Unit 11: Climate , Agriculture And Society
Climate , Agriculture And Society
In both East and West, the transition from old concepts has come through the enhanced capacity for scientific observation that has been achieved.
It has been realised that changes in the energy received from the sun may be of sufficient magnitude to exert a major bearing on climate.
These have a periodicity that is reflected in the pattern of climatic events associated with ice ages.
The present historic period follows the most recent ice age but is the period in which agriculture was initiated during the Climatic Optimum.
Short term fluctuations in the output of solar energy have only minor effects on climate.
But, long term fluctuations have had profound effects.
In China, changes have been observed to occur roughly at intervals of two centuries, and it was obvious from numerous records in the West that there are periodicities that are roughly two centuries in length in climatic tre
These changes cannot be regarded as fixed, they are of an approximate order.
But, we note that Chinese historians observed such periodicities and because Chinese thought was fixed on the past and on ancestral influence on patterns of behaviour, its historians were inclined to regard human existence
In China, the coincidence of periods of social instability, barbarian invasion, plague and pestilence occurred at times when Asia was dry and the continental influence pushed back the monsoons so leading to crop failures
The patterns of change in world climate have been quasi-periodic, not random.
The changes have affected agriculture, as has been substantiated from the coincidence of major changes during the period -8000 to 0 between climate and agricultural development.
There have been periods such as those highlighted by Bell when the coincidence of adverse climatic conditions had a catastrophic effect in social terms upon Empires that had been flourishing for long periods over extended are
These effects had significant impact upon such agriculturally based communities because their prosperity was closely dependent upon the supply of food.
Effects socially did not coincide exactly but were delayed, sometimes in the order of several decades.
In our own era, the rise and fall of civilisations in such far distant areas as Cambodia and Southern Mexico has reflected changes in climate of an intermittent character which can be related to the location and intensity of the intertropical convergence zone.
These are attuned to the occurrence of cold periods in higher latitudes and so reflect changes in the pattern of the Earth's heat distribution.
Similarly, countries situated astride the tropical and temperate zones, such as China and Australia, are greatly influenced by changes in the location of fronts between masses of cold and monsoonal air.
Such shifts may be for long periods of time (of the order of one or two centuries, but sometimes of millennia, as in the Climatic Optimum).
"Wobbles" in the circumpolar vortex influence the position of such fronts.