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Readers of The Long Summer may also be interested in The Little Ice Age: How Climate Made History, 1300-1850 also by Brian Faigen and reviewed at the Science Shelf.
In The Long Summer: How Climate Changed Civilization Brian Fagan notes that major changes in European military/political activities, and in their religious affiliates, may have been at least partially mediated by long-term weather changes. He points to the movement of the boundary between the "Mediterranean Air Mass" and the "Atlantic Air Mass", and the "Continental Air Mass". Since the boundary is a place where plants from both sides of that line grow well, an agricultural society gets richer on that boundary, called the "Ecotone", than elsewhere.
The ecotone of the Mediterranean air mass with the other two ran through N. Africa, Sicily, Greece, and Western Anatolia between 1200BC and 300BC. The wealth of Greece, Sicily, and Carthage rose to great heights in these years, starting from near zero. Then the ecotone started moving North, and the ability of the Mediterranean air mass to grow wheat (storable food for marching armies) was a great boost to the flowering of the Roman Empire. The ecotone stayed at the line from N.Spain, to Britanny, to the mouth of the Rhine, to the Frisian Islands and Jutland, and on East till about 300AD.
Then it began fluctuating, North to South, and South to North. The western roman provinces became unstable in their ability to hold back invaders, because wheat for feeding the legions was harder to get. In the period 535-540AD the ecotone was slammed back South by cooling from a major volcanic eruption. That was possibly in what became the Sunda Strait. The ecotone now ran from Tangier, to Carthage, to Cairo, to Basra. All hope of reunifying the Roman Empire ended, and waves of plague (potentiated by the temperature change because of the characteristics of the disease and its vectors) swept over Europe for the next 200 years. Islam expanded along the line of the ecotone, and became wealthy and powerful.
Then the ecotone began moving North, again, in 800-900AD, and the resurgence of Europe, starting with the eastern roman empire, began. As the ecotone kept moving North, it left the eastern empire behind, just as its most successful emperor, Basill II died. Within 50 years the empire was in deep trouble, from enemies on the west and east both. The ecotone once again settled on the line from N.Spain, to Brittany, to the Rhine's mouth, to Frisia. Northern Europe flowered, and thought they truly had "the favor of God". This is called the Medieval Warm period. It lasted till 1300AD.
Then the ecotone, and warm temperatures, and wealth, moved South. The plague spread again. This "Little Ice Age" caused many to wonder why they had "lost the favor of God", and thus religious heresies were fanned more easily where it was coldest. It took Gutenberg's printing press, and 200 years, but where people had the greatest contrast between the Warm Period and The Little Ice Age was where Protestantism won out between 1500 and 1700.
Climate affected long term politics because, in an agrarian society, climate was a major determiner of which nation had the best crops, and thus the most wealth.
This book has relevance to the current choices that society is making as replacements for fossil fuels. As we decide on our next set of energy sources for industrial society around the world, the idea that something affected by weather should be our primary energy source looks debatable. This is even more the case when considering that the average interregnum between glacial advances is shorter than we have already had this time around. Thus, biofuels look worse to me than other alternatives.