Heinrich Events. Evidence of Atlantic turnover circulaton shutdown...

The other events we have identified as rapid climate changes are Heinrich events (H1-5).

(Rahmstorf, 2002)

In 1988, Hartmut Heinrich noticed extreme layers of ice rafted debris (IRD) in his North Atlantic sediment cores which later became known as Heinrich events. Unlike Dansgaard-Oescher (D/O) events, Heinrich events are characterized to originate from Hudson Straight only (so the Laurentide ice sheet in North America), due to their very low amount of volcanic particles (which normally hints to Iceland) (Heinrich, 1988). 

The extremity of the layers strongly suggests some kind of catastrophic release of ice bergs into the North Atlantic over a period of 250-500 years (Hemming, 2004). MacAyeal’s model (1993) reproduced a released freshwater mass of roughly 1.25 million km³! That is about 55 times the size of the North American Great Lakes (with a Geat Lakes volume of 22,671 km³)! 

However, since it was entered over a time of up to 500 years, is that enough to shut down the THC? Most models say yes (Prange et al., 2004; Rocheet al., 2004; Ganopolski & Rahmstorf, 2001). In fact, some models show even less freshwater input to be enough for a full shutdown (Rahmstorf, 1995).

A more debated question is how that could have happened.

A large amount of scientists believe that it had to do with ice sheet instability. However, did the ice sheet bed become instable (MacAyeal, 1993)? Or was it maybe something like a collective joekulhaup explosion (Johnson and Lauritzen, 1995), similar to that we just had in Iceland? Maybe the simple thickness of ice increased the friction at the bottom which then produced enough heat to melt the bottom of the ice sheet (Clark et al., 1999)? We'll keep our eyes open and see what future scientists will find out!

Even though many questions are still unanswered, there is good evidence for a full shutdown of the THC during Heinrich events! Let's choose a slightly different approach to see whether this is true. Quickly think back to  the heat piracy and bipolar seesaw concept from the last post (heat is always transported northwards in the Atlantic partially via the Gulf Stream). If the THC really "turned off" during Heinrich events, then we should see a cooling in the North and a warming in the South.

Is there any evidence for that??

Yes there is! In the ice cores of Antarctica! And what do we see?? It warmed in the south!!!!! The evidence strongly suggests a full shut down of the THC during Heinrich events!

Seidov & Maslin (2001)

We can now say with fairly high confidence: It is actually possible for the Atlantic thermohaline circulation to stop, inducing an abrupt cooling in the north and a warming in the south.

Slightly scary that it actually is possible to turn off the ocean conveyor belt? Can it happen again?? Can WE make it happen?.......

keep reading the blog ;)

Dansgaard-Oescher cycles and the Thermohaline Circulation

Let's have a look at the last record again:

(Rahmstorf, 2002)

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.

...

When could the Thermohaline circulation have shut down during the last 100,000 years?

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:

(Rahmstorf, 2002)

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)