Study on Climate Change Offers Sobering Glimpse of Possible Social Upheavals

The latest study of climate changes and their impact on earlier societies provides a sobering signal of social disturbances that might occur in the future.

Megalith site in Amapa, Brazil. (Image credit: Dr. Frank Mayle, University of Reading, UK)

The prehistoric groups explored resided in the Amazon Basin of South America hundreds of years ago, prior to European contact, but the disruptions that happened may provide some insight for present-day life, says study co-author Mitchell J. Power, curator of the Garrett Herbarium at the Natural History Museum of Utah, University of Utah.

The paper titled “Climate change and cultural resilience in late pre-Columbian Amazonia,” was published on the Nature Ecology & Evolution website on June 17th, and it traces the effects in the Amazon before 1492.

Climatic conditions in the Amazon Basin experienced natural shifts during periods when much of the rest of the Earth was also affected. These times are called the Medieval Climate Anomaly, from about AD 900 to 1250, and the Little Ice Age, 1450-1850. In Amazonia, rainfall quantities and patterns varied, which impacted agriculture and subsistence patterns.

At present, climate change is affecting several parts of the world; but the current difference is that it is caused by man.

One of the biggest issues in the years ahead may be that climate extremes will harm a number of nations, and that their “climate refugees” will be forced to move from ancestral homes into more temperate and developed regions not as badly impacted by climate change. The migrations could cause significant stresses in the host nations, Power said.

The surprising findings of the study reveal that these types of crises happened during and after the first millennium in the Amazon Basin.

“Were we getting a window into that in prehistoric Amazonia? I think so,” said Power, who is also an associate professor of geography at the University of Utah. “So it’s kind of a one-two punch: if the climate doesn’t get you, it might be the thousands of bodies that show up that you have to feed because extreme drought forced them out of their homelands.”

Climate was a key factor in the cultural and social changes in ancient Amazonia, he stressed, but the research also exposes “more nuanced” effects due to cultural and subsistence practices as well as migrations. In particular, those cultural groups that subsisted with varied food resources or polycultures and agroforestry, avoided political hierarchies with an elite ruling class, and accepted an approach of making organic and charcoal-rich soil, called “Amazonian Dark Earth”, were most resilient to extreme climate deviations.

The researchers looked for signs of prehistoric climate and culture in six regions across the huge Amazon Basin during the last few thousand years: the Guianas Coast, Lianos de Moxos, and the Eastern, Central, Southwestern and Southern Amazon. Up to 8-10 million people were estimated to have resided in the Greater Amazon region prior to European contact.

Scientists synthesized archaeological, paleoecological, and paleoclimate studies by collating evidence of variations in natural vegetation and cultigens, deviations in precipitation and disturbance regimes as well as variations in population movements and cultural practices.

Rainfall estimates were retrieved by measuring the titanium percentage present in sediments deposited by runoff, as well as oxygen isotopes in cave speleothem records from across Amazonia. Botanical remains, including phytoliths (microscopic silica formations in plant tissue that are enduring in the soil), pollen and other plant fossil-based proof of cultigens, including manioc, maize, peanuts, squash, and cotton was used to rebuild subsistence approaches through time.

Another indicator of agricultural practices by some cultures was the presence or lack of Amazonian Dark Earth (ADE) formed by the buildup of organic materials, including charcoal, into soils through time, which offers a long-term investment in soil fertility, additional buffering against extreme variations in climate.

Archaeological remains that specified social structure and presence and absence of political pyramids were items such as intricate architecture, pottery, and earthworks, including mounds, canal systems elite burials, raised fields, as well as proof of fortifications and defensive structures. Whether regions were burned to aid agricultural production was another deliberation.

Since living plants absorb an isotope of carbon called C-14 that dissipates at a pre-determined rate after death, scientists amassed hundreds of radiocarbon dates from occupation locations across the Amazon basin. This helped determine the chronology of cultural change and show how people reacted to pressure from climate change and migration.

Paleoecological data was synthesized from a system of sediment cores across Amazonia from bogs, lakes and wetlands microfossil plant remains, including pollen, phytolith, and charcoal records offer information about the kinds of plants flourished at each site and whether fire was a main process.

A tool that was central to the research is the Global Charcoal Database, which is used to investigate connections among past fire histories, climate change and the role of man worldwide. Power helped put up the database while a post-doctoral student at the University of Edinburgh, Scotland, and is part of an international team, the Global Paleofire Working Group, that continues to add to a number of interdisciplinary studies such as this one.

After synthesizing paleo data with archaeological information on agricultural practices and cultures, the team learned that a minimum of two different social system paths were in place, and that frequently they had different results, based on flexibility.

“The flexibility, or lack thereof, of these systems explains the decline of some Amazonian societies and not others …” the report says. Societies that fell were at the end of eras of growth, accumulation, restructuring, and renewal. “Those societies had accumulated rigidities, and were less able to absorb unforeseen disturbances resulting in dramatic transformation.”

Complex societies with social hierarchies and widespread earthworks, including raised fields aided intensive agriculture of a restricted number of crops, but ultimately soil leaching and other factors left the villages at risk. Such settlements occasionally were able to make short-term enhancements; but then, as crises expanded further, such as a multi-decadal drought, they became vulnerable to collapse.

However, while certain groups endured huge reorganization, the paper says, “others were unaffected and even flourished.”

The report describes migrations and conflict that occurred potentially in reaction to extreme changes in climate. It records that the collapse of mound centers in the Guianas coast around the year 1300 CE, for instance, could have happened because of an extended drought that the scientists recorded – or the expansion of a culture called the Koriabo “could have been responsible for conflicts leading to the … demise, or at least accelerating a process triggered by climate change.”

On the other hand, societies that relied on “polyculture agroforestry,” that is, varying crops such as fruit-bearing trees, “in the long term, were more resistant to climate change.” These were the cultures that were also likely to produce ADEs.

Still under discussion is whether the creation of anthropic forests was deliberate or an outcome of people living in an area for centuries and disposing of seeds, nuts, and waste that just happened to spread necessary plants and offer a different food resource. Power does not take a stance on that, saying the combination of creating ADEs and polycultures and agroforestry were both long-term solutions to alleviating food scarcity that happened during times of extreme climate inconsistency such as during the Medieval Climate Anomaly.

Diverse agriculture linked with the dark soil, with inhabitants growing squash, maniocs, corn, and perhaps trees, made some groups better equipped to endure climate change. But these practices could not stop conflicts with others who were moving into their areas because of climate-induced collapse in neighboring regions.

The circumstances remind Power of conditions in Ethiopia, a nation from which he recently returned and is working on a parallel interdisciplinary project trying to comprehend the rise and fall of the Aksumite Empire. About 85% of the population currently takes part in agriculture production, which still depends on seasonal rainfall in many regions. Climate extremes can result in the wet season coming late some years, or not come at all.

This causes a ripple effect, motivating young generations to migrate, typically to Europe, he said.

Possibly, a similar thing took place with migrations in Amazonia in the era before Columbus. The newcomers were “like climate refugees,” Power said, “which is an interesting corollary to today’s problems.”

I believe the most important aspect of the research is showing how societies respond differently to climate change depending on several factors like the size of their population, their political organization, and their economy. We started the research expecting that climate change would have had an impact everywhere in the Amazon, but we realized that some communities were more vulnerable than others. To summarize one of the main ideas of the paper, those pre-Columbian peoples that depended heavily on intense and specialized forms of land use ended up being less capable of adapting to climatic events.

Jonas Gregorio de Souza, Study Lead Author, Universitat Pompeu Fabra

S. Yoshi Maezumi, also a co-author of the paper, said teams of researchers from different backgrounds helped confront questions from various angles, “each providing a piece of the puzzle from the past.” She is a lecturer at the University of the West Indies, Mona, Jamaica; a guest scientist at the University of Amsterdam, and an honorary research fellow at the University of Exeter, UK.

“Together, we have a better understanding of the long term changes in climate and human activity,” she said. “These long-term perspectives on how people responded to past climate variability, including droughts and increased fire activity may help provide insights into human adaptation and vulnerability to modern anthropogenic climate change.”

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