Plague and the Little Ice Age: A Harbinger of What is to Come?

BY PAIGE MAHONEY

For Europeans, the middle of the 1300s was marked by cold and death. What scientists and historians have now deemed the Little Ice Age started at the beginning of the century, bringing with it famine as crops failed due to poor growing conditions. At the same time, the plague was devastating the continent. In just seven years, the Black Death, which raged from 1347 to 1353, is estimated to have killed 40-60% of Europe’s population¹. It marked the beginning of the second plague epidemic, which lasted for four centuries and killed millions around the world before the bacterium faded away in the 1700s, causing sporadic illness until the next epidemic. While plague no longer causes massive epidemics that lead to countless deaths thanks to modern sanitation practices and antibiotics, its strong association with different climate events means that we might see more cases as our planet warms.   

The Little Ice Age and the Black Death have long been linked. It has been suggested that famine caused by the impact of falling temperatures on crops weakened Europe’s population through malnutrition, resulting in people that were much more vulnerable to severe illness when the plague arrived¹. Although this hypothesis holds much weight, looking at other climate trends during the decades leading up to the Black Death provides more information about why this disease flourished at the time—and gives insight into how modern climate change might influence plague’s future. 

Plague is caused by the Yersinia pestis bacterium. Cylindrical in shape, these bacteria enter the lymphatic system, where they travel to the lymph nodes, multiplying and wreaking havoc on the immune system. Y. pestis causes three types of plague: bubonic, which is the most famous and characterized by swollen lymph nodes called buboes; pneumonic, which is a result of the bacteria invading the lungs; and septicemic, the deadliest form that occurs when bacteria overwhelms the bloodstream, leading to blood poisoning². Without treatment, which consists of antibiotics and supportive care, mortality rates range from 50-100%, with pneumonia and septicemic plague being almost always fatal without prompt treatment². 

Yersinia pestis is transmitted by fleas, which feed on infected rodents and ingest the bacteria. During their next feeding, they transmit the bacteria into the bloodstream of whoever they are feeding on. The bacteria’s main reservoir—where it is found in the environment—is rodents: historically, the black rat and today, the gerbil³.

The Little Ice Age lasted from around 1300 to 1850. The transition into this period from 1280 until 1350 was marked by warmer and wetter conditions, notably in Central Asia³. This location is important because current research puts the origin of the second plague epidemic in this region. It is thought that Yersinia pestis traveled from Central Asia to Mediterranean ports around 1347 via the Silk Road trade system and was repeatedly reintroduced into Europe throughout the second plague epidemic as this trade continued³. 

Much of the research linking plague and climate variations is based on data from tree-ring growth⁴. By examining and estimating how much trees in a certain area grew in a year and matching that data up with weather records, scientists are able to form a picture of the climate during a certain time period. For example, since more tree growth is indicative of more rain, subsequent large rings suggest a period of increased rainfall.

One of these tree ring studies hypothesized that, based on increased tree ring growth from the decade before the Black Death, wetter conditions increased biomass in Central Asia, allowing for an explosion in rodent populations⁵. With more rainfall likely came more food sources, meaning that the environment was able to support more rats. However, within a few years, temperatures dropped as the Little Ice Age hit the area. As a result, the rodent population collapsed because food sources decreased and the climate became more inhospitable to rodents. Without as many rats, fleas were forced to find alternative hosts, leading them to settle among human populations, likely increasing spillover of plague. Thus, the rapid change in weather patterns might have been a key factor in the explosion of plague in the 14th century by instigating key human-flea interactions.

Although the Black Death may be the most famous plague outbreak, it is far from the only one linked to climate variations. In AD 536, in the middle of another mini ice age, plague swept across Eurasia⁶. Like the 14th century’s Little Ice Age, this cold period is thought to have weakened populations through famine, resulting in a continent that was much more susceptible to disease. Similarly, historians think that climate disruptions might have changed rodent migration patterns, creating more interactions between rats and humans, which might have resulted in plague spillover⁶. Ultimately, as geographer Francis Ludlow, who has studied this pandemic, says, “abrupt climatic events place great stress on societies” and can act as a tipping point for potential disasters like massive disease outbreaks—a phenomenon that has been seen time and time again with regard to plague⁶. 

Today, plague is endemic to rodent populations in Central Asia, Africa, South America, and the western United States⁷. It is found on all continents except for Antarctica and Oceania, but human cases tend to be most common in Africa ⁷. The World Health Organization recorded almost three thousand cases and over five hundred deaths attributed to Yersinia pestis between 2013 and 2018. More recently, a 2021 outbreak in Madagascar included 22 confirmed and 20 suspected cases of plague⁸. Although plague is endemic to Madagascar and a few cases are reported there annually, an outbreak of this size is worrying, especially considering that 19 of the confirmed cases were pneumonic, which is almost always fatal without treatment⁸. Thankfully, plague is a notifiable disease, meaning any cases must be reported to local and global health institutions. Therefore, groups like the WHO are well informed about any outbreaks. Importantly, this system makes it easier to identify any changes in disease trends. 

Today, due to increased sanitation and efforts to reduce the rat burden in urban areas, plague very rarely strikes densely populated areas and most cases result from close contact with infected animals⁹. Current numbers are significantly lower than the hundreds of thousands or even millions who were infected every year during the height of the second plague pandemic, but climate change has the potential to significantly increase modern case rates. 

A 2006 research study conducted by scientists from all over the world found that warmer springs and wetter summers increased Yersinia pestis prevalence in rodent populations¹⁰. Just a one degree celsius increase in spring temperatures could result in a fifty percent increase in plague prevalence in hosts. Additionally, warmer springs with less frost greatly benefited flea and rat populations as both animals were able to survive longer and produce more offspring¹⁰. In order for sustained plague transmission, like what was seen during the second plague pandemic, there needs to be a high enough saturation of rats and fleas in a certain region, meaning that increases in these populations substantially increases the risk of plague spillover into humans.

Recent climate data suggests that many parts of the planet are becoming more hospitable to fleas and rats. 2024 was the warmest year for several regions with plague reservoirs, including North America, Africa, and South America¹¹. These statistics are part of a trend of increasing temperatures worldwide. Additionally, global precipitation has increased by about 0.03 inches per decade since 1901, resulting in a warmer and wetter planet¹².  Given data from historic plague pandemics and present day studies, it seems likely that Yersinia pestis will flourish in the face of climate change. Immediately prior to the Little Ice Age, rodent populations exploded due to increased rainfall. When the planet suddenly cooled, the populations collapsed, leaving fleas searching for new hosts. Today, as the planet warms, we are seeing more and more extreme weather. Areas are flooded with rainfall before entering into a period of drought. Natural disasters are on the rise. These massive variations are likely to impact rodent populations, which may boom before collapsing, resulting in fleas searching for new hosts. The chances of those fleas finding humans will increase as deforestation continues to bring people and animals closer together. Plague—which today is commonly seen as a problem of the past—may thus accompany the rising temperatures and changing conditions of the next few decades.

Paige Mahoney is a first-year in Benjamin Franklin College.

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References

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