Plague. Famine. War. Disaster. In the sixth century AD a cataclysm occurred that changed the weather, destroyed civilisations and affected the course of world history for ever
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With some people it began in the head, made the eyes bloody and the face swollen, descended to the throat and then removed them from Mankind. With others there was a flowing of the bowels. Some came out in puss-filled swellings that gave rise to great fevers, and they would die two or three days later with their minds in the same state as those who had suffered nothing, and with their bodies still robust. Others lost their senses before dying. Malignant pustules erupted and did away with them. Sometimes people were afflicted once or twice, and then recovered, only to fall victim a third time and then succumb."

Thus wrote the 6th-century church historian Evagrius, describing the gruesome symptoms of the bubonic plague that devastated the Roman Empire and much of the wider world in the 6th- and 7th-centuries. The first area of the empire to be hit by plague was Egypt. The town where it first appeared was the Mediterranean port of Pelusium - traditionally the point of entry for Egypt's enemies for over 1,000 years. Persians, Syrians, Romans, Greeks - even Alexander the Great himself - had invaded Egypt through it. But this time, the enemy was not clad in armour. It was invisible, carried ashore on the backs of scuttling rats.

After devastating Pelusium, the disease quickly spread to Alexandria, then on to Constantinople and the wider Empire. Up to a third of the population died in the first serious outbreak - and in the capital, more than half of the inhabitants are thought to have perished. However, unknown to the inhabitants of the Roman Empire, their society was not the only part of the world being devastated at that time. For the mid- to late-6th-century also saw southern China tormented by famines and a terrible civil war; a bloodthirsty ethnic revolution on the Mongolian Steppes; freezing temperatures and disease in Japan; the fall of ancient Mexico's greatest city; and the demise of key civilisations in South America.

What lay behind this inventory of disaster and human misery? Was this all one huge, planet-wide coincidence - or was there a common cause generating mayhem all over the globe?

Scientific and historical evidence gathered over recent years suggests that the truth may be the latter option. I believe that the culprit was a massive volcanic eruption in south-east Asia, probably the volcanic island of Krakatoa in the Sunda Straits between Java and Sumatra. The mass of historical and scientific evidence I have gathered in four years of research strongly suggests that a huge volcanic explosion took place in the Krakatoa area in February 535AD. Certainly, historical evidence from Byzantium, western Europe, China, Japan and Korea - supported by data from the growth rings of trees - reveals that the world's climate became very disturbed at this time.

John of Ephesus, a 6th-century historian and church leader, chronicled the event. "There was a sign from the Sun," he wrote, "the like of which had never been seen or reported before. The Sun became dark and its darkness lasted for 18 months. Each day it shone for about four hours, and still this light was only a feeble shadow. Everyone declared that the Sun would never recover its full light again." Another 6th-century chronicler - a top Roman military official called Procopius - wrote that "it came about during this year that a most dread portent took place. For the Sun gave forth its light without brightness like the Moon during this whole year, for the beams it shed were not clear, nor such as it is accustomed to shed". And a third observer, Zacharias of Mytilene, said that "the Sun began to be darkened by day and the Moon by night".

In Ireland, drought conditions produced an appalling famine. In Japan, a civil servant recorded the King as saying that "yellow gold and 10,000 strings of cash cannot cure hunger. What avails a thousand boxes of pearls to him who is starving of cold?" In China, climatic conditions were so bad that the Emperor issued an edict which ordered that "in the capital, in all provinces, commanderies and districts, one should bury the corpses". Chinese sources also refer to a thunderous bang coming from the south- west, in the direction of Krakatoa - and evidence from an Indonesian chronicle suggests that Krakatoa did in fact erupt with unprecedented force in the middle of the first millennium AD.

Reconstructing the immediate sequence of events associated with a volcanic eruption which occurred one and a half millennia ago is a daunting task, but not an impossible one. Using the quasi-historical account in a Javanese chronicle, it is possible, assuming the account to be at least part genuine, to gain an insight into specific aspects of the eruption itself. And using geological and vulcanological knowledge of the area, and records of more recent large eruptions, it is possible to reconstruct what probably happened.

Between AD530 and AD535, there would almost certainly have been a long series of earthquakes in what is now western Java, southern Sumatra, and beneath the neighbouring seas. Earthquakes and accompanying seismically triggered tidal waves may well have seriously disrupted life in the region. Throughout the second-half of AD534, earthquakes would have struck at the rate of one or two per day. In the weeks immediately before the eruption, the rate would have accelerated to a peak of 50 per hour in the final 24 hours, mainly in the moderately powerful range of 1 to 3 on the Richter scale.

The AD535 eruption would have burst forth from a volcanic mountain located on what was almost certainly fairly low-lying ground. For several years, a 250-500 cubic mile mass of molten magma would have been moving closer and closer to the surface - probably at the rate of up to 30ft per month. This would have caused the land surface above to bulge upwards into a low dome increasing in height at up to 3ft per year over perhaps a five- year period. Then suddenly the pressure of the magma, between two to three miles below ground, would have proved too great: a crack would have opened up causing the eruption to start. A vast cloud of ash would have billowed out, followed by a column of red- hot magma.

Ultimately, dozens of cubic miles of magma, thousands of cubic miles of vapourised sea water, and vast quantities of ultra fine hydro-volcanic ash - generated by the interaction between magma and seawater - was hurled into the sky and a substantial percentage of it would have entered the upper part of the Earth's atmosphere. Like a natural equivalent of a nuclear winter, the eruption changed world climate.

In Mongolia, a severe drought killed off the horse-based economy - and with it the nomadic empire - of the ruling Avars, allowing their vassals, the Turks, to take over. This was the beginning of Turkic imperial civilisation - the start of the process that eventually produced the Ottoman Empire. The displaced Avars fled west to Europe, where they invaded the Roman Empire - and caused huge economic and social damage. The Romans lost vast amounts of territory (and tax base) to the Avars and their Slav vassals and associates.

The climatic disaster also caused ecological mayhem in East Africa - especially amongst the rodent population - and this triggered a massive outbreak of bubonic plague that swept northwards on board ships up the Red Sea, through the Roman equivalent of the Suez Canal, and into the Mediterranean. The Empire was devastated by repeated epidemics, and so was western Europe. The plague, the Avar invasions, and the rapid decline of the Roman Empire led Christians and Jews alike to believe that the end of the world was nigh. Indeed, in a sense it was.

In Arabia, the climatic problems had led to the destruction of one of the greatest engineering feats of the ancient world, the Marib Dam and its huge irrigation system. And the migratory, economic and geopolitical problems and changes caused by the demise of the dam (and the ravages of the plague in Yemen), together with the general apocalyptic atmosphere in the Mediterranean and Middle Eastern world, led to the emergence of a new religion: Islam.

In England, the plague destabilised the political and territorial status quo by hitting the Celtic west and centre more than the Anglo-Saxon east. It led, therefore, directly to Anglo-Saxon expansion and the foundation of England. Related processes took place in Gaul and the Iberian peninsula, and led to the establishment of a proto-France and a proto-Spain. In the Far East, the climatically-triggered famines and political chaos resulted ultimately in the emergence of a united China.

Thus, all over the world the eruption acted as midwife to the political structures that now dominate our planet. We may like to believe that the politicians and generals make history, but ultimately it is, I believe, natural forces and the weather that probably exert the lion's share of influence.

And that is a fact worth remembering, given that global warming in the 21st century is likely to present the planet with its biggest dose of climatic chaos since world history was resynchronised by the weather (courtesy of Krakatoa) 1,500 years ago.

David Keys' book, `Catastrophe, an Investigation Into the Origins of the Modern World', is published next week by Century, pounds 16.99. A documentary based on the book is on Channel 4 in August