Geoscientists are increasingly exploring an interesting trend: Climate change
has been affecting human society for thousands of years. At the American Geophysical
Union annual meeting in December, one archaeologist presented research that
suggests that climate change affected the way cultures developed and collapsed
in the cradle of civilization ancient Mesopotamia more than 8,000
Archaeologists have found evidence for a mass migration from the more temperate
northern Mesopotamia to the arid southern region around 6400 B.C. For the previous
1,000 years, people had been cultivating the arable land in northern Mesopotamia,
using natural rainwater to supply their crops. So archaeologists have long wondered
why the ancient people moved from an area where they could easily farm to begin
a much harder life in the south.
| The challenge to us
as paleoclimatologists is to develop much more detailed and well-dated records.
-Peter deMenocal, Columbia University
One reason could be climate, said Harvey Weiss, an archaeologist at Yale University,
at the meeting in December. The climate record in ancient Mesopotamia and around
the world shows an abrupt climate change event in 6400 B.C., about 8,200 radiocarbon
years before present. A period of immense cooling and drought persisted for
the next 200 to 300 years.
When the severe drought and cooling hit the region, there was no longer enough
rainwater to sustain the agriculture in the north, Weiss says. And irrigation
was not possible due to the topography, so these populations were left with
two subsistence alternatives: pastoral nomadism or migration.
Archaeologists first start seeing evidence of settlements in southern Mesopotamia
shortly after 6400 B.C. In the south, an area too arid to have sustained rain-fed
agriculture, irrigation from the Tigris and Euphrates rivers would have been
possible where the rivers flow at plain level, Weiss says. Irrigation farming
took three to four times the labor effort of rain-fed farming, but irrigation
agriculture would have made surplus production easier because the yield was
double that of rain-fed agriculture. Surplus production meant that people could
begin specializing in full-time crafts rather than relying exclusively on farming,
Weiss says, thus giving rise to the first class-based society and the first
"It's perhaps too extreme to say that climate change caused all of the
advanced society collapses," says Peter deMenocal, a paleoceanographer
at Columbia University's Lamont-Doherty Earth Observatory. "But it's also
too extreme to say that climate change has had no effect. The challenge to us
as paleoclimatologists is to develop much more detailed and well-dated records,"
The most fundamental question in Mesopotamian archaeology, Weiss concludes,
"is, 'why is there a Mesopotamian archaeology?'" Having already tied
the Early Bronze Age collapses from the Aegean to the Indus to the abrupt climate
change event 4,200 years before present, Weiss believes he can now tie the changes
of lifestyle and migration that were essential for early class formation and
urban life in Mesopotamia to an abrupt, multi-century shift toward drier conditions
which occurred near 8,200 years before present.
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