Geoscience Reference
In-Depth Information
4.4
INTERGLACIALS
Ice ages tend to end rather abruptly compared with their slow rate of formation
over many tens of thousands of years. A number of studies have been carried out
in an attempt to understand why this occurs, but none of them are entirely satis-
factory. Details on terminations of ice ages are presented in Section 10.2.3. The
durations required for the transition from glacial to interglacial conditions in the
last nine deglaciations are listed in
Table 10.5
.
On average, the duration of the
transition from glacial to deglacial conditions is about 6,000 years. There are
some indications that the Southern Hemisphere may be significantly involved in
the deglaciation process. However, as Wolff et al. (2009) said,
the detailed sequence of events that leads to a glacial termination
remains controversial. It is particularly unclear whether the northern or southern
hemisphere leads the termination
''
...
. The reason for the spacing and timing of
interglacials, and the sequence of events at major warmings, remains obscure.''
...
Interglacial periods are of great interest because we are presently in one, and
there is widespread concern that rising CO
2
, generated by human activity, may
amplify and extend this climate period with negative consequences for civilization.
It
is therefore relevant
to review data on past
interglacials with particular
emphasis on CO
2
levels and prevailing temperatures.
Holden et al. (2009) reported:
''Ice core evidence indicates that even though atmospheric CO
2
concentra-
tions did not exceed 300 ppm at any point during the last 800,000 years, East
Antarctica was at least 3-4
C warmer than pre-industrial (CO
2
280 ppm) in
each of the last four interglacials. During the previous three interglacials, this
anomalous warming was short lived (
3,000 years) and apparently occurred
before the completion of Northern Hemisphere deglaciation.''
Holden et al. (2009) presented a speculative theory to explain this based on
meltwater-forced slowdown of the Atlantic Meridional Overturning Circulation
(AMOC) during glacial terminations. Thornalley et al. (2011) provided evidence
that during the period ending the last ice age (16-10
kybp
) significant fluctuations
occurred in global overturning circulation. Although this paper received a great
deal of hype on Internet blogs (e.g., ''Dramatic ocean circulation changes
revealed''
1
), the results did not seem very impressive to this writer.
Sime et al. (2009) presented oxygen isotope data from three sites in Antarctica
showing that the change in isotope content was considerably greater in the
previous three deglaciations than in the present deglaciation. They analyzed
the relationship between isotope index and temperature at the three sites and
concluded ''that maximum interglacial
temperatures over
the past 340 kyr
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