Climate change had a deep impact on pre-Columbian societies. Population declines and climate-driven collapse of complex societies preceded the Columbian encounter throughout the Americas—from the pueblos of the U.S. Southwest, to the classic Mayan society in Mesoamerica, to the Tiwanaku state in the Andean highlands.
Little has been known—until now— about pre-Columbian responses to climate change in Amazonia. The region’s vast remote stretches have left scientists with significant knowledge gaps about what happened there in the millennia before the European encounter. The period prior to the Europeans’ arrival was beset by upheavals from droughts and increased rainfall.
Did the resulting fragility of some of these cultures contribute to their attenuation or demise before the Europeans brought with them both disease and violence?
Archaeologist Jonas Gregório de Souza, a Marie-Curie Postdoctoral Fellow at Pompeu Fabra University in Barcelona, and colleagues set about to find answers to that question using archaeological and paleoclimatic information for six geographic subregions within the Amazon.
The team expected to find that drought led directly to societal decline. But the data showed another picture. Some pre-Columbian cultures failed, while others endured.
Resilience in the face of climate change depended on the type of land use the groups practiced, as de Souza and his colleagues describe in the June 17 Nature Ecology and Evolution.
De Souza spoke with Scientific American about his findings and what they can teach us about our ability to cope with current climate change. (An edited transcript of the conversation follows.)
How big was the population in the Amazon region before the European encounter?
The estimates vary a lot. Some people are saying 10 million. More conservative estimates are six to eight million, and I think that's more reasonable.
And how were these people organized? Was it one or many cultures?
The Amazon is huge. You had a huge variety of peoples, and cultures there. Some of the groups were large, others we don't know, and others were not. The upper Xingu region [southern Amazon, part of contemporary Brazil] has massive fortified sites over a kilometer across. And you have thousands of people living in each of these settlements, all connected by roads. That's a very different case than the Geoglyph region [In the southwestern Amazon], where we have this monument but we haven't found a single habitation site yet.
Were they warriors?
You have these fortified [Xinguano] villages so they were certainly involved in some kind of warfare. You have the same phenomenon in the central Amazon around a thousand years ago. There were sites that were surrounded by defensive enclosures also in southwestern Amazon, in the Bolivian Llanos de Moxos.
And in this period, these groups had to face two climate events?
These two periods are well known in other parts of the globe. The Medieval Climate Anomaly appeared around a thousand years ago. It was a period in which temperatures were a lot higher during the Middle Ages. For example, this is when the Vikings started exploring Arctic regions. After that, around 1500 to about the 19th century, it started to get gradually cooler. And this is what was called the Little Ice Age.
In the tropics, in Amazonia, we didn't know until recently if and how those periods had significance. In the tropics, the temperature is not so much of an issue, the main factor is precipitation, whether it's wetter or drier.
How did you determine the effects of climate change, infrastructure, and political hierarchy in these societies?
We combined the archaeologic data, to know when the cultures started, when they were at their peak, when they ended. Archaeology can tell you a lot about things like settlement size, whether there's hierarchy or not. And then we also had paleoecological data that tells us a lot about land use: whether people were cultivating, whether they were burning the forest, what crops they were growing. We have pollen data, but also other proxies. We also have charcoal data that tells you a lot about the anthropogenic fires in the past. And most importantly we had the oxygen isotope records as a proxy for past variation in rainfall.
You studied six different regions; which surprised you the most?
Everything was a surprise because there didn't seem to be a pattern. You see these periods of drought and instead of seeing a society decline you see it apparently being unaffected. The Santarém culture in the lower Tapajós River [in the eastern Amazon, currently Brazil] of course was very surprising because they didn't seem to have been affected at all by the massive drought. At the same time the Marajoara [in the same region as the Santarém] were declining.
And you found that you could divide these groups into two types of societies, one that adapts to climate change and one that perishes?
Whenever we saw a period of extreme drought in the paleoclimatological records, sometimes we saw some societies declining, but at the same time, in the same region, you had other cultures that were apparently unaffected. We asked ourselves: Is there anything common among the societies that were flourishing and among the societies that were perishing in these events of climate change?
And then we brought several conceptual ways of explaining that. And one of them is the kind of land use system you had. So, you can divide the Amazonian societies of the late pre-Columbian period basically into these two groups that we defined. One of them adopted this agroforestry polyculture strategy, as we call it, that involves managing the forest and enriching the forest in the long term with useful species, [combining the cultivation of multiple annual crops and helping edible forest species to thrive] and gradually enriching the soils as well, forming the anthropogenic dark earth of the Amazon, the terra preta. This fits well with a category that is “landesque capital”' that is discussed in anthropology, where you are gradually enriching the environment in a way that might be beneficial for you, but over the long term it's going to bring permanent benefits for a future generation. The opposite of that would be the other model of land use, which is more intensive and is directed toward specialization and immediate production—for example, raised fields (for cultivation), or water-management systems.
You also found a relationship between the type of land use and how complex the society was?
That's also an important thing. Look at the Monumental Mound region [Llanos de Moxos], for example, where you have clear evidence of a ruling elite. Probably when you have this form of social organization, you need to have a more intense production of surplus in order to sustain the rulers, so the social organization and the type of land use may be connected.
You propose that the social and economic systems determined whether some of these Amazonian societies would be resilient to climate change.
The idea is that whether you are going to resist or not climate change depends basically on the kind of land use, on your economic strategy. Pre-Columbian societies were affected by climate change, but the way that they reacted and responded to climate change depended heavily on the kind of social organization they had and the kind of economies they had, and what was their system of land use.
What were the characteristics that made the agroforestry polyculture strategy groups more resilient to climate change?
They do not depend upon a single specialized way of using the environment. They are very adaptable, and they think in the long term. When you do polyculture agroforestry, you are mimicking the natural ecology of the rainforest, which is very different from clearing everything and planting a single crop, for example, which is what we do now.
And what does that tell us about our reality today?
I don't like giving lessons about what we should do for the future. But of course, I understand that there may be a message that some people may want to read, which is there are certain forms of land use which are simply not sustainable in the tropical forest. Perhaps the way we think about our economies nowadays, clearing large tracts of land, burning everything, and then planting single crops probably is not going to be a sustainable strategy in the long term, especially in a scenario of climate change which we are facing even now.