This blogpost concerns the apocalyptic crises that periodically afflict the world. I choose that A-word deliberately. We conventionally speak of the Four Horsemen of the Apocalypse, of famine, plague, death, and war, and during times of worldly catastrophe, people in many societies – not just Christian – turn readily to broadly apocalyptic movements and sects. Some endure long enough to become potent religious and social movements, and historians of religion neglect them at their peril. But as I will argue, those four menaces are actually much closer tied together, even inevitably so, than we have ever realized. The connection of climate-related disasters to plague, pandemic, and pestilence is very strong indeed. Understanding that dimension is vitally important for approaching our past, our history – but also, our futures, in an era of rapid climate change.

Repeatedly through history, civilizations and societies have collapsed more or less simultaneously around large sections of the world, forcing us to seek some explanation going beyond immediate wars and political crises.  Commonly, we look for some climate-related catastrophe, such as the effects of a super-volcano, or a megadrought, and often, such calamities cast a very long shadow on later concepts of religion. This was the theme of my recent book Climate, Catastrophe and Faith (Oxford University Press, 2021).

Such times of cataclysm usually correlate with outbreaks of pandemic disease, of plagues broadly defined. Twin super-volcanoes in the 530s AD set the scene for the Justinianic Plague that did much to finish off ancient civilization. One massive transcontinental crisis in the late 620s was promptly followed by a ruinous plague in the Middle East. The horrific climate disaster around 1320 was followed a generation later by the Black Death, which eliminated a large proportion of the population of Europe and Asia. So common are such linkages as to suggest that they go beyond mere coincidence, of troubles coming not as single spies but in diseased battalions. It is becoming increasingly likely that the climate meltdowns are directly driving the pandemics, through means that I will discuss shortly.

This point came to mind as I was reading a recent study of an era that has ling intrigued me. At the end of the third millennium BC, around 2200 BC, we witness the kind of simultaneous civilizational collapse that we know so well from other eras. Among other things, the Old Kingdom of Egypt collapsed (after a glorious half-millennium), as did the Akkadian realm, and we see all the signs of a sudden Dark Age – shrinking trade and communication, steeply declining populations, and general state failure. This is especially well studied in Mesopotamia, but signs of real trouble show up far more widely, as far afield as China and Western India. The best current theory points to a megadrought on an epic scale, which probably lasted for centuries. It all looked very much like the similar catena of horrors that befell the Mediterranean a thousand years later, and which precipitated the fall of the Mycenean and Hittite worlds.

Here is a lament on the collapsed Akkadian empire written not long after the event, in The Curse of Akkad:

For the first time since cities were built and founded, The great agricultural tracts produced no grain, The inundated tracts produced no fish, The irrigated orchards produced neither syrup nor wine, …

He who slept on the roof, died on the roof, He who slept in the house, had no burial, People were flailing at themselves from hunger.

I am quoting from Elizabeth Kolbert’s important 2008 series on “The Climate of Man,” in the New Yorker.

Something very bad was happening around 2200BC, during what is called the 4.2 kiloyear event – that is, 4,200 years before the present. Far from being a transient episode, that great third millennium event – the snappily named “4.2ka BP” – is now acknowledged as marking a decisive shift in global climate. It is the beginning of the portion of the Holocene era in which we have lived ever since, the so-called Meghalayan. It is quite conceivable that volcanic action or changing levels of solar energy might have been involved – or at least, something sufficient to transform the world’s oceanic currents. Just maybe, what about some extraterrestrial event, like a meteor impact?

We have one really infuriating problem here, in that we can easily find a very plausible candidate for the culprit, but the date as it stands just does not work. I am referring to the invaluable records preserved in the bristlecone pines of the US West. These map particularly severe years very effectively, and show us for instance that the globe was suffering particular horrors in 43BC, in the 530sAD, in 626AD, and other key dates. Almost certainly, the Something in question was some kind of volcanic action that darkened the skies, ruined crops, and induced far reaching famines. In these long biological arches, one date stands out very clearly as an utterly dreadful time to be alive, and that was 2036BC. It is a reasonable hypothesis that the trauma wrought on those Western trees was volcanic in nature, something at least on the scale of mighty Mount Tambora in 1815. If that was correct, it is scarcely surprising that civilizations would topple like dominoes. But in terms of explaining the collapses we have noted around the world, the date is just wrong, and it is off by two centuries. Four thousand years before the present simply is not 4,200 years before the present. Is it possible that the scholars looking at those bristlecone pines are in error about the exact dating? Could societies really have collapsed so utterly in 2200BC, and then have been hit again by a new and totally different cataclysm just a couple of hundred years later? I am frankly unable to resolve the contradiction.

But leaving aside that 2036 (??) event, let us agree that the collapse of the late third millennium was indeed climate-related. So far, so catastrophic. But now we turn to a brand new piece of research by a team of scholars headed by Gunnar U. Neumann, writing in Current Biology, in a piece entitled “Ancient Yersinia Pestis And Salmonella Enterica Genomes From Bronze Age Crete.” To over-simplify, the researcher report the surprising discovery of remnants of the bacteria associated with plague and typhoid, in Crete, and around 2000BC. The authors are admirably restrained in drawing conclusions, suggesting only that these diseases might have plated some role in the general social collapse. “Infectious diseases should be considered as an additional contributing factor; possibly in an interplay with climate and migration.” Or to quote a report of the project in Science Alert, “Extinct Pathogens Ushered The Fall of Ancient Civilizations, Scientists Say.”

This is all absolutely plausible, but the argument could very reasonably go further. For one thing, climate, migration and disease need not be separated from one another. A climate disaster like that of the years around 2200BC would assuredly drive migration, and would prepare the way for disease outbreaks. Think through the effects of climate change, whether a short term crisis or a longer term shift.

When studying such diseases, scholars use the concept of the epidemiological triad or triangle, which gives due attention not just to the agent directly causing the disease but also to the host carrying that agent and the environment in which it spreads. Sudden climate changes transform all three components of that equation. They cause the emergence of agents of infection by assisting the evolution of new pathogens or by spreading others that had previously existed only in very limited geographical regions. They also alter the behavior or distribution of hosts. New diseases often arise in animals before crossing over to human beings by various mechanisms of transmission; such diseases are called zoonotic, and the COVID-19 pandemic offers a notorious recent example. As climatic changes around the world transform ecologies and habitats, they have their impact on the flora and fauna with which humans come into contact.

The El Niño cycle illustrates the processes at work. Depending on the region, El Niño events are particularly linked to higher rainfall and floods, or to severe heatwaves and drought. Any or all of those changes can profoundly affect the spread of disease vectors, notably insects. In warmer climates, that especially means mosquitos, but a wide variety of pests can benefit from such changes. Not surprisingly, then, El Niño cycles often correlate with disease outbreaks. Long-term studies of Chinese history, for example, suggest a strong connection between climate and epidemic cycles.

Whether through changes in agents or hosts, successive climate phenomena of various kinds have caused the emergence and spread of such new or newly mutated diseases. The best known example is bubonic plague in the 1330s, but similar trajectories can be traced for typhus in the 1480s, influenza around 1560, and cholera after 1815. That last example is particular clear. Cholera began an evolutionary leap immediately following the Tambora eruption, and began a career that killed many millions over the next century.

My argument, then, is that climate-related disasters are very likely to have epidemiological consequences, and that something like THE THING that befell in the later third millennium BC would naturally be followed by new disease onslaughts over the following few decades, very much as the new Cretan evidence is suggesting. As the Current Biology authors suggest, “the massive droughts described in association with the 4.2 ka BP climatic event could have resulted in a shortage of clean drinking water and an immunologically weakened population with higher susceptibility to infectious diseases.” Just think of the role of all those climate refugees as disease vectors.

Powerfully reinforcing these lessons is a new article by Camilo Mora et al, “Over Half Of Known Human Pathogenic Diseases Can Be Aggravated By Climate Change,” in Nature Climate Change (2022). Here is the major takeaway:

It is relatively well accepted that climate change can affect human pathogenic diseases; however, the full extent of this risk remains poorly quantified. Here we carried out a systematic search for empirical examples about the impacts of ten climatic hazards sensitive to greenhouse gas (GHG) emissions on each known human pathogenic disease. We found that 58% (that is, 218 out of 375) of infectious diseases confronted by humanity worldwide have been at some point aggravated by climatic hazards; 16% were at times diminished. Empirical cases revealed 1,006 unique pathways in which climatic hazards, via different transmission types, led to pathogenic diseases.

It is foolish and short-sighted to write any kind of history of plagues and pandemics without paying full attention to the climate context, and the years around 2200BC or so illustrate this perfectly. Let’s see what happens in the Third Millennium, AD.

Climate change and pandemic, pandemic and climate change… never try to separate them.

Not for the first time, my thanks to Nachman Ben-Yehuda for keeping me up to date!

The evidence for this third millennium event was originally assembled in H. Nüzhet Dalfes, George Kukla, and Harvey Weiss, eds., Third Millennium BC Climate Change and Old World Collapse (Berlin/Heidelberg: Springer, 1997). See also:

Sturt W. Manning and Mary Jaye Bruce., eds, Tree-Rings, Kings, and Old World Archaeology and Environment: Papers Presented In Honor Of Peter Ian Kuniholm (Oxbow Books, 2009).

Karen Radner, Nadine Moeller, and D.T. Potts, eds., The Oxford History of the Ancient Near East. Volume I, From the Beginnings to Old Kingdom Egypt and the Dynasty of Akkad (Oxford University Press, 2020); and also Volume II: From the End of the Third Millennium BC to the Fall of Babylon (2022).

Matthew W. Salzer, Andrew G. Bunn, Nicholas E. Graham, and Malcolm K. Hughes, “Five millennia of paleotemperature from tree-rings in the Great Basin, USA,” Climate Dynamics 42 (2014): 1517–1526.

Harvey Weiss, ed., Seven Generations since the Fall of Akkad (Wiesbaden: Harrassowitz Verlag, 2012).

Harvey Weiss, ed., Megadrought and Collapse: From Early Agriculture to Angkor (Oxford University Press, 2017).

Full disclosure – every aspect of this account of the Third Millennium is of course subject to scholarly debate and revision, but what I am saying here reflects the best view as of present.