Adios

We have finally reached the end of reviewing the main theories for the Classic Maya collapse! Admittedly, before I got reading in depth, I was always biased towards the drought hypothesis as it seemed like a convenient explanation for such a societal and political change. I initially believed that drought triggered a chain of destructive events beginning with crop failure that eventually led to the demise of an already out-of-balance cultural system. The series of severe droughts most likely placed a great deal of stress on the elites, in turn affecting their systems and leading them to fight over tributary domains, as food and water resources continued to diminish with the advent of a natural disaster. Socio-political upheaval combined with environmental degradation that may have resulted from deforestation in the face of a hungry population ultimately led to a crumbling society.

However after reviewing all this in further detail, I now believe that they did not overshoot the carrying capacity, and while there may have been deforestation (some of which was necessary in order to build the large cities and construct magnificent monuments), with some areas more deforested than others, I do not think that this could have been a factor leading towards their collapse (McNeil, 2011). The evidence towards a severe drying trend is unequivocal, yet it did not seem to affect all sites. The drought theory still remains controversial among many archaeologists who advocate a blend of overpopulation, a weak economic base, and an internecine struggle for control among the elites, with a subsequent dramatic fragmentation of political power, as the main factors for causing the collapse (Aimers, 2011). McAnany and Negron also reject the drought hypothesis as the ‘prime mover’ of societal change.

After speaking to Dr Elizabeth Graham, an archaeologist and lecturer here at UCL, I think that economic change had a great role to play in the collapse. A bad economy can ruin a society, even today. We have seen that drought affected the Aztecs and Toltecs too, but their demise was ultimately attributed to other factors. If they were able to deal with the drought conditions, surely the Maya, who were the first complex Mesoamerican civilisation to exist were perfectly able to do so too, just as many societies can today. Yet in times of economic stress, divine rulers would not have been as resilient to a drought, adding to the socio-political problems.

 Economic change could also potentially explain the differential abandonment of dynastic centres that lasted over 125 years. As population declined in the south, the north experienced a large influx of population- at least temporarily. The first major dynasties to dissolve were those that ruled over landlocked centres like Calakmul. Those located strategically near the coast and major waterways such as Tulum (on the Caribbean coast of modern-day Mexico) continued to flourish well into the Postclassic period. Perhaps the coastal populations were the last to collapse because they were near to trading ports, and because they had more accessible water resources, unlike Copan and Calakmul that sat astride permanent water sources, which were highly dependent on rainfall. This leads me on to my final point. Did the end of the divine rulership actually qualify as an apocalyptic collapse, as identified by some movie producers (e.g. Mel Gibson) and writers (e.g. Jared Diamond), or did the so-called ‘failed’ civilisation just transform? After all, there are still 7 million descendants inhabiting the Yucatan peninsula today. 

 A prime example is Mel Gibson who produced the movie ‘Apocalypto’, in which he depicted the Maya in an unflattering manner. While it received good reviews, it was an extremely inaccurate portrayal of the rulers and priests as blood-thirsty savages (I have learnt this only after post 9!!). So if you do end up watching it, do not take it too seriously!

Nevertheless, that is another matter. We cannot ignore the fact that the civilisation was reduced to a mere 3% of its original size and the role of climate change as a destabilising factor should not be dismissed. Climate has long played a role in the rise and fall of many civilisations, including the disappearance of the Anasazi people of the American Southwest between ~1275 and 1300, as well that of the Akkadian Empire in Mesopotamia some 4,200 years ago, and the Mochica culture in Peru around 1,500 years ago, to name but a few (American Scientist, 2005). All these societal collapses have been attributed to droughts, evidenced by the increasingly precise high-resolution records of palaeo-climate.


You may be wondering how this is relevant for us today. Well, owing to the complexity of the climate system and consequently the significant uncertainties in our knowledge of climate change, one of the many challenges facing climate scientists and policy makers is predicting future impacts. Given current circumstances of global warming, it is of real interest to contemporary societies to understand how they can overcome the uncertainty in climate change in the coming years, using the past as a guide toward a better future. Ultimately, we are more interested in what changes will occur and how these changes will translate into impacts that matter to humans. There is already enough evidence that many of the potential impacts of global environmental change carry severe risks that can even be catastrophic. Yet knowing how ancient cultures responded to such changes may give us valuable lessons that can help improve humanity's future responses and policy-making. One thing we can learn from the Maya collapse, in particular, is the importance of water conservation and efficient management. Applying such principals of decision analysis to elements of current understanding and using models of virtual reality therefore seems like the most viable option to try and mitigate risks of climate change as soon as possible, to avoid the same fate as the Maya and other great cultures in human history. 

On a concluding note, I would like to thank you all for reading my blog! I have thoroughly enjoyed my time here on the blogosphere. I would also be greatly interested to hear your views on what caused the Classic Maya 'collapse', so feel free to comment! Adios for now amigos. 

References:

McAnany, P.A. and T.G. Negron (2010) 'Bellicose rulers and climatological peril?' In: McAnany, P.A. and N. Yoffee (eds) Questioning Collapse, Cambridge: Cambridge University Press, 142-175

Last but not least

In the next post I will be concluding the blog, after having exhausted the literature and attempting to gain both sides of the story for each theory, so to speak. If you look in the right-hand sidebar I have added a list of the theories I have discussed (although many of these are interlinked, for example agricultural failure and deforestation or agricultural failure, disease and diets). Hopefully this will simplify things and remind you of what has been mentioned over the past couple of months, so you too can come up with your own conclusions! 

Tree-mendously accurate?

This post may seem rather contradictory after the last couple, but I found a very recent paper that actually gives very robust evidence for drought, with the location and dating accuracy in check. Stahle et al. (2011) have discovered two Montezuma baldcypress trees in Mexico (Barranca de Amealco, Queretaro and Los Peroles, San Luis Potosi) that provide millennium-long, annually-resolved palaeoclimatic records (the first exactly dated records for Mesoamerica might I add). Not only do they reconstruct past rainfall, but they can also be used to date archaeological sites to a highly accurate level, especially in comparison to the proxies such as speleothems and sediment cores that have been constrained by age estimates.

The Queretaro record shows that the most severe drought of the past 1,200 years occurred between AD 897 and AD 922, which extended into the central Mexican highlands. Both records confirm the Terminal Classic series of droughts (Figure 1), indicating that they were centred at AD 810 and 860 – the same as the dates given by the Cariaco Basin core.  I have drawn up a table (below) showing all the key dates for severe dry periods (provided by the studies mentioned in this blog) to see how they all compare with this new reconstruction and to make it less confusing:

Location of proxy	Proxy used	Peak drought conditions
Queretaro and San Luis   Potosi, Mexico	Tree rings	AD 810, 860, 897-922
Macal Chasm cave, western Belize	Speleothem (luminescence, colour, δ18O and δ13C)	AD 754-798, 871 and 893-922
Lake Chichancanab, Yucatan, Mexico	Lake sediments (δ18O, δ13C and gypsum/calcite ratios)	AD 922
Cariaco Basin, northern Venezuelan coast	Laminated marine sediments (titanium content)	AD 810, 860 and 910

Did drought affect other cultures?

I know at the beginning I said I would briefly look at the other Mesoamerican cultures, but there has been so much on the Maya I haven’t had a chance yet. This paper actually documents the rise and fall of other civilisations throughout Mesoamerica including the Aztecs and Toltecs. So...did drought affect them? Before I answer this question, have a look at the map below to see what parts of Mesoamerica they inhabited:

Map of Mesoamerica showing the areas dominated by the Maya, Toltec and Aztec
Source: http://www.erin.utoronto.ca/~w3env100y/env/ENV100/hum/map.htm

Toltecs

The Toltecs dominated central Mexico during the early Post-Classic era; since they lived in the highlands where freezing conditions in the autumn shortened the growing season, early growing season drought would have affected them severely. The tree-ring reconstruction shows that a 19-year drought prevailed from AD 1149 to 1167, which may have easily caused famine. There is a possibility that drought played a role in their demise by pushing the Chichimeca population to migrate to the Toltec state, causing instability and eventual abandonment.

Aztecs

The collapse of the Aztecs is less complex than that of the Maya, as much of the population was decimated with the arrival of the Spanish in 1521, who used weapons unfamiliar to them and introduced new diseases and infections that they were not immune to. A drought during the Colonial era may have also contributed to the dramatic depopulation, weakening their society and making them more vulnerable. The new reconstruction indicates that it was the worst Mesoamerican drought since AD 771, lasting from 1378-1404 (as you can see in figure 1); despite this their collapse has ultimately been attributed to the Spanish conquest (McAnany and Negron, 2010). 


References:

McAnany, P.A. and T.G. Negron (2010) 'Bellicose rulers and climatological peril?' In: McAnany, P.A. and N. Yoffee (eds) Questioning Collapse, Cambridge: Cambridge University Press, 142-175

Explosive Eruption?

As we reach the end of the quest to find the cause of the collapse, I came across this blog post that posted an article by Richard Thornton, who talks about a study that places the blame on superheated volcanic gases and ash. Naturally I researched this further, but could not find the article mentioned. I did however find a similar article on the said Palenque Hydro-Archaeological Project. It is essentially a study of Palenque (as you may have guessed from the project name) – a large Maya city and the background of the blog- that has just been completed after 5 years, by the Foundation for the Advancement of Mesoamerican Studies (FAMSI). Located on the edge of the Chiapas Highlands, Palenque had a plentiful water supply, enabling it to become one of the most sophisticated and prosperous cities of the Maya region, yet inexplicably, it was one of the first to collapse. Palaeo-climate data provide very little evidence of a severe dry period that may have caused the abandonment of Palenque. Instead, a heavy layer of tephra (volcanic ash) dated for around 800 AD was found, along with evidence of destruction attributable to high heat.

Volcanic eruptions most certainly have the potential to ruin a society, causing massive causalities and property damage. The very recent awakening of the Popocatépetl volcano, near Mexico City, which prompted Richard Thornton to write his article, is a prime example. Several million lives could be lost if the volcano were to erupt, especially since last-minute evacuation of such a large metropolitan area is impossible.

There has been a long history of violent volcanic eruptions in Mexico, including that of Xocoteptl around 930 AD, in what is now northwest Mexico City. In fact, Mexico and the other Mesoamerican countries sit atop one of the most active geological zones in the world, containing numerous active and dormant volcanoes. For example, there is a chain of volcanoes (known to be active for the past two millennia) in the Chiapas state of Mexico and bordering Guatemala:

Location of El Chichon Volcano and surrounding cities.  The dotted line shows  the  areas most affected by the ashfall from the 1982 eruption. Source: Espindola et al. (2000)

The 1982 El Chichon volcanic eruption in this region was the worst known one in Mexico’s history, displacing over 20,000 people and causing at least 2,000 fatalities. This was partly due to the fact that people were not anticipating it, and so there was little time to escape once it had begun, and also because prior to the eruption it was not considered a hazardous volcano (De la Cruz-Reyna and Martin Del Pozzo, 2009). 

Before 1982: aerial photo of the summit of El Chichon volcano, Mexico

Post-1982 eruption: a kilometer-wide crater was formed, replacing the summit during the  most violent eruption in Mexico (known to humans) that killed 2,000 people and wiped out 9 villages.
Source: http://www.agiweb.org/geotimes/nov07/article.html?id=feature_danger.html

Espindola et al. (2000) suggested that such catastrophic eruptions may have been the prime mover of collapse of the Maya civilisation between 830 and 915 AD, after studies of past volcanic activity of El Chichon. They found that there have been at least 11 explosive eruptions in the past 8000 years, occurring around 550, 900, 1250, 1500, 1600, 1900, 2000, 2500, 3100, 3700 and 7700 years BP (note that the dates are rounded average calibrated radiometric age years). The 1250 BP eruption (i.e. centred around 676-788 AD) overlaps with the dates of the collapse of the western lowlands, including Palenque, which is located very close to the volcano (see map above). Had the Maya been unaware of the imminent danger of a volcanic eruption, the impacts would have been even more severe. Additionally, the impacts would not have been restricted to local areas due to the pyroclastic flows (a mix of ash, volcanic gases and lava) that would have reached distant sites, causing respiratory problems and potentially death. 


It seems volcanic eruptions may have been a primary cause for the collapse of Palenque, but I do not think it is possible to extrapolate this to explain the abandonment of the other cities. The small amount of literature on this particular subject is not sufficient enough for me to base a proper conclusion on, but for now I will discard this theory and begin to conclude the blog.  

Drought Doubt

Although the drought theory may be a popular explanation for the Classic Maya collapse, it assumes that the Maya were not able to adapt to such climate changes.  Many archaeologists believe that it fails to explain the complexity of the “collapse” and rightly so. Although there are a multitude of sites showing the occurrence of devastating droughts, many sites do not actually show this (see map below), in particular Laminai and other sites along the eastern Caribbean strip in Belize (Aimers, 2011).

Map showing sites with (in pink) and without (in blue) evidence of drought.
Source: Metcalfe (unpublished), provided by Dr. Elizabeth Graham, an archaeologist at UCL

It is therefore important to note that the palaeoclimate data that has been presented needs to be interpreted with care, keeping in mind that they are not always unambiguous. The location, chronology and the proxy used for past rainfall are all important when evaluating various records. For instance, the greater the distance from the area of interest (the Maya region), the less representative the archive will be – pretty self explanatory. So in the case of the Venezuelan marine sediment core record, does it actually inform us of what the climate was like some 2,700 kilometres away in the Maya lowlands? Figure 2 shows us how far away it really is.

Archaeological evidence from the sites marked in blue imply different rates of abandonment at different places. For example, in the Petexbatun region collapse occured in the 8th century, for Chichen Itza in the 11th century and for Mopan Valley possibly as late as the 13th century. Other sites like Laminai were not abandoned until the 1600s (after the Spanish invasion).
The sites marked in red have provided drought evidence, each with different (although still fairly similar) timings of occurrence. This contributes to the doubt surrounding the drought theory. 

Another important point to consider is how well the record is dated. Of course it will never be 100% accurate or precise, as there are numerous problems attached with all types of dating, but there are still some proxies that are far more accurate than others, especially tree rings. Other problems arise from calibrating a proxy and rainfall; we cannot always rely on it to be correct.

While there is enough robust evidence showing that a series of drought did occur, much remains to be understood. The difficulty in comparing palaeoclimate and archaeological records does not help matters and will require mutual cooperation from both fields in order to fill in the large gaps in our knowledge (Hodell, 2011). 

T minus 366 days

Just thought I would let you all know that according to the Mayan calendars we have exactly one year (366 days to be precise as 2012 is a leap year) to live before the world ends...

Deforestation Dispute

The last post advocates the theory of deforestation as a contributor towards collapse and a potential cause of the droughts. However (there is always a however) the hypothesis has been disputed by many who have observed present-day sustainable farming practices by the modern Maya, resisting the idea that the Classic Maya would have done things differently and willingly wreaked havoc on the land. If you cast your minds back to post 4, you may think this is very contradictory considering I commented on the fact that these sustainable methods were not utilised across the Maya region and only at a few sites, but recent evidence by McNeil (2011) has made me think otherwise...

Since the mid-1980s the archaeological site of Copan, Honduras, has been held up as the ‘type site’ for the deforestation hypothesis. It has regularly been used to exemplify how human ignorance of environmental limits  can destroy a city and has been used as a warning for modern populations, in particular by Diamond (2005).

However, recent analysis of pollen in a sediment core taken from the same pond as a previous study that suggested major deforestation occurred, does not support this (Figure 1). Indeed the 3000-year old record does show two periods of heightened deforestation during the Middle Preclassic and the Late Preclassic/Early Classic period (when the Maya began to construct their magnificent cities). Yet surprisingly, the landscape near the city centre was MORE forested during the Late Classic period than the Early Classic, contrasting to many predictions.  

Graph of pollen percentages from the sediment core from Petapilla Pond. 

Interpreting the diagram:

An abundance of the microscopic charcoal and the presence of Zea species of pollen imply that the deforested landscape is a product of human clearance and agriculture and not a product of natural grasslands. During the most dramatic level of deforestation (occurring at 512 cm into the core), 89% of the pollen record is composed of herb pollen that is indicative of human disturbance.

At the second peak of deforestation, coyol palm (acromcomia aculeate), a plant new to the pollen profile, is shown to increase in abundance, likely reflecting agroforestry practices.  

In contrast, during the Late Classic (shown between 270.5 and 250.5 cm) the pollen sequence indicates rapid RE-forestation where terrestrial arboreal pollen increases in composition from 59.8% to 89.8%. Herb pollen is actually much less than expected, especially in comparison to the biggest peak of deforestation.

The decline in herb pollen doesn’t support the environmental degradation hypothesis either; similar patterns have also been found in other pollen profiles from Lake Tamarindito and Coba, both of which were heavily populated during the Classic period. In fact there is ample evidence of how the Maya took care of the surrounding land, creating terraces to reduce erosion, wetlands to maximise their use of land and water along with raised fields and dams. Despite this, it is still widely believed that the Maya devastated their environment.

As you can see, there is great controversy surrounding the deforestation hypothesis, much like Easter Island (see Jenny’s blog for more information)!

Just to add...an interesting hypothesis is that the series of droughts may have actually led to re-forestation as populations starved and migrated elsewhere in search of water and food, allowing the forest to recover. OR perhaps this new evidence actually contributes to the mosaic pattern of collapse, where some cities were not deforested as much as others. Who knows?

All in the timing

We now know that the collapse was not uniform and neither was the change in climate. But how did the wetter south become abandoned 100 years before the drier north? It has been hypothesised that this regional and localised climate change occurred due to differences in water sources, tying in with Gill’s three distinct phases of abandonment that was already discussed last week. The map below also shows which areas were affected by what phase. To summarise:  

• Phase I (AD 760-810): initial abandonment of the western lowlands, where groundwater is scarce and rainfall was the primary source of water for Mayan cities
• Phase II (AD 811-860): abandonment of the southeastern lowlands, a region where freshwater lagoons provided at least some surface water supplies.
• Phase III (AD 861-910): large-scale abandonment of remaining cities in the central lowlands and in the north (areas with cenotes)

Thus, it seems that the South would have been affected much more by a decline in rainfall, even though the north was drier (Curtis et al., 1996). 

Map showing the three phases of abandonment: Phase  I (green), Phase II (pink) and Phase III (purple).
Source: American Scientist

Additionally, while there seems to be an obvious link between climate and collapse, the region affected was diverse in terms of geology, ecology and climates. Thus, there would have been distinct regional differences in response to single forcing factors, varying the effects on humans and environment alike, which could explain this mosaic collapse of the Maya area. Shaw (2003) suggested that previous theories do not truly explain the non-uniform pattern, and that anthropogenic deforestation also had a major role to play. Archaeologist Tom Sever told NASA that to make just one square metre of plaster required for the construction of their magnificent temples, monuments and reservoirs, the Maya had to cut down 20 trees - considering how much they actually built a lot of tree-burning must have occurred!  

According to the study, the Maya deforested the landscape that subsequently increased erosion dramatically, particularly in northern Guatemala (see posts from November). The different rates of erosion seemed to have led to different responses to climate change, where the major agriculturally intensified areas acted as a catalyst to the drought. Other areas that had significantly less tree removal were able to adapt better and continue to survive (albeit temporarily).

How can deforestation cause erosion?

Rainfall that would otherwise have been intercepted by the trees would fall to the ground, eroding the unsecure bare land. Because of drier temperatures, the land would easily be desiccated, making it easier for rainfall to runoff that can cause flooding. 

How can deforestation affect climate change?

Temperatures can increase, and evapotranspiration decreases and it therefore becomes drier at a local level. Climatologist Ben Cook from NASA's Goddard Institute for Space Studies (GISS) explains that with deforestation, the albedo (reflectivity) of the land would become higher, as lighter colours reflect more sunlight and absorb less radiation in comparison to darker colours characteristic of forests. This would affect precipitation, as there would be less energy available for convection. For a more in depth explanation (based on modern studies) as to why this is the case see Shaw (2003). These effects have found to be less extreme closer to the coasts where the ocean provides much of the moisture for the rainfall budget. This could explain why the interior of the Maya region was more susceptible to deforestation as it was dependent on rainfall provided by evapotranspiration.

Sever and his team actually showed how deforestation could also have exacerbated drought using computer simulations provided by 2 climate models: MMS (Mesoscale Atmospheric Circulation model) and CCSM (Community Climate System Model). The worst (100% deforestation) and best (no deforestation) scenarios were modelled, with the former causing a 2-3˚C rise in temperature and a 20-30% rainfall decrease. More recently, Ben Cook and his colleagues used a higher-resolution climate model and found that extensive agriculture and tree removal across the Maya region would have led to a 10-20% decline in summer rainfall (NASA, 2011). The simulations show that the most densely populated areas and consequently the most deforested areas would have experienced the greatest decline (as you can see on the map below). They also confirm that this was the case for the Aztec era.

New climate modelling showing how areas of widespread deforestation coincide with areas of declined rainfall, for the period between 800 and 950 AD. Source: NASA (2011)

 I recommend watching the short video in the NASA article, as it excellently summarises what I have just written (I wasn’t able to post it so here is the link: http://www.nasa.gov/topics/earth/features/ancient-dry.html)

Both modelling studies highlight the importance of forests and their role in maintaining high rainfall in the tropics. Ben Cook also suggests the potential for another mega-drought in the future if deforestation worsens again. 

Maya Collapse Match

Just out today is the Maya Collapse Match iPhone game, courtesy of TikTak Games. It is essentially a 'quality' game where the players (the archaeologists) must save the Mayan civilisation from destruction, with only 10 lives. It claims to have realistic scenarios based on current knowledge of their culture and Mayan sound effects... I will be sure to check this one out! 




Read more: http://itunes.apple.com/us/app/maya-collapse-match/id484007229?mt=8

Water water everywhere...Or not.

"Sunny days, in and of themselves, don't kill people, but when people run out of food and water, they die." –Richard Gill. 

Before we review this very fitting quote in more detail, it is important to note the water sources for the Maya, especially since this plus water management greatly influenced human settlement across the Maya region.

Centote with cool deep fresh water. The Maya believed that
this water purified and replenished their health 

Northern Yucatan: Because of the limestone surroundings, much of the surface water that accumulated during the rainy season was percolated down into the ground, forming magnificent caves and cenotes (sinkholes) (see photo on the right) that were sufficiently shallow to be available for use (Webster et al., 2007). These caves are amazing and if you ever visit Yucatan part of Mexico do not pass up the chance to swim in the cenotes to avoid missing a truly sublime experience!

Southern Lowlands: In contrast to the north, the water table would have been too deep for access, and so about 95% of the Maya centres depended solely on lakes and rivers for drinking and agriculture, with an average water supply for 18 months (National Geographic, 2003). Therefore much of the Maya social innovation was concentrated on excess water storage for times of greater need. For instance, they created artificial reservoirs designed to trap runoff from rainfall. There is plenty of evidence that the Maya recognised the importance of a wetland biosphere, and managed to turn their artificial reservoirs into constructed wetlands with a balance of pondweeds, smaller plants and algae that worked together to purify the water. Through this, they maintained a clean water supply throughout the year (Lucero et al., 2011).

Effect of a long-term drought

The Maya could obviously adapt well to extreme seasonal changes and short annual droughts, so what on earth went wrong? It seems that despite the elaborate schemes described above, they still had a huge dependency on rainfall that made them susceptible to longer-term droughts (Webster et al., 2003). It is plausible the three severe droughts may just have pushed them over the edge. 

Such a change in climate would have impacted all the regions and their populations, as the water supply that fuelled the growth of their civilisation could no longer be relied upon. Without sufficient rainfall, agriculture would have been affected and food supplies would have consequently been reduced (especially since maize is a relatively thirsty plant), opening up a whole Pandora’s box of effects (Lucero et al., 2011). Needless to say, an attempt to increase harvested food would have led to environmental degradation and perhaps further deforestation. Additionally, the associated decline in water quality and would have led to an increase in water-borne diseases and pests. The first few posts of this blog discusses such problems in more detail; drought would most certainly have exacerbated these. 

A Variable Sun

What could have caused the droughts?

We have so far seen some pretty compelling evidence of persistent droughts, but nothing on what could have caused it. Unlike the collapse itself, the timing of the dry intervals may actually be easier to explain...

Drawing on the findings from the Lake Chichancanab sediment core, Hodell et al. (2001) argued that the droughts were actually part of a cyclical pattern that occurred every 208 years on average. They linked this to subtle variations in solar activity that remarkably followed an almost identical 206-year cycle.  Changes in the number of sunspots and the sun’s brightness were documented in tree-ring records of cosmic-nuclide production, whereby fewer cosmogenic nuclides (including ¹⁴C and ¹⁰Be) are produced when solar activity is higher.

When compared to the δ¹⁸O records, these bi-centennial oscillations seemed to be in anti-phase for the past 2.6 millennia, i.e. implying that during times of greater solar activity, droughts were prevalent. This corresponded well with discontinuities in the Maya cultural evolution.

And this doesn’t seem to be a one-off; other δ¹⁸O records in sediment cores taken from the nearby Lake Punta Laguna show the same thing (Curtis et al., 1996). There are also other records from around the world too:

• Venezuela: Foraminifera sequences (another proxy for palaeoclimate) from the Cariaco Basin were affected by regional upwelling and intensity of trade winds, attributed to a 200-year cycle of solar forcing (Peterson et al., 1991)
• Sahel region: Evidence of drought (not surprising since the climates of the Yucatan and Sahel are linked by the seasonal migration of the Atlantic ITCZ)
• Equatorial East Africa: Similar relationship between lake sediment records and solar activity for the past 1,100 years (Stuiver and Braziunas, 1993)
• Oman: Similar oscillations in climate recorded in a stalagmite, around the same time (Kerr, 2001). 

But you may ask: how can such small variations in solar activity cause such large E/P shifts in the Yucatan? Although it is uncertain, it may be via an amplifying mechanism. Possibilities include:

• UV changes that can affect ozone production and the temperature structure in the stratosphere
• Cosmic ray variability that can affect cloud formation and therefore precipitation
• Changes in solar output that can affect global mean temperature, convection and intensity of the Hadley circulation in the troposphere, in turn affecting rainfall (actually implied by sensitivity experiments  conducted with global circulation models (GCMs) - Hodell et al. (2001)).

Other factors?

It is very much possible that solar activity could have triggered the droughts, but it may also have been a combination of a variable sun and ocean current changes. Additionally, a restricted northward movement of the ITCZ that may occur when the trade winds are displaced further south could have contributed to the droughts, since convergence and cloudiness would be reduced (Brenner et al., 2001). This may be a non-linear response to solar output changes or due to other mechanisms. The change in climate may also have been induced by shifts in the El Niño-Southern Oscillation (ENSO) tropical air mass (McAnany and Negron, 2010). Fact is, climate is inherently complex, and most of the time there is more than one cause of climate change due to the feedbacks. 


References:
McAnany, P.A. and T.G. Negron (2010) 'Bellicose rulers and climatological peril?' In: McAnany, P.A. and N. Yoffee (eds) Questioning Collapse, Cambridge: Cambridge University Press, 142-175

Present-day Climate

This post will not only be useful for you if you plan to visit the region (which I highly recommend!!), but also to compare what the climate was in the past and try to understand what may have caused such changes.

Northern Maya Lowlands (including much of the Yucatan)

Rainfall on the Yucatan is highly seasonal (the wet season lasts from May until October), influenced by the migration of the International Tropical Convergence Zone (ITCZ). This is essentially a band of clouds and thunderstorms that forms due to the convergence of the trade winds from both hemispheres. When it moves northward, it brings summer rains to the area under the influence of the Northeast Trade winds.  The Azores-Bermuda high-pressure system also migrates northward in the same season. During the summer, the sea warms to a toasty 26˚C in the Caribbean and tropical/ sub-tropical North Atlantic, providing plenty of moisture to fuel the violent, convective thunderstorms and occasional major hurricanes that hit the region every few years.

During the winter, the ITCZ and Azores-Bermuda high pressure system migrate down south; this combined with relatively low sea-surface temperatures (SST) allow dry conditions to prevail with low winter precipitation (Brenner et al., 2001).

Southern Maya Lowlands

The climate here is generally much wetter, covering tropical and sub-tropical zones, and so a greater number lakes can be found in comparison to the north (Haug et al., 2003). These include Lake Peten Itza and Lake Punta Laguna, which I will be mentioning in the next post. 

While the climate was not exactly the same a thousand years ago, the Maya still had to adapt to strong inter-annual variability of precipitation, often with a highly unpredictable onset of summer rains. Delayed summer rains had disastrous consequences to the farmers, many of whom depended on the slash-and-burn techniques, and so you can imagine what happened when there was a drought. 

Stalagmighty Evidence

Here is yet another paper that adds to the growing body of evidence for major droughts in the Maya Lowlands... Webster et al. (2007) used a stalagmite from Macal Chasm, a cave in western Belize to provide a 3300-year record of climate history of the region for the period from 1225 BC to present (so includes the Preclassic, Classic and Postclassic periods).

Location of Macal Chasm, along with other important Maya sites. Source: modified from Webster et al. (2007)

You must be thinking how on earth can a stalagmite record such changes? Well, a stalagmite can actually capture a cave’s response to changes in climate very well, by encoding the signals in various ways. The interior of a cave can in turn reflect outside temperatures and conditions and so it is an indirect way of inferring past climate. As this particular stalagmite was located near the entrance of the cave, it is thought to better reflect environmental conditions outside the cave.

By looking at the colour, luminescence and carbon (and oxygen stable isotope records encoded in the stalagmite, they showed how a series of significant droughts impacted the Maya, with the most sustained dry period (lasting from 700 to 1135 AD) – again coinciding with the Classic Maya collapse.

Luminescence: This can be used as a proxy for moisture availability, because it is produced by organic acids and therefore relates to productivity of soil and the amount of vegetation cover above the cave. The record generally showed a period of greater luminescence punctuated by intervals of lesser luminescence.  

Colour: This correlates with luminescence well, where greater values of the colour index correlate with lesser luminescence and vice versa. Higher values (darker colours) are indicative of dust or fine detritus that probably accumulated as a result of inadequate water flow that would have otherwise kept the tip of the stalagmite dirt-free (suggesting a dry interval). Basically, the more continuous flow of water meant the more translucent the stalagmite (see figure 1 below).

Figure 1.  Variations in grey colour (A) and in luminescence (D) with digital grey-scale images of visible colour (B) and UV-stimulated luminescence (C) in the upper-44 cm of the stalagmite. You can see how the two anti-correlate in the images quite well! 

Oxygen isotopes: Any increase in evaporative conditions would have led to greater oxygen isotopes (δ¹⁸O) values in the calcite. In contrast, greater rainfall characterised by lower δ¹⁸O values would have led to less evaporation and thus lower δ¹⁸O values in the calcium carbonate.  

Carbon isotopes: These are determined by the carbon isotopes (δ¹³C) of the CO₂ in the soil above the cave, affected by plant cover. Stable carbon isotopes will be lower when the plant cover is largely the C₃ vegetation typical of wetter climates (e.g. tropical forests and shrubs), than if the vegetation is predominantly C₄ found in hotter, drier climates (e.g. savannah grasslands). So, during dry periods with reduced plant growth, less soil CO₂ can be expected in percolating waters and therefore less exchange of soil C with C derived from the limestone and we end up with higher ratios of δ¹³C.

What did they show?

During a drought, we would expect a browner colour, lesser luminescence, and higher δ¹³C and δ¹⁸O ratios. These four records show exactly this, and considering they were all measured at different resolutions, they correlate fairly well (the R² coefficient was 0.82).

The highest isotope ratios combined with minima in luminescence are shown in the record,  highlighting four major droughts centred about 780, 910, 1074 and 1139 AD, the first two of which correspond with the Classic collapse (shown in Figure 2). The dates also match well with the 3 phases of abandonment hypothesised by Richard Gill.

Figure 2. Stable isotope, luminescence and colour records from the stalagmite are compared with  Maya cultural periods. 

Peak drought conditions are shown for 754-798, 871, and 893-922, with successive droughts increasing in severity. These correspond with the increasing decline of monument building.

The dates match well with the other two records reviewed last week, indicating that the droughts that affected the Maya were widespread rather than just local. An important thing to note is that these papers only review the physical evidence and do not explore the potential social factors arising from drought that can be applicable to contemporary situations (something that I plan to discuss later on in this blog).

The Drought Hypothesis

Until recently, it has been difficult to prove the drought hypothesis since the traditional palynological (i.e. pollen and macrofossil) studies used to reconstruct past climates have made it somewhat difficult to distinguish natural versus anthropogenic causes of changes in the assemblages.

Hodell et al. (1995) was one of the first papers to use other methods; they looked at temporal variations in ¹⁸O/¹⁶O ratios in a 4.9-m continuous sediment record, representing 9000 years, from Lake Chichancanab taken in 1993. It is a closed-basin lake (the largest in the Mexican part of the Yucatan peninsula), and so it loses a lot of its water through evaporation (there is not outflow via rivers into the ocean). Therefore, the oxygen isotopic composition of the lake water generally reflects evaporative conditions during the dry season very well, where past changes are encoded within gastropod and ostracod shells preserved in the lake sediments. 

Location of Lake Chichancanab, Yucatan, Mexico. Source: Hodell et al. (1995)

They proposed that during dry conditions with high E/P (evaporation/precipitation ratio), there would have been higher δ¹⁸O values; during wetter conditions, the opposite would have occurred. Through this, they found that the timing of the Classic Maya collapse was coeval to that of climate drying, providing the first unequivocal evidence of drought between 800 and 1000 AD (1,300-1,100 yr BP).  The record also shows that this was the driest period throughout the past 8,000 years. Gypsum/calcite ratios were also used to determine past changes in E/P, as they reflect changes in the hydrologic budget of the lake.

At present, the lake slightly exceeds saturation for gypsum (sulfur) and so gypsum remains in solution in the open water of the lake, as minerals generally only precipitate if the water is supersaturated.  Precipitation only occurs in the shallow areas around the edge, where evaporation is greater. When evaporation is high or rainfall is low, the lake volume is reduced and the gypsum saturation is exceeded. This leads to gypsum precipitation throughout the entire lake that is deposited on the lake floor and becomes preserved in the sediment record, providing a proxy of past increases in evaporation/precipitation ratios (E/P). SO, a lot of gypsum deposition (found in thick layers in the sediment core) is essentially a sign of past drought. This was evident in the sediment record during the Late Classic abandonment, where there were high gypsum concentrations present, suggesting optimal E/P conditions (see figure below). 

Of course, this one record cannot alone support such a theory; yet there has been a multitude of evidence since this paper from various proxies. The team actually returned to  the site five years later to obtain a more highly resolved sediment core that allowed them to look at variations in greater detail and pinpoint smaller-scale changes. They found that the droughts of the Yucatan increased in strength and frequency prior to 1100 AD, parallel to the observations from the northern Great Plains in North America and other parts of Central America.


However, the study also unveiled the effects of deforestation and soil erosion attributed to humans, and so we cannot fully rely on it to support the climate change hypothesis.

To further investigate whether climate change did impact the Maya, Haug et al. (2003) studied a ‘cleaner’ marine sediment core taken from the anoxic Cariaco Basin - just off the coast of northern Venezuela:

Both the Yucatan and the Cariaco Basin experience the same general climate, with distinct dry and wet seasons, a result of the migration of the ITCZ (an equatorial band of rainfall). During the winter it moves south of both places and in summer it moves north, encompassing both areas (shown by the dark green bands).
Why did they choose a record so far away? Because it is completely surrounded by a shallow continental shelf that prevents deep Cariaco waters from mixing with the open ocean. This deep water is anoxic and therefore does not support deep-sea organisms that tend to churn up sediment layers deposited each season. Hence, the undisturbed sediments  preserves a detailed record of past rainfall. Source: American Scientist

Layers of alternating bands of dark and light deposits were discovered in the undisturbed core, where the light colours were algae and other tiny fossils and the dark bands consisted of titanium. This bulk titanium content is thought to reflect changes in input from river banks and the hydrological cycle over the southern Caribbean region. During the rainy season, titanium would be washed into the sea via the rivers, and so thicker dark bands that indicate higher levels of metal, reflect a lot of rain. During drier periods and subsequently a weaker river flow, there would be less titanium washed in and so bands would be thinner. The seasonally resolved record of titanium shows exactly this; there was an extended period of dry conditions (that appears to have lasted from 760-930) around the same time of the Terminal Classic collapse, with three intense droughts centered around 810, 860, and 910 AD.

The bottom panel shows the titanium content for the past 2,000 years, encompassing the terminal Classic collapse as well as  manifestations of what have become known as the "Medieval Warm Period" and the "Little Ice Age". The middle panel shows the record in more detail for the period of interest (it has been smoothed for clarity). The top panel focuses on the Late Classic period, showing evidence of four multi-year droughts (indicated by low titanium content), separated by 40 to 50 years of more moderate conditions. Source: American Scientist 

Now, is it a coincidence that these dates coincide exactly with Gill’s suggested collapse and abandonment of the major cities and diminished monument building? From this evidence, I think it is safe to say that the Maya had never faced such drastic climatic changes, with the most severe drought of the past 7,000 years occurring at the pinnacle of their 1,500 years of existence.