Environment of Earth

May 24, 2015

Is Global Warming a scientific reality or hoax?

Filed under: Climate,Environment,Global warming — gargpk @ 2:26 pm

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April 12, 2012

CLASSIFICATION OF CLIMATE

Filed under: Climate,Environment — gargpk @ 10:43 pm
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The climate is most important factor controlling the environment. The type of soil and the native vegetation in a given region are basically the product of the climate of the area. The scientific study of climate requires a scheme of climate classification. Such schemes aim to categorize all the variations in the climates found in different areas into several clearly defined and easily distinguishable groups. Many useful systems of climate classification can be devised by taking different weather elements such as temperature, pressure, winds, precipitation as the basis of classification. the distribution of natural vegetation and soils may suggest still other types of classification schemes.

Temperature as basis of climate classification
The general parallelism of isotherms with parallels of latitude was perhaps the first basis of climate classification. With intelligent application, temperature has been made a fundamental factor in most of the schemes. Three major climate groups are clearly recognizable on the basis of this criterion:

  1. Equatorial tropical group:  characterized by uniformly warm temperature throughout the year and absence of a winter season.
  2. Middle-latitude group:  characterized by alternating summer and winter seasons.
  3. Polar-arctic group:  characterized by the absence of true summer season.

A common boundary between polar-arctic climates and middle-latitude climates is 10 degrees celsius (50 degrees F) isotherm of the warmest month (i.e. july in northern hemisphere). The boundary between middle-latituse and equatorial climates is marked by 18 degrees celsius (64.4 degrees F) isotherm of the coldest month.

The use of temperature alone as the basis of climate classification is unsatisfactory because humid and desert regions are not distinguished in such scheme.

Precipitation as basis of climate classification
The climate map in such a scheme would be same as the mean annual rainfall map. Such system may be refined by subdividing classes according to distribution of precipitation throughout the year, whether uniform or seasonal. However, such classification scheme fails because it groups cold arctic climates together with hot deserts of low latitudes controlled by air temperature. The cold climates, in general, are effectively humid with same meager precipitaion that produces very dry deserts in hot subtropics and tropics.

Vegetation as basis of climate classification
Different plant types require special condition of temperature and precipitation for their survival. Thus, plants form an index of climate and limits of growth of key plant types provide meaningful boundaries of climstic zones. There is much merit in such vegetational zones based climate classificstion scheme. However, vegetation is an effect rather than a primary cause of climate. Therefore, it can not give so satisfactory a climate classification as the one based on the primary cause(s) of climate.

Koppen climate classification system
After various attempts of classification schemes based on some single characteristic feature, it became evident that a meaningful system should devise climate classes that combine temperature and precipitation characteristics but also set limits and boundaries that fit into obviously known vegetational distributions. Dr. Wladimir Koppen (1918) devised such a system that was later revised and extended by him and his students. this Koppen climate classification system has become most widely used system for biogeographical purposes.

Koppen system is strictly empirical in nature. Each climate group is defined according to fixed values of temperature and precipitation, computed according to averages of the year or of individual months. No attention is paid to the causes of climate in terms of pressure belts, wind belts, air masses, air fronts or storms. A given place can be assigned to a particular climate subgroup solely on the basis of the records of temperature and precipitation of that area provided that the records are long enough to yield meaningful averages. Since air temperature and precipitation are most easily obtainable surface weather data, the system has great advantage that the areas covered by each subtype of climate can be computed or estimated for large regions of the world. This system thus, incorporates an empirical-quantitative approach.
Koppen system has a shorthand code of letters disignating major climate groups (A, B, C, D), subgroups within major groups(S, W, f, m, s, w) and further subdivisions to distinguish particular seasonal characteristics of temperature and precipitation (a, b, c, d, h, k).

Air mass source regions and frontal zones as basis of climate classification

Increasingly detailed studies of vsrious charateristics of weather elements, global circulation, air mass properties, source regions and cyclonic storms have yielded many principles related to the causes of weather patterns and their seasonal variations at global scale. Therefore, such knowledge has also been incorporated in the explanatory-descriptive system of climate classification based on the cause and effect. Thus, such system is based on the location of air mass source regions and nature and movement of air masses, fronts and cyclonic storms. Koppen code symbols have been incorporated into this system by interrelating both systems. This system simply provides a reasonable scientic explanation for the existence of Koppen’s climate groups.

June 7, 2011

Global maps from NASA Earth Observatory

Filed under: Climate,Environment — gargpk @ 2:54 am
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NASA satellites give us a global view of what’s happening on our planet. To explore how key parts of Earth’s climate system change from month to month, click on the following link and see the various global maps.

http://earthobservatory.nasa.gov/GlobalMaps/

March 9, 2008

CLIMATE THEORY AND ANALYSIS OF CLIMATE

Filed under: Climate,Environment — gargpk @ 3:00 pm
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The global climate is the result of innumerable interactions occurring amongst components of the global environment, chiefly amongst hydrosphere (particularly the oceans) and the atmosphere. These interactions result in particular meteorological conditions and spatial average fields of these conditions produce climatic regimes. The purpose of climate theory is to identify the average distribution of meteorological elements in space and time as well as their responsiveness to external factors. Numerical models of climate are devised for this purpose, which make it possible to calculate average fields of meteorological elements. With the development of high-power computers, it has also become possible to design numerical models that can reproduce non-average fields of meteorological elements and calculate such fields for prolonged time intervals thus allowing calculation of average fields describing climatic conditions. Various theoretical/numerical models have been developed in the past half a century. These allow description and/or study of present climate as well as changes in climatic conditions produced by natural or anthropogenic activities. For example, numerical model of climate given by Manabe and Bryan (1969) shows the influence of the circulation of the ocean waters on climatic conditions. The model given by Holloway and Manabe (1971) shows the distribution of basic components of heat and water balance at the Earth’s surface similar to that actually present on Earth. As has been pointed above, the numerical models of climate can be employed for the study of current climate as well as climatic changes. However, the models used to study climatic change have to meet more rigorous requirements than the models used to study current climatic regime. The important requirements for such models are:

  1. Model should not include empirical data concerning the distribution of individual elements of climate, particularly those that change substantially during the changes in climate.

  2. Model must recognize realistically all types of inflow of heat that influence the temperature field appreciably; in particular, the law of conservation of energy.

  3. Model must include major feedback relations among various elements of climate.

The third requirement is most important and has been discussed below.

Feedback Relations Among Elements Of Climate

Feedback relations among climatic elements are highly complex and interrelated. Such relations include both negative and positive feedback relations.

Negative feedback relationships

These relationships reduce the anomalies of meteorological elements and contribute to the approximation of the values of these elements towards their climatic normals. Thus, the climatic stability is maintained by negative feedback relationships among climatic elements. Major such negative feedback relationships are:

1. Long-wave radiation and temperature at Earth’s surface: Intensity of long-wave radiation increases with increase in temperature at Earth’s surface. This produces a greater expenditure of heat energy, which in turn inhibits further increase in temperature.

2. Heat transfer in atmosphere and air temperature gradient: The usual flow of heat into the atmosphere from a zone of higher temperature produces a smoothing process that eliminates the differences in temperature distribution.

Positive feedback relationships

Such relationships among climatic elements play major role in climatic change since these increase the anomalies in meteorological elements. Thus, positive feedback relationships reduce the climatic stability. Major such relationships are:

  1. Absolute air humidity and air temperature: Absolute air humidity increases with rise in air temperature. With rise in temperature, evaporation increases leading to comparative constancy of relative humidity in most climatic zones (except in dry continental regions). Increase in absolute air humidity decreases long-wave radiation. Thus, the increase in absolute air humidity with rise in temperature partly compensates the increased long-wave radiation attributable to increased temperature. Manabe and Wetherald (1967) showed that influence of the change in solar constant on air temperature at Earth’s surface is almost twice in the condition of constant relative humidity as compared to the condition when absolute humidity is stable. This particular feedback relationship is important in numerical models of thermal regimes employed in studies of climatic changes.

  2. Snow and ice cover and albedo of Earth’s surface: Positive feedback relationship between snow and ice covers on the albedo of Earth’s surface plays a very great influence on the patterns of changes in atmospheric thermal regimes. Ice or snow covers have high albedo and so reduce air temperature above them and climatic changes are intensified by formation and melting of ice. Available data shows that during summer months, albedo over ice cover in Central Arctic is about 0.7 while in Antarctica, it is about 0.8 to 0.85. In regions free of ice and snow, albedo of Earth’s surface does not exceed 0.15. This indicates that other conditions being same, snow and ice covers reduce the radiation absorbed by Earth’s surface by several times.

  3. Snow and ice covers and Earth-atmosphere system: The data shows that albedo of Earth-atmosphere system during summers in Central Arctic region is 0.55 and in Antarctic region is about 0.6. This is approximately twice the value of the estimated albedo for the planet as a whole which is about 0.33. Such large differences in values of albedo must exert considerable influence on the atmospheric thermal regime.

  4. Air temperature and Earth’s albedo: Ice and snow covers are created at the Earth’s surface due to reduction in air temperature. These covers cause a sharp decline in the absorbed radiation. This contributes to a further reduction in Earth’s temperature and consequently further increases the area under snow and ice. The reverse process may be effected by increases in temperature, which results in melting of ice and snow. Budyko (1968) showed that inclusion of this feedback relationship into a numerical model of atmospheric thermal regime invariably always exerts a very substantial influence on the distribution of air temperature at Earth’s surface. This influence can be shown by a simple example, which shows how average global temperature will change if Earth’s surface becomes fully covered by snow and ice and clouds in the atmosphere are absent. In such a condition, Earth’s albedo will change from its present value of 0.33 to the value for dry snow cover i.e. 0.8. Thi
    s increase in albedo will reduce the air temperature. The absence of clouds will further reduce the temperature of lower layers of atmosphere near Earth’s surface. In the present times, the average temperature in lower layers of atmosphere rises substantially almost everywhere at the Earth’s surface. This is due to greenhouse effect associated with absorption of long-wave radiation by water vapour and carbon dioxide present in the atmosphere. However, at very low temperature resulting in the formation of snow and ice covers at Earth’s surface, green-house effect would become insignificant and dense clouds which perceptibly change the radiation flows are not formed. Under such conditions, atmosphere will become more or less transparent to both short-wave and long-wave radiation. The average temperature of the Earth’s surface for such transparent atmosphere is determined by the formula

4/So(1- s) /4

where So = Solar constant; s = albedo of Earth’s surface; = Stefen’s constant.

The above formula shows that Earth’s average surface temperature at albedo value of 0.8 will be _ 87o C (186o K). Thus, the Earth’s average temperature will decline by approximately 100o C from its present value of 15o C if ice or snow were to cover the entire Earth even for a short period. Thus, the enormous influence of snow cover on the thermal regime of Earth becomes quite clear.

A number of studies have attempted to calculate the influence of sea polar ice on Arctic thermal regime. These studies are based on the available data of thermal balance in central regions of Arctic Ocean and approximate values of the proportions derived from a semi-empirical theory of climate. It has been established that polar ice reduces average air temperature in the Central Arctic during summer months by several degrees and by about 20o C in winters. It has been concluded from these studies that the Arctic Ocean could be free of ice in the present age. However, this state would be extremely unstable and it could develop an ice cover as a result of a relatively small change in climate.

Since a permanent ice cover exerts a substantial influence on the atmospheric thermal regime even when it covers only a small part of the Earth’s surface, this must be taken into account in studies of climatic changes.

Present-Day Climate

The climatic conditions of present century have been established on the basis of meteorological data collected from worldwide network of climatic stations. The data shows that elements of meteorological regime change perceptibly over time. These changes are both periodic (daily or yearly) oscillations as well as non-periodic oscillations of different time intervals.

Short interval non-periodic changes (of days or months) occurring in meteorological regime determine the oscillations in weather. These changes are not spatially homogeneous and are largely explained by the instability of atmospheric circulation. For longer time intervals (of several years), irregular oscillations of individual elements of meteorological regime occur along with long-term changes that are similar over large territories. Such changes characterize fluctuations in climate.

Since climatic fluctuations at present time are relatively modest, average values of meteorological elements over a period of several decades could be used in order to describe the climatic features of the present age. The use of such average values makes it possible to exclude the influence of unstable atmospheric circulation pattern. Following Table shows average temperature in January and July and also the average yearly atmospheric precipitation at various latitudes.

Data show that difference between average temperatures at Earth’s surface at various latitudes is almost 700 C. The Earth’s surface temperature is maximum at Equator and lowest at South Pole. Earth’s spherical shape exerts a substantial influence on the distribution of these temperatures by producing variations in the total radiation reaching the upper boundary of atmosphere. Further, permanent ice covers are found at high latitudes where air temperature does not rise above freezing point almost throughout the year. Apart from substantial changes in meridonial direction, average air temperature at Earth’s surface also changes substantially in most latitudinal zones at various longitudes. This is largely explained by the distribution of continents and oceans.

The influence of ocean’s thermal regime extends to a large part of the surface of continents on which maritime climate exists. This influence is characterized by relatively modest yearly oscillations in air temperature at middle and high latitudes. The amplitude of yearly temperature fluctuations increases sharply in those extratropical continental regions where influence of oceanic thermal regime is less pronounced, characterizing the continental climate.

The distribution of average latitudinal precipitation values produces a pattern in which the principal maximum value occurs in the equatorial zone, total precipitation declines at subtropical latitudes, two secondary maxima lie at middle latitudes and precipitation declines in polar latitudes. Changes in average precipitation at different latitudes are explained by the distribution of average air temperatures and by specific characteristics of atmospheric circulation. Other conditions being equal, total precipitation increases with temperature because it increases the volume of atmospheric water vapour. Vertical air currents that carry water vapour through condensation level producing clouds also have important role in precipitation.

The atmosphere’s overall circulation is closely associated with geographical distribution of stable pressure systems. Particularly important such systems are:

  1. low-pressure belt near the equator,

  2. high-pressure region at high tropical and subtropical latitudes

  3. the region of frequent cyclone-formations at middle latitudes.

Downward movements of air within high-pressure zones substantially reduce the precipitation. On the other hand, pronounced upward air movements increase the precipitation at equatorial latitudes and in several regions at middle latitudes.

Largest desert areas on Earth having negligible precipitation are found in the subtropical high-pressure zone. Total precipitation also declines in continental regions at middle latitudes, which are distant from ocea
ns because very small quantity of water vapour carried by air currents from oceans reaches these regions.

Thus in the continents, zones of humid climates are largely located at equatorial latitudes and in regions of maritime climate, at middle and high latitudes. Similarly, at high tropical and subtropical latitudes and in regions of continental climates, conditions of insufficient moisture prevail.

Table : Present-day latitude-wise air temperatures and precipitations

Latitude      January temperature (0C)        July temperature (0C)           Precipitation(cm/year)

90-80 N/S             -31/-13                                      1/-42                                        19/11

80-70 N/S             -25/-8                                        2/-30                                       26/25

70-60 N/S             -22/0                                       12/-12                                        52/67

60-50 N/S            -10/5                                        14/1                                           80/101

50-40 N/S             -1/12                                       20/8                                          75/108

40-30 N/S            -11/20                                     26/14                                         77/103

30-20 N/S             19/25                                     28/18                                         73/91

20-10 N/S             25/26                                     28/24                                      114/122

10-0 N/S               27/27                                     27/26                                       201/150

Stability Of Climate

Budyko (1968) first used a semi-empirical model of atmospheric thermal regime to study the single-valued character and stability of current global climatic regime. The study using distribution of average air temperatures among different latitudes showed that the present climate is not the only possible one for existing climate-forming factors. Aside from the existing climate, current external conditions may produce a climate corresponding to a ‘white Earth’ as well as other variants of climate. Relatively small changes in external climate-forming factors may greatly alter the existing climate. Numerous subsequent studies employing similar models of climate have confirmed this conclusion.

The study of Wetherald and Manabe (1975) about the stability of existing climate is especially interesting. Unlike studies using semi-empirical models of t
he distribution of average air temperatures among different latitudes, their study applied a three-dimensional model of a general theory of climate that includes a detailed consideration of dynamic processes occurring in the atmosphere. The model takes into account the influence of state transformations of water on thermal regime including the feedback relationships between the snow cover, polar ice and air temperature. The study showed that if solar constant increases by more than 2%, the average yearly air temperature shall increase by approximately 2o C at low latitudes, more at higher latitudes and by about 10o C at 80o N. The ice cover on Earth shall be reduced by such rise in average yearly air temperature. The conclusions of this study are similar to those obtained by using semi-empirical models of thermal regime of atmosphere. However, the calculations from such models show slightly larger changes in average yearly air temperature following increases in the solar constant by two percent i.e. 3o to 4o C at low latitudes and 12 to 14o C at 80o N.

Thus, it may be concluded that the contemporary climate of Earth is neither unambiguous nor highly stable. It is highly sensitive to small changes in inflows of heat arriving at the upper boundary of atmosphere.

References

Budyko, M.I. (1968) On the origin of Ice Ages. Meteorologiya I gidrologiya No. 11.

Holloway, J.L. & Manabe, S. (1971) A Global General Circulation Model with Hydrology and Mountains. Monthly Weather Review Vol. 99, No. 5

Manabe, S. & Bryan, K. (1969) Climate Calculation with a Combined Ocean- Atmosphere Model. Journal of the Atmospheric Sciences. Vol. 26, No. 4.

Manabe, S. & Wetherald, R.T. (1967) Thermal Equilibrium of the Atmosphere with a Given Distribution of Relative Humidity. Journal of the Atmospheric Sciences Vol. 24, No. 3.

Wetherald, R.T. & Manabe, S. (1975) The Effect of Changing the Solar Constant on the Climate of a General Circulation Model. Journal of the Atmospheric Sciences Vol. 32, No. 11.

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.

Climatic indices and biogeographical zones


Radiation balance (source of heat energy)

Radiation index of dryness (moisture conditions)

Less than 0 (Highly excessive moisture)

Between 0 and 1.0

Excessive moisture

Optimal moisture

Moderate dryness

Dryness

Excessive dryness

0 – 0.2

0.2 – 0.4

0.4 – 0.6

0.6 – 0.8

0.8 – 1.0

1.0 – 2.0

2.0 – 3.0

> 3.0

< 0 (High altitude)

I

Eternal snow

0 – 50 kcal/cm2/yr (South Arctic, sub-arctic and moderate latitudes)

IIa

Arctic desert

IIb

Tundra (with islands of sparse forests in south), bow forests in swampy areas

IIc

Northern and middle taiga

IId

Southern taiga and mixed forests

IIe

Deciduous forests and forest-steppe

III

Steppes

IV

Semi-deserts in temperate belts

V

Deserts in temperate belts

50 – 75 kcal/cm2/yr

(Subtropics)

VIa

Regions of subtropical semi-gilei with large swamp areas

VIb

Subtropical rain-forests

VII

Hard-leaved subtropical forests & brush, deciduous forests

VIII

Subtropical semi-deserts

IX

Sub-tropical deserts

> 75 kcal/cm2/yr

(Tropics)

Xa

Regions of equatorial forest swamps

Xb

High humidity equatorial forests with major swamps

Xc

Moderate humidity equatorial forests with moderate swamps

Xd

Equatorial forests, passing to light lropical & deciduous savannahs

XI

Arid savannahs, deciduous forests

XII

Semi-desert savannahs (tropical semi-deserts)

XIII

Tropical deserts