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

FUN FACTS: Christmas Edition

Where do researchers spend their winters?

There is an essential need to find answers to our climatological questions.

In some cases the location may give your project particular importance.

This beautiful uplifted coral atoll is one of Australia's main research islands. In 2001, Marshall et al. published new data on El Nino/La Nina influences in the Pacific. The sea surface temperatures were reconstructed from corals just off this coast.

The island and its surrounding ocean were identified as one of the world's major marine biodiversity hotspots (Hobbs et al., 2009; Hobbs et al., 2010; Hobbs et al., 2012). 

http://upload.wikimedia.org/wikipedia/commons/f/f1/Whale_shark_Georgia_aquarium.jpg

In the centre of the food web sits the Islands big star:  gecarcoidea natalis   or The Red Crab

Every year, it sets off on an enormous migration to the ocean...

 http://anim.viralnova.com/red-crab-migration/

...where their larvae feed the whole local marine life, including these big guys: Whale sharks (Meekan et al., 2009; Hobbs et al., 2009):

And what does this have to do with christmas? .... Well it all happens on Christmas Island! ;)

http://gpws.com.au/wp-content/uploads/2014/02/Christmas-Island4.jpg

The three modes of the Atlantic overturning circulation

The last 100,000 years of Earth’s climate history show that the THC has flipped between active and inactive states depending on the freshwater input in the Northern Atlantic and the temperature of the Earth. In fact, it seems like the Atlantic circulation switches between three distinctive modes of operation.

• warm mode: strong and active overturning circulation
• cold mode: weakened and slow overturning circulation, with deep water convection sites (the location where surface water is turned into deep water) moved south of the Arctic, somewhere north of Portugal. This leads to less cold and less dense deep water. Thus, it does not sink all the way to the bottom, but rather flows in the intermediate space. The deep current from Antarctica (AABW) can now flow all the way to the north
• Heinrich or “off” mode: the THC is fully shutdown. All deep water is coming from Antarctica and ocean mixing slows down. The result is an ocean with many stratified layers.

(Rahmstorf, 2002)

The first to notice this phenomenon was Stommel in 1961. Since then, many other scientists have accepted and expanded the hypothesis (Broecker et al., 1985; Rahmstorf, 2002).

One discovery was the hysteresis behavior of the Atlantic circulation. This means that changes are not always gradual. Instead, there are moments when only a very small forcing can lead to a big change in THC flow strength. This also means that, after the flow strength has fallen to a minimum, an extremely large backwards-forcing is needed to push the flow strength back to its normal flow rate (Rahmstorf, 1995; Ganopolski & Rahmstorf, 2001).

Figure produced by the blog author

Using a coupled climate model, Ganopolski & Rahmstorf (2001) showed that indeed the flow of North Atlantic Deep Water (NADW) seems to follow the hysteresis loop (see following figure). However, interestingly there seems to be a great difference in the shape of the loop depending on glacial or warm period. During a warm period (which we have today), the “fall” and “rise” of the hysteresis loop are much steeper than during the ice ages. This implies that changes in flow strength today may be much larger than during an ice age.

(a) Hysteresis reaction in the warm period; (b) Hysteresis reaction during the glacial period

Black lines: response for the high latitudes; red lines: response for the low latitudes

Ganopolski & Rahmstorf (2001)

However, as Hu et al. (2012) show, this might not be true. According to their study, the hysteresis effect becomes much greater when the Bering Strait (the Pacific inlet to the Arctic Ocean) is closed. This will only happen during ice ages (unless the continents crash into each other), since then frozen ice sheets will stop the flow. So maybe the hysteresis effect is not as pronounced in our warm world today, after all. To be certain about the hysteresis effect today, more research is needed in the future.

Some new interesting discoveries have been made. As we know the flow strength of the THC is strongly dependent upon the production of deep water in the Arctic. This happens in two main spots: west of Greenland in the Norwegian Sea + surrounding Seas, and east of Greenland in the Labrador Sea. 

http://www.climate.unibe.ch/main/jobs/Master/naoc/schematic_SPG.jpg

You might remember the North Atlantic subpolar gyre from one of the earliest posts. The North Atlantic subpolar gyre is a big counter-clockwise circulating mass of water. It is partially the reason why the Gulf Stream is pushed from the eastern North American coastline to western Europe and further past Iceland into the Greenland Sea. 

Schulz et al. (2007); Jongma et al. (2007)

Schulz etal. (2007) as well as Jongma et al. (2007) found out that deep water production in the Labrador Sea is part of a big feedback-loop. If ice is melting in the Arctic, a strong spinning subpolar gyre (SPG) will send a lot of freshwater to the Labrador Sea, due to its, counterclockwise rotation. This leads to over-freshening of the Labrador Sea and turns off local deep water production. This in turn weakens the whole conveyor belt and also the subpolar gyre. The spin becomes slow and weak and no more freshwater is imported into the Labrador Sea. In turn the subtropical gyre (STG) is now much stronger and pushes high salinity equator water preferably into the Labrador Sea, due to its clockwise rotation. So slowly salinity is restored and deep water convection/production resumes.

In contrast to the upper findings, Thornalley et al. (2009) present evidence from deep sea cores that the subpolar gyre may also buffer possible weakening in circulation strength by transporting salts between the Labrador Sea and the Nordic Seas. So if one spot reduces deep water production, the other may keep it up. However, this is an ongoing field of research and more answers are expected in the future.

To come back to the hysteresis theory, there is one particular question scientists have pondered about: how far is our current anthropogenic climate change pushing the thermohaline circulation? 

Will it never be strong enough to push the THC over the edge (a)? 

What if it can weaken the circulation? Will it be easy to bring it back to present day strength(b)? 

Or will it never be possible again to regain today’s flow speed (c)? 

How likely is the last/worst option?

Hysteresis response example from Stocker & Marchal (2000)

The bifurcation problem (option c) is not as unlikely as you might think. Already Stommel (1961) had spoken of a bifurication point, meaning a passing point of difficult return. Later, Rahmstorf (1995) modeled the NADW (North Atlantic Deep Water) flow response to freshwater input and found a possible “double loop hysteresis” in the high latitudes.

Upper graph: hysteresis loop for high latitudes; lower graph: hysteresis loop for low latitudes

Rahmstorf (1995)

Again the question where are we now and where are we heading?

Then again, Stocker & Marchal (2000) remind us, that model results are just as good as the model that computed them. Many responses of the climate system can be well reconstructed with the current models. However, some problems, such as ice sheet dynamics and cloud cover evolution, remain unresolved (IPCC, 2013).

I want to end this post with this very famous graph from the IPCC (2007):

Models can be overestimating. But we have to always remember, that they can as much underestimate reality. With arctic sea ice declining at such a rapid state… what will happen to our ocean conveyor belt?

Next time… 

Another obscure rapid climate change event: the Younger Dryas Cooling

We have talked about two distinct repetitive abrupt climate changes that are evident in the ice core records over the last 60,000 years: Dansgaard-Oescher (D/O) cycles and Heinrich events. 

However there is one particular cold event in the recent geological past that scientists are not able to identify as either one of them. This is called the Younger Dryas cold event (YD).

(Rahmstorf, 2002)

The YD cold event appears as an interesting rapid climate change event, since it has been detected during a phase of long-term warming. So why did climate all of a sudden drop to nearly ice-age like conditions? The interesting part is that geological evidence strongly suggests first a cooling, then a warming of roughly 4-8 degrees in Greenland within less than 20 years (Barber et al., 1999; Alley et al., 1993; Mayewski et al., 1993; Dansgaardet al., 1989)! 

The YD does indeed deserve the name RAPID climate change…

The most obvious mechanism known so far to drastically change northern hemispheric climate on such a short timescale is again the Atlantic circulation. In 1989, Richard Fairbanks researched coral cores from Barbados and calculated melt water discharge into the Atlantic Ocean from oxygen isotope records in the corals (see Science Fact for more information). He found two distinct events during the YD where freshwater entered at a maximum rate of 14 000 and 9 500 km3/year! To put that in relation: during Heinrich events models had calculated 1.25 million km3 of freshwater release, but in more than 250 years (see last post). That is a maximum rate of only 5000 km3/year. Today, the two big rivers flowing into the Atlantic (Mississippi and St Lawrence rivers) release barely 900 km3/year (Fairbanks,1989).

Hence, it is extremely likely that such a flush event of freshwater would have an effect on the thermohaline circulation in the Atlantic.

The question is what could cause such a sudden release of freshwater in such a short time?

In the late 1980s, Broecker and colleagues concluded that most likely a large freshwater lake emptied into the North Atlantic Ocean (Broecker et al., 1989; Broecker et al., 1988). They knew that the big Laurentide ice sheet on North America had been melting for a while (remember we are actually in a time of climate warming). Due to big moraines in the landscape it was possible to form two large lakes, Lake Agassiz and Lake Ojibway, located near the Great Lake area of today (Barber et al., 1999). Broecker and colleagues noticed that the normal outflow of the lake went southwards through the Mississippi river basin into the equatorial Atlantic. However at the same time of the YD onset, the outflow of the lakes changed, most likely due to a moraine breakdown that had served as a dam. Large amounts of water now catastrophically raced through the Hudson Straight into the North Atlantic (Broecker, 1989). A new study soon to be published in 2015 by Li &Piper shows that the Labrador Current (flowing along the North American coastline) did indeed speed up during the YD event probably due to all the extra freshwater input.

These findings led to a great increase in research about the YD and possible routes which the water could have taken. In 2010, Murton et al. presented evidence for old fluvial layers deposited in the Arctic Ocean. They argue that there was a second important outlet which formed during the same time and enabled large freshwater floods straight into the deep water formation sites. Hence, the Hudson Straight outlet might have been of only secondary importance in providing catastrophic freshwater flushes.

Outflow via Mississippi River and via Hudson Straight (Eastern Outlets) during the Younger Dryas cold event. notice that y-axises are inverted! (Broecker et al., 1988)

Both outflows from Lake Agassiz and Lake Ojibway (eastern & northern red arrows) 

(Murton et al., 2010)

In general, scientists are sure that this event led to a shutdown of the Atlantic circulation (Clark et al., 2001; Rahmstorf, 2002; Barber et al., 1999). Especially the release of freshwater directly into the areas of deep water formation, as it is the case with the Arctic outflow (Murton et al., 2010) will lead to fast reorganisations of  the circulation and stop new deep water formation if the freshwater force is strong enough.

Yet, there are many unresolved questions that scientists are still trying to work out.

One question deals with the mechanisms that can make a rather local disaster a global climate event. Here Rach et al.(2014) just found evidence that there was an obvious delay between climate cooling in Greenland and in Western Europe. This delay may show that not only cooling temperatures, but also changes in wind and rain patterns have influences on terrestrial climate (Rach et al., 2014).

Another question looks at CO₂ evolution during the YD. With the YD being a cold event, one would have expected low CO₂ concentrations. However, Steinthorsdottir et al.(2014) show an abrupt increase, then decrease of CO₂ at the beginning of the YD period suggesting that something had changed in the ocean circulation and forced it to “burp out” a cloud of CO₂. Did that change in ocean circulation maybe influence the YD event? We don’t know yet.

Short review:

After looking at three different distinct abrupt climate change events (Daansgard-Oescher cycles, Heinrich events, the Younger Dryas cooling event), we see that ocean circulation is not necessary the driver, but the amplifier of rapid climate changes. It almost seems like the Atlantic Ocean is operating in several modes... and what about today....?

Keep reading the blog ;)

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 ;)