Climate change has destroyed many civilizations in the past

6 Aug 2012. Michael Marshall. Climate change: The great civilisation destroyer?  War and unrest, and the collapse of many mighty empires, often followed changes in local climes. Is this more than a coincidence?  NewScientist

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Also see:Five civilisations that climate change may have doomed

Below are some excerpts from this article:

What caused the collapse of Mycenaean Greece, and thus had a huge impact on the course of world history? A change in the climate, according to the latest evidence. What’s more, Mycenaean Greece is just one of a growing list of civilizations whose fate is being linked to the vagaries of climate. It seems big swings in the climate, handled badly, brought down whole societies, while smaller changes led to unrest and wars.

Excavating in what is now Syria, Weiss found dust deposits suggesting that the region’s climate suddenly became drier around 2200 BC. The drought would have led to famine, he argued, explaining why major cities were abandoned at this time (Science, vol 261, p 995). A piece of contemporary writing, called The Curse of Akkad, does describe a great famine:

For the first time since cities were built and founded,
The great agricultural tracts produced no grain,
The inundated tracts produced no fish,
The irrigated orchards produced neither syrup nor wine,
The gathered clouds did not rain, the masgurum did not grow.
At that time, one shekel’s worth of oil was only one-half quart,
One shekel’s worth of grain was only one-half quart. …
These sold at such prices in the markets of all the cities!
He who slept on the roof, died on the roof,
He who slept in the house, had no burial,
People were flailing at themselves from hunger.

In 2000, climatologist Peter deMenocal of Columbia University in New York found more. His team showed, based on modern records going back to 1700, that the flow of the region’s two great rivers, the Tigris and the Euphrates, is linked to conditions in the north Atlantic: cooler waters reduce rainfall by altering the paths of weather systems. Next, they discovered that the north Atlantic cooled just before the Akkadian empire fell apart (Science, vol 288, p 2198). “To our surprise we got this big whopping signal at the time of the Akkadian collapse.”

It soon became clear that major changes in the climate coincided with the untimely ends of several other civilizations (see map). Of these, the Maya became the poster child for climate-induced decline. Mayan society arose in Mexico and Central America around 2000 BC.

Then the Maya civilisation collapsed.  Numerous studies have shown that there were several prolonged droughts around the time of the civilisation’s decline. In 2003, Gerald Haug of the Swiss Federal Institute of Technology in Zurich found it was worse than that. His year-by-year reconstruction based on lake sediments shows that rainfall was abundant from 550 to 750, perhaps leading to a rise in population and thus to the peak of monument-building around 721. But over the next century there were not only periods of particularly severe drought, each lasting years, but also less rain than normal in the intervening years (Science, vol 299, p 1731). Monument construction ended during this prolonged dry period, around 830, although a few cities continued on for many centuries.

When the climate becomes less favourable, less food can be grown. Such changes can also cause plagues of locusts or other pests, and epidemics among people weakened by starvation. When it is no longer feasible to maintain a certain population level and way of life, the result can be collapse.

In 2010, though, a study of river deposits in Syria suggested there was a prolonged dry period between 1200 and 850 BC – right at the time of the so-called Greek Dark Ages. Earlier this year, Drake analyzed several climate records and concluded that there was a cooling of the Mediterranean at this time, reducing evaporation and rainfall over a huge area.

What’s more, several other cultures around the Mediterranean, including the Hittite Empire and the “New Kingdom” of Egypt, collapsed around the same time as the Mycenaeans – a phenomenon known as the late Bronze Age collapse. Were all these civilizations unable to cope with the changing climate? Or were the invading Sea Peoples the real problem? The story could be complex: civilizations weakened by hunger may have become much more vulnerable to invaders, who may themselves have been driven to migrate by the changing climate. Or the collapse of one civilization could have had knock-on effects on its trading partners.

Around 900, the Tang dynasty began losing its grip on China. At its height, the Tang ruled over 50 million subjects. Woodblock printing meant that written words, particularly poetry, were widely accessible. But the dynasty fell after local governors usurped its authority. A study of lake sediments in China by Haug suggests that this region experienced a prolonged dry period at the same time as that in Central America. He thinks a shift in the tropical rain belt was to blame, causing civilisations to fall apart on either side of the Pacific (Nature, vol 445, p 74).

From 2500 BC until the 20th century, a series of powerful empires like the Tang controlled China. All were eventually toppled by civil unrest or invasions.  When Zhang compared climate records for the last 1200 years to the timeline of China’s dynastic wars, the match was striking. Most of the dynastic transitions and periods of social unrest took place when temperatures were a few tenths of a degree colder. Warmer periods were more stable and peaceful (Chinese Science Bulletin, vol 50, p 137).

Zhang gradually built up a more detailed picture showing that harvests fell when the climate was cold, as did population levels, while wars were more common. Of 15 bouts of warfare he studied, 12 took place in cooler times. He then looked at records of war across Europe, Asia and north Africa between 1400 and 1900. Once again, there were more wars when the temperatures were lower. Cooler periods also saw more deaths and declines in the population.

These studies suggest that the effects of climate on societies can be profound.

Trying to move beyond mere correlations, Zhang began studying the history of Europe from 1500 to 1800 AD. In the mid-1600s, Europe was plunged into the General Crisis, which coincided with a cooler period called the Little Ice Age. The Thirty Years war was fought then, and many other wars. Zhang analyzed detailed records covering everything from population and migration to agricultural yields, wars, famines and epidemics in a bid to identify causal relationships. So, for instance, did climate change affect agricultural production and thus food prices? That in turn might lead to famine – revealed by a reduction in the average height of people – epidemics and a decline in population. High food prices might also lead to migration and social unrest, and even wars.

The Khmer empire, centered in what is now Cambodia, began in 802 AD. It built the astounding temple of Angkor Wat, dedicated to the god Vishnu, in the 12th century. We now know that Angkor Wat was not, as long thought, a lone structure. It was the heart of a teeming city covering 1000 square kilometres, surrounded by even larger suburbs. Before the Industrial Revolution, Angkor was perhaps the world’s largest city. But it was sacked and abandoned in 1431 apart from the temple, which by then had been taken over by Buddhists. What made the Khmer abandon their metropolis? According to Brendan Buckley of Columbia University in New York, changes to the monsoon were a contributing factor. Buckley used tree rings to produce a yearly record of monsoon rainfall from 1250 to 2008. He found that the monsoon was weak in the mid to late 1300s. This was followed by a short but harsh drought in the early 1400s, just before Angkor fell. There were also a few years when the monsoons returned with a vengeance, causing severe floods.

Like many south Asian societies, the Khmer relied on the monsoon to water their crops. Canals and reservoirs channelled water to farms and homes in Angkor. Many are now filled with sand and gravel, carried in by floods, and Buckley showed the deposits in at least one canal date to the time of the collapse. This damage would have made it even harder to manage the water supply, at a time when it was already limited and unpredictable.

Between 300 and 500 AD, a people called the Moche thrived and established cities along the coast of Peru. Their farmers built a network of irrigation canals, and grew maize and lima beans. Their capital boasts the largest adobe structure in the Americas, the Huaca del Sol.   After 560, however, the Moche civilisation began to decline. By the time they abandoned the coastal cities around 600 and moved inland, their irrigation channels had been overrun by sand dunes.  The decline may have been triggered by changes in climate. Studies of ice cores suggest that an especially intense El Niño cycle around this time produced intense rainfall and floods, followed by a long and severe drought.

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