Americans have always loved to compare themselves to the ancient Romans. Our political language and ideology are suffused with Latin influences like “capitol,” “forum,” and “senate”; the neoclassical style is our federal architecture; our very model of a constitutional republic is deeply indebted to Rome’s example.

Naturally, the example of a great, seemingly indomitable power fading into ruin haunts the American imagination. The Roman Empire at its height stretched from the edges of Scotland to the sands of the Sahara, from the shores of the Atlantic to the hills of Syria. Economically, the Romans engineered one of the greatest “golden ages” of any preindustrial society. The empire was generous in granting Roman citizenship throughout its vast territory, and by making subjects into citizens, the empire helped to unleash the cultural potential of the provinces under Roman sway.

By the time of the first emperor Augustus (who ruled from 27 BC to AD 14), the Romans controlled virtually the entire Mediterranean shoreline, and they kept it for nearly half a millennium.

The empire reached its height in the middle of the second century. Although the great English chronicler of Rome’s fall, Edward Gibbon, described a long process of decline followed by piecemeal disintegration, today’s historians are skeptical of the idea of a slow decline. Rather, fiscal, social, and geopolitical challenges mounted and then suddenly overwhelmed the Romans.

The fall came in two parts: German kingdoms replaced Roman rule in the West in the fifth century, then Arab conquerors seized the prize parts of the Eastern empire in the middle of the seventh century. Of course, the underlying causes have always been hotly debated. Did the Romans tax too little or too much? Was there class conflict underneath the political regime?

But in recent years historians have also started to revisit the fall of the Roman Empire with an openness to the importance of environmental factors, including climate change and pandemic disease. Thanks to amazing new evidence from the natural sciences, we can now see that, while the human factors remain integral, they are sometimes just the surface effects of the deeper and more powerful forces of nature.

The story of Rome, ultimately, reminds us of the fragility of human societies in the face of nature and our precarious dependence on the fickle planet that is our home.

Here are six ways that the environment — physical and biological — brought down the mighty empire.

The Romans were enormously lucky when it came to climate. Then they got less lucky.

Today, greenhouse gas emissions are altering the earth’s climate at an alarming pace, but climate change is nothing new. Slight variations in the tilt, spin, and orbit of the earth change the amount and distribution of solar energy reaching its surface; the sun itself emits variable amounts of radiation; volcanoes spew ash that hangs in the upper atmosphere and reflects heat back into space. Historians have only recently begun to take into account the gold rush of new data about the climate in the classical world.

It turns out the Romans were lucky. The centuries during which the empire was built and flourished are known even to climate scientists as the “Roman Climate Optimum.” From circa 200 BC to AD 150, it was warm, wet, and stable across much of the territory the Romans conquered. In an agricultural economy, these conditions were a major boost to GDP. The population swelled yet still there was enough food to feed everyone.

But from the middle of the second century, the climate became less reliable. The all-important annual Nile flood became erratic. Droughts and severe cold spells became more common. The Climate Optimum became much less optimal.

The lesson to be drawn is not, of course, that we shouldn't worry about man-made climate change today, which threatens to be more severe than what the Romans experienced. To the contrary, it shows just how sensitive human societies can be to such change — now amplified in speed and scope by human activity.

Globalization brought great wealth — and disease

In the AD 160s, at the apex of Roman dominance, the empire fell victim to one of history’s first recorded pandemics — an event known as the “Antonine Plague” (after the family name of the ruling dynasty). It was unprecedented in magnitude. Death tolls are hard to come by, but the outbreak took the life of something like 7 or 8 eight million victims. By comparison, the worst defeat in Roman military history claimed around 20,000 lives.

Its cause remains debated, but the likeliest candidate is the smallpox virus or an ancestor of smallpox (a virus that may have evolved not long before this outbreak, most likely in Africa). The Romans traded throughout the Indian Ocean world, across the Red and Persian Seas; their ships reached India and the East African coast.

This trading network carried spices and precious metals and slaves — and germs. Unleashed inside the densely settled and interconnected Roman Empire, the new pathogen was devastating. The Roman Empire survived the Antonine Plague, but the social order was unsettled. From that moment onward, maintaining Rome’s dominance along the frontiers became a greater challenge.

A second pandemic pushes social institutions past the breaking point

The empire rebounded from the Antonine Plague behind the vigorous rule of an African-Syrian dynasty known as the Severans. But in the AD 240s, a ferocious drought struck. Close on its heels, another pandemic, known as the Plague of Cyprian, broke out. The biological agent of this pestilence remains a mystery (though genomic evidence may yet turn up), but its impact is clear. It wasted the population from one end of the empire to the other.

The resulting demographic crisis triggered a total meltdown of the entire imperial system, known as the “crisis of the third century.” Enemies poured across every border, piercing deep into parts of the empire which had not seen war for centuries. One emperor after another seized the throne.

The crisis is considered the “first fall” of the Roman Empire. The empire did reemerge, but with at least two profound changes. First, the empire was henceforth ruled by a different kind of emperor: A cadre of military officers from the provinces along the Danube seized control from the old, wealthy, Mediterranean aristocracy.

Second, the plague led to a crackdown on Christians that backfired mightily. At first, the Roman authorities blamed the pestilence on the Christian religious minority, and they set about trying to extirpate it. The church not only withstood the violent attacks but campaigned to care for the sick and bury the dead amid the pestilence — earning respect. Christianity grew more rapidly than ever in the aftermath of this trial.

Climate change prodded the Huns to move, setting up a chain reaction

The Roman Empire in the fourth century, led now by Christian emperors, enjoyed a kind of second golden age. But it was not destined to last. In the last decades of the fourth century and the first decades of the fifth century, the empire suffered a series of military defeats unlike anything in its history — at the hands of the Goths. But the Goths, in turn, were prodded to move against the Romans because of an incursion into Europe of Huns, from central Asia.

New paleoclimate evidence helps to explain why the Huns suddenly moved West. The Huns were nomads, native to the great belt of steppe that stretches from Hungary to Mongolia, an arid zone that depends on westerly mid-latitude storm tracks for rain.

Tree rings suggest that a megadrought in the middle of the fourth century might have made these nomads desperate for greener pastures. As they migrated West, they terrified the highly developed kingdoms, such as those of the Goths, that had long existed along Rome’s frontier. Partly because of this climate-caused upheaval, the Goths challenged Rome’s frontiers as never before. Rome’s Western territories ended up being carved up and reconfigured as Germanic kingdoms.

The Late Antique Little Ice Age

We rightly fear climate change in the form of global warming, but in the later Roman Empire, the greater danger was sudden sharp cooling. While the Western half of the empire fell, the Eastern, Greek half of the empire, now centered on New Rome, a.k.a. Constantinople, thrived.

In fact, during the reign of Justinian (who ruled from 527 to 65), the Roman Empire found new glory. In the first part of his reign, Justinian codified all of Roman law, went on the grandest building spree in Christian history (including erecting the Hagia Sophia), and took back Roman Africa and Italy.

But then came perhaps the worst environmental catastrophe yet: the dual blow of a little ice age and yet another pandemic. In the 530s and 540s, volcanic eruptions rocked the globe. We have long known that in the year 536 there was no summer; for about 15 months, the sun seemed to shine only dimly, unnerving people worldwide. In recent years, careful work on tree rings and polar ice cores has clarified what happened.

First, in AD 536, there was a massive eruption in the Northern Hemisphere. Second, in AD 539/40, a tropical volcano erupted. The result was not just a year of darkness but truly staggering global cooling: The decade 536 to 545 was the coldest decade of the last 2000 years, with average summer temperatures in Europe falling by up to 2.5 degrees Celsius. And this was no passing phenomenon. For a century and a half, colder temperatures prevailed across large parts of the Northern Hemisphere.

The first black death

Just as the climate started to turn colder, the plague appeared on the Southern shores of the Mediterranean — in AD 541. This was true bubonic plague, caused by the bacterium Yersinia pestis, the agent of the more famous medieval Black Death.

Thanks to remarkable analysis of its genome, the history of this bacterium is now well understood. The plague is at root a disease of rodents, and had been endemic among social burrowing rodents in central Asia. It probably traveled to Rome across the trading networks that carried silks from China to the Mediterranean. The plague first spread from one rodent species to another, carried by fleas — ultimately infecting black rats, which live in close quarters with humans. Once the bacterium reached the rats of the Roman Empire, it was mayhem.

This precursor to the more famous European “Black Death” of the middle ages may have carried off half of the entire population of the Roman Empire. The immediate (and insuperable) problem was disposing of the corpses; the longer-range problem was managing an empire with a severely weakened tax base and a serious manpower shortage — including in the army.

What’s more, the first pandemic inspired a wave of apocalyptic fervor. The pandemic not only wrecked Justinian’s dream of restoring Roman glory; it triggered a spiral of dissolution and state failure that stretched over the next century. One insidious aspect of plague is that it does not vanish after its initial work. It became permanently established in rodent colonies inside the Roman Empire and broke out repeatedly, every 10 to 20 years, unleashing new destruction each time. This helped push the Romans past the breaking point. By the middle of the seventh century, very little remained of the “eternal empire.”

Kyle Harper is professor of classics and letters, and senior vice president and provost at the University of Oklahoma. He is the author of the new book The Fate of Rome: Climate, Disease, and the End of an Empire.

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