Geoscience Reference
In-Depth Information
14
C date (yr B.P.)
Conventional
Cape Hallett
Edmonson Point
Gondwana St.
Terra Nova St.
North. Foothills
N. Adélie Cove
Adélie Cove
Inexpressible Island
Prior Island
Cape Hickey (N)
Cape Hickey (W)
N. Cape Day
Peninsula—Depot Is.
Cape Ross
Marble Point
Franklin Island
Beaufort Island
Cape Bird (Ross Is.)
Approximate Calibrated age (yr B.P.)
Figure 9-21.
Age distribution of modern and abandoned penguin nesting sites in Victoria Land, Antarctica. Modern
nesting sites are shown by horizontal rectangles (light gray). Nesting sites were particularly widespread during the
mid-Holocene climatic optimum (dark gray). Such favorable environmental conditions have not been repeated since.
Adapted from Baroni and Orombelli (1994, Fig. 2).
life. A popular concept exists, at least among the
general public, that climate should be stable and
that any change in climate is undesirable and
must somehow be related to human activity. On
the other hand, most paleoclimatologists have
come to the conclusion that the major shifts in
climate during the Holocene were driven prima-
rily by solar forcing, including variations in sun/
earth orbital geometry (Thompson et al. 2002)
and l uctuations in solar output (Eddy 1977), as
well as by episodic volcanism (Nesje and Dahl
2000).
Humans have played increasingly disruptive
roles in terms of habitat alteration during
the Holocene. Ruddiman (2005) argued that sig-
nii cant human impact on climate began with
the advent of agriculture some eight millennia
ago. Agriculture has a profound effect on natural
vegetation
-
forest, prairie, and wetland are
replaced in favor of various annual crops. The
net result is transfer of carbon previously stored
in the biomass into the atmosphere. Carbon
dioxide is derived mainly from forest clearing;
methane is a byproduct of growing rice and
raising cattle. Increasing levels of atmospheric
carbon cannot be absorbed into the oceans at
rates as fast as they were created by agricultural
expansion. Ruddiman (2005) concluded that the
natural trend for climatic cooling in the late
Holocene may well have been slowed or even
reversed by increasing levels of greenhouse
gases derived mainly from agriculture.
Certainly early man had an effect on local
and regional wetland conditions, primarily
through forest clearance, agriculture and irriga-
tion, but it is not yet clear that this human activ-
ity had signii cant inl uence on global climate
prior to the Industrial Revolution. In fact, Nesje
and Dahl (2000) concluded that pulses in vol-
canic eruptions played a major role in cooling
events in the northern hemisphere during the
Little Ice Age, and that rapid glacier retreat and
climatic warming in the late nineteenth and
early twentieth centuries were more related to
a quiescent phase of volcanism than to human-
caused emissions of greenhouse gases. This
