Geoscience Reference
In-Depth Information
to create palaeoclimate chronologies are a lack of long-living, climate-sensitive
trees in many parts of the world (such as Australia), the elaborate process of
field sampling, laboratory and statistical analysis necessary to produce accurate
estimations and the limited exposure of mainstream hydrologists to the field of
dendrohydrology (Loaiciga et al ., 1993).
Woodhouse and Overpeck (1998)analysed tree-ring data from the Great Plains
region in the central USA, to gain an accurate record of droughts extending
back to about AD 1300. They found that 20th Century droughts are not rep-
resentative of the full range of droughts that have occurred in the USA over
this period and probably the past 2000 years. Multidecadal 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 (see also Stahle et al. , 2000). The long-term
record also suggests that there has been a regime shift in droughts after the
13th Century (Stine, 1994). Droughts of the 20th Century have been charac-
terised by moderate severity and comparatively short duration. Droughts prior
to this time were at least decades in duration, whereas after this time (i.e.
20th Century) droughts were a decade or less in duration. The records also
highlight that there can be substantial periods of time between episodes of
truly severe drought. Woodhouse and Overpeck (1998)stressthat the same
is true of lake sediment, microfossil and isotope histories of droughts from
theUSA, Australia and Africa. Such records provide a warning that the short
historical records of droughts are often not a reliable guide to drought cli-
matology and 20th Century records are less than ideal as a guide to future
events.
Tree-ring records of droughts have also played a role in unravelling some
intriguing mysteries concerning the demise of some human populations. The
first group of English colonists to the USA landed on Roanoke Island, located off
theNEcoast of North Carolina in AD 1585. They did not last long and returned
to England a year later. A second group of colonists, arriving in 1587, mysteri-
ously disappeared and not a single colonist was found by the time additional
supplies were brought from England in 1591. The ring record of baldcypress trees
( Taxodium distichum )was used by Stahle et al .(1998)toreconstruct the climatic
history for this region. The record suggests that the most severe drought over the
past 800 years coincided with the disappearance of the Roanoke Island colonists
(Fig. 2.12). Stahle et al .(1998)also showed that a severe seven year drought
(AD 1606--1612) in Jamestown, the main settlement, may have been the cause
of a high death rate in the colony around this time. Only 38 of the original 104
colonists survived the first year (AD 1607) of the new settlement at Jamestown
Search WWH ::




Custom Search