Plankton living in the Mediterranean Sea some 2000 years ago have helped researchers to uncover a correlation between climate change and the spread of disease in ancient Roman Italy and into the early Middle Ages.
Using a sediment core recovered from the Gulf of Taranto, in the arch of Italy’s boot, Karin Zonneveld of the University of Bremen in Germany and colleagues reconstructed the regional climate from 200 BCE to 600 CE. The sediment record reveals that periods of rapid cooling and drying in the heart of the Roman Empire align with documented major disease outbreaks, the researchers report in a new study.
The core’s plankton fossils are from dinoflagellate cysts, also known as dinocysts. Dinoflagellates bloom in late summer and early fall, with thousands of species that thrive under varying surface temperatures and nutrient levels. By comparing the ratios of dinocyst species that flourish in warmer waters with those that flourish in cooler waters, researchers can precisely estimate historical temperatures. Dinocysts also respond to the water’s changing nutrient levels, which are controlled by precipitation. Rain and snowfall over the Italian Peninsula are channeled by rivers into the Adriatic Sea, where currents carry the nutrient-enhanced water southward around Italy’s heel and into the gulf.
The core was recovered from a location with an especially high deposition rate, with 1 cm of sediment deposited roughly every 10 years (compared with about 1 cm/1000 yr in the open Mediterranean Sea). That high deposition rate translates into the most detailed record to date of the regional climate at that time—with changes discernible down to a three-year resolution. The new data fill a gap in knowledge that other climate proxies were unable to. Tree-ring records for the area don’t go back far enough in time, pollen records are tainted by human agriculture, and cave stalactites don’t have a fine-enough temporal resolution. Ash and glass shards from nearby volcanic eruptions, combined with lead and carbon isotope dating, were used to constrain the ages of sediments in the core.
The researchers found that after a few hundred years of a warm, wet, stable climate known as the Roman Climate Optimum, a sharp downward temperature trend began around 130 CE and continued well past the arrival of the Antonine Plague in 165 CE. Temperatures never returned to the warmth or stability of the early Roman Empire. The Plague of Cyprian hit around 251 CE amid another rapid temperature decline that lasted half a century. The first plague pandemic arrived in roughly 541 CE, when temperatures were near the lowest measured over the entire record.
“This is a wake-up call,” says Zonneveld, who studies marine microfossils. She says climate change affects biodiversity and the migration of species—and of germs. Although the study doesn’t attribute a causal relationship between climate change and pandemics, the researchers note that climate impacts on agriculture and regional ecosystems could cause perturbations that trigger or exacerbate disease outbreaks. The results highlight the value of studying the complex relationship between climate change and infectious disease. (K. A. F. Zonneveld et al., Sci. Adv. 10, eadk1033, 2024.)