Geoscience Reference
In-Depth Information
Table 9-1.
Late glacial and Holocene climatic cycles of
North America. * Date is approximate beginning of
each climatic cycle in calendar years before present.
Adapted from Viau et al. (2002).
8.2 ka event in northern Europe. Another abrupt
change took place from about 6500 to 5000
years ago with cooling that coincided with a
second humid period. A third abrupt event was
a severe drought that occurred about 4000 years
ago. This drought lasted three centuries and was
the greatest recorded historically in tropical
Africa; it coincided with the First Dark Age,
when many early civilizations collapsed (Thomp-
son et al. 2002). This climatic record has major
implications for Holocene wetland conditions
across tropical Africa, the Middle East and
western Asia, especially in regard to lakes and
their associated features.
The Quelccaya ice cap in the Andes Moun-
tains of southern Peru preserves another unique
climatic record. Ice cores provide direct physical
evidence for the past 1500 years. Relatively
warm climate prevailed before about
AD
1550
and after 1900. The intervening cold period
represents the Little Ice Age (Thompson et al.
1986). This record compares favorably with late
Holocene temperature l uctuations in North
America (e.g. Campbell et al. 1998). Farther
south, Lake Titicaca rose signii cantly during
the sixteenth through nineteenth centuries as
a result of more humid, cooler conditions
( J. Argollo, pers. com.).
Still farther south, coastal Antarctica hosts
numerous penguin nesting sites. The number
and distribution of these sites through time is a
proxy for paleoclimate (Baroni and Orombelli
1994). Nesting sites were particularly wide-
spread during the Holocene thermal maximum,
4300 to 3000 years ago (Fig. 9-21). Timing of
the Antarctic HTM is noticeably later than
for the northern hemisphere, however, which
rel ects a signii cant lag time for the southern
ocean to respond to global climatic change.
Date*
Period
Climatic conditions
110
Modern
Modern climatic optimum
600
Little Ice Age
Coldest climate of Holocene
1650
Neo-atlantic
Medieval climatic optimum
2850
Sub-atlantic
Continued cooling
4030
Sub-boreal
Beginning Neoglaciation
6700
Atlantic II
Holocene climatic optimum
8100
Atlantic I
Continued warming
10,190
Boreal
Early Holocene warming
12,900
Younger Dryas
Cold late-glacial interval
13,800
Allerød
Warm late-glacial interval
the Little Ice Age. Evidence has accumulated for
several climatic cycles during the latest Pleis-
tocene and Holocene of North America (Viau et
al. 2002). Climatic oscillations, each lasting a
period of roughly 1650
500 years, took place
and caused changes in vegetation across the
continent (Table 9-1). These cycles may repre-
sent changes in atmospheric circulation with
global climatic consequences, which are docu-
mented in ice cores and marine sediments. The
origin of millennium-scale cycles is uncertain,
but many scientists consider solar forcing a
likely mechanism (see below).
±
9.4.3 Tropics and Antarctica
Moving to low latitudes, ice-core records from
tropical alpine glaciers provide unique climatic
data that may be compared with other climatic
proxies. Ice cores from Kilimanjaro glaciers, for
example, span the entire Holocene with high-
resolution oxygen-isotope and dust data that
corroborate other climatic indicators for equato-
rial eastern Africa (Thompson et al. 2002).
Warmer and wetter conditions marked the i rst
African Humid Period (
9.4.4 Holocene climate and early man
From this brief review of Holocene climate in
representative parts of the world, it should be
clear that signii cant changes in climate did
occur during the past several millennia. These
climatic shifts were broadly comparable world-
wide, but with distinct regional variations, which
had major impacts on local wetland habitats and
11,000 to 4000 years
ago), when lakes rose as much as 100 m above
current levels. Lake Chad, for instance, was com-
parable in area to the modern Caspian Sea. Lake
levels dropped abruptly 8300 years ago, which
marks a dry period and corresponds with the
∼
