24/02/2012 01:09 GMT

Maya Civilisations In Mexico And Guatemala Collapsed Because Of 40% Drop In Rainfall

Only a modest drop in rainfall led to the collapse of the classic Maya civilisation more than 1,100 years ago, a new study has found.

The demise of the civilisation, centred on present day Mexico and Guatemala, was sudden and has intrigued archaeologists for many years with theories of the collapse around 950AD blamed on social unrest, disease and extreme drought.

But the study has calculated for the first time just how much rain was lost and discovered that only a 25 to 40% drop was enough to help lead to the collapse of the sophisticated society of accomplished architects and mathematicians that flourished for around 600 years.

The research was led by Professors Martin Medina-Elizalde of the Yucatan Centre for Scientific Research in Mexico and Eelco Rohling of the University of Southampton.

Professor Rohling said: "Our results show rather modest rainfall reductions between times when the Classic Maya Civilisation flourished and its collapse - between AD 800-950.

"These reductions amount to only 25 to 40% in annual rainfall. But they were large enough for evaporation to become dominant over rainfall, and open water availability was rapidly reduced. The data suggest that the main cause was a decrease in summer storm activity."

The study combined records of past climate changes from stalagmites and shallow lakes to model 40% reductions in summer rainfall and reduced tropical storm activity over the region.

Professor Medina-Elizalde added: "For more than a century, researchers have related the demise of the Classic Maya civilisation to climate change, and especially to drought. No sound estimates had been made about the severity of this drought, but some have suggested extreme scenarios.

"New data made it possible to finally get detailed estimates. To do this, we developed a model that coherently explains changes in critical datasets of change in the region's balance between evaporation and rainfall."

Professor Rohling explained such modest rainfall reductions would have caused the disintegration of a well-established civilisation.

"Summer was the main season for cultivation and replenishment of Mayan freshwater storage systems and there are no rivers in the Yucatan lowlands. Societal disruptions and abandonment of cities are likely consequences of critical water shortages, especially because there seems to have been a rapid repetition of multi-year droughts," he said

The scientists also note that the reconstructed droughts during the demise of the Classic Maya Civilisation were of similar severity as those projected in the near future in the same region.

"There are differences too, but the warning is clear. What seems like a minor reduction in water availability may lead to important, long-lasting problems.

"This problem is not unique to the Yucatan Peninsula, but applies to all regions in similar settings where evaporation is high. Today, we have the benefit of awareness, and we should act accordingly," explained Prof Medina-Elizalde.

The work, published in the leading journal Science, was funded by the UK's Natural Environment Research Council.