What can ancient cities teach us about surviving climate change?


Aztec Tenochtitlan started out as a wet city in the middle of a swamp, but has managed to thrive through conquests, epidemics, droughts and floods to become one of the largest cities in the world today: Mexico City . As I took the students to see the Aztec ruins next to the Zocalo, I wondered how places like Tenochtitlan, Beijing or Rome (the “Eternal City”) managed to thrive for centuries, while other cities failed.

In my archaeological fieldwork, I have encountered many more failed urban sites than cities that have survived for centuries. Now is the time to take a look at these early cities to find out how some of them have successfully adapted to stresses and shocks, while others have not. Perhaps this knowledge can inform current work on urban adaptations to climate change. Researchers have identified a “knowledge gap” between what we need to know about planning cities for the future and what we know. Cities of the past can help fill this gap.

Much of the research, policy work, and government action carried out under the banner of ‘sustainability’ is best characterized as ‘ambitious sustainability’ – especially because we have little idea of ​​how things are going. unfold in the long term. True sustainability means that cities, societies and institutions will survive, not just five more years, but long into the future. How to design adaptation policies and strategies without understanding urban change over longer periods?

Archaeological and historical records are untapped sources of information on long-term urban adaptations. My colleagues and I have located thousands of ancient urban settlements that spanned several centuries, as well as thousands that lasted much shorter periods. Some of these cities still thrive today (like Rome and Mexico City), while others collapsed centuries ago (like Teotihuacan in Mexico and Angkor in Cambodia). Any city that lasts for hundreds of years has overcome resource problems, withstood shocks and stresses, and solved the collective action problems of people living and working together. In short, long-lived cities must have been well adapted to their social and natural surroundings – and we must study them to see if we can determine which factors contributed to long-term urban success in the ancient world, and which challenges have led to short-term failures. Archaeologists and historians have the data, but we have not yet done the scientific work to analyze it adequately. Perhaps this material will not contain any appropriate lesson. But, perhaps, it can add something unique to the knowledge and practice of today.

Historians and archaeologists have already begun to weigh in on adaptations to climate change. One popular genre is that of disaster stories like the collapse of Classic Maya (think Jared Diamond’s 2005 book Collapse, Where The next apocalypse, by Chris Begley, 2021). While ancient disasters make a good read, such stories are both too complicated and too limited to draw firm conclusions. They promote a biased view of ancient societies. People wonder what was wrong with the Mayan cities: why did they collapse? Yet those same cities have thrived for many centuries, much longer than any city in the United States has endured yet.

Groups of historians are beginning to move beyond this craze for collapse to make connections between past and present with titles such as “Lessons from the Past, Policies for the Future: Resilience and Sustainability in Past Crises”. These studies typically describe several case studies of short-term processes (for example, the Black Death or the Roman Conquest) and claim that they provide us with lessons today. The cases are anecdotes; they are not formally compared, nor analyzed quantitatively. Yet economists have found that when they look at cities from a longer historical perspective, they are remarkably resistant to plagues, wars, and other short-term disasters. After a first setback, cities almost always come back, economically, demographically and culturally. long duration?

The main strengths of archeology are a record of change over centuries, if not millennia, and a sufficiently large sample of sites and regions to create a quantitative and rigorous science of past adaptations to shocks and change. Archaeologists now systematically apply quantitative scientific methods to topics such as wealth inequality, city sizes and business models. It is time to extend these methods to urban adaptations to stresses and shocks, including climate change and other natural and social forces.

I am part of a research group at Arizona State University that has begun to use archaeological data to address urban survival and adaptation over long periods of time. (Disclosure: ASU is a partner with Slate and New America in Future Tense.) Unlike Ambitious Sustainability, we take a rather harsh view of ancient urban sustainability: If a city or colony managed to survive for many centuries, then it was durable. I recently looked back on the results of an archaeological survey I led in Mexico’s Yautepec Valley in the 1990s, to see if they could shed light on adaptations and sustainability to climate change today. . I think they do. My students and I scoured the landscape and located some 400 sites, parts of the landscape where people once lived. Our timeline is approximate, with periods lasting one or two centuries. Some sites have been occupied for a single period of time, while others have been occupied for several millennia. What was the difference between those who survived and those who did not?

One of the findings related to urban problems today concerns the effects of city size on adaptation and sustainability. The median persistence of settlements in the Yautepec Valley was 370 years, a high rate of urban success in all respects. A closer look at the data makes it clear that larger colonies generally lasted longer than smaller ones. Several reasons stand out. The first urban centers in this region were founded next to the best agricultural land in the first century BC. These great centers have flourished for over a millennium. Their superior locations have undoubtedly contributed to their persistence and success. The conquest of the valley by the Teotihuacan Empire (circa 150 CE), however, showed that major political transformations may be larger than the size of the city in explaining the persistence. This event sparked a wave of urbanization as new administrative centers sprang up across the valley. These towns probably helped administer cotton cultivation for Teotihuacan. When Teotihuacan withdrew three centuries later, these cities were abandoned. Their size (larger than other towns at the time) was of no help when their administrative focus ended. Does the more general role of size as a driver of sustainability extend to other old urban systems? If so, it can help academics and planners determine the optimal size of cities to ensure sustainability in the future. Planners are already associating higher urban population density with greater sustainability. If larger size is also more sustainable, then perhaps the current trend of shrinking cities in many areas needs immediate political help. Economists have shown that large cities generate more growth per capita than small towns, and this finding has been replicated for older cities. It would not be surprising if these city size effects also create cities that are better able to withstand shocks and survive into the future.

Two obstacles must be overcome before scientists and planners can use knowledge of ancient history and archeology. First, my colleagues and I need to reanalyze our data for persistence over time. What conditions (city size, infrastructure, institutions) have favored urban persistence in the past? Can such results be translated for urban adaptations today? It is not a trivial task. The simple reconfiguration of Yautepec’s data to examine the persistence of settlements over time required a dedicated workforce by a post-doctoral fellow and a graduate student. Second, urban adaptation specialists and archaeologists need to come together to make specific useful links between identifiable patterns and processes for ancient cities and the factors promoting or hindering the adaptation of cities to climate change today. Archaeologists can analyze many attributes of cities and settlement systems beyond their size and persistence; we appreciate any suggestions on what to do next.

Future Tense is a partnership between Slate, New America, and Arizona State University that examines emerging technologies, public policy, and society.

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