Extreme events over time - Extreme Events: A Physical Reconstruction and Risk Assessment - page 256

Dean (1997)likewise recognised severe droughts with a 400 year periodicity
in the 10 000 year long varve record of Elk Lake, USA. This periodicity is centred
at 1200--1000, 800--600 and 400--200 years BP and these episodes are interpreted
to represent episodes of windier and dustier conditions compared to the period
between 3000 and 1500 BP. Two other cycles of aeolian, and hence drought
activity, were recognised with periods of 1600 and 84 years. Based upon this
record, Dean (1997)stressed that the 20th Century droughts of the USA are not
representative of the full range of droughts that have occurred over the past
2000 years. Here, multi-decadal droughts of the late 13th and 16th Centuries
were more prolonged and severe than those of the 20th Century and few, if any,
major droughts have occurred in the periods between these episodes of severe
drought. The long-term record here also suggests that there was a regime shift
in droughts after the 13th Century. Droughts prior to the 20th Century were at
least decades in duration, whereas after this time droughts were a decade or less
in duration. As Woodhouse and Overpeck (1998)emphasise, the short historical
records of droughts from the USA, Australia and Africa are often not a reliable
guide to drought climatology and 20th Century records are less than ideal as a
guide to future events.
Non-atmospheric events
The resolution of the natural long-term records of earthquakes, land-
slides, volcanic eruptions and asteroid impacts with Earth is lower than the
long-term records of atmospheric events such as droughts and floods. These
events also tend to occur less frequently and there is usually a substantial period
of inactivity between them. Hence, they do not lend themselves to being reg-
istered in annual records as readily as some of the atmospherically generated
events. This is because the considerable time between events, such as four to
five centuries, is often longer than the lifespan of individual trees and some-
times even lakes, and the accumulation period of some speleothems. This does
not mean that these events cannot be recorded by these systems. As shown in
Chapters 6--9,non-atmospheric events have been recorded in the form of reac-
tion wood in trees from landslide events, and drowned forests and sediment
accumulations in lakes from subsidence and mass movements. The real value
of the prehistoric records of the non-atmospherical events is that they provide a
much more realistic guide of the return interval of these events rather than
atest of their randomness of occurrence over time. The prehistoric records
often do not provide a test for the latter because the chances are reasonably
high that these events will not have occurred, at least at extreme magnitudes,
within the timeframe of a short historical record. There is also a certain level