The question of what caused the decline of Classical Maya civilization in the southern lowlands during the 9th century AD is one of the most actively researched problems in American archaeology and one that raises broader questions about the vulnerability of complex societies to environmental change.

The traditional 'Maya collapse' narrative depicted a sudden, mysterious end to a flourishing civilization. Modern research has substantially revised this picture. The collapse was neither sudden (it played out over 150+ years) nor universal (the northern lowland cities continued to flourish) nor complete (Maya population and culture persisted, just without the monumental building programs and dynastic inscriptions that archaeologists use to define 'complexity').

Paleoclimate evidence — from lake sediment cores, speleothems (cave formations), and other proxies — now provides fairly strong evidence for a series of multi-year droughts in the Maya lowlands between approximately 800 and 1000 AD. The Maya agricultural system, which depended on rainfall rather than irrigation (unlike Mesopotamian agriculture), was particularly vulnerable to drought. Population levels in the Classic period had been pushing the limits of what the local ecology could support.

The political dimension is equally important. The intensification of warfare between Maya polities in the Late Classic period — documented in dynastic inscriptions that show an increasing frequency of 'star wars' (a particular kind of astronomical warfare) and the capture and sacrifice of rival kings — disrupted the agricultural organization and inter-regional trade that the palatial systems managed. The political and ecological stresses appear to have reinforced each other in a downward spiral.