Heinrich events - why can't we understand them yet?

[aspergers@40]

There's been so much research on the iceberg armada events (Wikipedia Heinrich events) since the classic 1988 paper by Hermut Heinrich himself. There's a host of different data concerning the events, so why is still such a mystery? Here's an excellent introduction to the research carried out in slide show format Paleo Slide Set: Heinrich Events: Marine Record of Abrupt Climate Changes in the Late Pleistocene.

Here's a 2009 report which links the events with abrupt changes in Indonesian waters http://www.agu.org/pubs/crossref/2009/2008PA001653.shtml. What's going on?

 

[Xnn]

Actually, we have a fairly good idea of what happened. In brief, a portion of the ice sheet covering North Eastern Canada near the Hudson strait broke up carrying a large amount of dirt and pebbles out into the nearby ocean. It was significant enough that it affected the sea level and global climate and was initiated by a rather subtle change. A tipping point was reached.

So, no surprise here; Ice sheets constrained by ice shelves are unstable. It doesn't take much and they will fall into to the sea. In fact, we have already recently observed several ice shelves breaking up surprisingly fast. The only difference is that Heinrich are much larger than what we have recently observed.

Now, as with most paleoclimatic data, the precise dating of events is inprecise and unreliable. So, it is never going to be difficult to point out a seeming contradiction in the data. This doesn't mean that we have no clue about what happened. However, it is important to understand these events very well as they provide potential clues to our future.

In particular, the West Antarctic Ice Sheet contains lots of ice constrained by enormous ice shelves. (Look up the size of the Ross Ice Shelf.) So, we know it is unstable and might collapse, thus generating a modern-day Heinrich like event in the Southern Hemisphere. Of course it won't the the same since it's on the opposite side of the World. However, the consequences of such a collapse would be dire: a rapid rise of several meters in sea levels around the world.

So, bottom line. It's not that we don't understand them as much as we need to understand them much better.

 

[aspergers@40]

So, bottom line. It's not that we don't understand them as much as we need to understand them much better.

I agree with this statement Xnn, but I'm not so convinced by your summations. It seems obvious that it is a huge global event, essentially a period of global warming that starts a sequence of events involving ice sheet discharge into the oceans. But what about the small clues:

1. Increased terrigenous runoff in Amazon fan
2. Increased grain size in wind-blown loess in China
3. Changes in relative Thorium-230 abundance, reflecting variations in ocean current velocity

It seems as though the world was generally much wetter, much windier with stronger ocean currents! How do you explain that?

 

[Xnn]

1. Increased terrigenous runoff in Amazon fan
2. Increased grain size in wind-blown loess in China
3. Changes in relative Thorium-230 abundance, reflecting variations in ocean current velocity

It seems as though the world was generally much wetter, much windier with stronger ocean currents! How do you explain that?

It's probably true relative to what it was before the event.

But again, be careful about Paleoclimate data as it is typically inprecise and unreliable.
Also, Heinrich events are from a particular ice shelve/sheet that broke up and reformed.
Response to a collapse of WAIS is unlikely to be exactly the same.

 

[aspergers@40]

It's probably true relative to what it was before the event.

Right, so what caused this then? An increase in insolation would do the job; more sunshine -> more evaporation -> more ppt + more wind. What are the other alternatives?

 

[Skyhunter]

Right, so what caused this then? An increase in insolation would do the job; more sunshine -> more evaporation -> more ppt + more wind. What are the other alternatives?

Since Heinrich and Dansgaard/Oeschger events, and the associated temperature shifts, are localized events, The effect on global climate is not as extreme. Increased insolation would have more of a global impact. It can also be ruled out by beryllium-10 isotopic analysis.

The theory with the greatest acceptance is that the glaciers grow until they reach a critical mass, at which point they discharge the accumulated ice into the Atlantic where it is picked up by the currents. The ice melts in the North Atlantic depositing refrigerator size boulders on the sea bed. The discharge of fresh water causes a slowdown or stopping of the Atlantic current.

This theory explains the ice rafted debris, the cyclical nature of the events, the abrupt cooling during the event, the rapid warming following the event, and the fact that these events only seem to occur during glacial epochs.

 

[Xnn]

This theory explains the ice rafted debris, the cyclical nature of the events, the abrupt cooling during the event, the rapid warming following the event, and the fact that these events only seem to occur during glacial epochs.

Right.

Strictly speaking Heinrich events involved the breakup of a particularly large ice shelf near the Hudson strait that forms during glacial periods. It lasted about 700 years. Since that ice shelf is long gone, strictly speaking it is not going happen again any time soon.

However, Antarctica has plenty of large ice shelves that are in the process of breaking up in a similar fashion right now. Just last week, an iceburg the size of Rhode Island broke free from the Wendall (Filchner-Ronne) shelf. Rhode Island is about 1% of the size of that ice shelf.

Ice burgs from earlier breakups have been sighted as far away as New Zealand (8000 miles).
More study will be needed to determine if this is the harbinger of a modern day "Heinrich" event or not. However, simple math shows that the Wendall (Filchner-Ronne) is breaking up at a rate comparable to such an event.

 

[aspergers@40]

Since Heinrich and Dansgaard/Oeschger events, and the associated temperature shifts, are localized events, The effect on global climate is not as extreme. Increased insolation would have more of a global impact..

Let's start at the beginning. The Heinrich events and D-O events ARE part of a global climate change, and are NOT just local (re-read the OP w.r.t to the Indonesian paper). Wikipedia:

The global extent of these records illustrates the dramatic impact of Heinrich events.

 

[Xnn]

Here's a quote from the paper:

global slowdown in thermohaline circulation during Heinrich events triggered by Northern Hemisphere cooling

So, basically what happens is that a slight amount of warming from a Dansgaard type event triggers the breakup of the large ice shelf near the Hudson Strait. This in tern allows all the glaciers behind it to flow into the sea at an accelerated rate. The result (over about 700 years) is a large influx of icebergs and fresh water into the North Atlantic that cools the Northern Hemisphere which in tern disrupts the thermohaline circulation on a global scale.

 

[aspergers@40]

Here's a quote from the paper:

So, basically what happens is that a slight amount of warming from a Dansgaard type event triggers the breakup of the large ice shelf near the Hudson Strait. This in tern allows all the glaciers behind it to flow into the sea at an accelerated rate. The result (over about 700 years) is a large influx of icebergs and fresh water into the North Atlantic that cools the Northern Hemisphere which in tern disrupts the thermohaline circulation on a global scale.

No, I don't agree on this point. What kind of temperature increase are you talking about, and what is the supposed mechanism? It's more intuitive to assume a relative amount of warming would lead to the same relative amount of cooling isn't it? Look at thumbnail #3 in the OP for more info.

 

[Andre]

Note that the relationship between Heinrich events and Dansgaard Oesgcher events is not that unambiguous:

http://www.ncdc.noaa.gov/paleo/abrupt/images/data3-gisp2-icecore.gif

source

 

[Skyhunter]

Let's start at the beginning. The Heinrich events and D-O events ARE part of a global climate change, and are NOT just local (re-read the OP w.r.t to the Indonesian paper). Wikipedia:

Any change in the THC would have global impacts on climate. But the impact would be amplified for the North Atlantic, as is evidenced in the GISP cores.

 

[Andre]

But it gets more complex when the Dansgaard Oeschger events also show up very distincively in the pacific as oxic-anoxic oscilations:

http://www.nature.com/nature/journal/v379/n6562/abs/379243a0.html

 

[aspergers@40]

But it gets more complex when the Dansgaard Oeschger events also show up very distincively in the pacific as oxic-anoxic oscilations:

http://www.nature.com/nature/journal/v379/n6562/abs/379243a0.html

Quite right Andre. The evidence quite clearly shows that the Antarctic also experienced D-O events, but the significance was a lot less; not even being noticeable the first time the cores were analysed. The situation is very different in the Greenland ice cores, showing a bigger effect. It seems reasonable to suggest a global warming of around a few degrees, but a local amplification mechanism has resulted in the Arctic polar region to become around 5 degrees warmer. What do you think?

 

[aspergers@40]

Any change in the THC would have global impacts on climate. But the impact would be amplified for the North Atlantic, as is evidenced in the GISP cores.

What are you proposing as the initial mechanism for the change in the THC though, and why with such a regular pulse?

Wikipedia: Dansgaard-Oeschger events:

Effect
In the Northern Hemisphere, they take the form of rapid warming episodes, typically in a matter of decades, each followed by gradual cooling over a longer period. For example, about 11,500 years ago, averaged annual temperatures on the Greenland icepack warmed by around 8°C over 40 years, in three steps of five years (see Alley (2000), Stewart, chapter 13) - 5°C change over 30-40 yrs more common.

Heinrich events only occur in the cold spells immediately preceding D-O warmings, leading some to suggest that D-O cycles may cause the events, or at least constrain their timing (Bond & Lotti 1995).

The course of a D-O event sees a rapid warming of temperature, followed by a cool period lasting a few hundred years (Bond et al.. 1999). This cold period sees an expansion of the polar front, with ice floating further south across the North Atlantic ocean (Bond et al.. 1999).

Also, rather interestingly, it quietly states:

The little ice age of ~400 to 200 years ago has been interpreted as the cold part of a D-O cycle, putting us (even without the effects anthropogenic global warming) in a period of warming climate (Bond et al.. 1999).

 

[Skyhunter]

What are you proposing as the initial mechanism for the change in the THC though, and why with such a regular pulse?

I am not proposing anything, just advancing the leading theory.

If I were to speculate based on the evidence I've seen, I would say that because the growth of the ice sheets is fairly constant, varies from year to year, but over the centuries it balances out. Once the sheets reach a critical mass, an as yet poorly understood flux in the solar cycle triggers a massive discharge of fresh water into the North Atlantic. This pulse of fresh water disrupts the thermohaline circulation, causing it to slow down, tropical waters get warmer because the heat is no longer being carried to higher latitudes. When the THC resumes, there is a rapid rise in temperature in the North Atlantic as the warmer than usual tropical waters are transported north..

This is what I suspect is the cause of the D-O events.

When the edge of the sheet collapses entirely during a D-O event,we see the ice rafted debris associated with Heinrich events.

But that is just my amateur musings.

 

[aspergers@40]

I am not proposing anything, just advancing the leading theory.

If I were to speculate based on the evidence I've seen, I would say that because the growth of the ice sheets is fairly constant, varies from year to year, but over the centuries it balances out. Once the sheets reach a critical mass, an as yet poorly understood flux in the solar cycle triggers a massive discharge of fresh water into the North Atlantic. This pulse of fresh water disrupts the thermohaline circulation, causing it to slow down, tropical waters get warmer because the heat is no longer being carried to higher latitudes. When the THC resumes, there is a rapid rise in temperature in the North Atlantic as the warmer than usual tropical waters are transported north..

This is what I suspect is the cause of the D-O events.

When the edge of the sheet collapses entirely during a D-O event,we see the ice rafted debris associated with Heinrich events.

But that is just my amateur musings.

Okay, so we agree on the general gist of things i.e. binge-purge model. The speculated additional local Arctic polar warming could be due to the loss of summer sea-ice imo. The loss of albedo would result in less sunlight being reflected back into space. This then would be the reason for heavy northern hemisphere iceberg discharge compared to light southern hemisphere iceberg discharge.

Quite rightly, the problem now seems to boil down to what causes D-O warming events. As you mentioned "an as yet poorly understood flux in the solar cycle triggers.." I was wondering whether you were referring to an increase in the radiation output from the Sun?

 

[Saul]

Reduced solar activity as a trigger for the start of the Younger Dryas?

http://cio.eldoc.ub.rug.nl/root/2000/QuatIntRenssen/

This paper is a good review of the data concerning the Younger Dryas. It is interesting to look at the development of the different hypotheses and mechanisms from a scientific historical standpoint as well as pure science. The authors postulated TSI variance mechanism is not correct.

The Thermal Haline Conveyor (THC) interruption theory postulated by Wally Broeker has been shown to be incorrect.

The THC does not flow as a tight conveyor.

The melt water pulse that was hypothesized to abruptly shutdown the North Atlantic Drift current occurred a 1000 years before the Younger Dryas event. (The timing of the melt water pulse is discussed in this paper.)

Seager et al showed planetary temperature affects associated with a complete interruption of the North Atlantic drift current is around a factor of 5 too small to account for the Younger Dryas Cooling.

The binge purge ice sheet theory has been shown to be incorrect. The ice sheets have been shown to simultaneously rush into the ocean. The ice sheets are geographically separate. There is no mechanism to regulate the building up of the ice sheets from geographical separate ice sheets.

 

[Saul]

This paper by Svensmark discusses the Polar See Saw. The Antarctic ice sheet warms repeatedly when the Greenland Ice Sheet cools and visa versa. (The paper includes ice core temperature from the Greenland and Antarctic ice sheets which shows the temperature changes on the two ice sheets are changing cyclically and 180 degrees out of phase. I have seen a number of papers that discuss a 1470 year cycle, 23 cycles counted.)

Svensmark postulates that the mechanism is cyclic changes to GCR, modulate by the solar magnetic cycle changes. The albedo of the Antarctic Ice Sheet is higher than clouds (he include ERBE data and a calculate of the specific amount) so an increase in cloud cover will cause the Antarctic Ice Sheet to warm rather than cool. The situation is different for the Greenland Ice Sheet as it is not isolated from the cooling and warming surround ocean by polar vortex. (The Antarctic Ice sheet is isolated from the surrounding Southern Hemisphere ocean by a polar vortex.)

The Younger Dryas appears to a special event that follows the solar magnetic cycle change that causes the warming. The Younger Dryas cooling was too abrupt and too long to be due to solar magnetic cycle modulate of GCR.

Did anyone notice the Greenland Ice Sheet was surging and has now abruptly stopped?

http://arxiv.org/abs/physics/0612145v1

The Antarctic climate anomaly and galactic cosmic rays

Borehole temperatures in the ice sheets spanning the past 6000 years show Antarctica repeatedly warming when Greenland cooled, and vice versa (Fig. 1) [13, 14]. North-south oscillations of greater amplitude associated with Dansgaard-Oeschger events are evident in oxygenisotope data from the Wurm-Wisconsin glaciation[15]. The phenomenon has been called the polar see-saw[15, 16], but that implies a north-south symmetry that is absent. Greenland is better coupled to global temperatures than Antarctica is, and the fulcrum of the temperature swings is near the Antarctic Circle. A more apt term for the effect is the Antarctic climate anomaly.


Attempts to account for it have included the hypothesis of a south-flowing warm ocean current crossing the Equator[17] with a built-in time lag supposedly intended to match paleoclimatic data. That there is no significant delay in the Antarctic climate anomaly is already apparent at the high-frequency end of Fig. (1). While mechanisms involving ocean currents might help to intensify or reverse the effects of climate changes, they are too slow to explain the almost instantaneous operation of the Antarctic climate anomaly.


Figure (2a) also shows that the polar warming effect of clouds is not symmetrical, being most pronounced beyond 75◦S. In the Arctic it does no more than offset the cooling effect, despite the fact that the Arctic is much cloudier than the Antarctic (Fig. (2b)). The main reason for the difference seems to be the exceptionally high albedo of Antarctica in the absence of clouds.

 

[Saul]

There is a set of large burn marks that are distributed throughout the Northern Hemisphere that have been dated to coincide with the Younger Dryas abrupt cooling event.

Firestone et al. postulated that the Younger Dryas abrupt cooling event was caused by an extraterrestrial impact. That does make sense as an extraterrestrial impact will only cool the planet for a few years. The Younger Dryas event cooled the planet for a 1000 years. (Planet when from interglacial warm back to almost glacial cold in less than 10 years.)

The extra terrestrial impact event also does not make sense as there were no craters left at the multiple burn mark sites.

This recent paper re-examined the concurrent multiple large burn marks and found no evidence of extraterrestrial micro particles.

http://www.pnas.org/content/106/43/18155

An independent evaluation of the Younger Dryas extraterrestrial impact hypothesis

Based on elevated concentrations of a set of “impact markers” at the onset of the Younger Dryas stadial from sedimentary contexts across North America, Firestone, Kennett, West, and others have argued that 12.9 ka the Earth experienced an impact by an extraterrestrial body, an event that had devastating ecological consequences for humans, plants, and animals in the New World [Firestone RB, et al. (2007) Proc. Natl. Acad. Sci. USA 104:16016–16021]. Herein, we report the results of an independent analysis of magnetic minerals and microspherules from seven sites of similar age, including two examined by Firestone et al. We were unable to reproduce any results of the Firestone et al. study and find no support for Younger Dryas extraterrestrial impact.

 

[Saul]

http://www.agu.org/pubs/crossref/2003/2003GL017115.shtml

This paper by Stefan Rahmstorf is one of the papers that notes the abrupt climate events follow a regular cycle which points to external forcing mechanism.

Timing of abrupt climate change: A precise clock by Stefan Rahmstorf

Many paleoclimatic data reveal a approx. 1,500 year cyclicity of unknown origin. A crucial question is how stable and regular this cycle is. An analysis of the GISP2 ice core record from Greenland reveals that abrupt climate events appear to be paced by a 1,470-year cycle with a period that is probably stable to within a few percent; with 95% confidence the period is maintained to better than 12% over at least 23 cycles. This highly precise clock points to an origin outside the Earth system; oscillatory modes within the Earth system can be expected to be far more irregular in period.