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<http://www.newscientist.com/article/mg21528761.600-climate-change-the-great-civilisation-destroyer.html>
 Climate change: The great civilisation destroyer?

   - 06 August 2012 by *Michael
Marshall*<http://www.newscientist.com/search?rbauthors=Michael+Marshall>
   - Magazine issue 2876 <http://www.newscientist.com/issue/2876>. *Subscribe
   and save*<http://subscribe.newscientist.com/bundles.aspx?promcode=6005&term=1Y&intcmp=SUBS-nsarttop>
   -

   [image: The palace of Mycenae today. All empires fall – but why?
<i>(Image: The Travel Library/Rex Features)</i>]

The palace of Mycenae today. All empires fall – but why? *(Image: The
Travel Library/Rex Features)*

1 more image<http://www.newscientist.com/articleimages/mg21528761.600/1-climate-change-the-great-civilisation-destroyer.html>

*War and unrest, and the collapse of many mighty empires, often followed
changes in local climes. Is this more than a coincidence?*

*See our gallery:* "Five civilisations that climate change may have
doomed<http://www.newscientist.com/gallery/climate-collapse>
"

1200 BC. The most beautiful woman in the world, Helen, is abducted by Paris
of Troy. A Greek fleet of more than a thousand ships sets off in pursuit.
After a long war, heroes like Achilles lead the Greeks to victory over Troy.

At least, this is the story told by the poet Homer around four centuries
later. Yet Homer was not only writing about events long before his time, he
was also describing a long-lost civilisation. Achilles and his compatriots
were part of the first great Greek civilisation, a warlike culture centred
on the city of Mycenae that thrived from around 1600 BC.

By 1100 BC, not long after the Trojan war, many of its cities and
settlements had been destroyed or abandoned. The survivors reverted to a
simpler rural lifestyle. Trade ground to a halt, and skills such as writing
were lost. The script the Mycenaeans had used, Linear B, was not read again
until 1952<http://www.newscientist.com/article/mg20227106.000-decoding-antiquity-eight-scripts-that-still-cant-be-read.html>
.

The region slowly recovered after around 800 BC. The Greeks adopted the
Phoenician script, and the great city states of Athens and Sparta rose to
power. "The collapse was one of the most important events in history,
because it gave birth to two major cultures," says anthropologist Brandon
Drake. "It's like the phoenix from the ashes." Classical Greece, as this
second period of civilisation is known, far outshone its predecessor. Its
glory days lasted only a couple of centuries, but the ideas of its citizens
were immensely influential. Their legacy is still all around us, from the
maths we learn in school to the idea of democracy.

But 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 civilisations 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.

The notion that climate change toppled entire civilisations has been around
for more than a century, but it was only in the 1990s that it gained a firm
footing as researchers began to work out exactly how the climate had
changed, using clues buried in lake beds or petrified in stalactites. Harvey
Weiss <http://www.yale.edu/archaeology/f.harveyweiss.html> of Yale
University set the ball rolling with his studies of the collapse of one of
the earliest empires: that of the Akkadians.

It began in the Fertile Crescent of the Middle East, a belt of rich
farmland where an advanced regional culture had developed over many
centuries. In 2334 BC, Sargon was born in the city state of Akkad. He
started out as a gardener, was put in charge of clearing irrigation canals,
and went on to seize power. Not content with that, he conquered many
neighbouring city states, too. The empire Sargon founded thrived for nearly
a century after his death before it collapsed.

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<http://dx.doi.org/10.1126/science.261.5124.995>).
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.*

Weiss's work was influential, but there wasn't much evidence. 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 <http://bit.ly/QvuLL6>: 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<http://dx.doi.org/10.1126/science.288.5474.2198>).
"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 civilisations (see
map<http://www.newscientist.com/data/images/ns/cms/mg21528761.600/mg21528761.600-1_1110.jpg>).
Of these, the Maya became the poster child for climate-induced decline.
Mayan society arose in Mexico and Central America around 2000 BC. Its
farmers grew maize, squashes and beans, and it was the only American
civilisation to produce a written language. The Maya endured for millennia,
reaching a peak between AD 250 and 800, when they built cities and huge
stepped pyramids.

Then the Maya civilisation collapsed. Many of its incredible structures
were swallowed up by the jungle after being abandoned. Not all was lost,
though - Mayan people and elements of their culture survive to the present
day.

Numerous studies have shown that there were several prolonged droughts
around the time of the civilisation's decline. In 2003, Gerald
Haug<http://www.climategeology.ethz.ch/people/ghaug>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<http://dx.doi.org/10.1126/science.1080444>).
Monument construction ended during this prolonged dry period, around 830,
although a few cities continued on for many centuries.

Even as the evidence grew, there was something of a backlash against the
idea that changing climates shaped the fate of civilisations. "Many in the
archaeological community are really reticent to accept a role of climate in
human history," says deMenocal.

Much of this reluctance is for historical reasons. In the 18th and 19th
centuries, anthropologists argued that a society's environment shaped its
character, an idea known as environmental
determinism<http://etherwave.wordpress.com/2010/09/17/human-geography-and-environmental-determinism-the-arguments-of-ellsworth-huntington-and-ellen-semple/>.
They claimed that the warm, predictable climates of the tropics bred
indolence, while cold European climates produced intelligence and a strong
work ethic. These ideas were often used to justify racism and
exploitation<http://www.jstor.org/discover/10.2307/215428>
.

Understandably, modern anthropologists resist anything resembling
environmental determinism. "It's a very delicate issue," says Ulf Büntgen,
also at the Swiss Federal Institute of Technology, whose work suggests the
decline of the Western Roman Empire was linked to a period of highly
variable weather<http://www.newscientist.com/article/dn19968-fall-of-roman-empire-linked-to-wild-shifts-in-climate.html>.
"The field is evolving really slowly, because people are afraid to make
bold statements."

Yet this resistance is not really warranted, deMenocal says. No one today
is claiming that climate determines people's characters, only that it sets
limits on what is feasible. 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. "Climate isn't a determinant, but it is an
important factor," says Drake, who is at the University of New Mexico in
Albuquerque. "It enables or disables."

Some view even this notion as too simplistic. Karl
Butzer<http://www.utexas.edu/cola/depts/geography/faculty/butzerkw>of
the University of Texas at Austin, who has studied
the collapse of civilisations <http://dx.doi.org/10.1073/pnas.1114845109>,
thinks the role of climate has been exaggerated. It is the way societies
handle crises that decides their fate, he says. "Things break through
institutional failure." When it comes to the Akkadians, for instance,
Butzer says not all records support the idea of a megadrought.

In the case of the Maya, though, the evidence is strong. Earlier this year,
Eelco Rohling of the University of Southampton, UK, used lake sediments and
isotope ratios in stalactites to work out how rainfall had changed. He
concluded that annual rainfall fell 40 per cent over the prolonged dry
period, drying up open water sources (*Science*, vol 335, p
956<http://dx.doi.org/10.1126/science.1216629>).
This would have seriously affected the Maya, he says, because the water
table lay far underground and was effectively out of reach.

So after a century of plentiful rain, the Maya were suddenly confronted
with a century of low rainfall. It is not clear how they could have avoided
famine and population decline in these circumstances. Even today, our
ability to defy hostile climes is limited. Saudi Arabia managed to become
self-sufficient in wheat by tapping water reservoirs deep beneath its
deserts and subsidising farmers, but is now discouraging farming to
preserve what is left of the water. In dry regions where plenty of water is
available for irrigation, the build-up of salts in the soil is a serious
problem, just as it was for some ancient civilisations. And if modern
farmers are still at the mercy of the climate despite all our knowledge and
technology, what chance did ancient farmers have?
Greek Dark Ages

While many archaeologists remain unconvinced, the list of possible examples
continues to grow. The Mycenaeans are the latest addition. The reason for
their downfall has been the subject of much debate, with one of the most
popular explanations being a series of invasions and attacks by the
mysterious "Sea Peoples". In 2010, though, a study of river
deposits<http://dx.doi.org/10.1016/j.yqres.2010.07.010>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 analysed several climate records and concluded that there was a
cooling of the Mediterranean at this time, reducing evaporation and
rainfall<http://dx.doi.org/10.1016/j.jas.2012.01.029>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 civilisations unable to cope with the changing
climate? Or were the invading Sea Peoples the real problem? The story could
be complex: civilisations 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 civilisation could have had
knock-on effects on its trading partners.

Climate change on an even greater scale might be behind another striking
coincidence. Around 900, as the Mayan civilisation was declining in South
America, 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.

Since the two civilisations were not trading partners, there was clearly no
knock-on effect. 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<http://www.newscientist.com/article/dn10884-collapse-of-civilisations-linked-to-monsoon-changes.html>was
to blame, causing civilisations to fall apart on either side of the
Pacific (*Nature*, vol 445, p 74 <http://dx.doi.org/10.1038/nature05431>).

Critics, however, point to examples of climate change that did not lead to
collapse. "There was a documented drought and even famines during the
period of the Aztec Empire," says archaeologist Gary Feinman of the Field
Museum in Chicago. "These episodes caused hardships and possibly even
famines, but no overall collapse."

Realising that case studies of collapses were not enough to settle the
debate, in 2005 David Zhang of Hong Kong University began to look for
larger patterns. He began with the history of the Chinese dynasties. 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<http://dx.doi.org/10.1007/BF02897517>
).
The Thirty Years war

Zhang gradually built up a more detailed picture showing that harvests fell
when the climate was cold <http://dx.doi.org/10.1007/s10584-005-9024-z>, as
did population levels, while wars were more
common<http://www.newscientist.com/article/mg19526135.500-climate-change-linked-to-a-millennium-of-war-in-china.html>.
Of 15 bouts of warfare he studied, 12 took place in cooler
times<http://dx.doi.org/10.1007/s10745-007-9115-8>.
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 <http://dx.doi.org/10.1073/pnas.0703073104>.

These studies suggest that the effects of climate on societies can be
profound. The problem is proving it. So what if wars and collapses often
coincide with shifts in the climate? It doesn't prove one caused the other.
"That's always been the beef," says deMenocal. "It's a completely valid
point."

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<http://www.newscientist.com/article/dn11645-climate-myths-we-are-simply-recovering-from-the-little-ice-age.html>.
The Thirty Years war was fought then, and many other wars. Zhang analysed
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.

He then did a statistical analysis known as a Granger causality test, which
showed that the proposed causes consistently occurred before the proposed
effects, and that each cause was followed by the same effect. The Granger
test isn't conclusive proof of causality, but short of rerunning history
under different climes, it is about the best evidence there can be
(*Proceedings
of the National Academy of Sciences*, vol 108, p
17296<http://dx.doi.org/10.1073/pnas.1104268108>
).

The paper hasn't bowled over the critics. Butzer, for instance, claims it
is based on unreliable demographic data. Yet others are impressed. "It's a
really remarkable study," deMenocal says. "It does seem like they did their
homework." He adds that such a detailed breakdown is only possible for
recent history, because older civilisations left fewer records.

So while further studies should reveal much more about how the climate
changed in the past, the debate about how great an effect these changes had
on societies is going to rumble on for many more decades. Let's assume,
though, that changing climates did play a major role. What does that mean
for us?

On the face of it, things don't look so bad. It was often cooling that hurt
past civilisations. What's more, studies of the past century have found little
or no link between conflict and climate
change<http://www.newscientist.com/article/mg20727774.200-civil-war-in-africa-has-no-link-to-climate-change.html>.
"Industrialised societies have been more robust against changing climatic
conditions," says Jürgen Scheffran of the University of Hamburg, who
studies the effects of climate change.

On the other hand, we are triggering the most extreme change for millions
of years, and what seems to matter is food production rather than
temperature. Production is expected to increase at first as as the planet
warms but then begin to decline as warming exceeds 3
°C<http://www.newscientist.com/article/dn11655-climate-myths-higher-co2-levels-will-boost-plant-growth-and-food-production.html>.
This point may not be that far away - it is possible that global average
temperature will rise by 4 °C as early as
2060<http://www.newscientist.com/article/dn17864-no-rainforest-no-monsoon-get-ready-for-a-warmer-world.html>[image:
Movie Camera]. We've already seen regional food production hit by extreme
heatwaves like the one in Russia in 2010. Such extreme heat was not
expected until much later this
century<http://www.newscientist.com/article/mg21528721.800-how-global-warming-is-driving-our-weather-wild.html>
.

And our society's interconnectedness is not always a strength. It can
transmit shocks - the 2010 heatwave sent food prices soaring worldwide, and
the drought in the US this year is having a similar effect. The growing
complexity of modern society may make us more vulnerable to collapse rather
than less<http://www.newscientist.com/article/mg19826501.500-why-the-demise-of-civilisation-may-be-inevitable.html>
.

We do have one enormous advantage, though - unlike the Mycenaeans and the
Mayas, we know what's coming. We can prepare for what is to come and also
slow the rate of change if we act soon. So far, though, we are doing
neither.
 The Khmer

The Khmer empire, centred 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<http://www.newscientist.com/article/dn12474-vast-ancient-settlement-found-at-angkor-wat.html>,
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<http://dx.doi.org/10.1073/pnas.0910827107>.
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 <http://dx.doi.org/10.1016/j.quascirev.2010.10.005>. This damage
would have made it even harder to manage the water supply, at a time when
it was already limited and unpredictable.
 The Moche

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.

Some of the people were giants for their time, reaching 180 centimetres,
and may have had a ceremonial role as "kneeling
warriors"<http://www.newscientist.com/article/mg18424771.700-the-curious-bones-of-perus-gentle-giants.html>who
were ultimately sacrificed to the gods. 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<http://www.newscientist.com/article/mg12517063.100-science-el-nino-events-devastated-two-ancient-civilisations.html>.
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.

*Michael Marshall is an environment reporter for New Scientist*


-- 
Art Deco (Wayne A. Fox)
art.deco.studios at gmail.com
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