Sebastian Wang considers “what we all know” about the rise and fall of the Roman Empire in the light of more recent research (not all of it politically motivated) into climate change:

The approximate extent of the Roman empire circa 395AD.

Before we get into climate, and for those who tend to the wholly ignorant spectrum of my readers, we need a quick sketch of Roman history. The Empire officially began in 27 BC, when Octavian — better known as Augustus — became the first emperor. It ended in the west in AD 476, when the last western emperor was kicked out. As said, the eastern half, based in Constantinople, carried on for another thousand years.

Broadly, we can divide Roman history as follows:

• 27 BC – AD 180: The golden age. Augustus and his successors took over and further expanded a huge empire. There was peace (mostly), trade flourished, and cities grew. People call it the “Pax Romana“.
• AD 180 – 284: Everything starts to fall apart. This is called the Crisis of the Third Century. Civil wars, foreign invasions, plagues, and economic collapse all hit at once.
• AD 284 – 395: The empire pulls itself together. Emperors like Diocletian and Constantine bring in reforms. But the empire is now divided for administrative convenience — east and west.
• AD 395 – 476: The west goes under. It’s invaded. It’s conquered and broken up. Very quickly, it disappears. Though, once again, a parochial view of history, we call this the Fall of the Roman Empire.

The standard histories still blame bad rulers or too many wars. That’s fair enough. There were some very bad rulers, and the wars without number. But if you look at the climate data — tree rings, ice cores, sediment levels — you start to see another pattern underneath what may be called the political and economic superstructure of Roman history.

When Rome came to greatness, the climate was unusually good. From around 200 BC to AD 150, there was a long phase of stable, warm, and mostly wet conditions. Scientists call this the Roman Climate Optimum. In Egypt, the Nile flooded regularly and well. That meant lots of grain. In the Alps, glaciers shrank. In northern Europe, people were growing grapes in places too cold for vineyards today. In the Middle East, the Dead Sea stayed high, showing good rainfall.

This kind of weather made everything easier. Crops were reliable. Surpluses could be taxed. Cities could be fed. Roads and aqueducts could be built and maintained. And because the army was well supplied, the Empire was protected, and could even continue a modest expansion. But, as McCormick and his team point out, the high phase of Nile flooding correlates exactly with the high point of Roman prosperity — and once those floods became less predictable, problems followed.

The good times came to an end. By the mid-second century, a wave of volcanic eruptions thew great masses of dust into the atmosphere, blocking sunlight. Solar activity dropped. The climate became less stable. Then came the Antonine Plague in AD 166. It probably started in the east and spread quickly. Some think it was smallpox. Whatever it was in terms of microbiology, it was almost certainly brought on by changes in the climate. It may have killed a third of the Empire’s population.

Worse was coming. By AD 200, climate records show more erratic rainfall and cooling. In Gaul and the Balkans, harvests became less predictable. Glaciers began to advance again. Speleothem data from Austrian caves shows sharp shifts in rainfall patterns.

At the same time, the empire started to shake. Between 235 and 284, Rome had over twenty emperors. Most were generals who seized power, then got killed. Civil wars broke out. Trade declined. Foreign tribes pushed harder at the frontiers. Coin hoards — money buried for safety — increased in number. That’s usually a sign of fear and instability. Cities shrank. The economy shrivelled.

Was this all because of climate? No — not wholly. A good definition of historical crank is someone who tries to explain everything in terms of one cause or set of causes. But as McCormick et al. argue, bad weather made everything worse. It weakened agriculture, strained supplies, and made people more likely to panic or rebel. In a world without modern logistics, you couldn’t afford bad harvests two or three years in a row.

The empire buckled in the third century, but didn’t collapse. And its survival probably was an effect of human agency. A line of competent Emperors rose from the army and stabilised the frontiers. This line culminated in the reigns of Diocletian and Constantine, who restructured the Empire. They fixed taxes. They reorganised the army. Constantine built his new capital in the east. His successors found Constantinople safer and more strategically useful than Rome.

This being said, around AD 290, climate records suggest a small rebound. Warmer temperatures and better rainfall returned — especially in the east. That helped the eastern provinces recover faster. They had stronger governments and better infrastructure. But climate helped. Dead Sea levels remained relatively high, which meant steady rain in the Levant.

The west wasn’t so lucky. Italy and parts of Gaul stayed unstable. In Britain, pollen records show that farmland was being abandoned. The archaeology matches this, with fewer building projects and shrinking urban centres. The killing shock for the west came in the fifth century. In Central Asia, a long drought began around AD 370. Steppe tribes like the Huns were hit hard. They migrated west, pushing other tribes like the Goths ahead of them. In AD 376, the Goths crossed the Danube into Roman territory. Two years later, they crushed a Roman army at Adrianople. This all happened in the eastern half. But greater wealth and better leadership allowed the government in Constantinople to push the barbarians west. Over the next century, the western empire was hit again and again.

Meanwhile, the weather got worse. Europe cooled. Rainfall patterns shifted. Flooding and crop failures increased. Volcanic sulphur levels spike in the ice core record from Greenland.

Rome was sacked in AD 410. Again in 455. Finally, in 476, the last western emperor was deposed. That was it. The western Roman Empire was gone.

The east survived. But was hardly untroubled. In AD 536, a huge volcanic eruption darkened skies around the world. The sun barely shone. Crops failed. Famines spread.

A few years later, the Plague of Justinian broke out. It probably started in Egypt and spread through trade routes. Some say it killed half the population in affected areas.

Climate and disease worked together. Hunger weakened people. Infection finished them off. As McCormick et al. put it, the event of 536 and the plague that followed created one of the worst demographic shocks in recorded history.