Geoscience Reference
In-Depth Information
On geological timescales, global climate has
undergone major shifts between generally warm,
ice-free states and Ice Ages with continental ice
sheets. There have been at least seven major Ice
Ages through geological time. The first occurred
2500 million years ago (Ma) in the Archean
period, followed by three more between 900 and
600Ma, in the Proterozoic. There were two Ice
Ages in the Paleozoic era (the Ordovician,
500-430Ma; and the Permo-Carboniferous, 345-
225Ma). The most recent Ice Age began about
34Ma in Antarctica at the Eocene/Oligocene
boundary and about three million years ago in
northern high latitudes. At present, we are
considered to be still within this most recent Ice
Age, albeit in the warm part of it known as the
Holocene, which began about 11.5ka. While
the total volume of land ice today (mostly
comprising the Antarctic and Greenland ice
sheets) is certainly much smaller than it was at
20ka, it is still substantial compared to other times
of the earth's past.
Major Ice Ages and ice-free periods can be
linked to a combination of external and internal
climate forcing (plate tectonics, greenhouse gas
concentrations, solar irradiance). The ice sheets of
the Ordovician and Permo-Carboniferous periods
formed in high southern latitudes on the former
mega-continent of Gondwanaland. Uplift of the
western cordilleras of North America and the
Tibetan Plateau by plate movements during the
Tertiary period (50-2Ma) caused regional aridity
to develop in the respective continental interiors.
However, geographical factors are only part of the
explanation of climate variations. For example,
warm high-latitude conditions during the mid-
Cretaceous period, about 100Ma, may be
attributable to atmospheric concentrations of
carbon dioxide three to seven times higher than at
present, augmented by the effects of alterations in
land-sea distribution and ocean heat transport.
Much more is known about ice conditions and
climate forcings through the Quaternary, which
began about 2.6 million years ago, comprising the
Pleistocene (2.6Ma-11.5 ka) and the Holocene
(11.5ka-present) epochs. It is abundantly clear
that this most recent Ice Age we live in was far
from being uniformly cold. Instead it was
characterized by oscillations between glacial and
interglacial conditions (see Box 13.2 ). Eight cycles
of global ice volume are recorded in land and
ocean sediments during the last 0.8-0.9Ma, each
averaging roughly 100ka, with only 10 percent of
each cycle as warm as the twentieth century
( Figure 13.4D and E ). Each glacial period was in
turn characterized by abrupt terminations.
Because of reworking of sediments, only four or
five of these glaciations are identified from
terrestrial records. Nevertheless, it is likely that all
were characterized by large ice sheets covering
northern North America and northern Europe.
Sea-levels were also lowered by about 130m due
to the large volume of water locked up in the ice.
Records from tropical lake basins show that these
regions were generally arid at those times. Prior
to 0.9Ma the timing of glaciations is more
complex. Ice volume records show a dominant
41ka periodicity, while ocean records of calcium
carbonate indicate fluctuations of 400ka.
These periodicities are linked to the Milan-
kovich forcings discussed earlier (see also
Chapter 3A). The precession signature (19 and
23ka) is most apparent in low-latitude records,
whereas that of obliquity (41ka) is represented
in high latitudes. However, the 100ka orbital
eccentricity signal is generally dominant overall.
The basic idea is that onset of glacial conditions is
initiated by Milankovich forcings that yield
summer cooling over the northern land masses.
This favors survival of snow cover through
summer, a feedback promoting further cooling
and ice sheet growth, leading to even further
cooling through slow feedbacks in the carbon
cycle discussed earlier. Onset of an interglacial
works the other way, with Milankovich forcings
promoting initial warming over the northern land
masses, setting feedbacks into motion to give
further warming and ice melt.
 
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