GLOBAL warming is one of the greatest threats to present day civilisation but work by a team of Scots scientists suggests the ancient Egyptians may have been earlier victims of climate change.
The pharaohs ruled their empire for hundreds of years, spreading culture, architecture and the arts before it collapsed into economic ruin. Why that happened is one of the great mysteries of history.
Now a team of scientists from Scotland and Wales believe the answer lies beneath the waters of Lake Tana, high in the Ethiopian Highlands, and the source of the all-important Blue Nile.
Samples taken over the past two years from sediments beneath Tana, which supplies the water which makes the lower Nile valley so fertile, reveal the lake may have almost dried up during the critical period around 4,200 years ago due to climate change.
According to the team's theory, the flow of water on which the farm-based ancient Egyptian economy thrived would have slowed to a trickle, causing a devastating famine that lasted for 200 years.
That would have been enough to destroy the Old Kingdom and its people, leaving only the pyramids and the Sphinx at Giza as their legacy to history.
The research is being carried out by a geological team from St Andrews University and the University of Wales, Aberystwyth.
Dr Mike Marshall, from the Institute of Geography and Earth Sciences at Aberystwyth, said that when the project began in 2003, the drought was only a theory, but the pieces of the jigsaw are now being unearthed.
"We have found evidence of drought events at several levels in the lake's sediments. That correlates with 4,200 years ago. Lake Tana at that time could have been at a very low level.
"It wasn't completely dried out, but the lake became less extensive. Parts of the fringes of the lake bed could have been exposed completely and so the water flow may have been much less than normal for long periods. This could have had a severe effect on water flows further down the Nile."
The Old Kingdom flourished between 2575BC and 2150BC and bequeathed the world some of its most iconic stone monuments. But historians have argued over what was to blame for the Kingdom's demise.
Theories include invasion from Asia or internal political conflict, but more likely are the consequences of repeated and damaging drops in the level of the Nile over decades. Although written archives from the era record famine resulting from drought, proof of what stopped the annual Nile floods from occurring has been hard to find.
Lake Tana feeds the Blue Nile, which joins the White Nile at Khartoum in Sudan. The Blue Nile provides two-thirds of the water in the Nile proper, which flows through Egypt to the Mediterranean. Annual monsoons in the Ethiopian Highlands have led to the yearly flooding of the Nile, which was so important to ancient civilisations in the area.
But although 53 miles long and 41 miles wide, the lake's greatest depth is 50 feet, which scientists believe would make it highly vulnerable to climate change.
Working from fixed and moving barges, the academics, backed up by Ethiopian drillers, have taken core samples from bore holes in the lake bed sediments dating back at least 18,000 years.
At that level, 79 ft down from the lake bed, they found strong evidence that Tana had completely dried out, suggesting a dramatic change in the Earth's climate.
Seismic surveys carried out by Dr Richard Bates from St Andrews' School of Geography and Geosciences have revealed the sediments are much deeper than previously considered.
"What we now know is that the lake dried out 18,000 years ago, which corresponds with the end of the last Ice Age. As we get better at what we are doing we should be able to detect the nuances of what went on in the lake after that time," Bates said.
The team returned to Ethiopia last month to continue their research.
Bates said: "The opportunity to establish such a long and detailed record of climate change at the heart of Africa has important implications, not only for trying to understand past, present and future climate change, but as we delve back into the past, it will also have important implications for the study of human development and the migration of early man from the cradle of mankind."
Heating planet: the theories
THERE is little debate that the globe heated up significantly during the 20th century - between 0.4 and 0.8 Celsius - but the reasons why vary.
The dominant theory, the greenhouse effect, is based on so-called 'greenhouse gases' such as carbon dioxide and methane trapping radiation from the sun below the earth's atmosphere.
A minority of researchers argue there is no conclusive proof that man is to blame, since the evidence is circumstantial. Some argue the warming is a result of natural variation, primarily due to cycles of radiation from solar flares. Others recognise the greenhouse theory , but note the high emissions from natural phenomena such as volcanoes. Most scientists agree man's impact has accelerated the process.