Collapse of Complex Societies: Did Drought Kill off the Mayans?



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When I wrote my blog about the effect of climate on the rise and demise of complex societies, I had no inkling that there was another paper on this subject in the works. But the latest issue of Science (9.XI.2012) has an article by Douglas Kennett et al., Development and Disintegration of Maya Political Systems in Response to Climate Change.

This article is actually better than the 2011 article I critiqued in my previous blog. Not only the authors present the climate data, but they also have quantitative data on social complexity and warfare. They use two proxies for social complexity: the total number of dated monuments and the  number of urban centers with dated monuments. These data are plotted at 25-year intervals and nicely show the trajectory of social complexity in the Mayan lowlands:

The graph also shows the number of war-related events per 25 years, reflected in the dated inscriptions, and the Inter-Polity Warfare Index (the proportion of war-related events among all recorded events).

So far so good. But after presenting all these neat quantitative data, the article inexplicably stops, and there is not a hint of statistical analysis that I could find, either in the article, or in the Supplementary Online  Material. Is this the case ‘as usual’ in Science, when all ‘hard science’ is lavished on constructing climate data, and none on determining whether there is a statistically defensible effect on social dynamics?! Yet there is nothing particularly difficult in performing such an analysis – if the authors send me the data, I’ll be happy to do it for them. Such an analysis would need to be done thoughtfully (you need to take into account temporal autocorrelations, and there are a couple of alternative model specifications I can think of), but it is routine for any competent statistician. Yet it was not done.

In the absence of statistical analysis I am reduced to eyeballing the data, and it doesn’t look good for the climate hypothesis. In particular, the period of growth in social complexity during the Late Classic era (600-800 CE) coincided with worsening climate. I could go on, but this is precisely the reason we need statistics – to reduce the subjective element in evaluating the hypotheses.

There are some other puzzling aspects of the argument in the paper. I know very little Mesoamerican history, but wasn’t there another cycle associated with Chichen Itza during the 11th and 12th centuries? I was puzzled why there was not another  peak in the data, until I read the supplementary materials, where I  discovered that the inscriptions data were quantified until only 1000 CE. I’d like to see where the heyday of northern Mayan lowlands (Chichen Itza and several other sites) would fall in relation to the eleventh century’s drought, the worst in Mayan history.

I am also curious about the great Mexican drought of 1535. I am not sure how relevant it is that it caused 1 million of deaths in Mexico – most of Mexico is in the much drier belt than Yucatan. Was there a drought in Guatemala in 1535?

The news piece accompanying the article cites archaeologist Andrew Scherer, who notes that the chief crop, maize, requires only 400-600 mm of rain, whereas the Mayan heartland gets 2000-3000 mm annually. Even a severe drought that reduces this amount of rainfall by 40% should  not have catastrophic consequences for Mayan agriculture. Additionally, the shift of Mayan complex societies to northern Yucatan (Chichen Itza etc), which gets less rain than the heartland around Tikal, does not make sense in light of the climate hypothesis.

Furthermore, the Mayan collapse in southern lowlands (around Tikal) resulted in depopulation of over an order of magnitude – 10-fold, perhaps 100-fold.

You cannot achieve such a decline by reducing the carrying capacity by 40%.

It is clear to me that social factors played a much more important role in the Mayan collapse, with climate, at best, a triggering mechanism and, at worst, irrelevant to civilizational dynamics in this part of the world.

So what is the alternative explanation? The truth is that I mostly know Old World history, and very little about Mesoamerica. But this is a blog, so here I go.

First, there was not just one Mayan collapse, but probably a whole series of them. Southern lowlands collapsed in the 9th century, northern lowlands in the 13th century. Before that there was El Mirador collapse around 150 CE. I would not be surprised if, when our data on Mayan cliodynamics gets better, we will find there were societal collapses every 2-3 centuries. In other words, I would expect that the Mayan cultural sphere, just as any other complex society, experienced secular cycles.

As I discuss in my book War and Peace and War not every secular decline leads to civilizational collapse. Such declines interact with longer dynamics involving the waxing and waning of asabiya – social solidarity underlying the ability of the society for concerted collective action. Let’s use as an example the case of Roman Empire, for which we have much more data, and which I wrote about extensively. During the early and middle Republic (especially fourth and third centuries BCE) the Romans were a nation characterized by high asabiya (for reasons explained in War and Peace and War). The Roman state went through several secular disintegrative periods – the crisis of the first century BCE, the crisis of the third century CE, and the final dissolution during the fifth century CE. By the fifth century CE Italy became an ‘asabiya black hole’, an area practically devoid of people capable of cooperating with each other to construct a functioning society.

The collapse of the Roman Empire was as bad in magnitude as the Mayan collapse, especially when considered in relative terms. At its peak in the 2nd century CE Rome (the city, not empire) had over 1 million people. By the seventh century, Rome’s population declined 100-fold, perhaps even 1000-fold.

Ancient Rome truly died. We tend to forget it, but when it got going again, it even started in a different location. The center of new Rome shifted to areas that were previously sparsely built – to Campus Martius (Field of Mars) and Vatican.

Returning to southern Mayan lowlands, the degree of societal collapse there is not particularly remarkable by historical standards (in addition to the Roman example I can cite dozens of others). According to the climate hypothesis, once rainfall amounts increased, we should have seen revival of civilization in southern Mayan lowlands. But it did not happen. Was it because the area around Tikal turned into an asabiya black hole? Asabiya black holes are highly persistent. Southern Italy is still a low cooperation area, fifteen hundred years after the Roman collapse.

In my opinion, when looking for causes and mechanisms of the rise and collapse of complex large-scale societies we should first pay attention to social factors, most importantly the capacity of the society for cooperation. Clearly the  resource base of the society is important and climate fluctuations affect it. But it’s a secondary factor – a modifier, rather than a prime mover.

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Charles Weber

Dear Peter Turchin,
Climate and culture are not the only factors that cause civilizations to collapse. Poisons, disease, soil erosion and warfare can have serious affects. Ten generations of Roman aristocracy died out probably from lead poisoning in plumbing and wine bottle stoppers. Opium more than likely damaged Chinese empires. The plagues had serious affects on Europeans. Soil erosion probably contributed considerably to the decline of Greece. Hitler and Stalin killing off the cream of Russian manhood probably ranks high on the reason for the unbelievable corruption in Russia, a corruption so pervasive that one can not even mail anything of value to them. I am afraid that the criminal removal of essential nutrients from and addition of poisons to food in our own society, which is causing horrendous medical costs, is a considerable part of the current decline in the USA. We are enormously productive, perhaps 20 times as productive as 200 years ago. As a result the awful effects from that disaster are much muted.
Of course droughts can have serious affects also. I believe a society in or near Ethiopia died out because of drought. Indeed, the drought can be caused by the actions of men in agriculture. The baring of soil to the sun’s rays is probably the primary cause of the current warming of the Earth (see ). Luckily there are ways to mute its affects, or if not, coping with them, for instance recharging ground water from floods (see the above URL).
My vote is for no more poison, no more erosion, no more war, very little bare soil, and definitely no more ruining food. Does my vote count?
Sincerely, Charles Weber

Charles Weber

PS There is a good chance that the Mayan civilization collapsed from diabetes caused by the import of chili pepper from Bolivia. Chili pepper contains capsaicin poison, which has dramatic affects on the nerves leading to the insulin gland (see ). It may account for a considerable part of the reason why the Central American Indians collapsed so quickly before a handful of Spaniards as well.
Sincerely, Charles Weber

Gene Anderson

I have some expertise here, as a Mayanist. The drought was certainly a factor, but, Peter, you’re dead right in your critiques. In particular, the central lowlands were already declining before the worst drought hit. A. Demarest has been arguing for years, with a lot of evidence, that warfare between the many small city-states got out of hand. It would have contributed to ecological overshoot, too. Also, yes, there were other local collapses in Maya history. Even so the 900s event was unique–you can see that most clearly in the forest tree pollen–the pollen record in the central lowlands goes from lots of grass and corn in the 800s to lots of forest trees and very little grass by 1000. The reason that drought is so devastating, even when it’s not extreme, is that the Maya insist on living on corn (75% of calories–from ancient times to conservative settlements today) and the drought hits mainly in June and July when the corn absolutely has to have rain. We are having this problem now in the communities I study: global warming leads to drier summers and more hurricanes, and either one totally wipes out the corn. Also it leads to more of what the Maya call “hot rains,” rains so warm that they stimulate the growth of fungus blights. So small climate shifts make the Maya area unproductive.
The point about the northern lowlands escaping the collapse, and going right on, has been made many times (see Yoffee and McAnany’s book on collapse). There was evidently less reduction in summer rain there, and also there is much more permanent water because of all the cenotes (water holes), themselves a product of the shattering of that whole area by the meteorite that killed the dinosaurs! Anyway–Chichen Itza declined because of local warfare–the action shifted to Mayapan, still a big city at conquest time.
Mayaland is not as subject to overall cycles as the Old World, because it was never united. It is too homogeneous and poor an environment to be unified easily. It isn’t really unified now! City-states would rise and fall, but only a major climate event would make them all fall at the same time.
A possibility that one of my students thought of is that the Classic Maya developed high-yield but sensitive maize varieties, which were more sensitive to drought etc. than the tough, hardy ones the Maya currently grow. NO way to test this, darn it!
Incidentally–re the other post–capsaicin is good for you. Far from being a poison, it’s very healthy.
best wishes, Gene Anderson

Charles Weber

Before we make statements such as “capsaicin is good for you” we had better perform experiments on primates. To feed it to millions of people and then perform experiments is crazy. Of course the primates I would like to see it performed on would be the CEOs of junk food corporations. Unfortunately that would be politically impossible.

Peter Turchin

Gene, thanks very much for your comment. Lots of interesting stuff here. Your point about the timing of rain being the critical variable is particularly interesting. This is new to me, although I should have thought of it, given my ecological background, since corn is basically a grass.

This reminds me of one thought that I had when reading the news piece in Science about Mongolia. The climate researchers there will quantify rainfall fluctuations by looking at tree rings. The resource for the livestock, on the other hand, is grass. As an ecologist I know that grass needs precipitation during the growing season, while trees, having deeper reaching roots, can utilize water stored in the soil since winter. This is by the way why areas with the same amount of annual rain can have very different biomes. If you have moderate amount of rain that peaks in winter, you tend to get mediterranean chaparral (brushy vegetation). But if the same amount of rain falls primarily in summer, you get grassy steppes. So the precipitation chronology based on tree rings can mislead you very much if you are primarily interested in the productivity of a grassland.

On your other point, that different areas within the Mayan lowlands were rising and falling asynchronously, and then most of them collapsed together. Tom Hall and I have a paper in JWSR where we discuss how exogenous perturbations (e.g., climate fluctuations) can synchronize cyclic systems, if they have similar periods, but otherwise are not connected:

Separate cycles can get entrained by this mechanism. This probably accounts for why the eastern and western ends of Eurasia would recurrently fall in synchrony, and may help to explain simultaneous collapse of the southern lowlands in the ninth century.


I thought the issue had been settled by Julian Jaynes.

Charles Weber

Dear Peter Turkin;
It is much more likely that a poison, capsaicin in chili peppers as imported from Bolivia, is what wrecked the Mayans. see the discussion below.
I suspect that a poison, capsaicin in chili pepper, is one of the causes of diabetes, especially in combination with a copper deficiency. I have published this in the 2008 Medical Hypotheses 71 p 323-324, entitled “Does capsaicin in chili cause diabetes?”. You may also see some discussion of this in . So it would be a good idea not to recommend chili in diets until such time as the matter is established on animal experiments, or to use capsaicin as a medicine. It is not impossible that capsaicin causes other problems as well.
Dr. Beale speaks of having a worm that one can do experiments involving diabetes on. Maybe one of your students could use such a technique. His URL is .
You may see how to increase copper in the diet in (chili probably operates synergistically with a copper deficiency).
You also may find a book about potassium nutrition as it relates to heart disease, gout, rheumatoid arthritis, high blood pressure, and diabetes, useful for your library. Its availability through Paypal along with its introduction and table of contents may be accessed in .
Sincerely, Charles Weber

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