|United Nations University - Work in Progress Newsletter - Volume 15, Number 2, 1998 (UNU, 1998, 12 pages)|
By Arie S. Issar
Could global warming have helped the Saracen armies take Jerusalem in 638 AD? This was one of the provocative thoughts advanced by Arie S. Issar, a water resources specialist, at the first lecture to be given at the UNU's new Global Environment Information Centre in November 1996. The whole issue of climate change is an integral part of the world's future water problems - with scenarios predicting lessened rainfall in parts of the world, increases elsewhere.
The environmental scientist can bring a new dimension to history. Employing the scientific tool of isotopic deterioration, in water science research for UNESCO, Professor Issar looked at past warming and cooling periods around the Sea of Galilee (Lake Tiberius). Heavier isotope composition in rock core carbonates, signalling the onset of warmer, drier temperatures, was found to date to the years just before the eastern Byzantine empire declined as Arab armies swept into the Middle East, in the 7th century AD. Archaeological records indicate declining Byzantine settlements at the same time - suggesting that the onset of desertification may have helped the Muslim conquerors in their Palestinian campaign.
This blend of chemistry, archaeology and history can provide valuable insights into the potential impact on society of climate change. When applied to China, for example, it indicates periods of warm and humid weather which opened up more land there to agriculture. This leads to the intriguing notion that global warming might, on balance, have a positive effect on China's capacity to grow food. In the Middle East, however, the greenhouse effect would likely cause a further drying up of already severe water shortages. Professor Issar is with the Water Resources Center at Jacob Blaustein Institute for Desert Research, Ben Gurion University of the Negev, Israel. The following article is based on his lecture "Climate Changes: Is It a Positive or Negative Process?" which is presented in Environmental Change and the Mediterranean (UN University Lectures 16,17), published in 1997. - Editor
During the research carried out for UNESCO, I adopted the principle of "the past is a key for understanding the future." I used data related to changes in the natural as well as socio-economic environment during the past 10,000 years.
Our research involved interpretation of isotopic deterioration -which is used to determine radiometric ages and conditions of formation. The data were taken from lacustrine carbonates in a core sample taken from the Sea of Galilee, along with stalagmite samples taken from caves in the Galilee of northern Israel. We further looked at archaeological data from a detailed study of Negev desert. Studies on the palaeo-levels of the Dead Sea and Mediterranean Sea were also incorporated.
Most importantly, we found that the information from the scientific analysis - both from the Sea of Galilee and the caved stalagmites - correlated with archaeological data derived from a detailed study of an area of 100 square kilometres in the plain of Beer-Sheva and Arad. The major conclusion is that climatic changes were the primary factors deciding the desertification of the semi-arid part of Israel (and not, as some historians have thought, the ravages of invading armies over the centuries.)
The Fall of Judaea
Around 700 BC, in the early Iron Age, the isotope composition becomes heavier. This may signal a new aridization phase, but cannot be correlated with the number of settlements. The decline in settlement only becomes obvious somewhere around 500 BC. At that time in history, the first kingdom of Judaea terminated, followed by a partial abandonment of the Negev. It is not clear whether this was due to the onslaught of conquering armies from Mesopotamia or to aridization.
Throughout the centuries of Byzantine rule in this part of the world, the isotopic readings suggest a cooler, more temperate climate. But then around 600 AD, our samples again record the isotopic composition becoming heavier - suggesting another period of desertification at about the time the Muslim forces began their conquests (Jerusalem fell to Islamic armies in 638 AD).
The Little Ice Age
From the isotope curves of the Sea of Galilee, another depletion can be discerned between about 1300 AD and 1500 AD, indicating another cooler, wetter period. This could correspond with the time of the Crusaders as well as to the later Little Ice Age, a period of colder weather from the 14th-17th centuries which brought shorter growing seasons in Europe and wreaked agricultural havoc. There is a problem in dating, however, for such short periods. These changes were not reflected in the resettlement of the Negev.
The Middle East and East Asia are linked climatically by the global weather systems that create monsoons. Warm and moist oceanic air masses are carried eastward to become the monsoons so essential to agriculture in that part of the world. Today, south-eastern summer monsoons dominate most of China.
Chinese Weather and History
We see similar correlations between climate changes and historical events in China to those we witness in the Middle East. In particular, weather in China appears to be connected to migrations within China. Chinese scientists have also attempted to link historical data, information from literature on Chinese lake evolution, the history of agro-cities in the Chinese deserts, and flood reports of the Yellow River.
The main migratory events in China coincided with social and political unrest, as well as invasions from the north by the Mongols. And five such migrations were linked with periods of severe climatic change, bringing cold weather.
The first period of migration observed was about 1000 BC, as nomads from the north-west moved eastwards, overthrew the Shang rule, and established the Zhou (Chou) dynasty. This migration could have been set in motion by the advance of glaciers and the snow lines to lower altitudes in the mountains of Tibet. Lakes in western and northern China became more saline and sand dunes appeared.
During cold and dry periods, the agro-husbandry boundary shifted southward.
In 300-500 AD, the boundary was two to four hundred kilometres south of the former border. These were the centuries following a northern invasion by the Huns, when China was divided between northern and southern dynasties. This was followed by warmer and more humid periods, as the Chinese kingdom was reunited and prospered.
Wet in China, Dry in Levant
When we correlate the data from the two regions, it can be seen that the cold, dry periods in eastern Asia correspond with the cold, wet periods in the Mediterranean. The reverse is true: warm and wet in China was warm and dry in the Middle East. When it began to get colder in China, around 1000 BC, it was beginning to get warm and dry out in the Middle East. Later, the dry and warm Muslim period in the Levant was warm and humid in China.
These observations and conclusions are in agreement with other results obtained by global climate simulation runs -indicating that the heaviest monsoons occur during interglacial conditions. Yet there is a price that the people of eastern Asia have to pay for the "good" monsoon years - the loss of life and damage caused by floods.
The main conclusion from this research - as it concerns the Middle East - is that the predicted global warming will cause the further drying up of the region. This would aggravate, most probably, the already severe problems of water supply - and could endanger the flimsy socio-economic balance in the region. To reverse the existing foreboding scenarios in the Middle East, scientists and engineers have to advance new and non-conventional methods for developing and managing water resources.
Ironically, the same greenhouse effect could be positive for China's agriculture. It seems that the climate change may have a positive impact especially on countries with monsoonal climates. But precautions must be taken in time to reduce the damage inflicted by floods and use the water in a positive way. The majority of the agricultural population of the world lives in these countries - thus, if the surplus of water is turned into a beneficial resource, the greenhouse effect may have its positive effects. What is truly needed, finally, is to stimulate non-conventional interdisciplinary innovations in all that relates to the development, management and methods of the use of water regarding the possible impacts of climate changes.