The historical record suggests that climate events have played a significant role in shaping human history. One example of this is the spread of the plague, or Black Death, which devastated Europe in the 14th century. Recent research indicates that the plague was reintroduced to Europe multiple times following Asian climate events.

Using tree-ring based climate records from Europe and Asia, researchers have identified periods of wet conditions, followed by droughts, across parts of Central Asia. These conditions were tough for rodents in the region, which are traditionally the hosts of the plague bacterium, and their numbers would plummet. Infected fleas would seek new hosts, often latching onto passing human traders or their camels.

It is not yet clear exactly how the plague made its way westward. However, we know that 14-16 years after the rodent-killing drought, plague would often be reintroduced into Europe. The delay may seem long, but following a dry period, it would take a year or two for the rodent population to crash and fleas to come into contact with humans. Then the disease would need to travel some 2,500 miles to Europe, covering about 250 miles per year.

The Black Death was not a one-off event. It marked the beginning of the second plague pandemic in Europe, which only disappeared from the continent after the Great Plague of London in 1665-1666. If the “reservoir” thesis were correct, we would expect plague outbreaks to be associated with local climate fluctuations, through changes in agricultural yields and primary productions in forests, affecting the number of urban and wildlife rodents, resulting in more plague.

These findings have important implications for understanding the potential impacts of climate change on human health. As the global temperature continues to rise, we may see the release of ancient bacteria from permafrost that has been buried for millennia. This could pose a serious risk to human populations, as these biological agents could trigger new outbreaks of disease that we are not equipped to handle.

For example, in the 1340s, a 1.5-degree movement of heating of the Earth’s atmosphere — probably because of solar flares or volcanic activity — changed the cycle of the Yersinia pestis bacterium. That 1½-degree difference allowed a small microbe to develop into the Black Death.

There are recent examples of melting permafrost presenting a serious hazard to people. In 2016, a 12-year-old boy died and over 40 people were hospitalized in Siberia after having been infected by anthrax released when high summer temperatures melted permafrost.

In conclusion, the historical record indicates that climate events have played a significant role in shaping human history. The spread of the plague, or Black Death, is just one example of this. Recent research suggests that the plague was reintroduced to Europe multiple times following Asian climate events. These findings have important implications for understanding the potential impacts of climate change on human health, and highlight the need for urgent action to mitigate its effects.