By Helen Sewell
BBC News Online science staff
Climate change was largely to blame for the collapse of the Mayan civilisation in Central America more than 1,000 years ago, research suggests.
Drought could have destroyed the civilisation
By the middle of the 8th Century there were up to 13 million people in the Mayan population but within 200 years their cities lay abandoned.
The Mayans built complex systems of canals and reservoirs to collect rainwater for drinking in the hot, dry summers.
Despite this there has long been speculation that the whole population was wiped out by drought, but there has not been enough evidence to support this theory.
Now research published in the journal Science suggests that climate change was indeed a major factor.
To investigate the Mayan decline, scientists studied the ancient build-up of sediment on the sea floor just off the northern coast of Venezuela.
They discovered layers of deposits in bands of alternating dark and light colours each about a millimetre deep. The light bands consisted of algae and tiny fossils, while the dark bands were due to sediments of the metal titanium.
The scientists say titanium was washed into the sea by rivers during the rainy seasons. Shallower dark bands, which indicate lower levels of the metal, show the rivers were flowing more weakly. The researchers say this was because there was less rain.
They have worked out that in the 9th and 10th Centuries, probably just before the Mayan civilisation collapsed, there was a long period of dry weather and three intense droughts.
Archaeological evidence suggests that one reason for the Mayans' initial success over other societies was that they controlled the artificial reservoirs.
If this is true, the scientists say the drought could easily have pushed the whole civilisation to the verge of collapse.
The German scientist who led the research, Gerald Haug, said this had serious implications for climate change today.
"A three-to-nine-year drought, which could be a failure of the monsoon systems in Africa or in India, and in particular the change in the background state of climate... is a very severe threat to modern humanity," he told BBC News Online.