Aquatic Ecosystem Variability and Climate Change – A Palaeoecological Perspective - Climate Change Impacts on Freshwater Ecosystems - page 23

Greenland ice core (Mayewski et al . 1997), and the record of ice-rafted debris
(IRD) from marine cores in the North Atlantic Ocean (Bond et al . 2001). As
these fluctuations were also in phase with the atmospheric residual 14 C variation
(Stuiver et al . 1998), a proxy for solar variability, he concluded that lake levels
in this part of Europe were responding to changes in effective moisture driven
by solar activity. He argued that the variability was of sufficiently high magnitude
to affect not just lake levels but also the livelihood of people, whereby periods
of relatively low solar irradiance might cause an increase in annual precipitation
and a shortening of the growing season (Magny 2004). An association between
solar activity and precipitation has also been proposed by Beer and van Geel
(2008), especially with respect to an abrupt event that occurred around 850 BC
that is recorded in natural archives in many regions of the world (van Geel &
Berglund 2000). They proposed that a shift to a cooler and wetter climate at
this time caused flooding, land abandonment and human migration in the
Netherlands but, at the same time, caused population expansion in south-central
Siberia where nomadic Scythians benefited from the conversion of hostile semi-
desert regions to more productive steppic grasslands as a result of more humid
conditions. Whilst these authors make little specific reference to changes in
freshwater streams and lakes associated with this event, it can be assumed that
aquatic plant and animal communities, especially those in littoral habitats,
would have been significantly affected.
In low latitudes, centennial- to millennial-scale changes in lake level over the
Holocene have been much more dramatic than in mid and high latitudes.
Superimposed on the multi-millennial orbitally forced changes described above,
relatively short-lived but extreme centennial excursions have occurred repeatedly
in the Holocene in low latitudes, with palaeolimnological evidence indicating
lake levels having risen and fallen sharply by tens of metres at least twice in the
early-(8400-8000BP) and mid-Holocene (4200-4000BP) (Street-Perrott &
Perrott 1991; Gasse 2000). These changes were probably related to significant
but, as yet, largely unexplained weakening in the monsoon system. The impact of
these climatic fluctuations was profound. The drought of 4200-4000BP, for
example, corresponded to the collapse of Old World societies from the Old
Kingdom in Egypt (deMenocal 2001), the Akkadian Empire in Mesopotamia
(Weiss 2000), the Indus Valley Civilization in India (Staubwasser et al . 2003) and
the Neolithic cultures in the Central Plain of China (Wu & Liu 2004).
In Africa, the mid-Holocene multi-millennial aridification described above
may also have been punctuated by significant centennial-scale variability. Although
the change in insolation was gradual, the climate response may have varied
strongly both temporally and regionally due to climate instability. Modelling
experiments demonstrate that, during this period, relatively small random
fluctuations in rainfall could trigger switches between different equilibrium states
in the climate system (Claussen 2008). Proxy evidence for this is provided by
Street-Perrott et al . (2000), who used lithological and palynological data from
lake sediment records in the Manga Grasslands of the Sahel to show a stepwise
process towards arid conditions, and by Jung et al . (2004), who used marine
sediment records from the Arabian Sea to argue for a similar stepwise transition
from humid to dry conditions.