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.

CAUSES OF CLIMATE CHANGE

Until recently studies of 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 which 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.

Continuos 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 astronomic changes occur in periods of tens of thousands of years. By comparison with 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 and Vashishcheva (1971) studied the climatic conditions during glaciation periods using a model describing the distribution of average latitudinal temperature in different seasons 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 of increased temperatures which attained a maximum in 1930s. From the studies of reasons explaining climatic change during this period it appears that increase in temperature has been caused by an increase in transparency of stratosphere resulting in increased inflow of solar radiation (i.e. increased meteorological Solar constant) entering the troposphere. This 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 the stratospheric aerosol and on the 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 transparency of stratosphere during first half of this century have been associated with the regime of volcanic activities and in particular with changes in the inflow of the products of volcanic eruptions (including sulphur dioxide) into stratosphere. There are grounds to believe that during last 3 to 4 decades, changes in climate have also begun to depend, atleast in some measure, on human activities.

HISTORY OF CLIMATE CHANGES ON EARTH

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. 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 extending over a large part of the land area which 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 Permocarboniferous 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 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 whose 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. Subsequently temperature increased once again and the last large-scale glaciations in Europe and North America disappeared between 5,000 to 7,000 B.C. At that time, post-glaciation temperature increases reached a maximum. 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 that 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.