And Global Warming that started in 900 lead to the collapse of the Byzantine Empire. To understand why that happened you first have to understand what that Empire survived the increase cold of the Dark Age Cold period.
The Roman Warm Period ended in the 200s, and that is the time period when the Roman Empire almost fell (and in many ways it, did, prior to 200, the Roman Empire was centered on Rome and what was later called the Latin Western Roman Empire, after the 200s, the Greek Speaking part of the Empire became dominant).
Anyway, during periods of declining temperatures, land became more and more marginal for agriculture and without food you can not maintain a rich urban life and a strong army. Furthermore, during such cold periods, people start to move south, leading to increase conflicts between peoples (Thus 300-600 is sometimes call the period of the Great migrations)
On the other hand certain areas actually boom during these cold snaps, do to increase rainfall, one of the biggest example of this is Asia Minor (Present day Asiatic Turkey). The reverse occurs during warm periods, these areas turn dry, again Asia Minor is the classic case of that.
In the 200s, Rome almost fell, various reasons for that failure are mentioned but it was the start of a Cold Period and Asia Minor and Syria boomed do to increase rainfall. This almost lead to the overthrow of the Roman Empire by the Syrian City of Palmyra and it lead to a strengthening of Persia, which became the first serious opposition to Rome since the Defeat of Hannibal in 202 BC and the subsequent defeat of Macedonia in 196 BC and Seleucid Empire in 188 BC.
https://en.wikipedia.org/wiki/Zenobia
The Palmyra revolt reached its peak in 271 AD, with almost a third of the Roman Empire under Palmyra control.
The Persian Sasanian Empire in 224 AD took over from the Parthians Control of Persia.
https://en.wikipedia.org/wiki/Sasanian_Empire
Thus the 200s saw a fundamental change in the world. Instead of a Latin Roman Centered Empire, that had been characteristic of the Roman Empire (and even the late Roman Republic), the Empire became more and more Greek and centered around Asia Minor. That center was next to its main rival, the Sassanian Persian Empire. Asia Minor, Syria, Iraq and Iran all saw increase food production, while you saw a decline in Gaul (modern day France), Spain and Italy. The Roman Empire survived the 200s, but by doing three things, first, it became Greek Dominated, Rome remain the Largest City in the World, but troops were raised in Illyricum (what use to be called Yugoslavia) and Asia Minor more then Gaul, Britain or even Italy.
Second, the Empire needed a central religion, Diocletian tired to bring back the Roman gods, but that failed for the Empire needed a means to get information from the peasants to the Emperor and from the Emperor to the Peasants and that required a meeting place to inform people of what the Emperor wanted. The old Roman Religion did not have such meeting places, worshipers went to a temple, made they offering to the priests, who then took it into the temple and came out and told them it was a good offering or not. No meeting places of a large numbers of worshippers. The Christian Church had no sacrifices, but they had huge meeting places where members went to listen to the Gossips. That is what the Empire needed to get messages to the people and Constantine embraced Christianity more for that reason then anything else.
Third, the Empire needed a secure place to raise troops, with the concentration of wealth in the Western Empire and in Egypt (it is believed only 8 people owned all of the land of Egypt in the 500s), the only area free of such massive concentration of Wealth was Illyricum (What use to be called Yugoslavia) and Asia Minor. Illyricum was where Constantine and Diocletian were from (and several other Emperors of the 200s). Illyricum provided those troops till the 400s when the Cold Snap returned and it became hard to feed troops using Mediterranean farming techniques.
The tendency for reduce grain production in the Western Empire, with increase grain production is Asia Minor, made the Eastern Empire more Powerful then the Western Empire, a reversal of the situation from the time of Hannibal till the cold snap of the 200s. It warmed up in the 300s, thus the Roman Empire stabilized after about 280 AD, but return to cold temperatures in the 400s. The Eastern Empire did well in the 400s, it survived almost intact. Various reasons for this is given, but the increase Rainfall in Asia Minor would provide the food to raise troops from that area. The decline in the ability to farm reduced the power of Illyricum for it joined Gaul (modern day France) in a drop in food production do to colder temperatures. Northern Italy saw a similar drop as did Spain. All tied in with the decrease in temperatures.
Egypt seems to have held its own, but Asia Minor boomed, Thus the Eastern Empire had a secure source of food, a secure source of place to raise troops and control over the Greek Speaking Christian Church. Western Europe saw more and more land become marginal, massive peasants revolts when they could not feed their families and even the abandonment of Britain as it became to cold to feed the troops (Cattle raising became the norm).
For all practical matters the Western Empire fell in 450 AD, when the German Commander of the remains of the Roman Army took over and named a nominal Emperor. In 476 a different German Commander of those same troops abolished the position of Emperor in the West, and swore alliance to the Eastern Emperor, an alliance that meet nothing except he said he would follow orders of the Eastern Emperor if he decided to do so.
The situation decline further in the 500s, while the Eastern Empire boomed enough to start a war to win back the Western Empire, but with a resurgent Persia on its border that effort was limited (and limited by the policy of Rome to return lands to its "Rightful" owners and demanding that the peasants on that land pay them back all the rent not paid since 450 AD. In the late 500s this lead to massive support of the Lombard invasion of Italy (Cold temperatures were a factor NOT the factor in the fall of the Western Roman Empire).
One other factor restricted the Eastern Roman Empire. The Slavic Invasions of the late 500s and early 600s. Unlike the earlier Germanic Invasions, this was driven by farmers, who had embraced the heavy plow. The Heavy Plow is NOT needed around the Mediterranean sea, Mediterranean farming techniques work quite well in that part of Europe (This was the use of Wooden plows to scratch the surface and plant crops). The heavy plow can dig under compacted soil and turn it over for planting. The Heavy Plow permitted farming to return to France and Britain and be introduced into Germany. You could do farming with the heavy plow in areas where the Mediterranean farming techniques just could not work. Thus even Illyricum could return to farm lands, but require the heavy plow which entered those areas with the Serbs, Slovenes and Croats or modern Serbia, Slovenia and Croatia (the Former Yugoslavia). After the Arab Conquest of the 600s, what is now the Ukraine replaced Egypt as the source of food for Constantinople for it had embraced the heavy plow (and to show you who the Romans feared the most, after the Arabs were push back from Constantinople in 700 AD, Rome moved against the Slavs NOT the Arabs).
The increase rain after 400 also affected Arabia, thus you saw increase food production even in the desert Kingdom. During this cold period, the Eastern Roman Empire (later called the Byzantine Empire, through NEVER by the residents of that Empire, till Constantinople fell to the Turks in 1453, that country claimed to the Roman Republic) fought the Persian Empire from 230 to 628 AD. In the final Roman Persian War, 603 to 628 AD both the Roman Empire and Persian Empire had destroyed each other and the border remained roughly what it had been since 230 AD. In 639 Rome was concerned about the Slavic movement across the Danube, when the Arabs moved against the Eastern Empire. To response to the Arab invasion, Rome recruited Slavs to fight the Arabs, but the Arabs won the battle and took over Syria to Egypt.
Syria was like Egypt, had a lot of peasants but most of the land was owned by Roman elites. When Persia took them over Syria (613) and Egypt (621) the peasants were kept on the land, but the Roman Elites had to escape to Constantinople. The Persians then supported the Christians in Egypt who had NOT accepted the results of the Council of Chalcedon (451 AD) as to the nature of Christ but the Persians then also took over the land as conquerors, making themselves the new owners of the land that the peasants worked. While the writers of the time period emphasis the religious angle, it appears that was because no one wanted to acknowledge the transfer of ownership. For over 8 years (the Persian Army did not leave Egypt till 629 AD) ownership rights where a product of PERSIAN Law not ROMAN LAW. On the surface, there was no difference, but since the Persians divided the land among themselves, paying the previous Roman Owners nothing, the Persians also permitted the peasants a higher percentage of food produced for their own use and to sell. The Roman Elites had become use to getting a much higher percentage of that food production to enrich themselves. The peasants liked this change. When the Persians left in 629, the Roman Elites returned and demanded not only the much higher percentage of food that they were entitled to, but also the money for the food produced in the eight years of Persian occupation. Thus we have Egyptian records calling these people tyrants. Thus when the Arabs hit 10 years later, the peasants remembered what happened under the Persians and while they did no fighting for the Arabs, they supported them with food and directions (Both of which they deny to the Roman Army trying to hold onto Egypt).
Rome decided to cut its losses and abandoned all attempts to retake Syria an Egypt till the 9th Century but retained its bread basket of Asia Minor.
More on the Council of Chalcedon:
http://www.newadvent.org/cathen/03555a.htm
Fall of the Byzantine Empire
Thus it was the greater amount of Rainfall that lead to increase food for troops in Asia Minor, Iran and even Arabia from 200 to 800 AD. It was one of many factors that saw the Eastern Roman Empire survive as a Greek Empire. The Byzantine Empire quickly recovered from its defeat by the Arabs and held onto to Asia Minor and defeated the Arabs in 700 AD. The Arabs took over Syria, Iran, Iraq and Egypt (and later Tunis and Carthage) as the Eastern Empire concentrated on holding Asia Minor. The Iron Plow permitted growth in population in Northern Europe, thus restricted what the Empire could go (i.e. it was boxed in, any effort to rebuild the Roman Empire ran across the problem that the Empire had to deal with the Eastern Slavs across the Black Sea, the Western Slavs across the Danube and the Southern Slavs as they moved into the Balkans, the Arabs holding Syria, Iran and Egypt, the Lombards and later the Franks holding Northern Italy). Thus by 700 the best that the Empire could do was to hold onto its bread basket, Asia Minor (which came under increasing Turkish attacks, as the Turks moved from Turkenstan to Iraq during this time period and embraced Islam).
Now starting around 850, the world started to warm up. Farm production increased in England, France, Germany, Poland, Sweden, Denmark and Norway. At the same time, rainfall fell in Asia Minor, Syria, Iran and Arabia. You do not hear of Arabia after 800 except as the homeland of Mecca. Like all warm period, people move north into areas with very little human population up to that point. Asia Minor saw more and more of its interior unfarmable. Greeks started to concentrate along the coasts. The Interior Themes (the Military and Administrative Units of the Empire from about 650 till its fall to the Fourth Crusade in 1204) saw food production drop, and with it the ability to support the troops that Themes had been providing since the 600s.
Into this mess, came Basil II, one of the greatest Greek Emperors. He expanded the Borders of the Empire till his death in 1025. At his death the Empire was the largest it had been since the Arab Conquest. With the drop in rainfall, the Central Themes had declines, but Basil decided to address that issue by providing support for those Themes and the peasants in those themes. Basil was to popular to overthrow do to his victories and conquests that his fellow elites found themselves getting the short stick, unlike the peasants who normally suffer that fate. Basil Conquests permitted him to address the problems of the peasants as those problems became worse do to the warming climate and the resulting loss of rainfall in Asia Minor.
Basil II died in 1025. He was succeeded by his brother for he had no children. His Brother was another member of the ruling elite and proceeded to undermine all of the programs that helped the peasants. It appears he even left in Turkic herders into the interior of Asia Minor as that area became to dry for farming. This was bad for such herders were more loyal to their Turkic leaders then the Greek Elite. Then Battle of Manzikert occurred in 1054. This Battle saw the Greek Army destroyed and the Seljuk Turks taking over central Asia Minor. These were herders not farmers. The increase dryness was tied in with the warming climate and made the area more fit for herders then farming, but that change also saw the breadbasket of the Empire slowly turning Turkish, as the herders moved into more and more marginal farm land. Thus the lost of food production in Asia Minor lead to a reduction in the Greek Army and with that reduction, the Seljuk Turks moved in.
In response to Greek requests for aid against the Turks, the Pope called for the First Crusade. The Greeks wanted it to help them take back Asia Minor, but the Crusaders wanted to go to the Holy land. The Greeks received some help in Asia Minor, but the Western Army went to Palestine NOT to Central Asia Minor. The Middle ages warm period was in full swing by 1100, thus Western Europe had surplus food and surplus troops to send East, while the same warming trend was weakening the Greek Empire. The Greek Empire supported the First Three Crusades, but then the Fourth Crusade occurred (it was the only Crusade condemned by the Pope as it was occurring for the Venetians had convinced them to take Constantinople NOT to go to Palestine).
The taking of Constantinople by the Fourth Crusade killed any hope of the Greek Empire surviving. The Greeks broke into at least three kingdoms, while the city of Constantinople was held by what is called the "Latin Empire". The City went into sharp decline after that siege. Thus by the time the Turks took it in 1453, it was a mere shadow of the city it had been in 1204. In 1204 Constantinople was full of people and life, the Latins had their own section outside of the city proper. Arabic was spoken in the streets and you even had Mosques in that city. By 1453, the City had been reduced to several large towns within the old walls, each of these towns had they own small walls. Herds of goats lived in the areas between these towns. Constantinople still almost defeated the Turkish attempts to take it, but the Turks did take it on their last attempt.
During that period, 1204 to 1453, Asia Minor became more and more like a desert. Unlike a true Desert, you could run herds of sheep over it and since that was a Turkish specialty, the Turks slowly took over more and more of the land, even while the Greeks held onto the urban centers. When the Turks took over such urban centers, they tended to leave the Greeks in Charge but used it as a base to move onto other lands. With the hostility between Greeks and the Latin West caused by the sacking of 1204, any hope to work to keep Constantinople died. The Greeks of Nicaeans retook Constantinople in 1261, ending the Latin Empire, but the other parts of the old Empire NEVER joined. The reason the city did not fall to the Turks in 1261, was the Mongols had destroy Baghdad in 1258 and every Moslem was dreading what to do about the Mongols. Asia Minor had become a series of mini-states by 1258, mostly of refugees from the Mongols AND the destruction of Constantinople in 1204. The destruction of both Constantinople, in 1204 and Baghdad in 1258 can be called the greatest calamities in world history, prior to the 20th century. Both cities had populations over a million when sacked. Both Cities were not only a center of Government, but Commerce and education (When the Mongols took Baghdad, they said it was the largest city they had ever taken, and they had moved deep into China by 1258, older citizens of Baghdad told them Constantinople had been larger before it was sacked in 1204).
The Mongols were NOT driven to conquest by any cold snap, they did it during the Middle Ages Warm Period and for the same reason Western Europe did the Crusades, the Warm period permitted increase food production, thus increase troops and all it took was one man to get everyone on the same page to do a world wide conquest. His attacks on China was Classic Asiatic steeps people attacks on China, but his attack on Central Asia was the result of him having access to food while Central Asia, like Asia Minor, Syria and Iran was undergoing a drop in rain fall.
Notice the Mongol move was driven by increase food on the Steeps and decrease food in Asia Minor to Central Asia. The movement of Turks into Asia Minor was aided by the drop in rainfall in Asia Minor tied in with the warming world wide temperatures.
I bring this up, for I suspect Asia Minor, Syria, Iraq, Iran and Central Asia will see further drop in rainfall, as rain goes further north. One report even suggest that the Sahara Desert may return to grass lands, as the heat in the Sahara draws in rainfall from the Monsoons that hit Western Africa to this day (5,000 years ago, the Sahara was more Savanna then desert as increase heat drew in the Monsoons from the South Atlantic). About 5,000 years ago, temperatures in the Sahara declined, and the Monsoons stop as they hit the South Coast of West Africa. Increase world temperatures MAY increase those temperatures and with it increase rainfall in North Africa.
More on Sahara being savanna:
http://www.space.com/10527-earth-orbit-shaped-sahara.html
Africa 8000 years ago:
http://www.esd.ornl.gov/projects/qen/afr(8-7.gif
http://www.esd.ornl.gov/projects/qen/nercAFRICA.html
Compare the above map with a map of Africa's climate today:
http://www.esd.ornl.gov/projects/qen/afr(5ky.gif
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