Extreme events over time - Extreme Events: A Physical Reconstruction and Risk Assessment - page 255

that floods in the Upper Mississippi valley were large between 600 and 300, 1000
and 750, 1800 and 1500, and 2500 and 2200 years BP. Either moderate or small
floods dominated the episodes between these periods. The key point about these
studies of long-term flooding history in the USA is that flood magnitudes and
frequencies have been highly variable over the past 7000 years. The causes behind
these variations in flood frequencies have been shifts in the climatic state where
large floods have tended to occur during warmer climatic intervals associated
with shifts in the position of the jet stream axis over the USA (Knox, 2003).
Of all the atmospherically-generated extreme events, long-term drought
records probably provide the best insight into the true behaviour of an extreme
event or hazard over time. Drought records generally have a higher resolution
than that able to be obtained for many other hazards or extreme events because
they can be recorded in annual sediment layers, tree rings and speleothem
layers. The higher resolution of drought records permits a more definitive test
of the hypothesis of randomness of a natural extreme event or hazard over time.
Indeed, the long-term drought records unquestionably show that these events
display variable frequency over time.
Using

13 Crecords from peat cellulose, Hong et al .(2001) showed that eight,
multi-decade to multi-century, drought periods occurred between 2200 BC and
AD 1200 in northeast China. After this time, the climate returned to wet and
cold conditions between AD 1200 and 1800, a period during which the frequency
or severity of droughts was much lower. Laird et al .(1996) used diatoms to recon-
struct a remarkable subdecadal record of lake level variations in the USA that
clearly depicts a dramatic change in drought regime after AD 1200. Prior to this
time droughts dominated the region following which few severe droughts are
apparent in the record.
Verschuren et al .(2000), in their reconstruction of the drought and rain-
fall history of tropical east Africa over the past 1100 years, showed that three
distinct drought episodes occurred in the early 1300s, between approximately
AD 1450--1550 and around AD 1700. Prior to AD 1200, drought appears to have
been consistently more severe. In east India, Nigram et al .(1995)founda77year
cycle of severe droughts over the past four to five centuries based upon foramin-
ifera records. Similarly, Mullins (1998)found five distinct phases of cooler and
drier conditions in Cayuga Lake, New York over the past 10 000 years that were
interpreted as episodes of severe drought that lasted several centuries. Mullins
(1998)suggests that the occurrence of these drier episodes followed a cycle with
aperiod of 1800 to 2200 years. Liu et al .'s (1997)stalagmite record from Shihua
Cave, Beijing showed seven major episodes of drought over the past 1100 years
and the occurrence of several climatic cycles with periodicities of 136, 50, 16--18,
11 and 5.8 years. There was also evidence for possible millennial scale variability.