Present climate and biological change - Climate Change: Biological and Human Aspects - page 209

2000). While detailed palaeoclimate records to test the climate-induced decline asser-
tion from Mesopotamia are rare, changes in regional aridity are preserved in adjacent
ocean basins. For example, in 2000 Holocene changes in regional aridity were determ-
ined using geochemical analyses of a marine sediment core from the Gulf of Oman.
(This is directly downwind of Mesopotamian dust-source areas and archaeological
sites.) These data suggest a very abrupt increase in dust, and hence Mesopotamian
aridity, around 4025 125 years ago that persisted for roughly 300 years. Further-

more, radiogenic (Nd and Sr) isotope analyses confirm that the observed increase in
mineral dust was derived from Mesopotamian source areas. This provides a direct
link between Mesopotamian drought and social collapse; hence drought was a key,
if not critical, factor contributing to the collapse of the Akkadian empire (Cullen
et al., 2000). The onset of the sudden drought in Mesopotamia around 4100 years
ago coincides with a widespread cooling in the North Atlantic. During this event,
sometimes called the Holocene Event 3, the Atlantic subpolar and subtropical surface
waters cooled by 1 ◦ Cto2 ◦ C.
Similarly, 4500-3500 years ago the Indus valley civilisation, and the principal
cities of Harappa and Mohrnjo-daro, ended due to drought. There is no evidence of
war but we do know that there was a major decline in rainfall across the Middle
East and northern India 3800-2500 years ago. The same drought may also have
affected civilisations around the eastern Mediterranean such as the Hittite empire,
the Mycenae and the Egyptians (whose longer-term fate may also have been climate
related, see below). I say 'may have' because their decline is sometimes attributed to
a raiding, aggressive, sea-going people.
Regarding climate's socio-economic importance to the success and failure of early
civilisations, it is interesting to note that the Roman Empire thrived over a period that
was coincident with a benign climate. Indeed it was a Roman, the writer Saserna,
who was one of the first to suggest that a biologically related human activity, viticu-
lture, was a proxy indicator of climate change. More recently dendrochronological
evidence (Buntgen et al., 2011) points to Central European summer precipitation
and temperature variability with wet and warm summers taking place during periods
of Roman and medieval prosperity. Conversely, increased climate variability from
approximately

±

250 to 600 coincided with the demise of the Western Roman
Empire and the subsequent European turmoil.
Meanwhile, in Western Europe the Argaric culture emerged in south-eastern Spain
around 4300 years ago. This civilisation, which inhabited small fortified towns,
was one of the first in Western Europe to adopt bronze working. But about 3600
years ago the culture died out. The area in which the Argaric people lived is one
of Europe's driest areas even today (indeed, especially today with recent warming)
and so the Argarics were living in a marginal or near-marginal environment. Pollen
analysis suggests that ecological change took place around that time. Part of this is
thought to be due to humans clearing forests, but there is evidence conditions were
becoming progressively arid from about 5500 years ago onwards as indicated by a
broad reduction in forest cover, the appearance of plants adapted to dry conditions
and a drop in lake levels. What it means is that the Argaric arose somewhere that
was already climatically stressed and this together with the pressure they themselves
exerted on the environment meant that their society was no longer sustainable.

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