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.