Droughts - Extreme Events: A Physical Reconstruction and Risk Assessment - page 33

may have caused the Indian monsoon to fail or occur with reduced intensity
at these times resulting in higher salinity levels at the mouths of rivers in
this region. Such a conclusion is not inconsistent with results from a range of
other studies that also suggest the Gleissberg solar cycle may have influenced
climate on Earth over the past few hundred years. Examples of this cycle have
been postulated to occur in Chinese rainfall records (Hameed et al ., 1983;Currie
and Fairbridge, 1985), the level of Nile River floods in Africa (Fairbridge, 1984),
temperature variations (Agee, 1980)andatmospheric

14 Cproduction (Stuiver,

1980;Stuiverand Quay, 1980).

Diatoms

Like foraminifera, diatoms can be used to infer past drought episodes.
Diatoms are small silicic plants that live in both marine and freshwater
environments and have species specific environmental ranges and tolerances.
Gaiser et al .(1998)developed a model of diatom response to hydroperiods in
small lakes along the US Atlantic coastal plain. Hydroperiods are defined as
ameasure of pond permanence and different diatoms will survive in a lake or
pond depending upon the water depth and chemistry. Gaiser et al .(1998)showed
that modern assemblages of diatoms accurately reflect the hydroperiod of lakes.
Their model has not yet been applied to prehistoric systems but it promises to
yield data that will accurately assess past drought episodes from fossil diatom
assemblages.
Verschuren et al .(2000)reconstructed the drought and rainfall history of
tropical east Africa over the past 1100 years based on lake-level and salin-
ity fluctuations of Lake Naivasha, Kenya using sediment stratigraphy and the
species compositions of fossil diatom and midge assemblages. They showed
significantly drier climate than today during the 'Medieval Warm Period'
( ∼ AD 1000--1270) and a relatively wet climate during the 'Little Ice Age'
( ∼ AD 1270--1850), which was interrupted by three prolonged dry episodes. Fig-
ure 2.6 shows three distinct drought episodes in the early AD 1300s, between
approximately AD 1450 and 1550 and around AD 1700. Interestingly, each of
these drought periods occurs immediately after episodes of sunspot minima.
Prior to AD 1200, drought appears to have been consistently more severe, which
like the episodes of drought over the following 500 years would have placed
substantial limitations on the prosperity of east African people.
Laird et al .(1996, 1998)alsoused diatoms to reconstruct the long-term hydro-
logical history of Moon Lake, North Dakota, USA. They found a remarkable sub-
decadal record of lake level variations that clearly shows a dramatic change in
drought regime after AD 1200. Figure 2.7 shows that prior to this time droughts