Last time we found a record showing temperature differences
from today over the last 100 000 years. Now let’s see whether we can find
possible THC-shutdown incidences…
On the first blink, the temperature record just looks like a
sequence of undefinable scribbles. However, we have to bear in mind that those
scribbles show temperature dropping and rising again within less than 100
years! From roughly 70 000 to 100 000 years ago, the scribbles are rather
boring and show no significantly extreme changes. But the time from 10 000 to
60 000 years ago shows a row of very extreme changes over only short periods of
time. These could possibly give us insights into ocean circulation changes.
The first question: do we see those extreme temperature
changes also in the Atlantic Ocean?
Yes we do! Look at this record found by Grootes et al.(1993) in the GISP2 Greenland ice core (blue line) and another one found by Sachs & Lehman (1999) in a subtropical North Atlantic deep sea sediment core (green line) for the last 60
000/30 000 years:
Looking at all those rapid climate change events more
closely, scientists have found out that there are two distinct happenings which
keep showing up in the record. They named one set Dansgaard-Oescher cycles
(event 1-20) and Heinrich events (event H1-H5).
Daansgard-Oescher cycles are characterized as being high
frequency climate oscillations (Maslin et al., 2002). The short warm phases
appear in the ice core records as 5-10 degree warming phases within only a few
decades. At first, cooling is happening gradually, then abrupt over less than
30 years (Rahmstorf, 2002). Both records show the D/O cycles, meaning that the
rapid warming/cooling was not confined to the North Atlantic, but happened
across the whole ocean. However in sediment records the cold phases are
recorded, since substantial layers of ice rafted debris (IRD, see INFO BOX)
show up in the record (Maslin et al., 2002). A study done by Voelker et al.(2002) shows that evidence actually exists throughout the globe making this a
significant global climate event.
Here we have our first candidate. Could it be possible that
changes in Atlantic thermohaline circulation caused these abrupt climate
events?
The next question is: how?Could you imagine how the THC can collapse? Post your ideas :)
( Info Box links: Bond et al. 1992; Bond & Lotti 1995; Alley & Macayeal 1994)
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