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Author: Karen Wright

Discover

Issue: March, 1998

Some 4,000 years ago, a number of mighty Bronze Age cultures crumbled.
Were they done in by political strife and societal unrest? Or by a
change in the climate?

Mesopotamia: cradle of civilization, the fertile breadbasket of western
Asia, a little slice of paradise between the Tigris and Euphrates
rivers. Today the swath of land north of the Persian Gulf is still prime
real estate. But several millennia ago Mesopotamia was absolutely The
Place to Be. There the visionary king Hammurabi ruled, and Babylon's
hanging gardens hung. There the written word, metalworking, and
bureaucracy were born. From the stately, rational organization of
Mesopotamia's urban centers, humanity began its inexorable march toward
strip malls and shrink-wrap and video poker bars and standing in line at
the DMV. What's more, the emergence of the city-state meant that we no
longer had to bow to the whims of nature. We rose above our abject
dependence on weather, tide, and filth; we were safe in the arms of
empire. Isn't that what being civilized is all about?

Not if you ask Harvey Weiss. Weiss, professor of Near Eastern archeology
at Yale, has challenged one of the cherished notions of his profession:
that early civilizations--with their monuments and their grain reserves,
their texts and their taxes--were somehow immune to natural disaster. He
says he's found evidence of such disaster on a scale so grand it spelled
calamity for half a dozen Bronze Age cultures from the Mediterranean to
the Indus Valley--including the vaunted vale of Mesopotamia. Historians
have long favored political and social explanations for these collapses:
disruptions in trade routes, incompetent administrators, barbarian
invasions. "Pre-historic societies, simple agriculturists--they can be
blown out by natural forces," says Weiss. But the early civilizations of
the Old World? "It's not supposed to happen."

Yet happen it did, says Weiss, and unlike his predecessors, he's got
some data to back him up. The evidence comes from a merger of his own
archeological expertise with the field of paleoclimatology, the study of
climates past. His first case study concerns a series of events that
occurred more than 4,000 years ago in a region of northern Mesopotamia
called the Habur Plains. There, in the northeast corner of what is
present-day Syria, a network of urban centers arose in the middle of the
third millennium B.C. Sustained by highly productive organized
agriculture, the cities thrived. Then, around 2200 B.C., the region's
new urbanites abruptly left their homes and fled south, abandoning the
cities for centuries to come.

Weiss believes that the inhabitants fled an onslaught of wind and dust
kicked up by a drought that lasted 300 years. He also believes the
drought crippled the empire downriver, which had come to count on the
agricultural proceeds of the northern plains. Moreover, he contends, the
long dry spell wasn't just a local event; it was caused by a rapid,
region-wide climate change whose effects were felt by budding
civilizations as far west as the Aegean Sea and the Nile and as far east
as the Indus Valley. While the Mesopotamians were struggling with their
own drought-induced problems, he points out, neighboring societies were
collapsing as well: the Old Kingdom in Egypt, early Bronze Age cities in
Palestine, and the early Minoan civilization of Crete. And in the Indus
Valley, refugees fleeing drought may have overwhelmed the cities of
Mohenjo-Daro and Harappa. The troubles of half a dozen Bronze Age
societies, says Weiss, can be blamed on a single event--and a natural
disaster at that.

Weiss first presented this scenario in 1993, when soil analyses showed
that a period of severe dust storms accompanied the mysterious Habur
hiatus. "I was thinking you can't have a microregion drought," he
recalls, "because that isn't how climate works. It's got to be much
bigger. And I said, 'Wait a minute, didn't I read about this in graduate
school? Weren't there those who, 30 years ago, had said that drought
conditions were probably the agency that accounts for all these
collapses that happened in contiguous regions?"' says Weiss. "Back in
the late sixties we had read this stuff and laughed our heads off about
it."

In 1966, British archeologist James Mellaart had indeed blamed drought
for the downfalls of a whole spectrum of third-millennium civilizations,
from the early Bronze Age communities in Palestine to the pyramid
builders of Egypt's Old Kingdom. But when Mellaart first put forth this
idea, he didn't have much in the way of data to back him up. Weiss,
however, can point to new paleoclimate studies for his proof. These
studies suggest that an abrupt, widespread change in the climate of
western Asia did in fact occur at 2200 B.C. Samples of old ocean
sediments from the Gulf of Oman, for example, show signs of extreme
drought just when Weiss's alleged exodus took place. A new model of
air-mass movement explains how subtle shifts in atmospheric circulation
could have scorched Mesopotamia as well as points east, west, and south.
And recent analyses of ice cores from Greenland--which offer the most
detailed record of global climate change--reveal unusual climatic
conditions at 2200 B.C. that could well have brought drought to the
region in question.

"I've got some figures I can show you. Figures always help," says
paleoclirnatologist Peter deMenocal, swiveling his chair from reporter
to computer in his office at Columbia University's Lamont-Doherty Earth
Observatory, just north of New York City. On the monitor, deMenocal
pulls up a graph derived from the research project known as GISP2 (for
Greenland Ice Sheet Project 2). GISP2 scientists, he explains, use
chemical signals in ice cores to reconstruct past climates. There are
two kinds of naturally occuring oxygen atoms, heavy and light, and they
accumulate in ice sheets in predictable ratios that vary with prevailing
temperatures. In a cool climate, for example, heavy oxygen isotopes are
less easily evaporated out of the ocean and transported as snow or rain
to northern landmasses like Greenland. In a warm climate, however, more
heavy oxygen isotopes will be evaporated, and more deposited in the
Greenland ice sheets.

By tracking oxygen-isotope ratios within the ice cores, the GISP2 graph
reflects temperatures over Greenland for the past 15,000 years. Near the
bottom of the graph, a black line squiggle wildly until 11,700 years
ago, when the last ice age ended and the current warm era, the Holocene,
began. The line then climbs steadily for a few thousand years, wavering
only modestly, until 7,000 years before the present. From then until
now, global temperatures appear relatively stable--"then until now"
comprising, of course, the entire span of human civilization.

"The archeological community--and actually segments of the paleoclimate
community--have viewed the Holocene as being climatically stable," says
deMenocal. "And so they imagine that the whole drama of civilization's
emergence took place on a level playing field in terms of the environment."

Until he met Harvey. Weiss, deMenocal wasn't much interested in studying
the Holocene; like most of his peers, he was more drawn to the dynamic
climate fluctuations that preceded it. In fact, the Holocene had
something of a bad rep among climatologists. "It was thought of as kind
of a boring time to study," says deMenocal. "Like, why would you
possibly want to? All the action is happening 20,000 years earlier."

Then a few years ago he read an account of Weiss's drought theory and
had an epiphany of sorts. It occurred to him that even the smallest
variations in climate could be interesting if they had influenced the
course of history. What if something was going on in the Holocene after
all? He looked up the 1993 paper in which Weiss had laid out the
evidence for the Habur hiatus and reported the results of the soil
analysis.

"I was pretty skeptical," says deMenocal. "I mean, what would you expect
if everyone left a town? It would get dusty. Especially in the world's
dustiest place. Big surprise."

Weiss, meanwhile, was getting a similar response from many of his peers.
But when he and deMenocal met at a conference in 1994, they hit it off
right away--largely because Weiss, too, was dismayed at the paucity of
his own evidence. "Peter was immediately sensitive to my moaning about
how we needed additional data, different kinds of data," says Weiss.
"And he immediately understood where such data could be obtained."

DeMenocal told Weiss that if a large-scale drought had in fact occurred,
it would have left a mark in the sediments of nearby ocean floors--the
floor of the Gulf of Oman, for example. Lying approximately 700 miles
southeast of ancient Mesopotamia, the gulf would have caught any
windblown dust that swept down from the Tigris and Euphrates valleys.
(The Persian Gulf is closer, but because it's so shallow, its sediments
get churned up, thereby confusing their chronology.) And deMenocal just
happened to know some German scientists who had a sediment core from the
Gulf of Oman.

Analysis of the gulf core is ongoing, but deMenocal has already
extracted enough information to confirm Weiss's suspicions. To track dry
spells in the sediments, he and his colleague Heidi Cullen looked for
dolomite, a mineral found in the mountains of Iraq and Turkey and on the
Mesopotamian floodplains that could have been transported to the gulf
only by wind. Most of the Holocene section of the core consists of
calcium carbonate sediments typical of ocean bottoms.

"And then all of a sudden, at exactly 4,200 calendar years, there's this
big spike of dolomite," says deMenocal--a fivefold increase that slowly
decays over about three centuries. The chemistry of the dolomite dust
matches that of the dolomite in the Mesopotamian mountains and plains,
verifying the mineral's source. And not only did deMenocal and his
colleagues figure out what happened, they may have figured out how.
Studies by Gerard Bond at Lamont-Doherty have shown that the timing of
the drought coincided with a cooling period in the North Atlantic.
According to a survey by Cullen of current meteorological records, such
cooling would have dried out the Middle East and western Asia by
creating a pressure gradient that drew moisture to the north and away
from the Mediterranean.

"The whole disruption, collapse bit, well, I just have to take Harvey at
his word," says deMenocal. "What I tried to do is bring some good hard
climate data to the problem." Why hasn't anybody seen this signature of
calamity before? Simple, says deMenocal. "No one looked for it."

Weiss's first hints of climate-associated calamity came from a survey of
his principal excavation site, a buried city in northeastern Syria
called Tell Leilan. Tell Leilan (rhymes with "Ceylon") was one of three
major cities on the Habur Mains to be taken over by the Akkadian Empire
around 2300 B.C. The city covered more than 200 acres topped by a
haughty acropolis, and was sustained by a tightly regulated system of
rain-fed agriculture that was co-opted and intensified by the
imperialists from the south. Weiss had asked Marie-Agnes Courty of the
National Center for Scientific Research in France to examine. the
ancient soils of Tell Leilan to help him understand the agricultural
development of the region. She reported that a section dating from 2200
to 1900 B.C. showed evidence of severe drought, including an
eight-inch-thick layer of windblown sand and a marked absence of
earthworm tunnels.

In his own excavations of the same period, Weiss had already found
evidence of desertion: mud-brick walls that had fallen over day floors
and were covered with, essentially, 300 years' worth of compacted dust.
And once he made the drought connection at Tell Leilan, he began turning
up clues to the catastrophe everywhere he looked. In 1994, for example,
Gerry Lemcke, a researcher at the Swiss Technical University in Zurich,
presented new analyses of sediment cores taken from the bottom of Lake
Van in Turkey, which lies at the headwaters of the Tigris and the
Euphrates. The new results indicated that the volume of water in the
lake--which corresponds to the amount of rainfall throughout western
Asia--declined abruptly 4,200 years ago. At the same time, the amount of
windblown dust in the lake increased fivefold.

Weiss came to believe that the effects of the drought reached downriver
to the heart of Mesopotamia, causing the collapse of the Akkadian
Empire. The collapse itself is undisputed: written records describe how,
soon after it tad consolidated power, Akkad crumbled, giving way to the
Ur III dynasty in--when else?--2200 B.C. The cause of this collapse has
been the subject of considerable speculation. But Weiss's studies of
early civilizations have convinced him that their economies--complex and
progressive though they may have been--were still fundamentally
dependent on agricultural production. In fact, he notes, one hallmark of
any civilization is that it requires a life-support system of farming
communities toiling away in the fields and turning over the fruits of
their labor to a central authority. The drought on the Habur Plains
could have weakened the Akkadian Empire by drastically reducing
agricultural revenues from that region. People fleeing the drought moved
south, where irrigation-fed agriculture was still sustainable. For want
of a raindrop, the kingdom was lost.

"Well, believe it or not, all my colleagues had not figured that out,"
says Weiss. "They actually believed that somehow this empire was based
on bureaucracy, or holding on to trade routes, or getting access to
exotic mineral resources in Turkey." But the drought itself is
documented, Weiss says, in passages of cuneiform texts. Images from a
lengthy composition called the Curse of Akkad, for example, include
"large fields" that "produced no grain" and "heavy clouds" that "did not
rain." Scholars had decided that these expressions were mere metaphor.

And many still stand by their interpretations. "I don't agree with his
literal reading of the Mesopotamian texts, and I think he has
exaggerated the extent of abandonment in this time period," says Richard
Zettler, curator of the Near East section at the University of
Pennsylvania's Museum of Archaeology and Anthropology in Philadelphia.
Zettler doesn't question the evidence for drought, but he thinks Weiss
has overplayed its implications. Although Tell Leilan may well have been
deserted during the putative hiatus, for example, nearby cities on the
Habur Plains show signs of continuing occupation, he says. As for the
Curse and other Mesopotamian passages describing that period, says
Zetder, "there are a lot of questions on how to read these texts--how
much of it is just literary license, whatever. Even if there is a core
of historical truth, it's hard to determine what the core of truth is."

Instead of backing down in the face of such commentary, Weiss has
continued to document his thesis. Echoing Mellaart, he points out that
2200 B.C. saw the nearly simultaneous collapses of half a dozen other
city-based civilizations--in Egypt, in Palestine, on Crete and the Greek
mainland, and in the Indus Valley. The collapses were caused by the same
drought, says Weiss, for the same reasons. But because historians and
archeologists look for internal rather than external forces to explain
civilizations in crisis, they don't communicate among themselves, he
says, and many aren't even aware of what's going on next door, as it were.

"Very few people understand that there was a synchronous collapse and
probably drought conditions in both Egypt and Mesopotamia," let alone
the rest of the Old World, says Weiss.

It didn't help Weiss's extravagant claims for third-millennium cataclysm
that his alleged drought didn't appear in the GISP2 oxygen-isotope
record. The graph in deMenocal's office, for example, has no spikes or
dips or swerves at 2200 B.C. just a nice flat plateau. That graph was
drawn from an interpretation of the ice-core data. But according to Paul
Mayewski of the University of New Hampshire in Durham, who is chief
scientist of GISP2, there are plenty of reasons a drought in western
Asia might not make it into the oxygenisotope record in the Greenland
glacier. Greenland might be too far away to "feel" the regional event,
or the drought may have left a different kind of chemical signature.
Only a climatologist like Mayewski could explain these reasons, however.
And no one asked him to.

"As a consequence, a lot of people called Harvey Weiss and said, `Well,
the GISP2 record is the most highly resolved record of Holocene climate
in the world. And if it's not in there, you're wrong, Harvey,"' says
Mayewski. "I didn't realize that poor Harvey was being abused for not
existing in our record."

Fortunately Mayewski, like deMenocal, is a curious sort with interests a
bit broader than his own specialty. When he happened upon Weiss's 1993
paper, he'd already lent a hand on a few archeological projects,
including one on the disappearance of Norse colonies from Greenland in
the mid-1300s. But he figured other scientists had already looked for
the Mesopotamian drought in the climate record. When he finally met
Weiss in 1996, he learned otherwise. Mayewski began reanalyzing his core
data with Weiss's theory in mind, and he uncovered a whole new Holocene.

"We can definitely show from our records that the 2200 B.C. event is
unique," says Mayewski. "And what's much more exciting than that, we can
show that most of the major turning points in civilization in western
Asia also correlate with what we would say would be dry events. We think
that we have found a proxy for aridity in western Asia."

Earlier interpretations of the GISP2 data had measured a variety of ions
in ice cores that would reveal general information about climate
variability. To look for the 2200 B.C. drought in particular, Mayewski
used tests based on 2.5-year intervals in the climate record instead of
50- to 100-year intervals. He also collected a broader set of data that
allowed him to reconstruct specific patterns of atmospheric
circulation--not only over land but over land and oceans. When Mayewski
focused on the movement of air masses over oceans, he found that air
transport from south to north in the Atlantic--so-called meridional
circulation--hit a significant winter low some 4,200 years ago. Mayewski
and deMenocal are studying how this event relates to drought in western
Asia.

"But it seems on the basis of the paleodimatic data that there is no
doubt about the event at 2200 B.C. says Weiss. "What the qualities of
this event were, and what the magnitude of this event was, that is the
current research frontier now."

Trouble is, even though the drought may seem like a sure thing, its
effects on Mesopotamia are still unproved, as Zettler points out. They
will remain controversial, Weiss admits, until archeologists better
understand the contributions of politics, agriculture, and climate in
the formation of ancient societies. That mission grows more urgent as
more archeologists seem ready to grapple with models of "climatic
determinism." In the past few years, drought and flooding have been
cited in the demise of several New World civilizations, including the
Maya of Central America, the Anasazi of the American Southwest, and the
Moche and Tiwanaku of Peru and Bolivia.

"Until climatic conditions are quantified, it's going to be very
difficult to understand what the effects of climate
changes--particularly controversial, abrupt ones--were upon these
societies," says Weiss. The precise constellation of forces that led to
the collapse of Bronze Age cultures around 2200 B.C. will probably be
debated for a very long time. But paleoclimatology has assured Mother
Nature a place in that constellation. And the notion that civilizations
are immune to natural disaster may soon be ancient history.

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