During Augustus’ reign in the first century A.D., the Roman Empire enjoyed a beneficial climate resulting in bountiful harvests and expanding birthrates. By the time of Marcus Aurelius, the global climate began to change. The world grew cooler wrecking havoc upon agriculture. The poorer harvests led to malnutrition, decreasing birthrates, and disease. As the population failed to expand, the government tax base and military pool decreased making it virtually impossible to defend the west against barbarian incursions.
By the mid-second century, the Roman population totaled at least 65 million people. Some researchers believe that the actual total topped over 120 million, but that is probably too high. At the same time, the climate grew cooler and food became more scarce forcing animals to forage closer to human populations. This led to a series of outbreaks in the latter half of the second century. The epidemics may have wiped out half of Europe’s population. Emperor Marcus Aurelius himself died of the plague.
By 200, the Roman population stood at 40 million. The lack of food led to disease and malnutrition. Both factors decreased birthrates. The demographic disaster weakened the empire by decreasing the tax base. In order to continue funding levels and provide bonuses for the military, the Severan Emperors devalued the currency leading to hyperinflation and destabilizing the government. In 235, the empire fell into chaos. Various claimants spent the next fifty years fighting each other for power. The power struggles and civil wars led to a complete breakdown of civil government.
The lower birthrates and civil wars stifled Roman population growth. By 310, the empire boasted a population of 55 million. However, 2/3 of that figure lived in the more prosperous and stable east. By this point, the empire split in two culturally and eventually politically. The split created a more provincial eastern empire further weakening the west. At the same time, emperors shrunk the size of the military in an attempt to deal with the declining population. This led to a more porous border for barbarians to punch through.
As emperors attempted to deal with the demographic disaster, the climate remained cool into the fifth century. The population did not recover as it declined to 51 million by 400. The Roman defeat at Adrianople in 378 marked a turning point. After Adrianople, the fighting grew more desperate and the empire struggled to replace dead soldiers. Additionally, the eastern empire made the tactical decision to reinforce their borders deflecting the nomadic hordes westward. The once powerful Western Roman Empire could no longer defend itself and stood alone against the invaders.
The Visigoths sacked Rome itself in 410 and the Vandals followed suit in 455. The Huns also wrought destruction in the 450s, but left following an agreement brokered by the church and a realization that an extreme famine left little to plunder. The western empire collapsed in all but memory in 476. Meanwhile, the global climate remained cool until about 800 when the planet entered the Medieval Warm Period.
The Roman Empire expanded as the climate remained favorable. Around 100, the planet cooled leading to disease, famine, and lower birthrates by the late second century. The empire never recovered from the population lag which led to shrinking government revenues and a smaller military. These problems collapsed the economy and created breaches in Roman border defense. In the end, the demographic disaster allowed the barbarians to conquer the empire.