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)

Why are we interested in ocean circulation? A short trip to the Earth's past.

Many people today are worried about the future of the earth when thinking of climate change. To most humans, climate and natural settings were considered the one stable concept in their otherwise unstable life. Hence, the thought of a 4˚C warmer Earth, as it is postulated by the IPCC and many newspapers often triggers communal fears of mass extinction that may even include the human race.

You may panic.

http://www.demonsinthedark.com/panicandalcohol.htm

However! Here is the interesting bit. During the last century, climate scientists have found evidence for long term climate changes throughout the earth’s history (e.g. Daansgard et al., 1982; Daansgard et al., 1993). Proxy data (eg by converting information found through biological and abiological indicators into climate variables) or even direct measurements (eg trapped air bubbles in ice cores) in geological records have shown that Earth has went through climates much more aggressive than the one we know.

-          Precambrian (600-3500 Ma): Snowball Earth: The whole of the earth’s surface was covered in ice (Hydeet al., 2000; Donnadieu et al., 2004)

-          Cretaceous (65-144 Ma): Greenhouse Earth: Ice did not exist. Fun fact: CO₂ pressure was above 1000 ppm vs today at 400 ppm (Barron et al., 1981; Barron,1983; Kuypers et al., 1999)

This shows that the Earth has a much greater climate span than we would have initially believed. Since our climate today includes ice on Earth (glaciers and permafrost) we are actually closer to the snowball earth climate than the Cretaceous version; meaning, we are stuck in an Icehouse Climate, while an ice free earth represents a Greenhouse Climate.

To get a more accurate understanding of our current climate situation, we should take a closer look at the recent past of our climate.

Vostok Ice core time series and Insolation, taken from Petit et al. (1999)

Petit et al. (1999) have managed to characterize climate for the last 400,000 years. We see that temperature (line b) has fluctuated strongly between cold glacial and warm interglacial periods. The Insolation curve (line e) suggests that those fluctuations are initiated by changes in solar energy on earth. Thanks to Milankovitch, we know today that climate over the last ice ages was indeed forced by three external variables (Eccentricity, Obliquity and Precession) with different oscillations (Zachos et al., 2001).

Now put yourself in the shoes of a climate change scientist and try to answer the following question using the graph above:

What is the difference between warming phases and cooling phases?

Post your answer if you like and see whether you were right next time J