Section 1.1: The Roman Warm Period

To comprehend how climate may have facilitated plagues in Rome, we must first examine the conditions that allowed the Empire to flourish. This period, known as the "Roman Warm Period" or the "Roman Climatic Optimum," coincided with unusually warm and stable weather across Southern Europe, creating optimal conditions for a cohesive society with extensive food production. The emergence of the Roman Empire around the Mediterranean Sea, which enabled rapid transportation of goods, is no coincidence. Scholars estimate the duration of the Roman Warm Period to be from approximately 100 BC to 500 AD, though specific ranges may vary among studies.

Scientific methods have confirmed this era was marked by exceptional warmth and stability. For instance, a 2020 article in Nature tracked the presence of specific plankton species in Mediterranean sediment cores. Globigerinoides ruber, a plankton sensitive to temperature changes, is often utilized by climate scientists. The ratio of Magnesium to Calcium found in their shells indicates seawater temperatures.

While land climate and ocean temperatures differ, they generally exhibit strong correlations, offering a glimpse into conditions experienced by Romans during this time. Historical events are reflected in sea surface temperature records, including the Late Antique Little Ice Age (LALIA), the Medieval Warm Period (MWP), and the Little Ice Age (LIA).

Various other records, such as pollen analysis from lake sediments, support the notion of a relatively warm climate. The favorable agricultural conditions allowed the Roman Empire to expand northward, including the notable invasion of Britain in 43 AD. However, this warming trend was not permanent; as temperatures began to decline, the regions suitable for crops shrank, undermining the Empire's ability to sustain its population of roughly 75 million, contributing to societal decline.

Section 1.2: The Impact of Climate on Disease

One of the most notorious pandemics in ancient history, the Plague of Justinian, struck after the fall of the Western Roman Empire, affecting the Eastern Roman Empire. This event was not isolated; it followed several earlier pandemics. Recent research indicates a strong correlation between cooler, drier periods and the timing of these outbreaks, including the Antonine Plague (~160 CE), the Plague of Cyprian (~250 CE), and the Plague of Justinian (~540 CE).

Environmental changes can create conditions conducive to pandemics. Shifts in climate exert evolutionary pressure, prompting species—viruses and bacteria included—to adapt and seek new hosts. These dynamics are complex and interconnected with societal and agricultural transformations.

While some studies suggest that the Plague of Justinian had minimal effects on agriculture, contributing factors remain debated. Reduced precipitation can hinder sanitation and hygiene, further exacerbating susceptibility to disease. Although the evidence suggests a connection, it remains inconclusive whether these plagues coincided with climate changes by mere chance.

A significant cooling event during the Plague of Justinian can be traced to multiple volcanic eruptions occurring within a decade. Ice core analyses revealing volcanic ash deposits from the 530s, along with historical accounts from that period, substantiate this.

"The sun gave forth its light without brightness, like the moon; men were free neither from war nor pestilence nor any other thing that brings death."

—Procopius, 536 A.D.

While the precise causal links are still under scrutiny, it is noteworthy that these volcanic eruptions coincided with the plague outbreaks. Identifying the specific volcanoes that erupted during this tumultuous period presents a challenge due to the variety of calderas and the complexities of dating eruptions.

The Antonine Plague, which occurred earlier around 170 AD, coincided with the decline of Pax Romana, a time of stability throughout the Empire. Climate analyses indicate that the Mediterranean was growing significantly colder and drier during this period. Although the precise nature of the disease remains debated, historians often suggest it was an early form of smallpox.

The question arises: did volcanic activity lead to these drier conditions? Likely, yes.

Volcanic activity records indicate an uptick in eruptions around 150 AD. Ash particles can circulate globally and impact climate. However, the correlation between these eruptions and the Plague of Justinian is less certain.

Recent studies have also highlighted the relationship between droughts and increased disease susceptibility. A comprehensive review of drought events found strong evidence that local droughts heightened the risk of illness among populations. Many of these droughts were linked to the El Niño Southern Oscillation, which influences global climate patterns.

In summary, climatic changes played a crucial role in both the rise and fall of the Roman Empire. Ill-timed volcanic eruptions were associated with deteriorating climate conditions and multiple devastating plagues. While debates persist regarding the extent of climate's influence, it is clear that Rome faced additional destabilizing factors, such as political turmoil and external invasions. Nevertheless, it appears that underlying climate changes tipped the scales, exacerbating the impact of these external pressures.