Video: The Ancient Maya Response to Climate Change: A Cautionary Tale


Ancient Maya civilization—known for its cities, monumental architecture, ceramics, hieroglyphic writing, and advanced understanding of mathematics and astronomy—suffered a major demise between the tenth and eleventh centuries. The causes continue to be investigated and debated. Paleoenvironmental research over the past twenty years has revealed that the demise coincided with a prolonged intensive drought that extended across the region, providing compelling evidence that climate change played a key role in the collapse of the Maya. B.L. Turner examines this evidence and the complex social and environmental conditions—including land use and landscape changes—that affected Maya societies. 

Gordon R. Willey Series Lecture recorded 2/27/20

About the Speaker

B.L. Turner II

Regents Professor and Gilbert F. White Professor of Environment and Society, School of Geographical Sciences and Urban Planning and the School of Sustainability, Arizona State University     

B. L. Turner II studies human-environment relationships from prehistory to contemporary sustainability. Focusing on the dynamics between society and land, his research has addressed the ancient Maya, smallholder agriculture in the tropics, tropical deforestation, and sustainability science. Dr. Turner is a member of both the U.S. National Academy of Sciences and the American Academy of Arts of Sciences, and serves as Associate Editor of Proceedings of the National Academy of Sciences. He has served on numerous national and international organizations addressing land, climate change, and sustainability. He holds a PhD in geography from the University of Wisconsin, Madison, and a MA and BA in geography from the University of Texas at Austin. 

Presented by the Peabody Museum of Archaeology & Ethnology and the Harvard Museum of Natural History in collaboration with the Mexico, Central America, and Caribbean Program at the David Rockefeller Center for Latin American Studies, Harvard University.


Read the Harvard Gazette article about the lecture.


The Ancient Maya Response to Climate Change: A Cautionary Tale

[00:00:10.08] Billie Turner II is a distinguished American geographer, member of the National Academy of Sciences, and prominent among the third generation of the Berkeley School of Latin Americanist geography. In August 2008, he took a position as the first Gilbert F. White Chair in Environment and Society at Arizona State University, where he was named a regents professor thereafter-- the highest faculty honor that you can get at ASU.

[00:00:41.34] For most of his career, though, he taught at Clark University in Worcester, which has the best cultural-geography program in the US of A-- little known fact in this end of Massachusetts, but it's quite true. And there he served as Alice C. Higgins and Milton P. Professor of Environment and Society, and was director of the Graduate School of Geography. To give you a flavor for his personality, he lists as one of his hobbies, entertaining graduate students. So this was very much in keeping with that. He's a highly-entertaining friend and companion.

[00:01:20.25] So our speakers is the first son of Billie Turner, noted botanist, who mentored the HMSC's very own Diana Munn Xochitl, where's Diana-- for her master's degree. So that's a family name around here. He has a BA and MA in geography from UT Austin, and received his PhD at the University of Wisconsin Madison in 1974, for work on ancient Maya agriculture-- really pioneering work. At the time, people didn't realize the degree to which the Maya participated in agricultural-intensification techniques.

[00:01:57.33] He focused on some agricultural terraces he discovered in the region of Rio Bec. That work, and Billy's studies of Maya agricultural intensification caught the eye of Harvard's distinguished Maya archaeologist Gordon R. Willey, who invited Turner to come to Copan to do research on human ecology on his own project-- the Copan Valley Sustaining Area Project-- from 1975 to '77. that is where he and I and Richard Leventhal-- Gordon's field director on that project, and also the sponsor of the Gordon R. Willey lecture series-- so we always tip our hat to Richard, for thinking of his alma mater on these events. That's where we first met Billy Lee Turner.

[00:02:45.12] His work was so compelling that he was asked by the director of the following government-sponsored project to do a complete environmental study and assessment of the valley and region. Turner's contributions to knowledge have evolved from his interest in human impacts on the natural world-- a universally-important topic. His early study was on the borders of archeology and geography-- sort of a border kind of person-- fueling his interest in agricultural pathways and livelihoods, more generally-- particularly, agricultural intensification.

[00:03:22.80] As an authority on agricultural systems, he produced several influential texts on the theory of agrarian change. So this is big-picture stuff. The environmental transformations that have accompanied population growth in Central America led to a broader engagement with cultural ecology and the human relationship with nature. A major initiative with colleagues at Clark University culminated in the volume he edited with Bill Clark and others, Earth Transformed By Human Action, published in 1990 by Cambridge University Press-- now a National Academy of Science classic. It was a major stocktaking of anthropogenic impacts on the planet.

[00:04:08.76] Over the past 20 years, Turner has led or participated in other research on the science and dynamics of global environmental change. He's written or co-edited a dozen different books and countless articles and book chapters, including the article that got me to thinking about him as the person that we should invite for this lecture. It was one that he co-authored with Jerry Sabloff, Gordon Willey's most distinguished former student, published in Science, on the classic "Maya, Collapse and Climate Change."

[00:04:42.72] Turner is a member of the National Academy of Science-- as I mentioned, a fellow of the American Academy of Arts and Sciences, and the American Association for the Advancement of Science, and the Center for Advanced Behavioral Studies at Stanford. The list goes on and on-- many distinctions and honors over a very productive career. His interest in specific impacts of populations and societies on land use, change, and alterations and land cover led to a return to fieldwork in Central America, supported by several large research grants and working with a number of PhD students. Again, that graduate student theme, very important to him.

[00:05:23.45] The specific focus was to understand contemporary patterns of land use, informed by social investigations and GIS and remote sensing. He is associated with the development of what's called LUCC analysis-- Land Use/Cover Change-- and ways to assess it, as concern grows about tropical deforestation and agricultural expansion. He's also promoted the emerging field of sustainability science, which is focused now at Arizona State University. I could go on all night, but I'd much rather, as I'm sure you would, hear what he has to say. So please give me a lot of warm welcome for our dear colleague and friend, Billy Lee Turner.

[00:06:06.60] [APPLAUSE]

[00:06:12.17] Before I move on to the lecture, I do want to say something about this wonderful gentleman and wonderful scholar, Gordon R. Willey. The last time I lectured at Harvard on the Maya was 1985, invited by Gordon to come give a small lecture to the archeology program. But he ultimately was very important in my career. As Bill pointed out, he asked me to join the Copan Project. As a graduate student, I had a paper that made it to Science. And I know he was deeply involved in approving that for that particular journal. When I came up for promotion and tenure, he wrote a very short, two-paragraph letter, but it was terribly powerful. People were sort of in awe. And he subsequently did a lot of things that helped my career in multiple ways.

[00:07:09.13] I never had any sort of negative with the Gordon, other than one. And it was in 1985, just before I was to give the lecture that he invited me. This was a time period in which they were searching for his replacement. And as we were walking out of his office, he tapped me on the shoulder. And he said, Billy, you can't take my place. And that was the end of that. [LAUGHS] And therefore we move on.

[00:07:36.34] All right, a little bit of something about myself, even though you heard. I am a human environmental scientist, or I call myself a human-environmental scientist. I work at the intersection between the social and the environmental. The problems are framed in terms of the social and environmental. And the way to proceed to the analysis is the interaction of the social and environmental.

[00:07:58.77] So what does that mean? Those people at the intersection often are truly not horribly in-depth in one or the other. And here I am talking about the Maya. I'm not a classical Mayanist, and I'm not a classical paleo environmentalist. And I'm going to try to bring those things together regardless.

[00:08:20.19] So what am I going to talk about? Well, I'm talking about the societal demise and depopulation that takes place in the Maya lowlands about 850 to 1000 Common Era. Part of my title was, cautionary tale. Why the cautionary tale? For those for reasons that you see there.

[00:08:40.83] Initially, and many people may not know this, the first explanation that I can come across for the collapse-- I mean demise and depopulation, although from time to time I'll use the word collapse, because that's what we've long called it-- the collapse. One of the initial explanations was climate change. And then we went 70 years in which climate virtually was verboten when it came to assessments of climate. And then the paleo environmentalists hit us over the head with, my god, look at what we found climatically. But, as we'll learn from those people that do social environmental systems, or sustainability science and whatnot, those kinds of interactions are invariably complex, and it's extremely difficult to pinpoint one forcing function that creates the situation.

[00:09:32.01] So that's the cautionary tale element. Now a little bit more about what I'm talking about. I'm talking about the demise and depopulation in this area that you see in yellow. I think it looks yellow anyway. This is what's often called the elevated interior, or interior uplands of the Maya lowlands. It is a rolling karstic terrain, reaching elevations about 350 meters max, and typically with groundwater at 100-or-150 meters below the surface. It was this area that before the demise and depopulation was the most densely settled and most landscaped-transformed area in the Maya lowlands. If we put up a map of all the big sites, you would see more clustered in this region that I'm pointing out than anywhere else in the Maya area.

[00:10:30.36] Once the demise takes place, the area is virtually abandoned-- and for quite some time, as we'll see. And you almost had a rewilding-- rewilding in the sense that the forest comes back in a significant way. So there is a shot of some of the new laser data, showing the density of population across these areas. And that whole population leaves for whatever reason, leaving us with this-- what we would call before a millennial long wave of population growth and decline.

[00:11:05.50] Now, why do I raise that with you? Because having one millennial long wave of population ascendancy and decline for an area as large as we're looking at is rather unusual. It's very difficult around the world to find such situations. Invariably, you find two millennial long waves. In the case of this area, we not only have just the one. Even today, we haven't even begun to reach the level of density that the Maya entertained before the demise.

[00:11:39.48] Now, the first explanation that we've had that inserted climate was 1912, by this gentleman, Ellsworth Huntington-- Yale University. He went to Copan. He looked at stratigraphic along the riverbank, and he concluded the following-- the Maya entered the area when it was relatively dry. They rose to ascendancy during that period. Then it got wet. When it got wet, disease vectors came through and wiped them out. And so this became the, if you would, initial-- at least that I can find-- cause of the climate-- just the opposite of the climate we're talking about today.

[00:12:20.11] So he was part of a large body of luminaries who espoused the geographic factor. Ultimately the term we use is environmental determinism, as opposed to the geographic factor. And I'm sorry, I'm at Harvard, so I'm going to say it. The father of that was William Morris Davis, a geologist, who I guess was in this building, or part of this building at one time in his life. He is considered the father of American geography, even though there's no geography program at Harvard today. You got it in geology.

[00:12:59.28] But the point is, he defined for the entire American system what geography was at the time. And that was the quote that you see below, the "inorganic control over the organic." That is, how the biophysical world affects the social world. And that then gave rise to all these people that started looking at climatic data around the world and relating it to society in one way or another.

[00:13:27.27] Well, for 70 years after Ellsworth made his proclamation-- really based on minimal research, by the way. It was just, I'm going to go observe stratigraphy in the Rio Copan River. Climate or the geographic factor environmental determinism becomes mute within the Maya understanding. I remember as a graduate student talking about environment in the Maya and it was simply, oh, be quiet about that. You shouldn't do this. Why? Because my mentors were afraid that if you raised climate that would raise the notion of environmental determinism. We can begin to explain things in a way that we know is dangerous in one way or another.

[00:14:11.81] Now understand, during those 70 years, from the time Ellsworth made his claim to the time I'm going to bring you to, we really had few integrated programs of field studies in Maya area. Now, what do I mean by that? I don't mean that you had a great archaeological project and there was a paleo environmentalist associated with it, or whatnot. I'm talking about projects who define themselves by the intersection between the social and the environmental world, in which you have many, many people from both sides-- both archaeological and paleo-ecological people working together on a defined problem in the area. The programs that we did have that developed in those seven years, at least as we got into the '60s and '70s and whatnot, were really focused on how big the population was and what was the agricultural base that supported that population. That was sort of the integrated that comes to my mind.

[00:15:05.50] We also have to understand, during this time period we did not have the strategies, the tools, the techniques, and methods that we have today to begin to address the paleo-ecological record. Nonetheless there was data coming out, by one individual here, one individual there-- molluscan data, pollen analysis, stratigraphy and whatnot-- all suggesting that something had happened in this area. There was something environmental that went on. And that induced an argument about whether this change that we're looking at was meteorologically, or a product of what the Maya had done.

[00:15:48.34] And then just before all the data comes in about climate, we have people elsewhere-- in this case, South America-- finding significant climate change in certain of their records, and then using teleconnections-- the notion that if it's happened here those processes can go across the Caribbean, across the Gulf of Mexico, and played in the realm of the Maya.

[00:16:18.73] So that's where we were. The only thing you could show us was from teleconnections. And then, bam, the boom came in the 1990s. An explosion of paleo environmentalists invading-- wow, that sounds like they're bad people-- invading the Maya lowland area anyway, doing their work. And there's many of them, all of them very good. But the two groups I really want to point out was a University of Florida team that really smashed this on the head with the work from Chichancanab that climate change was there, and we better pay attention to it. And more lately, the University of Texas team-- husband-and-wife team-- that take the climate data, add to it paleo-geomorphological and hydrological work, to grapple with the kinds of landscape features that took place as the demise and depopulation was taking place.

[00:17:10.13] So what I'm going to do in the first part is I'm going to walk you through, in my simplistic way, the data that forced us to pay attention to climate. First, it takes place in the north, in the northern part of the Yucatan-- Lake Chichancanab, Punta Laguna, and at a cave, Tecoh Cave. And then it's going to move south. And we're going to look at Macal Chasm, Yok Balum, and Lake Salpeten. I'm not going to go into great detail here. Bear with me.

[00:17:39.62] But before we get to what they had to say, they were very honest, good, paleo-environmental researchers. And they pointed out to us the following. Although we're making the claim about climate, you must understand a few things. First of all, we're suspicious of the teleconnections. Because all of our work tells us the further distance is in the teleconnection, maybe the least robust it is for the area you're trying to apply it to.

[00:18:06.68] Second, their argument was that their initial data really didn't have the fine temporal baseline that they needed. That had to be improved. You don't want to talk in hundreds of years. You want to talk in ten-year, five-year time periods when you're dealing with these things. And third, that they're going to use proxies. But proxies don't always associate a nice one-on-one with what happens to precipitation. And what we're going to find is through the early '90s and up through, let's say, 2010, they're going to solve these problems, in my mind, for the Maya region.

[00:18:41.15] So here we go, Lake Chichancanab-- what did this study do? It was looking at oxygen and hydrogen isotopes in a core taken out of the bottom of the lake. And basically, what it was showing is pronounced desiccation in the climate from the Late-Classic through to the early Post-Classic period. The TC down here stands for Terminal Classic, just in case of what we don't know those terms.

[00:19:12.85] And you can just look at the lines. The further they go up, the more arid the situation is. You can find these long time periods then-- half centuries and centuries in scale-- in which we get 41% to 54% of a decrease in precipitation. And the two big areas that take place are 750, 850, and 950, 1050 Common Era.

[00:19:37.28] I'm not going to go into the Punta Laguna work, but it basically shows the same thing-- massive drought in the north during those time periods. But it's not fine-tuned in terms of the temporal dimension. And then along comes the work from Tecoh Cave. Now, here we do have very fine-tuned dimension, from a stalagmite.

[00:19:59.72] And you can look in the bottom graphic. And you look at the gray zone, and you see the eight different dry-period interludes that take place. So now we can get down to estimates of down to three years, 18 years, 20 years, and whatnot, in terms of when these interludes are taking place, how long they last. And in this particular case, the argument is the desiccation arranged on the order of 36% to 52% of the average normal rainfall for the area, and that it basically takes place from about 800 to 950 Common Era. I hope we're seeing that there's a general trend developing in these studies. This study, by the way, did use oxygen isotopes.

[00:20:44.70] Well, that was the north. What about in the south? Some people argued, maybe the south was different. Well, the work started to come out and by and large, it seemed to show things that were very common to the north. Now, here's a different study-- a stalagmite from Macal Chasm. And here we're going to use luminescence. I'm certainly not an expert on luminescence. But the luminescence that you get in a stalagmite reflects organic acids that are released in soil and vegetation, which are linked to precipitation. And therefore, you can use this luminescence as a way to begin to grapple with what's happening in the environment. And what does it show-- and I marked it for you in the yellow-- from 750 to 1150? Significant desiccation of the region.

[00:21:34.10] What I want you to keep an eye on, because I'm going to come back to it later, is there were significant drought interludes long before this demise and depopulation. Yes, the ending one was quite large. But we want to think about how the Maya responded to those that occurred earlier. Two more studies from the south came up-- lakes: Lake Salpeten and Yok Balum Cave. And Salpeten not only used the hydrogen and carbon isotopes that some of the other studies had used, but it used yet another prox-y. And this proxy being leaf-wax lipids.

[00:22:13.01] Essentially, the wax on leaves basically can tell you something about precipitation in the environment. Was it more moist, less moist, and whatnot? And what does it show? This is the top graphic that we're looking at now. The two arrows are designating this time period, from the Late-Classic through to the Post-Classic period, of significant desiccation in the environment. And what you're looking at here is the green is coming from the south Lake Salpeten and the orange is the Lake Chichancanab. And what they're trying to show you in the graphic is that it was just as intense in the south as it was in the north, this desiccated period.

[00:22:54.00] The other point that was trying to be made here-- maybe I should use this pointer. I'm not very good at using pointers. I don't think they look very good. Anyway, the other point was that for the south, it may have been a little bit moister than the north when this extreme desiccation hit. But I'm not going to really go through that in great detail. Well, the south didn't have fine-tuned temporal data. Well, Yok Balum Cave provided it. Because again, now we have a stalagmite, and people are working on it. And they're getting fine-tuned temporal resolution, that you see in the bottom picture. And you see the two arrows pinpointing this extreme aridity, that's going to take place between about 820 and 870, with the extreme case taking place in the Terminal Classic period.

[00:23:47.58] Well, Sheryl Luzzadder-Beach put this all together for us in this wonderful graphic. And when she said, let me take all the data that we have from all the different sites and put it together, and let's see what it tells us. Well, what you see are seven large drought periods, that take place throughout a long time period of Maya occupation, from the Pre-Classic into the Historic period. Pay no attention to the last two. That's Historic. What's good about that, however, is that we actually have some documentation of people talking about how dry it was during that time period. Where I want you to pay attention is these two. These are the two big dry interludes that we're concerned with today, because they are the ones that are going to be bandied about as a factor in the demise and depopulation of that elevated interior.

[00:24:42.15] The other thing I want you to keep an eye on is that dark orange, or burnt-orange line at the top, is the South American climate change, . And I want you to look very carefully. You'll see that in its case it does not show a consistent dry signal that's consistent with the Maya dry signal. In other words, the teleconnections doesn't work as well as the paleo environmentalists were telling us. So summing all this up, we have this situation, that we now have pretty good climatic data across the entire occupation of the elevated uplands-- in fact, the entire Maya area, not just the elevated uplands.

[00:25:27.12] And the three problems that are raised before have been solved, or largely solved. We now have fine-grained temporal dating from the north and from the south, giving us more or less the same signals in that dating-- in its linked to dessication. That we have relatively consistent signals in all the data, and we don't need the teleconnections to tell us anything. We have the data from the Maya region. And because we have a large number of different proxies, and they're all giving us the same interpretation, it gives us confidence that this desiccation was real. It was severe and real.

[00:26:12.03] So what we now have now, I would argue, is a pretty much consistent agreement among the community that drought evidence syncing with the demise and depopulation is pretty real. It was present. It needs to be accounted for. It may even be that we're at the point where we can look at the variation in the kind of different Maya areas and Maya sites as it links to the specificity of the dessication that's taking place.

[00:26:43.14] All right, cautionary tale-- we weren't going to say anything about climate. We couldn't avoid it now. All The paleo environmentalists have slapped us in the face and said, let's get to it. The question is, was climatic change-- and I'm going to use the term climatic change to mean independent of whatever the Maya are doing. This is something that's happening, let's say, at the global scale. Was it sufficient? Is it just sufficient to know that we had this desiccation in the climate of the Maya area?

[00:27:15.97] And before I get into that, I want to pay attention to a great colleague-- he just passed away recently-- Karl Butzer. Karl Butzer did research from geology to geography to archeology. I think he wrote a famous book called Archeology As Cultural Ecology, or something like that-- very, very deep into it. Anyway, he paid a lot of attention to societies around the world and their response to various environmental perturbations and degradations. And just read the one sentence. "Social environmental dynamics and societal collapse are invariably complex, involve independent variables, and are difficult to predict."

[00:28:01.53] So this was his shot at all those people that want to say, OK, we had climate. And we have to pay attention to climate. And perhaps that's all we need to be paying attention to. He also told us-- and in most cases, he looked out around the world historically, you had more adaptation to whatever the big disturbance was from environment than you had anything else.

[00:28:24.99] So now let's take that understanding-- there's complexity-- and begin to ask a series of questions. The first one was-- remember that graphic showing you past drought interludes in the Maya area? So we had significant past-droughty time periods. In each case, the Maya not over only overcame them-- what results is a larger population and a materially more advanced social arrangement. I guess I'm not saying that right. The amount of the buildings they build become more. The things we think as monumental architecture growed and so forth. In other words, they respond. They adapt to those past changes. So now we have to begin to ask that particular question.

[00:29:12.55] The question then that raises is well, why didn't they respond to the last one-- to the two periods I'm showing you at the end, in which we had this very, very severe mega droughts taking place? And I suspect that they really did. And what we're going to find out is, yes, we're beginning to have insights that they did. But-- and this is what Jeremy Sabloff and I were attempting to articulate-- was there something peculiar about the social-environmental system that the Maya had created in their particular environment that made it different from the past? Their environment had become maybe too vulnerable to big hazards affecting them-- less resilient? Or, did their environment actually amplify and create the climate change? All three of those are could be happening, and it's that that I'm going to address now.

[00:30:08.41] What were the conditions at the end of the Classic period when this intense drought period came on? Well, the Maya had transformed-- they had a huge population. I think we've all resolved that particular issue. I mean, we could argue over what the actual density number was, but it was significant, with sites all over the place. They were urban, in my sense of the term of what urban means. They had large populations-- urbanized populations.

[00:30:39.76] And they had made fundamental changes in the entire landscape of the elevated frontier. They had denuded much of the forest. They had agriculture growing up dry lands. They had huge areas of wetlands. And in these wetlands, they had manipulated them as well. They had water reservoirs all over the place. They did have managed forest. And I won't go into all of them. But essentially, you had a landscape that looked a little bit more, in my mind, driving across Ohio than driving across the Yucatan today.

[00:31:14.05] What do we know now? We know, with the new laser data coming out from Carnegie, Smithsonian, and others, that that was right. That what we had in 1970 and called-- some people call it, anyway, the new orthodoxy of the Maya, in terms of number of people and landscape changes and whatnot-- the new data is showing that. It's just reifying what we already knew.

[00:31:37.00] Now, that's why I get a little bit aggravated when I see The New York Times put that up, that somehow the lasers have blown our mind. Well, they didn't blow my mind. That's what we had already said was taking place. And they're just demonstrating, in fact, that it was probably true. And that even includes, in the second little graphic-- the one that says B-- that the expansion of the amount of agricultural systems that we did not know specifically-- like, for this particular location and the lasers are-- so it's terribly important. Don't get me wrong. I just wish we wouldn't hype it so much. But maybe that's what sells the papers and the online reading and whatnot.

[00:32:20.80] So the Maya had a highly-intensive, manicured landscape, providing them food, fuel, and water. Jerry and I's suggestion is that this made them highly vulnerable and lowly resilient in a number of ways, which I'm going to articulate. But if you want to see it in detail, you can see that particular graphic and all the arguments about it in a proceedings of the National Academy of Sciences in 2012.

[00:32:54.40] As the drought was taking place, we now-- and this did follow. So here is good news from the laser scans, I'm going to say. It led us to a recognition by the Texas duo that do this work that the Maya in the elevated interior moved slightly. I don't mean they moved location, but they shifted agriculture slightly to the wetlands-- a little bit lower in elevation from the elevated interior. Because these wetlands still had water, and that allowed them to use a system that's depicted in the picture.

[00:33:29.65] The issue here, though, is their work is suggesting it didn't last too long. And it probably didn't last too long, they think, because the desiccation was so severe that even these wetlands didn't have sufficient water. And so the amount of food and whatnot that you wanted from the system wasn't persistent. Now, we don't know this for sure yet, but that seems to be where the Texas duo are going in their ongoing work in the area.

[00:34:00.93] What did this landscape do-- this transformed landscape? As global environmental change came on board over the last 25 years, one of the works that has really been striking is the degree to which the meteorological and ecological community have demonstrated that if you change the land cover-- the land surface of the Earth-- in significant ways, it has forcings on the climate. You can think, that's not too hard for anyone to think through. You've cut down a lot of trees, and you don't have evapotranspiration anymore, at least in that particular area. But what they were able to show is that whatever is happening climatically that is independent of the local area, that local area itself will have some interaction and feedback with the climate change that's going on.

[00:34:56.37] Nobody has actually attempted that work in the Maya area, but there are several models that have attempted to say, could you get the level of desiccation that we're finding in all those studies I showed you if you only relied on climate change-- natural climate change? And their answer is, no. The only thing that would give it is if you had forcings from land-atmosphere interactions. In other words, the Maya had cut down so much of that vegetation and changed it in so many ways that they were amplifying the aridity that already was present. Things were getting drier. They were making things drier, in tandem with the climate change that was ongoing. That modeling work, I think, is pretty good. We need more fieldwork, however, to begin to assess it better.

[00:35:50.04] What else happened in this configuration that you see? Well, we had the land forcing affecting the climate change. We of course had tremendous water difficulties that were going on. The Maya had built big reservoirs-- all sorts of systems to feed water into the reservoirs and whatnot. And there was just an insufficient amount.

[00:36:09.41] What we have not paid enough attention to are the next three bullets you see, because they have a lot to do with food production. The critical limiting nutrient in the soils is phosphorus, in this particular area. Where does the phosphorus come from? Well, one part of the phosphorus comes from capturing phosphorus and dust in the atmosphere through the canopy. You cut the canopy, you've lost the ability to pull that phosphorus out of the atmosphere, to the same degree than having some other kind of vegetation. So that becomes a significant problem through time, and would have been ascending to its heart of scale at the time period we're talking about. It would have increased soil temperature and decreased soil moisture. Well, we know this. We have our field work that demonstrates this case-- again affecting crop production.

[00:37:04.46] One that we haven't paid attention to in any of the archeological work is that this is an area where you cut that forest and that bracken fern invades it instantly. Bracken fern is very thick, grows very rapidly, and just essentially chokes out all other vegetation. Now, the question becomes, how do the Maya deal with that? We don't know. How do they deal with it today? Today, Mayan people will just burn the bracken fern. But what does that do? That just makes it denser when it comes back again. So the question is, how did the Maya confront this constant problem of things are getting drier? Bracken fern is all over the place. Did they expend all the labor to actually pull it out? Maybe they had some technique we don't know. But it's something that people need to be paying attention to.

[00:37:59.64] And of course, what happened that they had done to the landscape, they had degraded their biomass for fuel. And there's all sorts of work coming out from my Mayan sites now that show, they had this much-- They were using big timber, then less timber, and then they were using little trees that nobody else would use in the past-- because they have denuded so much of that particular landscape.

[00:38:22.68] So they had transformed this landscape. That leads me to a series of critical questions. So here's the first critical question. Was the transformed state of this elevated interior, interacting with climate change, sufficient to generate a tipping point in the social-environmental system? And led to the demise and whatnot? Clearly, this transformed landscape and its interaction with climate was significant, but perhaps insufficient. Recall what Butzer keeps telling us about how complex SES situations tend to be.

[00:39:00.47] Were there any options that the Maya could have entertained to combat this drought? Well, we now know they were trying to expand wetland agriculture, but it seems to have been insufficient-- or insufficient giving the desiccation that was taking place. But again, that's another area of work that research still needs to add. Or, and this the third, was the capacity to invest in mitigation strategies reduced? And this is where Jerry and I go off the board and say, yeah, we have increasing evidence that that's the case. And here's what it is.

[00:39:41.09] We know that when the elevated interior dominated the Maya area, they controlled trade crossing the peninsula-- more or less this east-west arrow that you see. And if you want to know how can it be, you actually can take rivers up to the elevated interior, cross it a bit, and then go down to more rivers and get to the other side. When the demise takes place, this is what happens. We have big canoe trafficking going around the peninsula, instead of across the peninsula. In fact, Christopher Columbus ran into one of these canoes trying to go across to Columbus to do trading.

[00:40:19.79] The problem we have here is a chicken-and-egg issue. Did the Maya collapse and the collapse caused the loss of one trade route, and therefore they shifted to another? Or, did the beginning of a shifting trade route, or the amount going around the peninsula, help trigger a loss of commerce in the elevated interior-- and hence precipitate part of the demise? We have a new theory out that said, whoa, this area may have been a big area for cultivation of cacao, or chocolate.

[00:40:55.41] Wait a minute, I think I have a picture for you-- there we go-- of chocolate. You've heard the famous dissertation, when money grew on trees. And this is when money grew on trees. And the argument here is that the drought was so intense that the control that the elevated interior had over cacao production was lost, because the cacao production became too difficult. Once they lost that, they lost a big part of their trade mechanism. And then the trading then circumvents the peninsula, rather than going across.

[00:41:30.09] So there now is a new major challenge, I would argue. And that challenge is, we have to improve our spatial and temporal dynamics of Maya trades and trade routes. Can we handle that in some way, so that we can get a notion of, were they simultaneous, the drought and the shifting trade? Did one come first? Did one come after? And to me, that's a big problem that needs to be considered. And people are addressing it in very inventive ways.

[00:41:57.08] So here's a study that came out recently. That's the study they're on the bottom, that says, EG network analysis, and network analysis of trading in parts of the Maya area. And what this study basically says, in the trade-network analysis, is that trading was going across the peninsula and then shifted and started going around the peninsula. But the point I'm trying to show you this is that no one in my time would ever use a social-network analysis to do a study of the Maya. Now, we have all these very inventive ways that people are beginning to address the problem.

[00:42:35.69] So what Jerry and I basically offered in 2012 was the notion that some combination of commerce and severe drought, which included the landscape transformation that the Maya made interacting with climatic drought, may-- just may have been the complex mechanism that created the stress on the system and led to a tipping point. And when this tipping point came, basically it was just much more efficient to get up and move-- maybe to the literal along the coast. There you did have water. The groundwater was much closer to the surface. And you had trade flowing around the peninsula. But it did lead to a depopulation and demise of the elevated interior.

[00:43:20.02] Now, there is another part of the story, because the environmental subsystem returns. The social subsystem never does. Now, I'm not trying to offend people who tell me, yeah, there were Maya that were-- yes, there were. But the level of the density just wasn't there. The material achievements simply were not there. So about 80-to-120 years or so, the forest comes back. The soil recovers as much as up to a 280-years afterwards. These are pretty good soils for the tropical region. Nobody invades it and takes it over again-- remember that millennial long wave in population.

[00:44:03.97] Now, there was minimal resettlement, but it was minimal. And I recall everybody the words of Bernal Diaz. Bernal Diaz was with Cortes, when Cortes for some reason decided he would march from Mexico City to Honduras. Can you imagine that? From Mexico City to Honduras, to quell an uprising in Honduras. And they march right through the elevated interior. And Bernal talks about they're hacking their way through the forest, because there aren't trails. They almost die from a lack of food because they can't find villages to force them to feed them. And it wasn't until they stumble out on Lake Peten Itza and there's Tayasal there, and they get food and water, et cetera, et cetera, they can advance on and make it to Honduras. So yeah, there were Mayan there, but they were very minimal compared to what was.

[00:44:55.94] So the question becomes, that environment returned. The people didn't. A very interesting question. So the another issue to raise is, why didn't early on, the Maya-- and even today-- move into this area? And I'm showing you pictures here only to show you that there's so much forest still there today that it becomes part of the Maya Biosphere Reserve and whatnot.

[00:45:22.62] In case you don't know, the one factor that's changing the landscape there a lot now is narco deforestation. And it's a very, very significant problem. Anyway, was the problem that we never had sufficient land pressures along the literal, that people never had to go back in big numbers into the interior? We don't know. The one that always I kind of like is the notion that the investment to go back into the interior was too large.

[00:45:50.41] You had a pretty nice living along the coast. You had a nice standard of living. You were going to have to invest a lot of money to have the same if you were going to move back into the interior. There's too much infrastructure that had to be reconstructed. Or, were there some unknown environmental or social factors at work? We just don't know.

[00:46:09.02] I've heard rumors, through the talking grapevines, that there is a study out there that says there was a change in the seasonality of precipitation. It hasn't been published yet. It's only been rumored at one talk that I've heard, so I can't speak to whether or not that isn't one of these issues that we need to be paying attention to. I'm sure we'll learn about that in the near future.

[00:46:30.29] So the next major challenge, I would argue, is we have to improve our understanding of why people didn't go back into the elevated interior in any numbers. And if we can answer that, or improve our understanding of it, we may begin to have better insights about the collapse in general.

[00:46:50.29] OK, I'm almost done. Here's the key element. We did have a depopulation and a material demise of the elevated interior. In fact, I call that elevated interior, in some articles, the heartland. And the heartland because it had the most people in it.

[00:47:07.96] So we know that took place. We know that there were severe long-term climatic drought, and it was real. But we have some suspicion that that climate drought alone was likely insufficient to bring down the elevated interior. But we also know that they had an enormously transformed landscape. Very high cost to maintain it, because the infrastructure was so large. And it too had environmental impacts, beyond those that I'm going to talk about in a moment. And that those two linked together to amplify the drought-- plus, all the other problems that are going on in this transformed landscape. Then on top of that, if you've lost the trade, if you've lost the commerce-- I don't know if it's cacao. I don't know what else it was.

[00:48:01.04] But you've lost that commerce going across, and now it's going around the peninsula. That then leads to that. And that's sort of the argument, although I've added onto it, that we made in 2012. And it seems to me the evidence has not overturned in any significant way to date, although some people will argue about x, y, and z.

[00:48:27.55] I want to think all these kinds of people that have helped me through the years. I want to thank those four archeological projects you see there, that I worked on in my first 20, 25 years. The last project at the end was a social-environmental systems project, that gave us a lot of insights about the phosphorus and whatnot, that I just gave to you.

[00:48:48.67] And I'll end with something, because I like to embarrass people. And so I thought this would embarrass Bill. But then I saw him for the first time in 25 years, and he looked just like he did when we were at Copan in 1977. This is Richard Leventhal, who was also a Willey student. I think he's at University of Pennsylvania now. Anyway, I hope that was comprehensible, and thank you.

[00:49:13.08] [APPLAUSE]