Environment
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?
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.
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 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). 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:
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 civilisations (
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. 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
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.
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.
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.
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 of the University of Texas at Austin, who has
studied the collapse of civilisations,
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).
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
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 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 was to blame, causing civilisations to fall apart on either side of the Pacific (
Nature, vol 445, p 74).
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).
The Thirty Years war
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.
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).
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.
"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.
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.
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,
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.
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"
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.
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
. We've already seen regional food production hit by extreme heatwaves like the one in Russia in 2010. Such extreme heat 
