We see that all Dansgard-Oescher (D/O) cycles
happen during the big glacial period. So one hypothesis that scientists came up
with believes that the triggering of the cold drops could be due to ice bergs.
The large amounts of ice rafted debris (IRD, see last posts info box) on the ocean sea floor suggest that big pieces of sea
ice broke off the big Laurentide ice sheet sitting on top of North America and
the big Scandinavian ice sheet located on top of Scandinavia and Northern
Europe, and floated southwards on the Atlantic Ocean. Of course ice sheets are
much colder than liquid water, so melting must have happened fast. Hence, large
amounts of freshwater were released to the North Atlantic changing the salt
content within the water.
From the last posts, we know this implies a reduction of North
Atlantic deep water formation and hence a weakening of the whole Atlantic THC!
Models indeed show that input of freshwater to the North
Atlantic leads to a weakening of the Atlantic circulation. However, many
factors, such as amount, rate and location of freshwater input, seem to
influence how drastic this weakening is (Ganopolski &, Rahmstorf, 2001;
Clark et al., 2002; Hu et al., 2008). Also, many model runs suggested other
earth compartments to play a role in the global distribution of cooling by
changing wind, rain and evaporation patterns (Clark et al., 2001).
So, sadly the answer is not so simple. If models show
something, does that mean it really happened?
Blunier et al. (1998) may have found the missing link when
they were comparing ice core records in the Arctic with the Antarctic. They
simply plotted both temperature curves on the same time scale (not as simple if
you have to do it….) and saw that both records unexpectedly did not line up.
When the arctic temperatures were cold, the Antarctic temperatures were warm
and vice versa. This was not the case for all D/O cycles, but very distinct for
few. How could that be possible?
During the same year, Stocker (1998) proposed the solution:
the Atlantic circulation. We all profit from the heat the Gulf Stream
constantly transports to the north. However, we tend to forget that the heat is
actually stolen from the southern Hemisphere. If we compare heat transports in
other oceans, heat north of the equator goes north, while heat south of the
equator goes south. Contrary, in the Atlantic heat goes north no matter where
it is located, due to the THC.
What Stocker is implying for the D/O events is that a
cooling in the North Atlantic will lead to more ice bergs melting and more
freshwater input. This will weaken the THC and slow down heat piracy from south
to north. As a result, the southern hemisphere will end up with more heat,
leading to a warming in the south, while there is a cooling in the north. This
process of the thermal bipolar-see saw (or sea saw) can be found in actual
climate models (i.e. Stenni et al., 2011) as well as in climate models (Seidov& Maslin, 2001).
As a summary we can conclude that obviously
Atlantic THC played a major role in forming the D/O cycles. However what actually induced climate to change is still discussed. Some say solar insolation gave the first initial forcing (i.e. Cruz et al., 2005), some say the changes in Atlantic circulation can explain the climate changes (Seidov& Maslin, 2001).
Still, then what changes the THC?
Ice volume… what changes ice volume? ..... You see the problem.
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