An Encyclopaedia of Indian Archaeology v1.c12.s3: 7.3 Palaeoclimate
An Encyclopaedia of Indian Archaeology v1.c12.s3
- Language
- English
7.3 Palaeoclimate
v1.c12.s3.1
The physical framework of the climatic system comprises the atmosphere, the hydrosphere, the cryosphere (ice masses and snow deposits), the lithosphere (land masses) and the biosphere (plants, animals, man and their habitat). The energy which drives this system comes from the sun and is intercepted by the earth and its atmosphere. This radiation interacts with clouds and air, land and sea, snow and ice, while the remainder is reflected back into space. Winds and ocean currents are generated by the forces set up by these complex interactions, which in turn are responsible for converting the sun's radiation into different energy forms and ultimately into long-wave terrestrial radiation which is emitted back into the outer space. ‘Weather’ is the end result of these complex activities when viewed on short time-scale, and ‘climate’, when the time-scale is long.
v1.c12.s3.2
During the life span of 4500 million years of the earth it has experienced several ice ages, each lasting several million years. In the intervening periods there were practically no polar ice caps. The most recent Ice Age started around 1·8 million years and still continues. During this period ice sheets over the poles and high mountains have extended and contracted in complicated patterns. Colder periods are termed ‘glacials’ and warmer ones ‘interglacials’. At the peak of the Last Glacial the ice sheets extended as far down as 40° N. in N. America and 52°30′ N. in Europe and, as a result of locking up of vast amounts of water in these ice sheets, the sea-level went down to 100 to 150 m below the present level.
v1.c12.s3.3
Due to various uncertainties involved the estimates for each interglacial vary from 5000 to 25000 years. The Last Glacial began around 120000 and reached its peak around 20000 B.P. and ended by 10000 B.P.—the onset of the Holocene [9.6]. Broadly speaking, the climatic cycles in the two hemispheres were in phase. It has been estimated that in *c.* 18000 B.P. the land-based ice sheets in the n. hemisphere reached a thickness of 3 km. In the s. hemisphere there was a corresponding increase in the sea ice. The surface waters cooled to a global average of 2·3 C. Corresponding to these glacial maxima, the tropics registered an intensely arid phase reflected in the lake levels of Africa. The early Holocene interglacial warming of the n. latitudes is matched by an increase of precipitation by 150% (over the 1931-60 mean) between 24° N. and 8° S. in Africa.
v1.c12.s3.4
The climatic changes are generally ascribed to the changes in the eccentricity of the earth’s orbit, obliquity of the axis and precession of the equinoxes, and to some extent to the ‘sun-spot cycle’. Recent searches on the marine sediments do reflect periodicities of 23000 years (corresponding to precession index), 42000 years (obliquity of the axis) and 100000 years (orbital eccentricity) which have given added support to Milankovitch’s original astronomical theories of climatic change. Considerable controversies however still exist in this field.
v1.c12.s3.5
Manifestations of climatic change are multifold. Changes in sea-level, extent of former ice sheets, lake levels, fluvial sediments of rivers, extent of deserts—all reflect changes in palaeoclimate. These are also manifested in the increase/decrease of cold- warmth-loving flora and fauna in the geological terrestrial and marine sediment records [9.8.1]. For example the cooling of the Pleistocene is marked by the disappearance of discosters and the appearance of cold-loving species like *Globorotalia truncatulinoides* and *Globorotalia menardii* complex (sinistral) at about 1·9 million years. There is a general correlation between the interglacial phases of the n. and the pluvial phases of the tropics. This can also be detected in the changes in the pollen record. For example the onset of deglaciation in Toshmaidan, Kashmir, is datable to *c.* 15000 B.P. and the advent of the early Holocene pluvial is marked in the pollen records of Rajasthan salt lakes at *c.* 10000 B.P. [7.4].
v1.c12.s3.6
Ericson, D.B., & Wollin, G., 1968, in *Science*, 162, pp. 1227-34 (for Pleistocene climates and deep-sea sediments). Frenzel, B., 1973, *Climatic Fluctuations of the Ice Ages*, London. Hays, J.D., Imbrie, J., & Shackleton, N.J., 1976, in *Science*, 194, pp. 1121-32 (for earth's orbit and ice ages) Lamb, H.H., 1971, in special issue of *Palaeoclimatology*, 10, 2-3, pp. 83-6. Lamb, H.H., 1972, *Climate: Present, Past & Future*, 1, London. *Report of a Committee on Climatic Change*, 1976, Australian Academy of Science, Canberra. Somayajulu, B.L.K., 1977, in Agrawal, D.P., & Pande, B.M., ed., *Ecology & Arch. of w. India*, pp. 5-16. Street, F.A., & Grove, A.T., 1976, in *Nature*, 261, pp. 385-90 (for late Quaternary lake-level fluctuations in Africa).—D.P. AGRAWAL