Droughts - Extreme Events: A Physical Reconstruction and Risk Assessment - page 37

became gradually drier culminating in episodes of drought which have largely
persisted to the present. Hong et al .(2001)were also able to identify eight,
multi-decade to multi-century, drought periods between 2200 BC (4200 year BP)
and AD 1200. At the same time Europe experienced diminished rainfall and
desertification of the Saharan region was occurring. Other evidence also sug-
geststhat northeastern China experienced severe droughts over this 3000 year
period. Historical records show the widespread use of astrology and meteorol-
ogy to predict rainfall throughout China at the same time and prayers for rain
were also recorded on bones and tortoise shells around this time. The

13 C peat
record shows that a return to wet and cold conditions occurred from AD 1200 to
1800 after which dry and warm conditions have largely persisted to the present
day.
Hong et al .(2001)werealsoabletousethe

13 Crecord to demonstrate a cor-
relation between past precipitation, temperature and solar activity. Solar activity
(sunspots) changes the abundance of 14 CinEarth's atmosphere through varia-
tions in the amount of solar radiation reaching the Earth. Weak solar activity
results in higher production of 14 CinEarth's atmosphere and strong solar activ-
ity lower concentrations. Hong et al . showed that warm and dry episodes over
thepast 6000 years in China correspond to strong solar activity (small

14 C

14 Cvalues).
Solar activity changed from a weak period around 2200 years BC, which was
characterised by six strong peaks of

values) and cold and wet periods to weaker solar activity (large

14 C, to a stronger period of solar activity

18 Olevelsmarking the tran-
sition into a dry warm phase. After AD 1200, solar activity weakens again and
there isacorresponding fall in

13 Candrise in

which corresponds to a fall in

13 Casclimate changed back to a wet phase.
Spectral analyses showed that these climatic variations correspond to periodic-
ities ranging from 70 to 1061 years which Hong et al .suggest were likely to be
due to perturbations in patterns of atmospheric circulation and transport of
moisture principally induced by variations in solar activity.
Hong et al .(2003)usedthe

13 Crecordfromapeat bog and Carex species at
Hongyuan on the Tibetan Plateau to reconstruct the Indian Summer Monsoon
over the past 12 000 years. The record shows abrupt variations in the monsoon
over this period and reveals a clear correlation with ice rafting events of the
North Atlantic (Fig. 2.9). While this record does not focus specifically on droughts
or other natural hazards it does highlight how variable the monsoon has been
over this time period. This of course emphasises the variability of nature over
time and particularly the relative rapidity with which the system can oscillate.
As the monsoon is a major controller of the frequency and intensity of floods
and droughts, and tropical cyclone occurrences too, these oscillations in the
monsoon highlight that the occurrence and intensity of extreme events will
also be highly variable over time.