What was the point?!

Having read previous posts, you might be forgiven for thinking to yourself: 'What's the relevance of all this stuff?' or 'Why should I care about what happened over 1,200 years ago?'. These are very valid questions and while they have in-part been answered before, hopefully I can do it once-and-for-all here.

The crux of the matter is that understanding past climate changes helps us to predict future ones and potentially mitigate any great damages or losses that they might bring. Discovering that a serious drought occurred across Mesoamerica at approximately 900 AD helps us to become aware of the potential dangers that inhabitants face today. What's more important, though, is understanding why the drought occurred when it did and the main process through which it happened. This allows us to gain greater understanding of the environment in which we live and predict when future changes might occur. This knowledge may even be able to be applied to different areas of the globe.

Understanding the role that humans play is also important. As mentioned in this blog, for example, it has been modelled that there is great potential for anthropogenic deforestation to increase the severity of drought conditions in Central America. This finding was based on data from the past and can help direct environmental management policies that should be implemented today. This example is especially relevant as the area in question does see high levels of deforestation that could prove extremely costly in the future.

Hopefully the point of the research covered in this blog has now become a bit clearer.

Environmental Deterioration: Conclusions

Looking at the evidence, some research has suggested that degradation, through deforestation and soil erosion, may well have played an important role in Classic Maya collapse. Abrams and Rue (1994) correlated deforestation and resultant soil erosion to the time of Mayan population decline in Copan, Honduras. They stated that such environmental degradation served as a primary factor in, what they saw was a gradual, Mayan demographic decline. Shimkin (1973) established the possible impact that such degradation might have as eventual shortages in fuel wood, necessary for heating and cooking, may have led to increased endemic respiratory and gastrointestinal diseases, contributing to a higher mortality rate and local population decline.

On the other hand, according to other research, it is apparent that, at their peak population, the Maya were actually very adept at conservation and degradation was not an influencing factor in Classic Maya collapse. Beach et al. (2006) discovered that, in many areas, terracing was employed and soil erosion was lower than at earlier times in the history of the Maya. McNeil et al. (2010) found that prior to the time of the Mayan population collapse, forest cover was actually increasing. They also suggested that erroneous chronology, found using poor dating methods, and an insufficient sediment core helped to disprove Abrams and Rue’s (1994) findings.

These conflicts revolve around whether deforestation was actually occurring around the 900 AD and not whether deforestation had the potential to cause demographic decline. The link between deforestation and its potential impacts on the climate and Mayan society were highlighted by Oglesby et al. (2010). They found that widespread deforestation does have the potential to induce drought conditions that may well have impacted the Maya and bolstered a natural drought cycle. However, without substantial evidence for widespread deforestation across the region, this paper does not prove that the potential of this activity was realised and actually served as a factor in the population decline of the Classic Maya.

It therefore seems that forest clearance had the potential to, at least, exacerbate drought conditions. However, in order to establish this capability was actually realised, more robust evidence that deforestation actually occurred is needed. 


References


Shimkin, D. B. (1973). 'Models for the downfall: Some ecological and culture-Historical considerations.' In Culbert, T. P. (ed), The Classic Maya Collapse. University of New Mexico Press, Albuquerque, pp. 269-299.

Collapse due to soil erosion?

Ignoring the deforestation debate, Beach et al. (2006) concentrated solely on the levels of soil erosion across the temporal and spatial extents of the Mayan empire.

Their paper provided new data from two sites (Blue Creek and Cancue´n) and synthesised more than a decade of the authors’ research in Guatemala, Belize, and Mexico. These research projects analysed more than 100 excavations in upland and depression sites, cored lakes and wetland sediments, and studied sediments in the field and laboratory using radiocarbon dating, a battery of soil chemistry tests, stratigraphic analysis, magnetic susceptibility, elemental analyses, and artefact identification. Their objective was to date when sedimentation and soil erosion occurred, identify stable surfaces, and correlate them with the state of knowledge about past land use.

Beach et al.’s (2006) findings indicated three general epochs of accelerated soil erosion. These occurred in the Pre-classic period (c. 1000 BC to AD 250), the Late Classic (AD 550 to 900), and in the last several decades.

At some sites (the Petexbatun, the Three Rivers, and the Belize River) a higher than expected soil erosion in the Pre-classic period, due to the region’s first pioneer farmers, was found. These sites also showed less than expected soil erosion in the Late Classic when population peaked and land use was the most intensive. It is stated that this resulted from the wide diffusion of many types of terracing that may have conserved soils when ancient Maya populations were greatest, but may also be partly the result of sediment exhaustion, in which erosion may have already removed the readily erodible component of upland soils.

In other regions like Cancue´n, Guatemala, however, most soil erosion occurred during the Maya Late Classic (AD 550–830). Erosion here was intense but short-lived, whereby depressions record 1-3 m of aggradation in two centuries.

Beach et al. (2006) conclude that though there is some isolated evidence for a rise in soil erosion during the Classic Mayan period when population pressure was highest, in fact erosion had started before this time. It is apparent that there is also ample evidence for lower soil erosion at several sites during the periods of highest human populations and intensive land uses. Though the Mayans did cause widespread geomorphic change, both in terms of soil erosion and soil conservation, Beach et al. (2006) state that their impacts started near the beginning of their civilization, which persisted more than a thousand years beyond the start of the population decline.

It therefore seems unlikely that soil erosion could account for the devastating population decline that was experienced by the Maya at around 900 AD.

Deforestation: Evidence challenged

McNeil et al. (2010) directly challenged the results obtained and conclusions reached by Abrams and Rue (1994).

Their first criticism stems from the longevity of Abram ad Rue’s core, where the oldest sediments date to A.D. 1010 , at least 100 years after the well-documented collapse of Copan’s population and political system. McNeil et al. (2010) also found the “gradual collapse” thesis to be problematic because scholars have demonstrated that the method of dating used incorrectly models the complex weathering processes undergone by the dated artefacts. They state that this erroneous modelling produced erroneous dates that created false indications of extended collapse. In addition, McNeil et al. (2010) state that a slow demographic decline is not supported by other lines of archaeological evidence at the site. Thus, it is concluded that Abrams and Rue’s (1994) sediment core is from the Post-classic period and does not provide information concerning the environmental impact of human populations during the Classic period.

In order to disprove Abrams and Rue’s (1994) findings completely McNeil et al. (2010) analysed a longer sediment core taken from the same pond. It demonstrated that forest cover actually increased from A.D. 400 to A.D. 900, with arboreal pollen accounting for 59.8% - 71.0% of the pollen assemblage by approximately A.D. 780 - 980. The highest levels of deforestation were actually found about 900 B.C. when, at its peak, herb pollen made up 89.8% of the assemblage. It was suggested that this event likely coincided with the widespread adoption of agriculture, a pattern found elsewhere in Mesoamerica.

McNeil et al. (2010) conclude that deforestation was not widespread in Copan around the time of Classic Maya collapse and, thus, it could not have caused population decline.

Evidence of deforestation

Abrams and Rue (1994) analysed the link between deforestation and Maya collapse in Copan, Honduras (Figure 1). They state that the population of Copan grew by 5,000 people from 550 AD to 700 AD and then by a further 20,000 by 850 AD, when it was at its peak. However, during the period from 850 AD until 1000 AD the population declined dramatically by 50% and then by a further 50% during the following 150 years. Copan became completely uninhabited by 1200 AD and was not occupied again until the 19^th Century. 

Figure 1: Map of Central America showing location of Copan, Honduras (Source: Adapted from Image)

Evidence for deforestation was gained from palynological data obtained from a core extracted from the Agua-da de Petapilla, a small bog located just north of Copan. Figure 2 shows the expected decrease in arboreal species during the Late and Terminal Classic periods and a gradual reforestation period associated with an increasingly reduced population. It can be seen that the significant reduction of arboreal species, particularly that of the dominant pine, Pinus oocarpa Schiede, correspond temporally to the Late Classic period.

Figure 2: Pollen profile (Source: Abrams and Rue 1994)

  

It was estimated that at least 23 km² of the upland pine forest may have been completely cleared by the end of the Late Classic period. An assessment of human needs for arboreal resources suggests that deforestation was the result of extensive clearing from the foothill zone for agricultural and habitational purposes and from the upland forest zone for domestic purposes of cooking and heating. Abrams and Rue (1994) believed the data suggested that three was not a sudden collapse in population but a more gradual decline

Having correlated deforestation with the time of Maya collapse, Abrams and Rue (1994) then studied its effect on the soils. It was found that surface runoff, sediment and nutrient loss greatly increased following the burning of pine from upland slopes. In fact, vegetation cover was discovered to be the primary factor in affecting these rather than slope angle. It was concluded that these factors led to the reduced productivity of the agricultural infrastructure and thus the decrease in population at Copan.

Aside

Interestingly, Abrams and Rue (1994) alluded to some lessons that may be learnt from the Mayan system that may prove very beneficial today. They state that the process of deforestation is best considered as a consequence of the broader process of urban growth, evidenced by increasing population size and density at Copan and other Late Classic centres. In this way, the relatively dispersed catchment areas of pre-urban settlements formed a larger single urban catchment area, leading to increased exploitation and denudation concentrated around centres. Many urban areas in less developed countries are evolving in this way and, it is warned, that if unchecked such processes could result in the same way as with the Maya.

Environmental deterioration as a cause for Mayan collapse

Aside from the climate-based theories for the collapse of the Classic Maya are those that deem environmental deterioration, as a result of rising population densities, as a major cause. It is said that such ecological mismanagement would have led to reduced productivity of the agricultural system, which in turn would have been largely responsible for the depopulation of urban centres. Among these, deforestation has occasionally been cited as a playing a major role.

Deforestation is the long-term reduction in the aerial extent of arboreal forest through the removal of tree species, either for the trees themselves or four some other resource(s) associated with the forest (Oldfield, 1981, p. 280). It is necessary for the rate of removal to exceed the rate of regrowth in order to be considered as deforestation.

The link between deforestation and the collapse of the Mayan empire was first alluded to by Cooke (1931), who stated that the rate of soil erosion was enormously accelerated when forest was cut and the cultivated soil was exposed to the torrential rains. This model gained support over the years, but the first empirical evidence was presented by Sanders (1973). Studying an area of Guatemala, he found that over 40% of soils had high to very high susceptibility to erosion and of these 37% had high fertility and thus attractive to an expanding agricultural system. It was added that these soils are found in areas which, prior to cultivation, were covered with forest. Despite this evidence for the link between deforestation and agricultural decline, however, the rate, absolute chronology and specific causes of ecological deterioration remained unclear. Furthermore, it remained necessary to qualify the relationship between agricultural decline and demographic collapse.

Shimkin (1973) suggested that the eventual shortages of fuel wood, necessary for heating and cooking, may have led to increased endemic respiratory and gastrointestinal diseases, contributing to a higher mortality rate and local population decline. 

References

Cooke, C. W. (1931). 'Why the Maya Cities of the Peten District, Guatemala, were abandoned.' Journal of the Washington Academy of Sciences 21(13): 283-287.

Oldfield, M. L. (1981). 'Tropical deforestation and genetic resources conservation.' In Sutlive, V. H., Altshuler, N., and Zamora, M. D. (eds.), Blowing in the Wind: Deforestation and Long-Range Implications. Studies in Third World Societies Pub. 14, College of William and Mary, Virginia, pp. 277-346.

Sanders, W. (1962). Cultural ecology of the Maya lowlands (Part I). Estudios de Culture Maya 2: 79-121.

Shimkin, D. B. (1973). 'Models for the downfall: Some ecological and culture-Historical considerations.' In Culbert, T. P. (ed), The Classic Maya Collapse. University of New Mexico Press, Albuquerque, pp. 269-299.

Conclusions: What caused the drought?

Past research (analysed in previous blogs here, here and here) does not provide a definitive reason for the drought that may well have caused the Classic Mayan collapse. Hodell et al. (2001) discovered a very convincing correlation between periods of drought and high levels of solar activity in a 206-year cycle. However, no explanation of the processes behind this link were proven and so this theory still remains in doubt.

One of Hodell et al.'s (2001) proposed mechanisms was through the movement of the Hadley cell. The potential for this to cause drought was picked up by Gill et al. (2007). It is stated that the southwest–northeast travel of the North Atlantic High, a product of the Hadley cell, has the potential to bring drought to Mesoamerica. However one major contradiction prevents the two theories being complementary. Gill et al. (2007) state that movement of the Hadley cell would result from periods of reduced solar activity, but Hodell et al. (2001) found correlation between high levels of solar activity and drought. This disagreement means that no clear conclusion can be drawn as to whether the Hadley cell plays an important role in inducing drought in Central America.  

Moving on to potential anthropogenic causes of drought that might compliment the natural ones, Oglesby et al. (2010) investigated the possible role of deforestation. Using a climate model, it was found that increased deforestation in Central America would lead to stabilisation of the atmosphere and a reduction in precipitation. As no dated evidence was assessed, the actual reasons behind the drought acannot be proved using this discovery. However, it does highlight the potential of anthropogenic environmental degradation, and deforestation in particular, to strengthen natural drought. 

While it is yet to be known exactly what caused the Mayan drought, it seems likely that solar forcing played some role in the process due to its striking correlation with dry periods.  Due to the complexity of the climate system, the mechanisms by which fluctuating solar activity is tranformed into times of reduced rainfall are still contentious. More research is needed if this link is to be totally understood. However, it does appear that increased levels of deforestation, if they existed, may well have complemented natural drought conditions and, even if only slightly, exascerbated the impact of the dry period.