Social Collapse in History [SCIENCE mag]

                Science --  Weiss and Bradley 291 (5504): 609
        

ARCHAEOLOGY:  What Drives Societal Collapse?

Harvey Weiss and Raymond S. Bradley*



The archaeological and historical record is replete with evidence
for prehistoric, ancient, and premodern societal collapse. These
collapses occurred quite suddenly and frequently involved
regional abandonment, replacement of one subsistence base by
another (such as agriculture by pastoralism), or conversion to a
lower energy sociopolitical organization (such as local state
from interregional empire). Each of these collapse episodes has
been discussed intensively within the archaeological community,
commonly leading to the conclusion that combinations of social,
political, and economic factors were their root causes.

That perspective is now changing with the accumulation of
high-resolution paleoclimatic data that provide an independent
measure of the timing, amplitude, and duration of past climate
events. These climatic events were abrupt, involved new
conditions that were unfamiliar to the inhabitants of the time,
and persisted for decades to centuries. They were therefore
highly disruptive, leading to societal collapse--an adaptive
response to otherwise insurmountable stresses (1).

In the Old World, the earliest well-documented example of
societal collapse is that of the hunting and gathering Natufian
communities in southwest Asia. About 12,000 years ago, the
Natufians abandoned seasonally nomadic hunting and gathering
activities that required relatively low inputs of labor to
sustain low population densities and replaced these with new
labor-intensive subsistence strategies of plant cultivation and
animal husbandry. The consequences of this agricultural
revolution, which was key to the emergence of civilization,
included orders of magnitude increases in population growth and
full-time craft specialization and class formation, each the
result of the ability to generate and deploy agricultural
surpluses.

What made the Natufians change their lifestyle so drastically?
Thanks to better dating control and improved paleoclimatic
interpretations, it is now clear that this transition coincided
with the Younger Dryas climate episode about 12,900 to 11,600
years ago. Following the end of the last glacial period, when
southwest Asia was dominated by arid steppe vegetation, a shift
to increased seasonality (warm, wet winters and hot, dry summers)
led to the development of an open oak-terebinth parkland of woods
and wild cereals across the interior Levant and northern
Mesopotamia. This was the environment exploited initially by the
hunting and gathering Natufian communities. When cooler and drier
conditions abruptly returned during the Younger Dryas, the
harvests of wild resources dwindled, and foraging for these
resources could not sustain Natufian subsistence. They were
forced to transfer settlement and wild cereals to adjacent new
locales where intentional cultivation was possible (2).

The population and socioeconomic complexity of these early
agricultural settlements increased until about 6400 B.C., when a
second postglacial climatic shock altered their developmental
trajectory. Paleoclimatic evidence documents abrupt climatic
change at this time (3), the last major climatic event related to
the melting continental ice sheets that flooded the North
Atlantic (4). In the Middle East, a ~200-year drought forced the
abandonment of agricultural settlements in the Levant and
northern Mesopotamia (5, 6). The subsequent return to moister
conditions in Mesopotamia promoted settlement of the
Tigris-Euphrates alluvial plain and delta, where breachable river
levees and seasonal basins may have encouraged early southern
Mesopotamian irrigation agriculture (7).

By 3500 B.C., urban Late Uruk society flourished in southern
Mesopotamia, sustained by a system of high-yield cereal
irrigation agriculture with efficient canal transport. Late Uruk
"colony" settlements were founded across the dry-farming portions
of the Near East (8). But these colonies and the expansion of
Late Uruk society collapsed suddenly at about 3200-3000 B.C.
Archaeologists have puzzled over this collapse for the past 30
years. Now there are hints in the paleoclimatic record that it
may also be related to a short (less than 200 year) but severe
drought (9-11).

Following the return to wetter conditions, politically
centralized and class-based urban societies emerged and expanded
across the riverine and dry-farming landscapes of the
Mediterranean, Egypt, and West Asia. The Akkadian empire of
Mesopotamia, the pyramid-constructing Old Kingdom civilization of
Egypt, the Harappan 3B civilization of the Indus valley, and the
Early Bronze III civilizations of Palestine, Greece, and Crete
all reached their economic peak at about 2300 B.C. This period
was abruptly terminated before 2200 B.C. by catastrophic drought
and cooling that generated regional abandonment, collapse, and
habitat-tracking. Paleoclimatic data from numerous sites document
changes in the Mediterranean westerlies and monsoon rainfall
during this event (see the figure), with precipitation reductions
of up to 30% that diminished agricultural production from the
Aegean to the Indus (9-11).


[Figure Here]
Climatic effects. High-resolution lake, marine, and speleothem
cores and tephrochronostratigraphy document abrupt aridification
and linkage with Akkadian empire collapse at Tell Leilan, Syria
(9-11).



These examples from the Old World illustrate that prehistoric and
early historic societies--from villages to states or
empires--were highly vulnerable to climatic disturbances. Many
lines of evidence now point to climate forcing as the primary
agent in repeated social collapse.

High-resolution archaeological records from the New World also
point to abrupt climatic change as the proximal cause of repeated
social collapse. In northern coastal Peru, the Moche civilization
suffered a ~30-year drought in the late 6th century A.D.,
accompanied by severe flooding. The capital city was destroyed,
fields and irrigation systems were swept away, and widespread
famines ensued. The capital city was subsequently moved
northward, and new adaptive agricultural and architectural
technologies were implemented (12). Four hundred years later, the
agricultural base of the Tiwanaku civilization of the central
Andes collapsed as a result of a prolonged drought documented in
ice and in lake sediment cores (13). In Mesoamerica, lake
sediment cores show that the Classic Maya collapse of the 9th
century A.D. coincided with the most severe and prolonged drought
of that millennium (14). In North America, Anasazi agriculture
could not sustain three decades of exceptional drought and
reduced temperatures in the 13th century A.D., resulting in
forced regional abandonment (15).

Climate during the past 11,000 years was long believed to have
been uneventful, but paleoclimatic records increasingly
demonstrate climatic instability. Multidecadal- to
multicentury-length droughts started abruptly, were unprecedented
in the experience of the existing societies, and were highly
disruptive to their agricultural foundations because social and
technological innovations were not available to counter the
rapidity, amplitude, and duration of changing climatic
conditions.

These past climatic changes were unrelated to human activities.
In contrast, future climatic change will involve both natural and
anthropogenic forces and will be increasingly dominated by the
latter; current estimates show that we can expect them to be
large and rapid (16). Global temperature will rise and
atmospheric circulation will change, leading to a redistribution
of rainfall that is difficult to predict. It is likely, however,
that the rainfall patterns that societies have come to expect
will change, and the magnitude of expected temperature changes
(17) gives a sense of the prospective disruption. These changes
will affect a world population expected to increase from about 6
billion people today to about 9 to 10 billion by 2050. In spite
of technological changes, most of the world's people will
continue to be subsistence or small-scale market
agriculturalists, who are similarly vulnerable to climatic
fluctuations as the late prehistoric/early historic societies.
Furthermore, in an increasingly crowded world, habitat-tracking
as an adaptive response will not be an option.

We do, however, have distinct advantages over societies in the
past because we can anticipate the future. Although far from
perfect and perhaps subject to unexpected nonlinearities, general
circulation models provide a road map for how the climate system
is likely to evolve in the future. We also know where population
growth will be greatest. We must use this information to design
strategies that minimize the impact of climate change on
societies that are at greatest risk. This will require
substantial international cooperation, without which the 21st
century will likely witness unprecedented social disruptions.


References and Notes



1.   H. Weiss, in Confronting Natural Disaster: Engaging the Past
to Understand the Future, G. Bawden and R. Reycraft, Eds. (Univ.
of New Mexico Press, Albuquerque, 2000), pp. 75-98.

2.   O. Bar-Yosef, Radiocarbon 42, 23 (2000).

3.   F. Gasse, Quat. Sci. Rev. 19, 189 (2000).

4.   This flooding may have altered thermohaline circulation
(THC), although there is as yet no direct paleochemical data
demonstrating a shutdown or reduction in THC at this time.

5.   A. N. Goring-Morris, A. Belfer-Cohen, Paléorient 23,
71 (1997).

6.   S. K. Kozlowski, The Eastern Wing of the Fertile Crescent
(BAR Intl. Series 760, Oxford, 1999) [publisher's information].

7.   R. M. Adams, Heartland of Cities (Univ. of Chicago Press,
Chicago, 1981).

8.   www.science.widener.edu/ssci/mesopotamia

9.   H. M. Cullen et al., Geology 28, 379 (2000) [GEOREF].

10.  M. Bar-Matthews et al., Earth and Planetary Science Letters
166, 85 (1999) [ADS].

11.  G. Lemcke, M. Sturm, in Third Millennium BC Climate Change
and Old World Collapse, H. N. Dalfes, G. Kukla, H. Weiss, Eds.
(Springer, NATO ASI 49, Berlin, 1997), pp. 653-678 [publisher's
information].

12.  I. Shimada et al., World Archaeol. 22, 247 (1991).

13.  A. Kolata et al., Antiquity 74, 424 (2000).

14.  M. Brenner et al., in Interhemispheric Climate Linkages, V.
Markgraf, Ed. (Academic Press, New York, 2001), pp. 87-103
[publisher's information].

15.  J. S. Dean et al., in Themes in Southwest Prehistory, G. J.
Gumerman, Ed. (Schl. Amer. Res. Press, Santa Fe, 1993), pp. 53-86
[publisher's information].

16.  www.grida.no/climate/ipcc/regional

17.  The leaked Summary for Policy Makers of the upcoming Third
Assessment Report by the IPCC gives estimates of 1.5 deg to 6.0
deg C.

* H.W.'s research was supported by the National Endowment for the
Humanities, NSF, Malcolm H. Wiener Foundation, Leon Levy, Raymond
Sackler, and Yale University, and R.S.B.'s research was supported
by the NSF and the U.S. Department of Energy. We thank H. F.
Diaz, M. K. Hughes, M. Moseley, and E. J. and D. S. Bradley for
comments.



H. Weiss is at the Departments of Anthropology and Near Eastern
Languages and Civilizations, Yale University, New Haven, CT
06520, USA. E-mail: harvey.weiss@yale.edu. R. S. Bradley is at
the Department of Geosciences, University of Massachusetts,
Amherst, MA 01003, USA. E-mail: rbradley@geo.umass.edu

     
     
     Volume 291,
     Number 5504,
     Issue of 26 Jan 2001,
     pp. 609-610.