinal belt that shifts to the north in summers and to the south in winters. This corresponds to the most favourable conditions in the subtropics in the winter and spring and at tropical latitudes in the summer. In East Asia and North America, the structure of moisture regime differs considerably from the first scheme because of substantial differences in circulation processes.

(iv) In most regions and at all longitudes there is either insufficient moisture (regimes 4 and 5) or insufficient heat (regime 1 & 2) during a major part of the year. Only in the narrow belt close to Equator, conditions corresponding to regime 3 exist throughout the year. Evidently under regimes 2 and 4 and especially under regimes 1 and 5, productivity of natural plant cover is reduced.

(v) Under conditions of insufficient heat the type of biogeographical zone is determined by climatic regime of the period in which productivity of natural plant cover is greatest even if that period is relatively short. For example, the zonal landscape of tundra is determined by conditions of the warm season, which may last no longer than 1/5th to 1/4th of the year. In such cases, climatic regime of the cold season, which extends over most of the year does not determine the landscape’s zonal character.

(vi) In regions of insufficient moisture also a regularity similar to the above one is observed. The most humid period of the year plays a determining role in establishing the type of zone, even though it may be shorter than the period with insufficient moisture. In natural zones with insufficient moisture, there may be short periods of either sufficient or excessive moisture which do not result in development of typical forest growth.

The above described regularities relating the type of biogeographical zone to seasonal changes in the climatic factors determining the zonality, in fact, complement the concept of the influence of climatic factors on biogeographical zonality described earlier in the Law of Geographical Zonality.

RADIATION BALANCE OF EARTH

Filed under: Environment — gargpk @ 2:19 pm
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In every component of the global environment, radiation is constantly coming and a portion of incoming radiation is constantly going out of it. The algebraic sum of radiation fluxes reaching and leaving a place is called its radiation balance. In the study of global environment, following three types of radiation balances are important:

(a) Radiation balance of Earth’s surface (R)

It is the algebraic sum of radiation fluxes reaching and leaving the surface of Earth. Its magnitude is equal to the difference between the amounts of direct and diffused short-wave radiation absorbed by Earth’s surface and the long-wave effective radiation and is given as:

R = Q(1 – A) – I

where, R = Radiation balance of Earth’s surface; Q = Total direct and diffused short-wave radiation reaching Earth’s surface; A = Albedo of Earth’s surface given as a fraction of unity; I = Effective radiation.

Due to ‘green-house effect’ of atmosphere, the average radiation balance of Earth’s surface is always positive. The radiation balance of Earth’s surface is linked to the radiation balance of atmosphere.

(b) Radiation balance of Earth-atmosphere system (Rs)

It is the algebraic sum of radiation fluxes reaching and leaving Earth-atmosphere system as a whole. This it is equal to the difference between radiation solar radiation reaching the upper boundary of atmosphere i.e. incoming radiation (incident solar radiation) and long-wave radiation leaving atmosphere’s upper boundary i.e. outgoing radiation. Its magnitude may be given as:

Rs = Qs(1 – As) – Is

where, Rs = Radiation balance of a vertical column extending from upper boundary of atmosphere to the Earth’s surface; Qs = incoming radiation; As = Albedo of Earth-atmosphere system and Is = outgoing radiation.

The outgoing radiation of Earth-atmosphere system includes that part of Earth’s surface radiation which passes through atmosphere unaltered and goes out of atmosphere’s upper boundary plus radiation of atmosphere itself going out of its upper boundary. The outgoing radiation is much influenced by clouds. In the absence of clouds, Earth’s surface radiation within wavelength range of 900-1200 nm plays important role. In completely cloudy conditions, the radiation from the upper surface of clouds becomes very important and this radiation depends on the temperature of that surface. The temperature at the upper surface of clouds is usually much lower than the temperature at the Earth’s surface. Therefore, clouds substantially reduce the amount of long-wave radiation into the outer space.

(c) Radiation balance of atmosphere (Ra)

It is equal to the difference between radiation balance Earth-atmosphere system and the radiation balance of Earth’s surface i.e. given as:

Ra = Rs – R

or, substituting the expressions for Rs and R, as:

Ra = Qs(1 – As) – Q(1 – A) – (Is – I)

The magnitude of the radiation balance of atmosphere is negative and equal to the absolute value of radiation balance at Earth’s surface. This negative radiation balance of atmosphere is compensated by:

(i) inflows of energy from condensation of water vapor during cloud formation and precipitation and

(ii) flow of heat from Earth’s surface associated with turbulent heat conductivity of lower atmospheric layer.

Geographical distribution of radiation balance

The radiation balance at Earth’s surface is not uniform but varies at different geographical locations. Following two factors have important effect on the variation of radiation balance over Earth’s surface.

1. Relationship between latitude and irradiance: The radiation reaching certain surface area (B) at Earth’s surface depends on the latitude and is given by Lambert’s cosine law:

QB = Qo cos z

where, Qo = Irradiance of solar beam at upper boundary of atmosphere i.e. equal to solar constant; z = solar zenith angle.

Zenith angle of direct solar beam may be defined under any condition of varying latitude, solar declination and solar time is given by:

cos z = sin sin + cos cos h

where, = latitude; = angle of solar declination (angle between solar beam and equator which varies between -23.45 degrees on 22 December and +23.45 degrees on 22 June); h = hour angle of Sun (measure of time from solar noon where one hour equals to 15 degrees).

Maximum monthly range of solar irradiance shows considerable increase with latitudes and is also related to variations in photoperiod (time in hours between sunrise and sunset P). Value of h at both sunrise and sunset may be derived from solar declination and latitude of site by:

cos h = -(tan tan )

so that the photoperiod (P) is gives as:

P = 2/15 cos-1 h

2. Optical effects of atmosphere on solar irradiance: The ideal pattern of solar irradiance at the upper boundary of atmosphere is not realized perfectly at Earth’s surface because of the optical effects of atmosphere on solar beam passing through it. Following three features of the interaction between solar irradiance and the atmosphere determine solar irradiance at the Earth’s surface.

(i) Path length (m): Assuming that the thickness of atmosphere over Earth’s surface is uniform, the length of path, which solar beam traverses from upper boundary of atmosphere to the Earth’s surface i.e. the path length (m) is given as:

m = 1/cos z

The path length (m) is shortest at the place where solar beam and Earth’s surface are perpendicular to each other. The above relation is found to be correct upto zenith angle (z) of about 70 degrees (Robinson, 1966). At greater zenith angles, curvature of Earth and atmospheric refraction cause increasing overestimation of the value of path length.

(ii) Atmospheric transmittance: The radiation of solar beam passing through atmosphere may be absorbed, reflected and scattered by various gases and aerosols in the atmosphere. The effects of these phenomena on the solar irradiance are reflected in mean atmospheric transmittance (). If all the effects of atmosphere on the solar beam are assumed to be constant throughout the depth of atmosphere, then depletion of the irradiance of solar beam at upper boundary of Earth’s atmosphere (Qo) will be simple function of path length (also termed air mass) and mean atmospheric transmittance. Then the solar irradiance reaching Earth’s surface (Q) will be given as:

Q = Qo m

The value of strongly depends on dust and pollutants present in the atmosphere and may vary from 0.4 in polluted atmosphere to 0.8 in very clear and dry atmosphere.

(iii) Cloud cover: The solar beam passing through Earth’s atmosphere is also affected by the cloud cover present in it. Various empirical relationships have been derived which relate solar irradiance reaching Earth’s surface to some measure of cloudiness. No such relation is perfect because of the variations in optical properties of different cloud types. A general relationship derived from global observations at 88 well separated meteorological stations is given as:

Q = Qo (0.803 – 0.34f – 0.458f2)

Where, f = monthly fractional cloud cover.

The variations in mean atmospheric transmittance have not been included in this relationship and the above relationship will show changes with such variations.

The salient features of the geographical distribution of radiation balance of Earth’s surface keeping in view above effects on solar irradiance at upper boundary of Earth’s atmosphere are as follows:

1. Yearly total radiation: It varies substantially within the range of less than 60 to more than 220 kcal/sq. cm/year.

(i) At high and middle latitudes, distribution of total radiation is zonal in nature while in tropical latitudes the distribution deviates from zonality.

(ii) Total radiation is greatest in high-pressure belts in Northern and Southern Hemispheres, especially in desert areas of continents. Highest total radiation is found in Northwest Africa. It is due to almost total absence of clouds in that region. However, total radiation is reduced in low latitudes near equator, in regions of monsoon climates and in certain other regions due to increased cloudiness.

(iii) Total radiation also varies seasonally i.e. in different months of the year. This may be illustrated by considering its values in June and December i.e. the months in which average height of Sun is highest and the lowest in Northern Hemisphere and vice-versa in Southern Hemisphere.

During December, zero isocline passes somewhat to the north of Arctic Circle. At latitudes above it, Sun never rises above the horizon and total radiation is zero. To the south of zero isocline, total radiation increases rapidly. Its distribution in region below Arctic Circle and above tropical latitudes is largely zonal. In low tropical latitudes, the zonal character of distribution is absent and total radiation is determined by the degree of cloudiness in different regions. Further, average zonal changes in total radiation are relatively small from low latitudes in Northern Hemisphere throughout entire Southern Hemisphere. Continuous increase in length of day towards South Pole in Southern compensates for reduction in average height of Sun and so there is negligible reduction in total radiation towards higher latitudes in Southern Hemisphere.

During June, similar situation exists with regard to distribution of total radiation. In Northern Hemisphere, total radiation shows relatively little change except for desert regions where its value is high. At middle and high latitudes of Southern Hemisphere, total radiation decreases with increasing latitudes.

2. Yearly total radiation balance: Yearly total radiation-balance at Earth’s surface is positive over entire surface of oceans and land. Negative yearly sums of radiation balance are found only in regions of permanent snow or ice cover. The yearly radiation balance shows sharp changes from land to ocean areas. As albedo values of ocean surface are lower, radiation balance of ocean areas is usually higher than that of land areas at same latitudes.

On ocean surface, distribution of radiation balance is generally zonal in character. However, some regions particularly where warm and cold currents operate, some deviations from zonality are found. At tropical latitudes, radiation balance of ocean surface shows small changes while in the middle latitudes, there is rapid reduction in corresponding balances from lower to higher latitudes. Greatest value of radiation balance of Earth’s surface is 140 kcal/sq. cm/year, which occurs in Arabian Sea.

On the land surface, changes in yearly radiation-balance values are also partly zonal in nature. However, in certain regions deviation from zonality is found due to difference in their moisture conditions. In the dry regions, radiation balance is lower in comparison with regions of sufficient or excessive moisture at same latitudes. The low radiation balance in dry regions is due to:

(i) Reflection of short-wave radiation

(ii) Higher expenditure of radiation energy on effective radiation owing to high surface temperature, low cloudiness and relatively low air humidity. Thus alongwith general reduction in radiation balance at higher latitudes, there are also found regions of further reduced radiation balance in areas of dry climate. This reduction is particularly observed in Sahara, deserts of Central Asia and in many other deserts and arid regions. In monsoon regions, yearly radiation balance of Earth’s surface is also somewhat reduced due to intense cloudiness during warm season. In humid tropical regions, highest yearly radiation balance values are found on land but even these just reach 100 kcal/sq. cm/year which is quite less than corresponding maximum value for the ocean.

SOLAR RADIATION ON EARTH

Filed under: Environment — gargpk @ 1:37 pm
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The Sun of planet Earth emits radiation at a temperature of about 6000 degrees K. Average radiation emitted by Sun at its surface (Sun’s radiant emittance) is 73×106 W per square meter. Spectrum of this solar radiation shows distinct emission lines indicating that it comprises of radiation of different wavelengths. The intensities and thus the magnitudes of the radiation of different wavelengths are also different. The intensity of far ultra-violet radiation is very low due to absorption of radiation by outer photosphere of the Sun. Further, extreme ultra-violet and X-ray parts of solar radiation are emitted from the chromosphere and corona regions of the Sun. These regions have temperatures as high as 1 million K. The solar radiation falling at the upper boundary of Earth’s atmosphere is termed incident solar radiation. Its average magnitude over the Earth is given by (Solar constant x r2)/4r2, where r = radius of Earth.

The Solar constant is the irradiance on an area at right angle to solar beam and outside the Earth. Its value is 1353 W per square meter. The average incident solar radiation at upper boundary of Earth’s atmosphere is approximately 11 x 109 J/m2 /yr. Important factors that affect this solar radiation received by Earth are:

  1. Spherical shape of Earth: Earth is a sphere and, therefore, the angle at which incoming solar radiation strikes the upper boundary of atmosphere is not same at all points. The radiation strikes Earth at right angle in the center but the angle gradually becomes more acute towards periphery. As a result, the amount of solar radiation reflected back from the upper surface of atmosphere is zero at the center and increases gradually towards periphery. Thus, the amount of radiation penetrating atmosphere and entering Earth-atmosphere system is maximum in the center and gradually decreases towards periphery.

  2. Orbit of Earth: The orbit of Earth around Sun is not perfectly circular but is slightly elliptic. Sun occupies one focus of this elliptic orbit. The mean distance between Sun and Earth is about 150 million kilometers. However, the orbit of Earth is elliptical and so the distance changes during different times in the year. Earth is closest (about 91.5 million miles) to Sun on about January 3, at which time it is said to be in perihelion. It is at greatest distance (about 94.5 million miles) from Sun on about July 4 when it is said to be in aphelion. These differences in distance also cause some difference in amount of solar radiation received by Earth. However, the ellipticity of orbit is not the reason of seasons on Earth. An important fact related to the Earth’s orbit around Sun is that the geometry of Earth’s orbit is not constant. The orbit at present is very slightly elliptical being nearly circular but the shape of Earth’s orbit changes cyclically from almost circular to markedly elliptical and back with a periodicity of 100,000 years. The solar constant is the mean solar irradiance on an area at upper boundary of Earth’s atmosphere perpendicular to incoming solar beam, which is about 1353 W per square meter. In the present state of Earth’s orbit being nearly circular, the difference between perihelion and aphelion is about 3.5% and the difference in solar constant at these two points is about 6.66%. In the state of most elliptic orbit, difference in solar constant at perihelion and aphelion may be as large as 30%.

  3. Inclination of Earth’s axis of rotation: The axis of rotation of Earth is not perpendicular to the plane of ecliptic i.e. the plane in which Earth’s orbit and Sun lie. Earth’s axis of rotation makes an angle of about 66.5 degrees with the plane of ecliptic and is tilted 23.5 degrees from the line perpendicular to plane of ecliptic. The Earth’s axis although always makes an angle of 66.5 degrees with plane of ecliptic, also maintains a fixed orientation with respect to stars. The Earth’s axis continues to point to the same spot in the heavens as it makes its yearly circuit around Sun. This inclination of Earth’s rotational axis alongwith its fixed orientation throughout the whole orbit around Sun causes different seasons on Earth. Between September 23 and March 21, North Pole of Earth’s axis is tilted towards the Sun and South Pole is away from the Sun. During this period, Northern Hemisphere has summers and Southern Hemisphere has winters. In this period, daylength and, therefore solar radiation received increases towards North Pole and decreases towards South Pole. From March 21 to September 23, South Pole is tilted towards Sun and North Pole is tilted away from it, North Hemisphere has winters and South Hemisphere has summers during this period. In this period, daylength and, therefore, solar radiation received increases towards South Pole and decreases towards North Pole. Maximum tilts of North Pole and South Pole towards Sun occur on June 21 and December 22 respectively and these dates are termed summer solstice and winter solstice respectively. Midway between the dates of solstices, twice the Earth’s axis is at right angle to the line drawn from Sun to Earth and neither pole is tilted towards Sun. This condition occurs on March 21 or 22 (vernal equinox) and on September 22 or 23 (autumn equinox). Two important cyclic changes related with inclination of Earth’s axis of rotation have been noted. First is the wobbling of Earth’s axis of rotation with a periodicity of 21,000 years. This causes continuous and cyclic hemispheric variation in the solar constant. Second change is cyclic variation in the angle of inclination of Earth’s axis of rotation within the range of 21.8o and 24.4o (23.45 degrees at present) with a periodicity of about 40,000 years. Therefore, the distribution of solar irradiance at Earth’s two hemispheres varies continuously with this 40,000 years cyclic periodicity.

1TRANSFORMATIONS OF SOLAR RADIATION

The general fate of the incident solar radiation in Earth-atmosphere system is as below:

(a) Absorbed in stratosphere (mainly by Ozone) = 3%

(b) Absorbed in troposphere by:

(i) Carbon dioxide = 1%

(ii) Water vapor = 12%

(iii) Dust = 2%

(iv) Water droplets in clouds = 3%

(c) Reflected from clouds = 21%

(d) Scattering back into atmosphere 6%

(e) Reflected back from Earth’s surface = 4%

(f) Received at Earth’s surface as:

(i) Direct radiation = 27%

(ii) Diffused radiation via clouds or downward scattering = 21%

Solar radiation received by Earth i.e. incident solar radiation undergoes various transformations after entering the uppermost boundary of atmosphere. The solar radiation is absorbed by atmosphere, hydrosphere, lithosphere and biosphere. Some part of solar radiation absorbed in a component provides energy for the dynamic functions of that component. The remaining part of absorbed radiation is re-emitted from the component as long-wave radiation. Two important features in the study of the transformation of solar radiation are albedo and effective radiation, which are discussed below.

1.1Albedo

The fraction of solar radiation received by a body that is returned back from it forms the albedo of that body. In general, the radiation reflected back from clouds (21%), scattered back into atmosphere (6%) and reflected back from the Earth’s surface (4%) together constitute Earth’s albedo. The average value of albedo of Earth as a whole comes to about 33% or 0.33 (represented as fraction of unity).

The albedo of Earth as a whole has two components:

(a) Albedo of Earth’s surface: It is that fraction of radiation received at the Earth’s surface, which is returned back from the surface. Its value varies depending on the extent of snow cover, vegetation and the soil characteristics. Average albedo values for the snow range from 0.7 to 0.8 and may be as low as 0.4 to 0.5 in case of wet or dirty snow. In the deserts that have light sandy soil and are without vegetation, surface albedo values are typically 0.4 to 0.5. Albedo of damp soil is usually less than that of the corresponding dry soil. In case of damp chernozem soils the albedo values may be as low as 0.05. Albedo of natural Earth surface covered with thick vegetation cover generally ranges from 0.1 to 0.25. Areas covered with coniferous forests have lower albedo than those covered with meadows.

The height of sun in the sky determines the absorption of radiation in the water bodies, mainly the oceans. When sun is relatively high, radiation reaches water surface at high angle. A large part of the incoming radiation penetrates upper layers of water body and is absorbed. When sun is low, the radiation reaches the water surface at low angles and most of it is reflected. Thus it does not penetrate much and the albedo value of water surface increases sharply at low sun. However, in case of diffused radiation, albedo of water surface is much less variable and is about 0.1.

(b) Albedo of Earth-atmosphere system: It is more complex in nature than that of Earth’ surface. Its value is largely determined by the presence-absence, nature and thickness of clouds. In the absence of clouds in the sky, albedo of Earth-atmosphere system depends largely on the albedo of Earth’s surface. If clouds are present, a large portion of solar radiation reaching atmosphere is reflected back from the upper surface of clouds and albedo value of system increases. Albedo of clouds is usually 0.4 to 0.5. In the presence of clouds, albedo of Earth-atmosphere system is usually greater than that of Earth’s surface, except where surface is covered with relatively clean snow.

1.2Effective radiation

It is the difference between the amount of Earth’s radiation from Earth’s surface and the amount of long-wave counter-radiation from atmosphere. Most important factor connected with effective radiation is ‘green-house effect’ due to presence of atmosphere. The atmosphere has various gases viz. carbon dioxide, methane, water vapor etc. which selectively absorb long-wave radiation. Due to this Earth’s atmosphere is relatively more transparent to short-wave radiation than to long-wave radiation. Since long-wave radiation from the Earth’s surface is trapped in the atmosphere, average effective radiation from Earth’s surface as a whole is much lower than the short-wave radiation absorbed at the surface.

The effective radiation of Earth’s surface largely depends on the temperature at Earth’s surface, atmospheric humidity and clouds. Experimental data has shown that radiation of Earth’s natural surfaces is generally quite close to the radiation of black body at corresponding temperatures. Further, a significant part of long-wave radiation lost from Earth’s surface is compensated by long-wave counter-radiation from the atmosphere. This counter-radiation mainly depends on the amount of atmospheric water vapor i.e. air humidity and clouds and so these factors affect the effective radiation of Earth’s surface.