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The worst epidemics in human history have a climatic dimension

Interview with Kyle Harper

Kyle Harper is a historian. He is interested in the complex relationship between humans and nature over the centuries. Initially a specialist in the Roman Empire, his latest work focuses on the history of climate change and infectious diseases. He describes their impact on societies of the past, shedding light on our present and its environmental challenges.

For the year 2023-2024, he has been invited to occupy the annual Avenir Commun Durable chair, which is supported by the Collège de France Foundation and its major patrons the Covéa Foundation and TotalEnergies.

We tend to think that the evolution of societies is solely caused by human decisions, but many of your works deconstruct this idea. In your view, climate is also an essential factor in determining people's fate. To what extent has it influenced ?

Kyle Harper : Thanks to research over the last few decades, we have a growing body of evidencefrom" natural archives ", such as ice cores and tree rings, that gives us new insight into the profound relationship between human societies and climate. We're still sorting out all this new data, but we can clearly see that our destiny has long been inseparable from that of the environment. This is true on different time scales, from very long-term changes, such as the transition from the Ice Age to the warm Holocene period, to very abrupt changes brought about by events such as major volcanic eruptions. We have learned, for example, that these often produce extremely rapid cooling of the climate. In the past, this has had unpredictable effects, from crop failure to conflict.

It has become clear that, although societies react very differently to environmental challenges, climate change has been a major factor in destabilizing them. Societies could adapt to a certain level of stress, but beyond that, serious social consequences awaited them, such as mass migrations, conflicts, health crises and so on. From this point of view, a climate change of several degrees in the space of a century presents enormous challenges.

You have studied the climate of Roman antiquity in particular. What were its characteristics, and what climatic events did the people of that era have to contend with ?

It was while studying the Roman Empire that I became convinced that we needed to reflect on the importance of natural factors in human history, and understand how they interact with social factors. Roman history is a huge canvas : itoffers many perspectives on the interaction between climatic stress and human adaptation. At certain points in their history, the Romans took advantage of a relatively favorable and stable climate to experience demographic and economic growth. Climatologists were the first to name this period the " Roman Climatic Optimum ", which stretches from the last centuries BC to the first centuries AD. Of course, there is no such thing as a perfect climate ; in very general terms, this phase of Holocene history was exceptionally stable. At other times, the Romans met the challenges of a variable and unstable climate by developing their resilience, both technologically and socially.

Their experience can help us to understand the importance of what are known, in the language of modern climate policy, as " compound and contagious risks ". Climate change is often more destabilizing when it interacts with other risk factors. During the crisis of the third century, for example, the combined pressure of monetary crisis, war, constitutional crisis and abrupt climate change proved overwhelming for Roman society. Later, in the sixth century, the interdependent factors of climate change and pandemic were absolutely devastating.

You also demonstrate the major role played by infectious diseases throughout history. To what extent are they linked to climate, and what impact have they had ?

This is an important and urgent question ! We'd like to understand the relationship between climate and disease in a more robust way than we do at present. We know that the climate system is deeply linked to the biosphere. It affects the biology of pathogenic microbes. It also influences the biology of animal hosts and vectors, such as mosquitoes, which transmit them to humans. Finally, climate influences the vulnerability of human societies to epidemics. Starving and war-torn societies are highly susceptible to infectious diseases. What we are beginning to understand more clearly is that there is a causal link between rapid climate change and epidemic mortality throughout human history. Climate change has been both a trigger and an amplifier of infectious disease, and has been implicated in some of the deadliest diseases in human history. The most striking example is probably the link between bubonic plague and climatic factors. We're still trying to better understand the mechanisms of interaction, but it's pretty clear that the worst epidemics in human history have a climatic dimension.

Although modern societies have many tools at their disposal to mitigate the threat of a pandemic, Covid-19 reminded us how difficult it is to contain one. Historical data therefore encourages us to consider disease as a major risk factor in future climate change scenarios.

Environmental disasters shake societies, but do not necessarily bring about their downfall. How can we explain a society's varying degrees of resilience ?

Resilience is the ability of a society to survive challenges and adapt in the face of stress. The science of resilience has much to learn from the past. Historically, resilience has had both technological and social dimensions .

In social terms, for example, institutions capable of resolving tensions in times of stress are important. Faced with an unstable climate, the inhabitants of the Roman Empire developed remarkable irrigation and water supply systems, as well as agreements on water rights in times of drought. And above all, they created an inter-regional food system, which cushioned climatic stress in a given region : these food distribution systems ensured that people didn't starve in times of food crisis. This last example seems to have been crucial over the centuries of modern Europe.

I think one of the key lectures is that it takes both technological solutions and social change to successfully adapt to changing environments.

If we usually separate the humanities from the natural sciences, your research blithely combines them. You combine the historian's traditional sources with data obtained from paleoclimatology^[1], dendrochronology^[2], vulcanology, microbiology, etc. What does this interdisciplinarity allow you to do?

Interdisciplinarity enables us to explore the best-known periods of the past with fresh eyes. It allows us to ask new questions that are not bound by the perspective and limitations of conventional sources. Historians have long debated : should we try to see the past through the lens of the present, or should we try to understand the past in its own terms ? This is in fact a false dichotomy : both are possible. We can enthusiastically ask ourselves very modern questions, informed by the challenges of our world, while at the same time trying to approach societies of the past in the same way as an anthropologist approaches a different culture, being respectful and open to learning about another way of seeing the world.

Of course, we're concerned about future climate change, so we look to the past to learn how our ancestors responded to the challenges of a changing environment. However, this concern is also generating new interdisciplinary knowledge. The natural archives of paleoclimatology are not the fruit of a disinterested, context-free curiosity about the Earth's past. Rather, it is the urgent challenge of human-induced climate change that motivates the effort to reconstruct the history of the climate system. For historians, the result is a remarkable abundance of new and unexpected data on the environment of past societies. This new type of data does not call into question the traditional expertise of historians and other specialists in the humanities and social sciences. Quite the contrary, in fact. More than ever, we need the tools to help us understand the diversity of human responses throughout the past.

The environment impacts on human beings, but they are, of course, themselves agents of ecological change. This is the subject of your next book, The Last Animal. What will it be about, specifically ?

Biologist Barry Commoner once said that the first law of ecology is that everything is interconnected. Simple but brilliant. He meant that organisms in the biosphere are all connected by networks of energy flows. You can't change one part of an ecosystem without affecting all the others. I think this is a powerful prism through which to think about humanity's rapid expansion towards ecological domination. After writing about the history of climate and the history of invisible microbes, I wanted to write a book about the history of man's impact on animal biodiversity. There are some excellent books on the current biodiversity crisis , which has been dubbed the " sixth extinction ". As a historian, I wanted to know about its origins. Where does this crisis come from? When did it start ? I'm still in the middle of writing, but I defend the idea that the significant alteration of biodiversity by man is an integral part of our past. And yet, the last fifty to seventy-five years (the so-called " great acceleration " of population growth, energy use and consumption) are truly unprecedented and disturbing.

Faced with the climate changes we are experiencing and will experience in the future, what readings can we draw from the past ?

We live in a very different world, much more technologically sophisticated. We have much more efficient food production systems. We have a scientific understanding of nature. We should therefore be much more resilient, and yet the challenges we face are, in many ways, much greater. One of the lessons of the past is that a climate change of a few degrees in less than a century is absolutely enormous : when a phenomenon of this order has occurred in history, it has often been synonymous with serious crises for human societies.

A second observation is that change in human systems is not linear. If climate is a complex system characterized by feedbacks and tipping points, so are human societies. We can absorb stress up to a certain point, but beyond that point, change can be dramatic and cascading.

Thirdly, the most significant impacts of climate change are not felt directly - like the direct cost of sea-level rise considered in isolation - but in the interaction between climate stress and social stress. Historically, it is the interaction between climate change and wars, migrations, food crises and contagious diseases that has been most significant.

This year, you've been invited to occupy the annual Avenir Commun Durable chair at the Collège de France. What does this invitation mean to you, and what do you hope to convey to your audience ?

First and foremost, it's a great honor. I also have the onerous task of doing justice to the remarkable work of my colleagues, and I hope to bring the latest knowledge in this field to the Collège de France . I also feel a sense of urgency to communicate the most important historical knowledge on the nature of climate risk, resilience and fragility of societies in this lecture. When I write my lectures, I almost always consult the latest IPCC assessment report. I try to imagine what I would like to emphasize or add to it, armed with my historical perspective on climate change, and as we work collectively to solve one of the greatest challenges humanity has ever faced.

I believe that a historical perspective can broaden our perception of what is possible, in both good and bad ways. It reminds us that history is full of devastating collapses that have often occurred suddenly and unexpectedly. It is also full of examples of adaptation and change in the way humans live and relate to nature. History is, ultimately, a humanistic discipline, and how we tell the story of past events will shape our perception of who we are and what is possible.

Interview by Salomé Tissolong