Dr. Dagomar Degroot, Georgetown University
Until recently, it was notoriously difficult to connect today’s extreme weather with the gradual trends of climate change. Scientists shied away from saying, for example, that catastrophic droughts or severe hurricanes reflected the influence of anthropogenic global warming. Yet today, scientists use big data from satellites and weather stations to inform supercomputer simulations that reveal the extent to which warming trends have raised the odds for previously unusual weather. Scientists now report, for example, that the drought that crippled Syria between 2006 and 2009 was between two and three times more likely in today’s climate than it would have been a century earlier. They feel comfortable concluding that the rains of Hurricane Harvey were perhaps 20 times more likely now than they once were. Armed with these statistics, many scholars and journalists now conclude that events like the Syrian Civil War, which unfolded in the wake of that devastating drought, can be convincingly connected to climate change.
Yet how can we link past climate change – change that happened before the advent of big weather data – to human affairs? Many historians and archaeologists favor qualitative methods. They identify weather events in surviving documents, or in paleoclimatic proxy data (such as tree rings, ice cores, or lakebed sediments) that register the influence of temperature or precipitation. Next, they carefully study texts or ruins to determine how these weather events influenced human activities – such as farming, hunting, or sailing – that clearly depended on favorable weather. By looking at enough of these relationships, over a long enough timeframe, they ultimately reach conclusions about the influence of weather trends – that is, climate change – on the human past.
Environmental historians might be most familiar with these qualitative methods. They inform a raft of new books and articles in climate history, on diverse topics that range from the fall of Rome to the colonization of Australia; from the origins of apocalyptic Norse mythology to the travails of Arctic whalers.
But these qualitative methods are much less influential beyond the historical profession. Today, there is a large and rapidly growing “quantitative” school of climate history that instead relies on statistical means to discern the impact of climate change on human history. Papers in this school are cited more frequently in the latest IPCC assessment report, for example, than publications written by historians who prefer more qualitative means of doing history.
Natural scientists, economists, and historical geographers in the quantitative school of climate history quantify diverse social variables in particular regions, then graph their highs and lows over decades, centuries, even millennia. Next, they develop or make use of temperature or precipitation reconstructions for those same regions across identical timescales. Finally, they use statistical methods to find covariance between their graphs of social and climatic trends.
Most published work in this vein finds statistically significant correlations between these trends. In study after study, Chinese historical geographers have found striking correlations between climatic cooling and the wars, rebellions, and dynastic transitions of Imperial China. European scholars have found equally impressive correlations between cool, wet conditions and conflict in northwestern Europe over the past five centuries. In southeastern and central Europe, by contrast, correlations exist between conflict and warm, dry weather. Another, even more ambitious study finds strong correlation between European wars and climate changes over 2,600 years.
Quantitative climate historians often focus on China and Europe, and not only because most of them live in these regions. People across much of China and Europe have long relied on rain-fed agriculture, which should have been especially sensitive to fluctuations in temperature or precipitation. They also kept unusually detailed, and unusually continuous, records of their activities. Yet a growing group of quantitative researchers now concentrates on the much more recent history of sub-Saharan Africa, where millions continue to rely on rain-fed agriculture. Many studies correlate warming, drying trends across Africa to twentieth-century civil wars, although some emphasize that these correlations only existed under the right socioeconomic conditions.
The great appeal of quantitative approaches to climate history is that they seem to replace the messiness of the historian’s craft, and the subjectivity of the qualitative findings, with scientific objectivity and certainty. Quantitative historians have used statistical correlation not only to confidently explain the past, but also to predict the future. Already in 2007, historical geographers concluded, for example, that Chinese “war-peace, population, and price cycles in recent centuries have been driven mainly by long-term climate change.” Two years later, another group controversially concluded that the frequency of civil wars in sub-Saharan Africa would likely increase as the continent warmed, since a regional correlation between temperature and violence existed in the past.
But have quantitative scholars really found a better way of doing climate history, one that at last permits predictions of the kind that always remain frustratingly out of reach for historians and archaeologists? Well, not quite. On close examination, the soaring claims made by many quantitative scholars in fact rest on assumptions that remain frustratingly subjective . . . and at times, simply misguided.
Most importantly, the correlations identified in quantitative work are meaningless unless their trends reflect the right data. Some studies of this kind use weather observations in surviving documents to graph centuries-long trends in temperature or precipitation. Decades ago, the great meteorologist Hubert Lamb relied on much the same method to identify a hot climatic regime that he called the “Medieval Warm Period.” The medieval centuries, Lamb concluded, were at least as warm as the late twentieth century.
While some deniers of anthropogenic global warming still use Lamb’s graph, scholars have changed the name of his period to the “Medieval Climate Anomaly.” It turns out that Lamb, unversed in the art of reading historical sources, simply took medieval references to weather at face value. When the legitimate weather observations examined by Lamb are read more carefully, and used alongside climate reconstructions compiled with more reliable tree ring or ice core data, they reveal a period of modest but erratic warming in the high medieval centuries. Nothing comparable, in other words, with late twentieth-century warming.
The lesson here is that references to weather in ancient documents do not always simply reveal the state of the atmosphere in a particular time and place. The problem is much more acute when considering very long timescales. Before the instrumental era, even seemingly reliable weather observations over decades or centuries are really the product of many observers, some of whom might use different methods to record weather. Moreover, sources that may seem especially dependable at a glance – such as many European chronicles – in fact refer to weather metaphorically, or use fabricated weather events to justify the course of human affairs.
Researchers should therefore strive to use weather observations in historical documents as a starting point – only a starting point! – in a long process of reconstructing a region’s climate. Where possible, documentary evidence should be used alongside climate reconstructions compiled with tree rings, ice cores, lakebed sediments, and the many other proxy sources in natural archives. The best climate reconstructions often use the most proxies. Of course, many excellent reconstructions have now been published for most parts of the world. There is often little need to develop a regional climate reconstruction from scratch.
In quantitative climate history, multi-proxy climate reconstructions should also reveal climatic trends on the same spatial scale as the social variables under consideration. Even some scholars who do use so-called “multiproxy” climate reconstructions to find their correlations go on to match trends of global or hemispheric temperature or precipitation with trends of local or regional historical events. Yet before the onset of anthropogenic global warming, climatic trends rarely unfolded at the same time in every part of the globe. A general cooling trend across the northern hemisphere, for example, did not always lead to colder temperatures in China.
If quantitative climate historians face problems when choosing which climate reconstructions to use – or how to make them – these pale in comparisons to those that bedevil their attempts to quantify social variables. Quantitative studies of war, for example, have used makeshift and now defunct websites to determine when wars began and ended. Others have relied on historical scholarship that is well over a century old. It is as though the historical geographers, political scientists, natural scientists, and economists who typically write quantitative climate history do not recognize that the disciplines of history and archaeology are as rigorous and dynamic as their own. Naturally, correlations that rely on obsolete or untrustworthy data about the human past can tell us little about the influence of climate change on human history.
Jan de Vries, Philip Slavin, and I have also flagged a second big problem faced by quantitative approaches to climate history. Studies that find correlations over centuries, let alone millennia, rarely appreciate that social variables change through time. A statistically significant correlation between warming and economic growth in the high medieval centuries, for example, does not necessarily hint at the same kind of relationship between climate change and human affairs as a similar correlation several hundred years later. Over the course of those centuries, the cultural, economic, social, and political pathways by which climate change affects human life may have fundamentally changed, and the individuals who control those pathways will have obviously died. The question becomes: what are quantitative approaches to climate change really measuring?
That gets us into a third, and related, problem of quantifying the human past. It is one thing to quantify a particular kind of agricultural production over long timespans. Though agricultural practices can change dramatically over those timespans, even in pre-modern societies, scholars may still find correlations between agricultural yields and climatic trends that can suggest something new about the human past. Yet it is quite another matter to quantify the number or intensity of a major social event, such as a war.
Attempts to link the number of wars by decade to decadal temperature or precipitation, for example, face the challenge of quantifying long and complex wars: precisely the kind of war that often placed the greatest strain on agricultural resources also affected by climate change. Scholars might consider the Thirty Years’ War, for example, as either a single war or a series of wars, and their subjective choice would determine the correlation identified in a study between seventeenth-century climate change and European conflict. In some of these studies, the early seventeenth century may look like a time of relative peace in Germany!
Scientists have also used arbitrary numbers to decide when violence amounts to a war. Does violence rise to the status of war when at least 1,000 people have died, as some studies assume? Presumably the standard would be higher in very populous societies and lower in less populated ones, but this distinction is never made in quantitative studies. Graphing wars by quantity can also lead scholars to misrepresent changes in quality. Scholars might easily count the First and Second World Wars as only two wars, for example, yet of course their material and human costs dwarfed those of any previous conflict. If problems of this nature plague the superficially simple task of correlating the number of wars to temperature trends, imagine the challenges of determining similar correlations with, for example, economic development or cultural efflorescence!
It turns out that quantitative approaches to climate history often obscure more than they reveal. Far from providing a more objective, “scientific” way of understanding the impact of climate change on the human past, they really rely on assumptions that are every bit as subjective as those made in more qualitative work. Yet unlike many qualitative climate historians, they leave those assumptions unacknowledged.
I am convinced that quantitative climate historians could fruitfully address at least some of these problems by interacting more with qualitative scholars, most of whom work in the humanities. Unfortunately, many historians, at least, have not heard of quantitative approaches to climate history, while most quantitative scholars have little inkling of qualitative approaches to their subject. Remarkably, I have never seen a work of qualitative climate history cited within a paper that aims to identify correlation. Part of the problem is that quantitative and qualitative scholars often work in different media. While historians prioritize books, most scholars value short, multi-authored studies.
Yet collaboration is surely possible, and if so it would undoubtedly prove productive. Quantitative scholars have recently used statistical means to identify not only how climate change might be correlated to human activities, but also how it might have partly accounted for – that is, caused – those activities. Such studies have yielded models that are really variants of models that qualitative scholars had already developed. What if they had worked with qualitative scholars from the start? Meanwhile, qualitative scholars often use statistics to support their conclusions, without always understanding what those statistics actually reveal (or what they don’t). What if qualitative scholars consulted colleagues in more quantitative disciplines while developing these statistics?
At the Climate History Network, we will strive to incorporate more quantitative scholars within our ranks. Perhaps we will be able to build a shared community in the coming years, one that will yield a more comprehensive kind of climate history.
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