Geoscience Reference
In-Depth Information
Interestingly, modest IRD events have also recurred through the Holocene
(Bond et al.,
1997
), when (excepting Greenland) there was much less continental
ice in the Northern Hemisphere. Using high-resolution Holocene data, Bond et al.
(
2001
) found that these IRD events show a rough recurrence on two time scales, the
approximate 1,500 +/− 500 year scale seen during glacial periods, but also around
400-500 years. There is a relationship between these cycles and inferred solar vari-
ability. Whether the apparent solar relationship seen in the Holocene records can be
extended back into the last glacial cycle is not known. This inferred solar variability
is not linked with Milankovitch cycles but to forcing such as those associated with
sunspot cycles and altered fluxes of ultraviolet radiation.
R. Alley, S. Anandakrishnan, and P. Jung (
2001
) and S. Rahmsdorf and R. Alley
(
2002
) propose “stochastic resonance” on a 1,500-year time scale. This involves
interactions between a weak periodic climate forcing, climate “noise” and a non-
linear amplifier. The basic idea is that one has an amplifier, namely the THC, which
does not respond to signals below a certain threshold. However, if climate noise
(e.g., random fluctuations in freshwater fluxes) combines correctly with a weak
periodic forcing, stochastic resonance can occur, seen as a strong response of the
THC. The idea finds support in results from a so-called model of intermediate
complexity (Ganopolski and Rahmsdorf,
2001
). This study showed that in contrast
to “on” and “off” modes of the THC, such as associated with the salt oscillator, or
catastrophic events such as the draining of proglacial lakes, a more subtle expla-
nation for D-O events is geographical shifts in NADW production. And it seems
that such geographical shifts in NADW production can be triggered fairly easily by
climate noise. When this model is driven by random noise of realistic amplitude,
combined with a weak periodic (1,500-year) climate signal, D-O events emerge
that are very similar to those seen in the Greenland ice core records. The origin of
the weak periodic 1,500-year climate forcing has not yet been clearly identified,
but the solar variability link postulated by Bond et al. (
2001
) must be considered
as a contender.
A final comment regards interhemispheric correlations. The major aspects of
the glacial/interglacial cycles as seen in the Greenland ice cores correspond well
to those in Antarctic records, pointing to global-scale signals. However, inter-
hemispheric relationships between millennial-scale D-O cycles are more complex.
Evidence from ice cores indicates that when the North Atlantic warms abruptly,
Antarctic temperatures drop, a phenomenon known as the bipolar see-saw. This see-
saw implicates the AMOC because this circulation is known to orchestrate how heat
is redistributed between the Northern and Southern hemispheres. The idea is that a
reduction in the strength of the AMOC, such as caused by a massive input of fresh-
water to the North Atlantic, gives rise to a rapid decrease in northward heat transport
in the northern hemisphere, compensated by an increase in southward ocean heat
transport in the southern hemisphere. Records examined by S. Barker et al. (
2009
)
indicate that rapid temperature changes in the South Atlantic during the last degla-
ciation were, within dating uncertainty, instantaneous and of opposite sight to those
that occurred over the North Atlantic.
