Geoscience Reference
In-Depth Information
Moving north, the Anasazi, Ancient Pueblo People or Ancestral Puebloans were
an old native American culture, in the south west of what is today the USA, that
arose around 1200
and who are best known for their stone and adobe dwellings
built along cliff walls. The period
bc
700-1130 saw a rapid increase in population
due to consistent and regular rainfall patterns. Yet the cliff settlement was aban-
doned by
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1300. What is known is that after approximately 1150 North America
experienced climatic change in the form of a 300-year drought, which also led to
the collapse of the Tiwanaku civilisation around Lake Titicaca. The contempor-
ary Mississippian culture also declined around this time. This was just one of the
'mega-droughts' the south-west USA has seen in the past 1000 years. A number
of these and their social impacts were outlined by Peter deMonocal from Columbia
University in a review paper in
Science
in 2001. In this he notes that that a com-
prehensive dendrochronological analysis (see section 2.1.1) of hundreds of tree-ring
chronologies from across the USA has established that there was a series of summer
droughts extending back to
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1200. Furthermore, this chronology points to there
being much more persistent droughts before the 1600s than subsequently. These so-
called mega-droughts were intense and persisted over many decades, recurring across
the USA south west roughly once or twice every 500 years. This dendrochronolo-
gical evidence has also been corroborated by lake sediment records. Peter deMonocal
further points out that palaeo-oceanographic data indicate that these events were
associated with changes in subpolar and subtropical surface as well as deep-ocean
circulation.
This picture became clearer in 2011 when the first dendrochronological record
going back over a thousand years was constructed. Through the deduced drought
timings this showed that the south-western USA drought of 1150 extended down to
Mexico (Stahle et al., 2011). (This dendrochronological record also shows a mega-
drought in
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897-922 and readers will note that this too is coincident with the drought
in northern South America that affected the Mayans in
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750-900: American mega-
droughts may have covered a wider area than previously thought.)
Turning to the eastern hemisphere, work by Pingzhong Zhang, Hai Cheng, R.
Lawrence Edwards and colleagues in 2008 used an isotope record from Wanxiang
Cave, China, to correlate the Asian monsoon history over the past 1810 years. The
summer monsoon correlates with solar variability, northern hemisphere and Chinese
temperature, alpine glacial retreat and Chinese cultural changes. The Asian monsoon
was generally strong during Europe's medieval warm period (MWP). Conversely, it
was weak during Europe's Little Ice Age, as well as during the final decades of the
Tang, Yuan and Ming Dynasties, all times that were characterised by popular unrest.
It was strong during the first several decades of the Northern Song Dynasty, a period
of increased rice cultivation and dramatic population increase.
This 1810-year timeline from China sees changes that chime with the timing of
(sometimes different) changes in climate represented in the central European dendro-
chronological data of Ulf Buntgen et al. (2011) and other proxy records. Exceptional
climate variability is reconstructed for circa
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250-550, and coincides with some
of the most severe challenges in Europe's political, social and economic history in
the early medieval period. Distinct drying in the 3rd century paralleled an aforemen-
tioned period of serious crisis in the Western Roman Empire marked by barbarian
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