SCIENCE FACT: The RAPID/MOCHA project

www.rsmas.miami.edu/users/mocha/mocha_files/SJ08_video_0002.wmv

Check out the video!

The RAPID project is an amazing intercontinental project with US and Europe heavily involved.
The big goal is the continuous measurement of the meridional overturning circulation strength at 26.5°N. To do so, a whole array of measuring buoys were lowered down to the Atlantic ocean floor, each of the buoys longer than several times the Eiffel Tower!

Since then, a scientific crew drives out there, locating the buoys with high accuracy gps and collect the measurements.

The project made possible a whole new understanding of the meridional overturning circulation in the Atlantic.

Before, only point-in-time measurements existed about circulation strength. According to those, the AMOC or THC must have weakened. But now, scientists were able to notice the extremely large variability of the AMOC. This was totally unexpected outcome!

Still, many scientists see a reduction, even in the longterm data (see below). Whether that reduction is significant or just part of the multi-year variability is still unknown.

Still, the project has proven to be reliable in measuring the AMOC over time! Hence, it has been extended twice already since 2004. Let's see whether they extended again!.....

http://www.sams.ac.uk/stuart-cunningham/rapic-moc-mocha

http://www.rapid.ac.uk/rapidmoc/

http://www.rsmas.miami.edu/users/mocha/

The Scare of Rapid Climate Change for our near future…how much is true...

Current global warming has been proven to lie outside the range for the earth’s natural variability (IPCC, 2013). So we know that the state of current warming is likely abnormal and that it might have influences on the Atlantic meridional overturning circulation (AMOC), also known as the thermohaline circulation (THC).

You have seen the melting rates in the Arctic on the bottom of the last post… in coupling with warmer temperatures, waters in the North Atlantic will become fresher and warmer, which reduces their density and slows down deep water formation in the North Atlantic. No more water pushes southward, slowing down the AMOC in return.

Scientists were quite worried about the force of global warming on the AMOC. In 2002,Vellinga & Wood used a HadCM3 model (a coupled ocean-atmosphere model) to investigate the global impact that awaits us, if the AMOC were to shut down. The results are frightening. Within only 20 years, Europe would cool by 1-3°C, and the northwest Atlantic up to 8°C! Even North America and Asia would suffer under cooling of 2°C. The numbers might seem small, but Vellinga & Wood(2002) note that a cooling of >1°C has never been observed since 1659 (the onset of direct air temperature measurements in the UK).

If this gives you an unwell feeling, think about the currently projected warming due to anthropogenic greenhouse gases: 4°C by 2100, if we do not cut our emissions quickly (IPCC, 2013). This implies a warming of 1°C in 20 years, similar to the cooling in the little Ice Age.

In response to the “shut down scare”, many studies investigated the likelihood of current AMOC or THC shut down. However, Stouffer et al. (2006) note that global warming may increase the freshwater input to the North Atlantic, but only by the order of 0.14 Sv (see INFO BOX). To shut down the conveyor, at least 1.0 Sv are needed which is highly unlikely to occur. Similarly, Wood et al. (2003) state a shut down to be highly unlikely under current CO₂ projections.

Model studies are helpful in understanding system behavior. Their problem: models are only as good as the understanding of the system during the time the model was written. Anything we do not know, we cannot imply in a model and cannot reprocess. One such variable are thresholds. As the three modes of the THC show, it is likely that ocean circulation presents threshold behavior. Knutti& Stocker (2002) conclude in their ocean model analysis that today’s models are insufficient for finding the AMOC’s threshold points, mostly due to missing information. Thus, they are unable to surely predict the changes in ocean circulation under climate change scenarios.

Paleoclimatology is unable to help in this case, since CO₂ has not been this high for more than 2 million years…

So the only possibility to get a better insight into the AMOCs behavior is to measure it directly...

…and the RAPID program was born!



http://www.rapid.ac.uk/index.php

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)

The 1 million "currency" question:

The one question everyone is currently worrying about in terms of global warming:

Could our release of greenhouse gases and the current increase in global mean temperatures lead to a shutdown of the THC and would this send the whole of Europe and North America into ice age like conditions?

 

(btw if you think this "freeze" is completely overexaturated.... check out the FUN FACT about Brinacles! ;) )

To investigate this question, we will take a paleoclimatic approach and look back in time to see whether the THC has shut down before, under which circumstances it does this, and what the consequences were for the North Atlantic region.

But then, where do we search? 1000 years ago? 100 000 years ago? 1 million? When the Earth was created??....

Let’s review what we know about the THC (see other posts):

-          It is sensitive to temperature and salt content (deep water production)

-          Heat is transported from south to north via currents

-          Currents may flow very fast

-          The mixing time for the whole world ocean is roughly 1000 years

-          We believe that changes called rapid climate changes are connected to ocean circulation

This gives us the hint that turning off the THC probably will happen on a rather short geological time scale. So luckily, we do not have to travel back to the Earth’s first birthday. However, 1000 years might be too short aswell, since one water drop needs this long to have gone through the whole conveyor belt system. So we expect to find something in the 100 000 year range!

Now we just need data! And if we look around the world, there are an uncountable number of archives that have recorded the last 100 000 years of climate! Especially important are ice cores! (for more information see SCIENCE FACT). Ice cores on Antarctica go back 800 000 years (Luthi et al.,2008)! Greenland ice does not reach back that far, but still enough to cover our period of interest (Svensson et al., 2008). The great thing about ice cores is that little bubbles of air have been trapped in the ice (Alley, 2000). With careful extraction, you can get an air sample from the 400 000 year old atmosphere! You gotta admit... that's pretty cool!

And this is what we are going to look at. Here you see "real" temperature values for the last 100 000 years:

(Ganopolski & Rahmstorf, 2001)

Let me know what you see/feel/think/notice!

Downwelling and deep water formation – what drives the THC

As we have seen in previous posts, ocean water reaches higher densities the colder and saltier it gets. Thus, to find significant areas of downwelling, we have to look for places on the globe, where ocean water is made particularly cold and salty. This leads us to the poles.

Both the Arctic and Antarctic have the potential to cool water to minimal temperatures. In the Antarctic, waters under ice sheets may lose so much heat, that the process is referred to as super-cooling. Through ocean gyres [info box], water is brought to the surface and cooled via convection by releasing heat to the atmosphere. Due to its lower temperature, the water mass will increase its density and sink to the bottom, where it is pushed away from the creation center by following water masses. The conveyor belt is moving.

Simply cooling water down will not lead to particularly dense waters. Parallel to the temperature loss, salinity needs to be increased. This is mainly possible by taking away water, but leaving the salts behind. Hence, the left behind water mass will become more saline.

There are two main processes that will accomplish the above: evaporation and ice formation.

Through evaporation, water will be removed from the oceans and enters the atmosphere as vapor. Since most salt particles are too heavy, they will be left behind. Ice formation leads to a similar process. By freezing ocean water, fresh water is taken out of the water mass, while the salts stay behind. This process is referred to as brine rejection.

Deep water formation in the Antarctic: Antarctic Bottom Water (AABW)

There are several places around the Antarctic continent where deep water formation takes place. The most famous one is the Weddell Sea, where the Atlantic Ocean hits Antarctica. Deep water formation in Antarctica is mainly connected with heat loss and brine rejection. Large year round ice sheets cool the surrounding ocean water to up to minimum temperatures of -2.2°C and increase their salinity by constantly freezing more water. This leads to the AABW being the coldest and densest water mass on earth.

Deep water formation in the Arctic: North Atlantic Deep Water (NADW)

In the Arctic, most downwelling is happening in the Barents Sea, Greenland Sea and Labrador Sea. Here, convection and mixing are the two most important processes. As explained above, gyres transport water to the surface and cool it there. In difference to the Antarctic, the Arctic ice is purely sea ice with no underlying continental mass. Thus, many areas experience a great fluctuation in ice amount with no ice during summer and little during winter. The cold open oceans lead to extreme heat loss (no sea ice that protects the upper water layer) that rapidly cools down water masses. In a complicated mixing process, many different water masses with different densities form the NADW which leaves the Arctic to flow southwards as the Atlantics deep water flow. When it reaches Antarctica, it mixes with the Antarctic Circumpolar Current, which flows all around the South Pole and the Antarctic’s AABW. From there, the new water mass intrudes other ocean basins and connects the Atlantic with other world’s oceans.

You might have noticed that there is no particular process to enhance salinity in the Arctic. Under certain circumstances the mixing of all those different water masses may lead to higher salinity, but the most important process is actually happening long before the water reaches the Arctic: Evaporation of large amounts of water at the equator and the subtropics.

Due to the Hadley Cell and the Coriolis force [info box], these large amounts of water vapor are transported east across the Atlantic and across Middle America. The flat topography of Middle America allows the water masses to be exported straight into the Pacific, which means that the Atlantic loses large amounts of water which are not coming back (Richter & Xie, 2010). The only way to counterattack this water export is by importing fresh water through river outflows. However, looking at the Atlantic, only few large rivers (e.g. the Amazon) enter the Atlantic with significant freshwater inputs. When calculating the difference of input and output, we see that the Atlantic is losing more than it gains. Thus, the water masses flowing northwards become saltier. 

By the time they reach the Arctic deep water formation places, the salt content is high enough to form deep water merely by lowering temperature.

[All of the above named processes are well described in van Aken (2007) The OceanicThermohaline Circulation, An Introduction. Atmospheric and Oceanic SciencesLibrary, Vol. 39 or Talley et al. (2011) Descriptive Physical Oceanography: An Introduction, 6.Ed.]

What drives Ocean Circulation: The Thermohaline Circulation

We were interested in the factors that drive and control Broecker's ocean conveyor belt. The most influencial process is called...

The Thermohaline Circulation (THC):

What is that? If you've come across some latin terms you might have figured out that thermo obviously refers to temperature, while haline refers to salinity. Thus, you have just named the two most important factors that drive the ocean conveyor belt.

Here is how it works:

Density of salty water happens to be positively correlated with salt content and negatively correlated with temperature. This means that the density of cold and salty water is higher than that of warm and fresh water and the less dense water will swim ontop of the denser water.
In the oceans, this phenomenon leads to large water masses being "sorted" by their density, with coldest and saltiest waters on the bottom of the ocean and warmest, freshest waters at the surface (Broecker, 1997).

http://omp.gso.uri.edu/ompweb/doee/science/physical/cipatt1.htm

Are you the kind who doesn't like jumping into the salty ocean during your beach holidays??... well be happy you're not a diver ;)

The upper figure shows that under normal circumstances there is no mixing between the layers. However, imagine you change the density of one water parcel in the corner of the figure by making it colder, then the parcel would sink and push away the last water parcel in the bottom layer which is forced to rise. Due to the rise it heats up slightly and becomes less dense, rising even better. And just like that you end up with a cycle that gets your water layers to rotate. In principle, the ocean conveyor works just like that. At upwelling areas bottom deep water comes to the surface, while downwelling areas surface waters sink to the bottom (Broecker, 1997).

graph produced by the author


To fully understand the process, let's take a trip to the most presigious downwelling areas on Earth: ARCTICA and ANTARCTICA

wooooooo.... you'll need your mittens!

http://www.pinterest.com/tristess/aleut-inuit-pantheon-people-of-the-pacific-northwe/

What is Ocean Circulation and how does it work?

During your last read you have found out that Ocean Circulation may answer your question of how rapid climate changes can occur.In the next few sessions an important and famous scientist will help us understand how Ocean Circulation and Climate are interconnected.

Nothing in nature is static. Everything moves, forms and reforms constantly throughout the geological timescale. Constant movement is also found in the world’s oceans. Due to wind, rain and evaporation (e.g. Richter & Xie, 2010; Clark et al., 2002) water is being moved in, out and within each large mass of water. In certain location this movement becomes directed and forms large ocean currents flowing past landmasses and through continent openings. This whole system consisting of many directed flows of current is referred to as ocean circulation.

In 1982, a scientist by the name of W.S. Broecker published his first idea of what the global ocean circulation may be like. He thought of ocean circulation as a conveyor belt transporting water along a route through all world oceans and back to its original position. This means that oceans throughout the earth are interconnected. Thus, changing the climate in one place will automatically influence the whole Earth.

The Great Ocean Conveyor Belt, taken from W.S. Broecker's "The biggest chill" (1982)

However, as young critical scientists you have to ask Mr. Broecker one particular question to verify his hypothesis: what are the important factors that drive and influence the ocean conveyor belt?

If you have an idea, post your answer below.

Here is the Answer: Abrupt Climate Changes

Last posted question:

What is the difference between warming phases and cooling phases?

Here is the answer:

During cooling phases, the Earth tends to gradually cool down over a period of thousands of years. In contrast, warming phases happen over a few centuries! 

This discovery has put scientists infront of a difficult question: Slow climate changes are explainable with extraterrestrial forcing through the Milankovich cycles, but how do we explain those observed rapid climate changes?

When reconstructing climate, it is important to look not only at one component (eg the atmosphere) or one resource (eg ice cores), but many possible variables.
Thus, let’s have a look at a different climate record that highlights another component: coral reefs in the ocean:

Sea level fluctuation time series reconstructed from coral reef terraces, taken from Siddall et al. (2003)

When looking at past sea level stands we see almost identical fluctuation structures in the ocean compared to the fluctuations of temperature or greenhouse gases in the atmosphere.

Thus, mechanisms for changing climates may not only be found in the atmosphere, but also in the oceans.

And here is the big clue: Ocean Circulation is believed to be one possible mechanism that may trigger these otherwise unexplainable abrupt climate changes!

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

Now what?

Ok now what? 

Have you ever wondered about the actual scientific consensus on the doom stories known about climate change and ocean circulation?

Well you have found the right place! Together with me and many other people, you will find out about the scientific base that lies underneath the ocean circulation theory.

Come by each week and learn about ocean circulation, the conveyor belt model, the threshold hypothesis, future projections and ways of doing climate research.