DURING the past two months, the world has been stifled by exceptional temperatures. Deadly wildfires have ripped through parts of the United States and Greece; Japan has declared its heatwave a natural disaster; drought in Britain has led to hosepipe bans and caused farmers to slaughter their cattle for lack of feedstock. As the planet warms, global heatwaves and associated droughts will become increasingly common. History offers numerous cautionary tales about the effects that these weather-related shocks can have on society and politics.

One such lesson is how drought affected the stability of the Roman empire 1,500 years ago. In a new paper published in Economics Letters, Cornelius Christian of Brock University and Liam Elbourne of St Francis Xavier University identify a strong association between rainfall patterns and the duration in power of Roman emperors. The academics hypothesise that lower precipitation reduced crop yields, leading to food shortages and eventually starvation for soldiers stationed at the empire’s frontiers. As a result, troops were more likely to stage mutinies and assassinate their emperor.

The academics combine data on assassinations—some 25 emperors were assassinated, roughly one-fifth of the total—with precipitation data collected from rainfall-sensitive oak-tree rings across the Roman frontier in France and eastern Germany. They find that a one-standard-deviation decline in annual rainfall (a 20% reduction from the average) was associated with an increase of 0.11 standard deviations in the probability that an emperor would be assassinated the following year. The Gordian dynasty from 235 AD to 285 AD was particularly tumultuous: 14 of the 26 emperors who ruled were assassinated during this period. Of course hungry troops were not the only cause of the demise of emperors. This period was also marked by plague, invasions and economic depression.

It might be easy to dismiss the lessons from 1,500 years ago. Ancient Rome had little ability to store grain for long periods or irrigate crops. Yet, to this day, dictators rely on an obedient army to retain power. And more broadly, it has been long established that adverse weather causes economic shocks that lead to unrest, and even to civil war. For example, droughts are widely cited as a cause of civil war in Sudan, and of the rise of Boko Haram in Nigeria. In 1,500 years little has changed: extreme weather events, caused by climate change or not, tend to increase political instability.