How drought helped drive a mighty civilisation to extinction

The Maya disappeared more than 1,000 years ago, leaving pyramids, a unique form of writing and a network of lost cities in the deserts and jungles of Central and South America.

For two centuries archaeologists have speculated why their civilisation collapsed, with theories ranging from civil war to overpopulation and environmental degradation.

Today, an international team of scientists publishes powerful evidence pointing to a series of devastating droughts over a period of 150 years, which played a pivotal role in ending one of the most intriguing cultures in human history.

Cores drilled into the muddy sediments of the Cariaco Basin of the southern Caribbean Sea have identified three intense droughts around AD810, AD860 and AD910, which correspond to the periods where the Maya are thought to have abandoned some of their cities.

The scientists, led by Gerald Haug, professor of geology at the GeoForschungSzentrum institute in Potsdam, Germany, conclude in the journal Science that a change in the climate pushed the Maya civilisation into terminal decline. "These data suggest that a century-scale decline in rainfall put a general strain on resources in the region, which was then exacerbated by the abrupt drought events," they say.

The Mayan civilisation, which existed for about 2,000 years, disappeared around AD900. The Mayans mastered a system of counting, were able to chart the movements of the stars and planets and established a rich commercial trade and ritual tradition based on bloodletting and human sacrifice.

Their disappearance went unnoticed until the early 19th century when an American explorer, John Lloyd Stephens, discovered the lost Maya city of Copan deep in the South American jungle.

The civilisation stretched across much of what is now southern Mexico, Belize and Guatemala. Its history extends over a period of two millenniums, with the culture blossoming and the population growing to between 3 million and 13 million people in the "classic" period between about AD250 and AD750.

But Professor Haug said the civilisation experienced a "demographic disaster" between AD750 and AD950. Many of the densely populated cities were suddenly emptied. Eventually the civilisation collapsed, with only a fraction of the Maya population surviving.

Some archaeologists suggested that the demise was caused by civil war between competing Maya groups. Others suggested migration, disease, over-farming, or a combination of these factors.

The sediments from the Cariaco Basin off the coast of Venezuela contained seasonal layers of titanium deposited into the basin by local rivers – making the amount of the titanium an accurate indicator of regional rainfall.

Previously, archaeologists estimated the years when Maya cities were abandoned by analysing the last dates carved into the local monuments. Professor Haug found a remarkable match between this archaeological evidence and the dates of the worst periods of drought estimated from the lowest levels of titanium in the Carioca Basin.

Water was an important natural resource for the Mayan people– they went to great effort to collect rainwater in cisterns and to build an extensive network of canals and irrigation channels.

For small villages, a relatively short drought of even a few years would not perhaps have caused mass deaths but for a large urban centre with a dense population such droughts could easily prove terminal, Professor Haug said.

Decline and fall - end of great empires

Lost civilisations and abandoned cities are the stuff of historical fiction but there are many true-life examples. The monument of Great Zimbabwe, right, is the most famous stone building in southern Africa. It is thought to have been built over a period beginning in 1200 and ending around 1450 but not everybody agrees on who was responsible. The most likely are the Karanga people.

For hundreds of years the lost city of Angkor Wat, left, in present-day Cambodia, was legendary. Peasants told French colonists of "temples built by gods or by giants". Stories told of a lost civilisation but archaeologists realised the city was the centre of a Cambodian civilisation, built more than 2,000 years ago but abandoned after invasion by the Thais.

The mystery of the huge statues on the Easter Islands, right (now called Rapa Nui), can also be explained by the rise and fall of a "lost" civilisation. When people first arrived on Rapa Nui, they quickly exploited the palm forests and planted banana trees and taro root. The population flourished until the 17th century but the depletion of natural resources led to a severe fall in numbers, raising questions over who was responsible for the statues and how they were moved to these remote islands.