What does archaeologist tom sever think caused the mayan downfall?

Archeologists presented that the fall of the Mayan is caused mainly by drought and deforestation. Tom Sever said that it was all caused by the Mayans unplanned ways of agriculture that led to their doom. He said that they should have planted on highlands during the wet seasons and planted at their bajos (swamps, lowlands) during the dry season. He added that they should have tried to till the soil of the vast bajo lands. This might be their solution to stop deforestation that disturbed the climate and caused the drought.  

Forced the british to retreat