Droughts - Extreme Events: A Physical Reconstruction and Risk Assessment - page 23

would have grown and stabilised the dunes in response to greater precipitation.
Periods of landscape instability are recognised by intervals of aeolian erosion
and deposition when vegetation was reduced or absent and when soils were
unable to form. Increased wind velocities can cause reactivation of dunes. Once
one part of a dune is activated by strong winds the moving sand can smother
vegetation resulting in total reactivation of the dune. It is important to ascertain
whether low precipitation, strong winds, or both, may have been responsible in
thedie back of the stabilising vegetation. Several studies have argued that dune
reactivation is more likely to be a function of reduced vegetation cover due to
adecrease in available moisture as opposed to increased wind strength (Hesse
and McTainsh, 1999).
Chronologies of dune landscape stability and instability have been used
to estimate past climate variations in the Great Plains, USA. There, Forman
et al .(2001)investigated the link between past phases of dune reactivation and
megadroughts. Megadroughts are suggested to last from decades to centuries
and are likely to be initiated when climatic conditions cause a reduction in veg-
etation cover below a threshold level of ∼ 30%. The aeolian record in the Great
Plains shows that numerous megadroughts occurred during the Holocene (last
10 000 years) including several discrete events during the past 2000 years. In the
central and northern Great Plains up to three events occurred in the last 1000
years and each of these was more severe than the devastating droughts of the
Dust Bowl period during the 1930s to 1950s. The most recent of these appears
to have occurred after the 15th Century. Figure 2.1 shows the stratigraphy and
chronology of one of the dune reactivation sites in this region and highlights a
period of dune stabilisation when a soil, radiocarbon dated at around 500 years
BP, developed on the then dune surface. Another soil layer, higher in the strati-
graphy identifies the termination of a subsequent megadrought. Such droughts,
if they were to reoccur today, would bring unprecedented economic hardship to
the USA.
Forman et al .(2001)suggest that megadroughts would have probably devel-
oped during La Nina periods when sea surface temperatures (SST) in the tropical
Pacific Ocean, and later the tropical Atlantic Ocean, cooled resulting in weaker
cyclogenesis in North America. Cooler SSTs in the Gulf of Mexico may have also
reduced the funnelling of moisture northwards into the lower troposphere over
theUSA forcing the jet stream further north, producing near continent wide
droughts.
Dune activity, indicative of past drier conditions, has also been noted in
numerous other locations around the world including northern India, China
and Australia (Bowler, 1976;Wasson, 1986;Nanson et al ., 1992). While many
studies of dune activity can only obtain chronologies that indicate broad time
spans of drier conditions, and not necessarily specific episodes of drought, these