Geoscience Reference
In-Depth Information
likely to change in the future in ways we cannot yet predict. The regions indicated
in
Figure 23.1
as the ones most affected by ENSO during the last 100 and more
years of meteorological records will remain susceptible in the future, with a very
strong likelihood of increased variability in both temperature and precipitation (IPCC,
2007a
). Nicholls (
1989
) has suggested that in regions strongly influenced by ENSO
events, the plants and animals have developed adaptive strategies to cope with the
rainfall variability.
The Nile provides an exceptionally long record of past floods and droughts. ENSO
events are generally well-reflected in the last 500 years of Nile flow records (Ortlieb,
2004
). In addition, low Nile flows were common between 950 and 1250 AD, roughly
coeval with the Medieval Warm Period (MWP) in Europe. There were also severe
droughts in East Africa during the MWP which caused major human migrations in this
region but do not appear to have been in any way caused by anthropogenic changes
in land use (Verschuren et al.,
2000
). Very severe droughts also afflicted the western
United States during the MWP between 900 and 1300 AD (Cook et al.,
2004
).
ENSO forcing may be apparent in a five-year cycle that persisted through periods of
high and low Nile flow (De Putter et al.,
1998
). Fraedrich et al. (
1997
) discerned eight
almost synchronous abrupt changes in minimum and maximum Nile floods, many
of which were associated with thirty-five- to forty-five-year persistence time scales.
The 75.9 year periodicity identified by De Putter et al. (
1998
) from records of high
flood levels during 950-1250 AD may be similar to the roughly 90 year periodicity
evident in the Lake Lisan laminations from the Dead Sea Rift (Prasad et al.,
2004
).
The increase in the frequency of high Nile floods after around 1250 AD may itself be
linked to the increase in the south-west Asian monsoon inferred from proxy records
of
Globigerina bulloides
collected from box cores from the Arabian Sea (Anderson
et al.,
2002
). The reasons behind these periodicities remain obscure but appear to
reflect changes in global atmospheric circulation linked to solar and other external
controls (e.g., volcanic aerosol forcing) that were modulated by changes in SST in
the equatorial Pacific and were reflected in ENSO events.
23.5 Volcanic eruptions and droughts
Using tree ring data from more than 300 sites across Asia, Cook et al. (
2010
)have
established a precise chronology of very severe droughts and monsoon failure in that
region. Four of the worst droughts were those of 1638-1641, 1756-1768, 1790/1792-
1796 and 1876-1877. There are two matters of particular interest concerning the
timing of these droughts. The first is that these severe droughts were also synchronous
with wetter phases in other parts of Asia, so that there was considerable spatial vari-
ability across Asia (Cook et al.,
2010
; see also the commentary by Wahl and Morrill,
2010
). The second matter, not discussed by Cook et al. (
2010
), is that each of these
major drought intervals coincides very broadly with times when the volcanic dust
