From the Publishers Weekly review:
In Keys's startling thesis, a global climatic catastrophe in A.D. 535-536--a massive volcanic eruption sundering Java from Sumatra--was the decisive factor that transformed the ancient world into the medieval, or as Keys prefers to call it, the "proto-modern" era. Ancient chroniclers record a disaster in that year that blotted out the sun for months, causing famine, droughts, floods, storms and bubonic plague. Keys, archeology correspondent for the London Independent, uses tree-ring samples, analysis of lake deposits and ice cores, as well as contemporaneous documents to bolster his highly speculative thesis. In his scenario, the ensuing disasters precipitated the disintegration of the Roman Empire, beset by Slav, Mongol and Persian invaders propelled from their disrupted homelands. The sixth-century collapse of Arabian civilization under pressure from floods and crop failure created an apocalyptic atmosphere that set the stage for Islam's emergence. In Mexico, Keys claims, the cataclysm triggered the collapse of a Mesoamerican empire; in Anatolia, it helped the Turks establish what eventually became the Ottoman Empire; while in China, the ensuing half-century of political and social chaos led to a reunified nation. Huge claims call for big proof, yet Keys reassembles history to fit his thesis, relentlessly overworking its explanatory power in a manner reminiscent of Velikovsky's theory that a comet collided with the earth in 1500 B.C. Readers anxious about future cataclysms will take note of Keys's roundup of trouble spots that could conceivably wreak planetary havoc.
Rome collapsed by AD 450, long before this, and even Christendom (lands in the former Roman Empire) was breaking up, by doctrinal schism of Donatists (a very strict sect primarily in the Middle East and Africa which denied the legitimacy of other parts of the church) Arians (overcome from controlling Roman Catholicism, but still around, especially in central Europe) and the Bizintine/Eastern Orthodox churches (widdening their cultural/lingual differences from the Roman (Catholic) West) as well as the various un-unified tribes and such that controlled the former Empire made the Palestine, Egypt and Northern Africa—cultural centers in late Rome—but now ill-defended outposts, easy pickings by the AD 635+ Moslem armies.
There’s no need to point to some drastic climate change to blame civilization’s change from monolithic (& very exploitive) Rome, to the feudal Middle Ages.