Droughts - Extreme Events: A Physical Reconstruction and Risk Assessment - page 24

Chronology
(14C yrs BP) Stratigraphy

464 - 146
560 - 320
644 - 507
913 - 695
701 - 554
714 - 556
910 - 650
1056 - 792
1170 - 910
1502 - 1301
1510 - 1300

Soil layers

2380 - 2150
3080 - 2710
3450 - 2750
3630 - 2280

Alluvium Aeolian

7020 - 6390

7900 - 7580

8950 - 8780

20 260 - 18 960

Figure 2.1. Sand dune chronology, Great Plains USA (from Forman et al ., 2001). The
soil layers represent periods of dune stability when the climate was wetter.

studies can nonetheless highlight the rapid transitions into relatively dry phases
of climate during otherwise wetter conditions. For example, Gillespie et al .
(1983)found that the early Holocene in Ethiopia was marked by shifts into
arid phases during a phase of climate which was generally becoming wetter
towardsthe Holocene climatic optimum (HCO) ( ∼ 5000 years ago). The HCO was
aperiod when temperatures and precipitation globally were generally higher
than today. These marked excursions into more arid conditions occurred during
theintervals 11 000--10 000, 8500--6500 and 6200--5800 years BP. The period from
11 000 years onward was generally marked by higher lake levels in Africa when
precipitation is estimated to have been at least 25% higher than today (Street-
Perrot and Harrison, 1984). These early Holocene conditions are often seen as
an analogue of future enhanced greenhouse conditions for some regions of the
globe. The Ethiopian record highlights that relatively rapid departures into pro-
longed episodes of severe drought are possible and may last between 10 2 and