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I saw at one time ?a leaflet that asked people to come together in stopping climate change. It seems that many are not aware that the climate changes all the time and that the change is not stoppable. Climate changes, however, differ in their timing and magnitude and are a result of many factors, such as the distance between the sun and the equator, which contributes to the heat budget of the Earth, and the difference in the temperature of the equator from that of the cooler poles due to deviations in Earth's orbit, or variations in solar radiation.

The differences in temperature lead to air currents which in turn influence rainfall. On the scale of tens of thousands of years, the Earth experienced numerous episodes of glacial cooling, alternating with warmer intervals. Following the last major glaciation which began 110,000 years ago, a transition to warmer conditions from 16,000 to 11,500 years ago was characterized by frequent climatic oscillations. Bands of foragers in climatically sensitive habitats, such as semi-desert regions in Southwest Asia, North Africa and China responded with a variety of social and food-extractive technologies. These included intensive utilization of wild grasses and managing animal games, the manufacture and use of grinding stones, trapping, the use of bows and arrows, as well as food preservation. While some continued to elaborate their hunting gear, others settled down to maximize the gain from wild grain resources. The most successful groups lived in the Eastern Mediterranean where wild wheat and barley were abundant.

The Invention of Agriculture

From 11,600 to 8,200 years ago, the climate became warmer and in the Eastern Mediterranean, wetter. It was during this period that successive generations of foragers, who took advantage of the well-watered habitats, adopted farming as their dominant mode of obtaining food. This marked the most remarkable revolutionary achievement of humankind -- the invention of agriculture.

Life has never been the same since. Villages coalesced to form corporate village communities governed by councils or chiefs. Afterwards, conglomerates of farming communities merged into kingdoms, while those who managed cattle, sheep and goats became herders and roamed the rain-fed grasslands outside the river valleys preferred by farmers.

The effect of climate change on humanity under this new agrarian regime with its politically more complex organization assumed a new turn. This has been mostly due, in part, to the nature of the agrarian ecology and economic growth potential. Agricultural yields fluctuated annually, in part because of interannual variability in rainfall, but more importantly, they also varied responding to decadal and centennial changes in climatic conditions, which influenced both the flow of rivers and rainfall in the grasslands. These problems were tackled by digging irrigation canals to parched lands, drains to dispose of excess water and building dykes to protect fields and settlements from the ravages of floods.

Great Civilizations

As agrarian communities expanded, they developed into complex political States with a hierarchical management. State officials, clerks, and priests who deployed rituals and myths to promote and buttress the policies of the State led to an increase in the demand for greater food production. These demands were met by extracting tributes from the farmers who had to work harder and expand their fields to meet the growing demands of the State functionaries.

By the 5000 B.C. the early agrarian States had developed into the world's first great civilizations in Egypt, Mesopotamia, and India. But by around 4200 B.C., an abrupt turn of climate led to dramatic changes all over the world. On the banks of the Nile, the Egyptians had established a centralized State. Successive dynasties constructed imposing pyramids for four hundred years from 4600 to 4200 B.C., before a sudden, unanticipated series of reduced Nile flood discharge spelled disaster. The government collapsed. Famines ravaged the rural population, violence erupted and the whole country slipped into a state of chaos.

In Mesopotamia, early state societies emerging by the beginning of the fifth millennium B.C. depended heavily on irrigation to overcome the recurrent droughts and floods of the rivers Tigris and Euphrates. They invented the shaduf water lifting device, and used canals, drainage channels, weirs, dykes and reservoirs. Unlike Egypt, the soils of the floodplain were prone to salinization, which reduced productivity. The rulers resorted to warfare as early as 4,500 years ago, which ultimately led to the rise of the militaristic Akkadian Empire 200 years later. The Akkadians extended their rule over rain-fed regions to increase their income from tributary sources. However, the high expenses of keeping a military empire, the loss of productivity as fields and farmers were overworked, and a growing dependence on products from marginal rain-fed lands placed the Akkadian Empire at the risk of collapse from any internal or external perturbations. After no more than a century since its rise, the empire fell prey to three consequences of the global climatic event of 4,200 years ago.

First, the flow of the Tigris and Euphrates was vastly reduced, undermining the productivity from the valleys. Second, the yield from rain-fed farming suffered from droughts. Third, the Gutian nomadic tribes living in the Zagros mountains and suffering from droughts that affected pastures, took advantage of the weakening Akkadian Empire and its internal strife: they made travel unsafe, disrupted the economy and undermined tribute collection, thus depriving the empire from its vital resources.

Further East, in China, rice and millet cultivation became the dominant mode of subsistence since 11,600 years, due to abundant monsoonal rain. However, the droughts of 4,200 years ago led to the abandonment of settlements in the middle reaches of the Yangtze River as those rains were failing. Along the Yellow River, shifts in its course, as well as soil erosion as a result of climatic fluctuations and farming, contributed to the rise of hierarchical complex societies before they were threatened by the climatic cooling event in 4200 B.C., which not only caused droughts, but also led to a decrease in the number of frost-free days, thus hurting agricultural productivity. This is indicated by the decline of the Longshan culture in the Yellow River Valley around 4000 cal. yr BP, and the agrarian societies around Central China. It also seems that the droughts may have encouraged political integration and cooperation to overcome famines, leading to the emergence of the first dynasty to be described in historical sources, the Xia Dynasty of China (about 4,100 to 3,600 years ago) in the western area of Henan Province and southern Shanxi Province.

Since 4,200 years, many kingdoms and empires rose and fell, mostly as a result of the predominance of warfare to secure land and labour in order to expand production. More often than not, in the long run, internal rivalries, the costs of controlling vast territories inhabited by disgruntled heavily taxed peasants, as well the cost of endless military battles, led to a rapid turnover of dynasties.

By the first century A.D., many of the previous kingdoms and empires were overrun by the Roman Empire. But it was not long before this vast empire, almost global in its outreach, began to suffer from the same problems that had previously hastened the demise of earlier empires. Climate change during the third and fifth centuries A.D. provided the final blow. Here again, as in the case of the Gutians and the Akkadians, successive droughts led to the nomadic, horse-mounted archery attacks of the Huns on the Germanic tribes, who in turn attacked the Romans. The effect of climate change on the grasslands of the deserts and semi-deserts was again responsible for the rise of the attacks by the Mongols in the twelfth century A.D., whose ancestors now live in Mongolia, China, (Inner Mongolia), Russia, and a few other central Asian countries. Not only would that climatic instability encourage the Mongol nomads to attack settled communities, but it also weakened the settled kingdoms and empires making them vulnerable.

The climatic events that contributed to the expansion of the Mongols were global -- known as the the Medieval Climatic Anomaly. They had a profound effect on Europe during the medieval period. They were also influential in many other parts of the world, including North America. In Egypt, droughts from the ninth to the early part of the thirteenth century A.D., led to severe famines and political upheaval.

The end of the glorious Tang Empire of China (907 A.D.) is now believed to have been hastened by climate change. Historic shifts in the annual monsoon cycle in China may have pushed the Tang Dynasty into terminal decline: a prolonged drought and poor summer rains fuelled peasant uprisings that brought about the dynasty's end. Archaeologists found evidence of stronger winter monsoon winds leading to migrations of rain associated with the Intertropical Convergence Zone from 700 to 900 A.D. A similar climatic event contributed to the collapse of the Classic Maya in Central America.

It is important to stress that climatic change is only one of the many causes of the collapse of civilizations, and that perhaps more important factors are those related to how societies are governed. Perhaps the effect of climate change on the Tang Empire would have been averted had the aging Emperor Xuanzong not been complacent or indifferent to State affairs; or had he not appointed wicked chancellors who corrupted the political order and by 755 A.D. possibly unable to prevent the enemy troops to menace the empire??Is not a military imperial system vulnerable to climate change? Would a system without oppressive military rule evaded the rise of separatist forces during the eighth and ninth centuries A.D.? Finally, would a more equitable and charitable policy by the Tang Emperor towards peasants prevented their large-scale uprising in 859 A.D.?

Let us not single out climate change as the principal cause for the rise or collapse of civilizations. Then and now, climate change can be handled by systems of management which ensure that unjust actions do not impair the vitality of the natural habitat, that rulers do not overexploit the masses for the benefit of a few, and do not resort to military power to colonize or plunder the resources and peoples of other nations.

Just like many previous social systems that have become extinct, we are extremely vulnerable. Indeed, the situation is now worse because the economies of all nations are closely interconnected and because the cumulative effect of polluting the planet by industrialization over the last 200 years is now threatening a global climatic upheaval. The planet has also become overpopulated, overcrowded, and over-urbanized. Within the last 50 years, the demand for water, to single out one of our vital resources, has risen, depriving more than the one billion people from access to clean drinking water. In my opinion, there is no solution for resolving our global crisis without putting aside short-sighted, nationalistic policies, and financial enterprises and forging a global managerial institution. Such an institution would unify and coordinate the know-how, the financial and human resources of all nations, to rehabilitate our threatened environments, and promote and disseminate new, safe technologies to reduce hunger and poverty. Global warming -- to which industrial countries have heavily contributed -- is a wake up call, not just to the threats of climate change, but more importantly to the current deficiency in our political and social institutions and values.