For their study, the researchers reconstructed temperatures and precipitation for the period from 200 BC to 600 AD, with a resolution of three years. This means that two data points cover a period of three years – for paleoclimate researchers this is extremely high resolution. The period extends from the so-called Roman climate optimum to the late antique Little Ice Age. This period also included three major pandemics that are known from records: the Antonine Plague (around 165 to 180 AD), the Cyprian Plague (around 251 to 266) and the Justinian Plague (from around 540).
Each of these pandemics followed a climate change: The Antonine Plague occurred during a cold snap that followed several decades of cooling and drought. The Cyprian Plague coincides with a second period of severe cooling. Finally, the Justinian Plague follows an extreme cooling in the 6th century. “There was always a parallel,” explains first author Prof. Karin Zonneveld from MARUM and the Department of Geosciences at the University of Bremen. “A period of climate change was followed by a pandemic outbreak.”
To reconstruct past temperature and precipitation patterns, Zonneveld and her colleagues used so-called dinoflagellates. The single-celled organisms live in the sunlit upper part of the sea and form cysts that are deposited as fossils in the ocean floor. Dinoflagellates have different preferences for their environment, with some living only in colder waters and others only in warmer waters. Some prefer waters rich in nutrients, while others can only live in very clean, nutrient-poor waters, explains Zonneveld. “As conditions change in the upper waters, the composition of the types of cysts that accumulate on the seafloor also changes.” This creates a very high-resolution archive that goes back further than, for example, tree rings in this region.
The samples were taken from a core originating from the Gulf of Taranto. In southern Italy, volcanoes regularly erupt and erupt – the most prominent example being the eruption of Mount Vesuvius in 79 AD, which destroyed Pompeii. The ash emitted rises into the atmosphere, trickles down onto the water and then sinks to the sea floor. There it remains preserved in a thin layer of ash, the so-called cryptotephra. “Volcanic ash contains many small glass particles that can be easily seen with a polarizing microscope,” explains Karin Zonneveld. “The elemental composition of the glass particles in the ash of each volcano is unique and can even be different in individual eruptions of the same volcano. Using tiny needles, we were able to pick out individual glass shards in collaboration with the Bremen volcanologist Andreas Klügel and analyze their elemental composition.” This meant that the deposits could be precisely linked to volcanic eruptions from Mount Vesuvius and volcanoes on the island of Lipari and the deposits could be dated.
To find the missing piece of the puzzle, a coincidence brought Zonneveld together with her co-author, the historian Prof. Kyle Harper from the University of Oklahoma (USA). He, too, has long suspected a causal connection between climate and pandemics. Together, it was possible to precisely date and compare climate and weather data as well as glass particle analyzes with historical events.
The researchers come to the conclusion that climate-related stress could trigger a pandemic outbreak or increase disease outbreaks – for example because food is scarce or people are more vulnerable. Harper and Zonneveld agree that this could hold important information for the future: “We have a completely different society at the moment than in ancient times, especially because of modern science and everything that goes with it – germ theory, antibiotics, vaccines, clean water. But there are also parallels. Similar to Roman times, climate is still an important factor affecting fundamental aspects that affect our well-being, including agriculture, access to clean water, biodiversity, geographical distribution and “Migration of organisms. Studying the resilience of ancient societies to climate change and how climate change and the emergence of infectious diseases are related could give us better insight into the climate change-related challenges we face today.”