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)

SCIENCE FACT Where does all that knowledge for past climates come from? And what on Earth are isotopes??

Most of us know that the continuous measuring of climate variables (such as temperature, rainfall, ...) didn't start until roughly 150 years ago (see data sets on KNMI Climate Explorer). So then how are climate scientists able to know about climates in the past?... simply because the Earth has recored its climate history in various natural archives. Our job is it to look at these archives and understand the codes in which the different climate variables are encrypted... much like a detective.

The most common and famous natural archives are:

ice cores
sediment cores (either from the ocean or from lakes)
stalagtites + stalagmites
fossilized pollen
tree rings
corals

Why those?
All of the above named natural phenomena have one similarity: they all come with a record of time. Without a record of time, I will never be able to reconstruct the past. In all, time is represented as layers (except for pollen, which are normally found in a certain layer of a terrestrial core). If the layers are well kept, I simply have to go ahead and count backwards, thinking of each line as a year.
In the case of sediment cores, layers might not present an annual resolution meaning the lines here represent bigger time pieces. In that case I can use big global events that may be seen in the core and give those layers a date (i.e. Sarna-Wojcicki et al., 1985; Drexler et al, 1980; Machida, 1999). An example for such a big event is the explosion of the Yosemite Volcanoe. The event was so huge, that Yosemite ashes can be found in almost every record that goes this far back. In the more recent history, we can easily date the 1950s and 60s, due to the large amounts of atmic bombs that were tested. The material from those bombs is also visible in most modern archives (Picciotto & Wilgain, 1963; Eichler et al., 2000) .

Here are some examples of natural archives:

Ice core:

http://upload.wikimedia.org/wikipedia/commons/thumb/3/31/GISP2_1855m_ice_core_layers.png/384px-GISP2_1855m_ice_core_layers.png

Coral core:

http://www.fws.gov/coralreef/images/core_sample.jpg

Ocean deep sea sediment core:

http://ocean.si.edu/sites/default/files/photos/cores.jpg

Cave Stalagmite:

https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhdpIZNfwWE4gElOx3WXiK7_V9Bn4Ykenrwqu1JQMiRcKz1cX5G97S7ODZUuwg2D-gAeAAe0TsjRPZwh0JuBADhYZsNUL22ky2tZ1NTsEG88BzcuGr4lKFR5Si_jR8d1b8XFbFUcEeEAjUz/s320/StalagmiteCoreSlice_12_MNJensen72.preview.jpg

Soil core for Pollen:

http://www.sciencephoto.com/image/214061/350wm/G3550193-Soil_core_for_pollen_research-SPL.jpg

Tree rings:

http://cdn.desktopwallpapers4.me/wallpapers/photography/1680x1050/1/8788-tree-rings-1680x1050-photography-wallpaper.jpg

How can scientists extract data out of this? The isotope method:

The whole core is important for dating. The actual climate information however is extracted in the most genious and creative ways. The ones listed here are just a few ways of how one can do it.

One way is to look at the thickness of layers. Tree rings or ice cores for example (depending on where they are from) may differentiate between rain/snowyy vs dry years.

Another way is looking at isotopes. Whats that?

--> Every atom has electrons, protons and neutrons. The amount of electrons and protons (amount of electrons = amount of protons) defines what it is (either oxygen, or carbon, or iron...). The neutrons are defining how heavy it is. More neutrons = heavier. Less neutrons = less heavy.

So an oxygen atom with 16 neutrons is lighter than an oxygen atom with 18 neutrons.

Exactly this is what the scientists use.

Let me use the deep sea sediment core to show you how (bare in mind that this is just a simple way of explaining... incase you are a climate scientist):

Step 1: drill the deep sea sediment core

Step 2: count the layers and have an idea how old each layer is

Step 3: go to the layer you're interested in and look for little animals that lived in this time, then died, sank to the ocean floor and now are located in your core. Most scientists use little living beings called foraminifera. They build a calcium (chemically: CaCO3) shell and thus can provide you with carbon and oxygen atoms.

Step 4: get the atoms out of the little foraminifera

Step 5: count how many heavy oxygens and how many light oxygens you can find and calculate a ratio: all oxygens with 16 (16O)/ all oxygens with 18 (18O).

You are done! wow. what does that tell you about the climate?? a lot! Let me give you a little background information:

We know that 16O is lighter than 18O. So imagine you are at the equator. It is nice and warm and a lot of water from the ocean evaporates into the air.

What happenes: of course the lighter particles evaporate first meaning lots of 16O leaves the ocean and circles in the atmosphere.

Now imagine we travel to the poles. It is very cold and lots of moisture is in the air. It snows. All the 16O that we just collected from the oceans falls down as snow and gets stored on the big ice sheets. Do you see: the Atmosphere is a SORTING MACHINE for oxygen isotopes!!!!

Now if you search the isotopes in your sediment core: you see that there is layers with lots of 18O and layers where 18O = amount of 16O. From what we learned above: the layers with lots of 18O show ice ages (lots of snow keeps lots of 16O out of the ocean), while the layers where 16O and 18O are almost the same show warm periods (most 16O rains back into the ocean and only very little is turned into snow and stays on the poles).

That is quite amazing! Now you know why scientists love isotopes! :D

If you want to read it from the "discoverer of isotopes" himself: Shackelton (1987); Chappel & Shackelton (1987); Gat et al. (1981)

Good informatin about ice core sampling can be found here: Alley (2000)



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