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.