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18
O for four ice cores across the Tibetan Plateau plotted from the
top to the bottom with decreasing elevations (a) Dasuopu, 7200 m asl [from Thompson et al., 2000]; (b) Guliya, 6200 m asl
[Thompson et al., 2003]; (c) Puruogangri, 6070 m asl [from Thompson et al., 2006b], reprinted from the Annals of
Glaciology with permission of the International Glaciological Society; and (d) Dunde, 5325 m asl [Thompson et al., 2003].
The isotopic enrichment with increasing elevation in the twentieth century is illustrated by the mean isotopic values for the
50 year period from 1937 to 1987.
Figure 12.
The 1000 year history of decadal averages of
δ
change, including insolation variability, volcanic eruptions,
and interactions and feedback between the oceans, atmo-
sphere, biosphere, and cryosphere currently are augmented
by the radiative gas-driven climate forcing caused by the
activities of almost 7 billion people. The increasing global
surface and lower to middle tropospheric temperatures and
the subsequent rapid disappearance of glaciers and ice caps
that have existed for hundreds to tens of thousands of years
certainly fits the description of abrupt climate change as
stated by the National Research Council. As was the case for
the prehistoric Peruvian cultures, many societies in develop-
ing countries are still dependent on agriculture for their
survival and would be (and some indeed have been) devas-
tated by sudden and prolonged decreases in precipitation.
Even highly developed, prosperous nations would be hard-
pressed to cope economically with an event like the 5.2 ka B.P.
cold reversal or the 4.0
-
4.5 ka B.P.
“
megadrought,
”
espe-
cially if the onset of such an event were as sudden and
extreme as the paleoclimate records indicate and concentrated
in the low latitudes where the vast majority of people live.
The drivers of these events, and in fact the causes of short-
term variability we experience today such as El Ni
ñ
o, are still
not fully understood.
The history of rise and fall of past highland and coastal
cultures in Peru before the rise of globalization would indi-
cate that population migration such as has occurred since at
least the 1940s from the highlands to the coastal plains is
inconsistent with the longer-term trends. This may be
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