MECHANISMS OF CLIMATIC CHANGE

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

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.