Equilibrium Climate Sensitivity

[chubbs]

I'm a little lost.  The other GHGs' absorb energy at different wavelengths, so I don't see how they are interchangeable (if that's what the point was).  Rather I thought the other GHG's were feedback mechanisms that were already included in the TCR.  Its been the feedback mechanisms that I considered to be the least understood and weakest link in GCM.

Let me try to clarify. Many other GHG are increasing due to mans activities: CH4, N2O, CFCs (now decreasing), HFCs, SF6 and many additional minor gases.These gasses are emitted during combustion and many other activities including industry, farming, refiigeration, fire suppression etc. While these gases may absorb in different IR bands than CO2 they are all well mixed in the atmosphere like CO2 and their.overall ghg effect is similar. The increase in these gasses is not feedback from increasing CO2 but a direct result of man's various actvities. The impact of these gases is often converted to a CO2-equivalent basis  to put all the GHG on a common basis using the radiative properties of each gas. As discussed above currently all man-made GHG have ghg forcing equivalent to a concentration of CO2 alone of 478 ppm. When evaluating TSR or ESS using historical data or when projecting future impacts these gasses need to be accounted for since their impact can not be separated from CO2. Only in models can CO2 be varied independently of the other gasses.

[StudentOfClimatology]

At this stage, I'd put climate sensitivity at 3.5-4K, based on isotope analysis covering the Pleistocene...ESC cannot possibly be determined due to the presence of harmonic thresholds. Look at large global events like the Younger Dryas, for example. Rapid swings like these are found throughout the Holocene, as well...without any obvious trigger..

I personally doesn't understand the linear projections in either TCR or ESC. Paleoclimatological evidence suggests the system is highly non-linear, and that sensitivity and/or resonance-governed feedback to CO^2 will vary based on large scale boundary state(s) that may be governed by long-term orbital parameters..

"Dumbing down" the climate system to a simple cause-effect operation will never work..

[chubbs]

At this stage, I'd put climate sensitivity at 3.5-4K, based on isotope analysis covering the Pleistocene...ESC cannot possibly be determined due to the presence of harmonic thresholds. Look at large global events like the Younger Dryas, for example. Rapid swings like these are found throughout the Holocene, as well...without any obvious trigger..

I personally doesn't understand the linear projections in either TCR or ESC. Paleoclimatological evidence suggests the system is highly non-linear, and that sensitivity and/or resonance-governed feedback to CO^2 will vary based on large scale boundary state(s) that may be governed by long-term orbital parameters..

"Dumbing down" the climate system to a simple cause-effect operation will never work..

 

The plot posted above indicates that so far the relationship between increased GHG and GISS ST has been linear. This doesn't rule out non-linear behavior in the future.

[nflwxman]

The plot posted above indicates that so far the relationship between increased GHG and GISS ST has been linear. This doesn't rule out non-linear behavior in the future.

Thank you for the excellent linear regression above with GHG.  I think linear behavior will generally occur unless there is some type of major tipping point passed (that I believe SOC is referring to).   A few points:

 

-The loss of arctic sea ice is actually only adding a very small amount of heat to the system, thus it's impact will probably appear linear.

-Wildfires, ocean acidification, and other carbon releasing sources will also appear linear due to system inertia.

-Perhaps a major collapse of an ice sheet or methane release will cause a non-linear relation with GHGI.  

 

SOC, what has caused non-linear behavior in the past aside from orbital changes?

[ORH_wxman]

Thank you for the excellent linear regression above with GHG.  I think linear behavior will generally occur unless there is some type of major tipping point passed (that I believe SOC is referring to).   A few points:

 

-The loss of arctic sea ice is actually only adding a very small amount of heat to the system, thus it's impact will probably appear linear.

-Wildfires, ocean acidification, and other carbon releasing sources will also appear linear due to system inertia.

-Perhaps a major collapse of an ice sheet or methane release will cause a non-linear relation with GHGI.  

 

SOC, what has caused non-linear behavior in the past aside from orbital changes?

 

 

The Younger Dryas was likely due to a mass exodus of fresh water into the North Atlantic with the melting of the Laurentide Ice Sheet...recommencing another ice age for about 1,000 years. Though this has often been debated in the literature.

 

The other is super volcanoes. Toba 74,000 years ago almost put humans extinct.

 

I'm sure there's others that I'm forgetting and some we don't really understand yet.

[StudentOfClimatology]

The Younger Dryas was likely due to a mass exodus of fresh water into the North Atlantic with the melting of the Laurentide Ice Sheet...recommencing another ice age for about 1,000 years. Though this has often been debated in the literature.

This is actually an outdated hypothesis that is now mostly fueled by the media and blogosphere...though some (very vocal and influential) scientists still cling to it like gospel. It was once a mainstream hypothesis, until the sharp cooling was found across the tropical domain, well into the Southern Hemisphere, as our methods of proxy extraction and interpolation have improved dramatically. This is discussed heavily in the latest literature...the AMOC cannot explain the YD...in fact you'd expect a warming within the tropical domain-space under the influence of slowing meridional/overturning ocean cells.

The other is super volcanoes. Toba 74,000 years ago almost put humans extinct.

Volcanism may indeed have been an initial trigger to a cascade of feedbacks. But this is still highly speculative and also supports the existence of non-linear "thresholds".

I'm sure there's others that I'm forgetting and some we don't really understand yet.

I agree. I think sometimes we tend to "drum up" forcings that appear simplistic and/or easy to model within a given set of parameterizations. Problem is even our modeling works within a scale of inter-working singularities...which may not be true in reality, based on what we know about climate change of the past..

[StudentOfClimatology]

The plot posted above indicates that so far the relationship between increased GHG and GISS ST has been linear. This doesn't rule out non-linear behavior in the future.

Casting judgement based on 50-70 years of data will, in my view, lead to inevitable failure. Paleoclimatological evidence is, at this point, is extensive enough that we can fully discount a linear relationship between forcings and internal feedbacks. Of course, this is a problem that we cannot (and may never be able to) adequately model, so there is resistance to this new reality, and any divergence between modeling and observation is "fixed" through tuning preferred external forcing parameters.

What we've seen in this regard, so far, has been minor and easily "explainable", via current knowledge. However, this will not be the case forever. Soon, we will be forced to realize it...the question is, when?

[ORH_wxman]

This is actually an outdated hypothesis that is now mostly fueled by the media and blogosphere...though some (very vocal and influential) scientists still cling to it like gospel. It was once a mainstream hypothesis, until the sharp cooling was found across the tropical domain, well into the Southern Hemisphere, as our methods of proxy extraction and interpolation have improved dramatically. This is discussed heavily in the latest literature...the AMOC cannot explain the YD...in fact you'd expect a warming within the tropical domain-space under the influence of slowing meridional/overturning ocean cells.

 

 

 

So what is the latest theory on the Younger Dryas?

 

There was an impact theory I remember a few years ago, but that seemed to be dismissed.

[StudentOfClimatology]

So what is the latest theory on the Younger Dryas?

There was an impact theory I remember a few years ago, but that seemed to be dismissed.

That's the problem...we don't have one. One of my peers has dedicated her doctoral thesis to this question because it's essentially open-season now. I'm definitely not smart enough to take this on..haha.

The impact theory can be discounted as a clear dating mismatch was discovered amongst the most important 11 proxy sites containing black-mat deposits. The one interesting fact we have uncovered is a shift in the levels of several isotopes (namely beryllium and CE) which occurred at both poles simultaneously and preceded the YD by 150-400yrs..suggesting a large excursion in the magnetic field. However, while this would explain the nano-diamonds in the samples, it is probably unrelated to the YD as we have no mechanism to connect the two..at least directly.

[sokolow]

I guess I had thought that the freshwater melt hypothesis was not only still plausible, but also still retains its status as most likely forcing mechanism for the YD cold event? IDK if there's a good broad field review that summarizes the state of the field (your friend's lit review?)

[bluewave]

I guess I had thought that the freshwater melt hypothesis was not only still plausible, but also still retains its status as most likely forcing mechanism for the YD cold event? IDK if there's a good broad field review that summarizes the state of the field (your friend's lit review?)

 

The latest  that I read was that it was Greenland melt which was responsible and not Canadian melting.

 

http://sciencenordic.com/greenland-icebergs-may-have-triggered-younger-dryas

 

Now the latest research indicates that it was not meltwater from Canada that triggered the  Younger Dryas cooling (see Factbox). According to a new study, this was caused by icebergs and meltwater from Greenland.

 

Here's another study on melting leading to abrupt climate change:

 

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3856815/

 

 

roc Natl Acad Sci U S A. Dec 3, 2013; 110(49): 19713–19718.

Published online Nov 18, 2013. doi:  10.1073/pnas.1304912110

PMCID: PMC3856815

Earth, Atmospheric, and Planetary Sciences

Spontaneous abrupt climate change due to an atmospheric blocking–sea-ice–ocean feedback in an unforced climate model simulation

Sybren Drijfhout,^a,b,1 Emily Gleeson,^c Henk A. Dijkstra,^d and Valerie Livina^e

Author information ► Copyright and License information ►

 

Go to:

SIGNIFICANCE

There is a long-standing debate about whether climate models are able to simulate large, abrupt events that characterized past climates. Here, we document a large, spontaneously occurring cold event in a preindustrial control run of a new climate model. The event is comparable to the Little Ice Age both in amplitude and duration; it is abrupt in its onset and termination, and it is characterized by a long period in which the atmospheric circulation over the North Atlantic is locked into a state with enhanced blocking. To simulate this type of abrupt climate change, climate models should possess sufficient resolution to correctly represent atmospheric blocking and a sufficiently sensitive sea-ice model.

Keywords: climate modeling, thermohaline circulation, Great Salinity Anomaly

 

[StudentOfClimatology]

I guess I had thought that the freshwater melt hypothesis was not only still plausible, but also still retains its status as most likely forcing mechanism for the YD cold event? IDK if there's a good broad field review that summarizes the state of the field (your friend's lit review?)

She hasn't been reviewed yet so I can't say too much attm. Completion and commencement probably this coming Spring

The freshwater hypothesis was initially theorized to explain the cooling over the North Atlantic Region(s), specifically. However, it has been subsequently discovered that the cooling was largely global and synchronous in nature, precluding the North Atlantic as an initial forcing. Plus you would expect a warming over the tropical Atlantic under than scenario, which is not observed.

This is one of the first papers to take note of the shift in the production of various isotopes:

http://m.geology.gsapubs.org/content/23/10/877.short

[sokolow]

The latest that I read was that it was Greenland melt which was responsible and not Canadian melting.

Thanks. I hadn't seen that particular one but FWIW I always wondered if the fixation on Lake Agassiz was some kind of reverse Bretz, where he had a channeled scabland evidencing massive outburst floods, but no source for the water until they reconstructed Missoula -- and with Agassiz here we have a lake but disputed evidence for the location of its outlet(s) and / or breach. So the fixation is building in the antiholistic assumption that a catstrophic, sudden eastward draining of Agassiz is the only mechanism of freshwater injection.

She hasn't been reviewed yet so I can't say too much attm. Completion and commencement probably this coming Spring

Congrats to your friend in advance!

The freshwater hypothesis was initially theorized to explain the cooling over the North Atlantic Region(s), specifically. However, it has been subsequently discovered that the cooling was largely global and synchronous in nature, precluding the North Atlantic as an initial forcing. Plus you would expect a warming over the tropical Atlantic under than scenario, which is not observed.

This is one of the first papers to take note of the shift in the production of various isotopes: http://m.geology.gsapubs.org/content/23/10/877.short

I guess I don't know I'd found an arguement for synchroneity of cooling off of moraine radiocarbon and surface exposure dating of valley glacier fluctuations

[StudentOfClimatology]

Thanks. I hadn't seen that particular one but FWIW I always wondered if the fixation on Lake Agassiz was some kind of reverse Bretz, where he had a channeled scabland evidencing massive outburst floods, but no source for the water until they reconstructed Missoula -- and with Agassiz here we have a lake but disputed evidence for the location of its outlet(s) and / or breach. So the fixation is building in the antiholistic assumption that a catstrophic, sudden eastward draining of Agassiz is the only mechanism of freshwater injection.

Congrats to your friend in advance!

I guess I don't know I'd found an arguement for synchroneity of cooling off of moraine radiocarbon and surface exposure dating of valley glacier fluctuations

At this point, it's nearly impossible to deny the global nature of the Younger Dryas. We now have evidence of it all the down to the Southern Ocean:

http://www.sciencedirect.com/science/article/pii/S0012821X03005569

Both records demonstrate the existence of cooling events in the Southern Hemisphere, which are synchronous with the Northern Hemisphere Younger Dryas cold reversal (between 12.9 and 11.5 kyr BP). Such evidence for the spatial distribution and timing of abrupt climatic fluctuations is essential data for groundtruthing results derived from global climate models.

It is also observed in Africa, Australia, and South America:

http://www.nature.com/nature/journal/v366/n6451/abs/366146a0.html

THE last deglaciation was interrupted by an abrupt cooling event, the Younger Dryas, at 11,000 & 10,000 yr BP (uncalibrated radiocarbon timescale). Originally recognized in climate records from northwest Europe, the Younger Dryas has now been identified in marine and ice-core records worldwide. In the tropics, a broadly contemporaneous change in climate is recorded by decreases in water levels and increased salinity of lakes, indicating a period of arid climate caused by a reduction in ocean-to-land moisture flux. The exact timing of these changes in relation to the Younger Dryas event in high-latitude records has remained unclear, however. Here we present climate records based on analyses of diatom assemblages, geochemistry and magnetic mineralogy of radiocarbon-dated sequences of laminated lake sediments from Lake Magadi in the East African rift. These records provide a detailed record of climate change in lowland equatorial Africa throughout the last deglaciation (12,800 & 10,000 14C yr BP). We find that lake-level and humidity maxima coincide with the most rapid phases of ice melting in the Northern Hemisphere, and that the climate changes, including the Younger Dryas event, were synchronous at low and high latitudes. Thus, the effects of abrupt climate change appear to be felt at both high and low latitudes without a significant time lag.

Once the cooling and subsequent warming was discovered in worldwide equatorial climates as well as throughout the Southern Hemisphere, the idea of a local, North-Atlantic based cooling was no longer plausible

I honestly don't know why people cling to that explanation..it makes zero sense

[sokolow]

I guess my comment there is given a YD signal in the Andres core that's not just "broadly contemporaneous" or "synchronous within the limitations of the dating method employed" but indeed a temporally well-resolved regionally representative YD signal with necesary magnitude and diachronic "shape" that is highly synchronous with YD records in the northern hemisphere isn't necessarily a deathblow to a North Atlantic meltwater injection framework -- that doesn't invalidate the causal chain.

And AFAIK theirs is one of relatively few such southern hemisphere paleorecords, such that marine records from offshore NZ and Oz are open to alternative explanation and sometimes standing in complex relation to terrestrial records. My impression was that "complex" is the word for southern hemisphere YD temperature signals in terms of chronology, magnitude, shape, and regional coherency -- that's my impression because the NZ glacial complex the Andres team cites has in years since proved to be a giant pain in the butt, datingwise, and it would be nice to have a check to bring back to chronologies of glacial fluctuations in the Southern Alps.

[StudentOfClimatology]

Some of the older papers from the 80s/90s especially could not find synchronicity or were skeptical of it..all this for various reasons. Our techniques have improved since then, and several miscalibrations have since been realized. The most recent literature essentially confirms the global nature of the YD...paleoclimate is the focus of my postgraduate studies..I wouldn't speak with such certainty if I did not believe it to be true.

As for the North Atlantic..it can't be ruled out 100% (and may have amplified the temperature swings in that domain). However, if it were the cause, then you'd expect a subsequent warming of the tropical Atlantic. A decrease in the efficiency of meridional overturning cell(s) requires a reduction in heat transport and exchange. We have several cases where the AMOC did slow down, and the proxies reflect the expected temperature profile.

[StudentOfClimatology]

I just want to say that, yes, there were significant changes in oceanic-atmospheric dynamics over the North Atlantic domain during the YD. However, the same can be said for most of the planet and it's not clear what the initial trigger was.

That said, if you're interested in the North Atlantic, this recent study is a good read. It investigates the chicken-egg problem, and explains how the cooling may have been amplified over the North Atlantic due to shifting wind patterns:

http://www.nature.com/ngeo/journal/v1/n8/full/ngeo263.html

The Younger Dryas cooling 12,700 years ago is one of the most abrupt climate changes observed in Northern Hemisphere palaeoclimate records. Annually laminated lake sediments are ideally suited to record the dynamics of such abrupt changes, as the seasonal deposition responds immediately to climate, and the varve counts provide an accurate estimate of the timing of the change. Here, we present sub-annual records of varve microfacies and geochemistry from Lake Meerfelder Maar in western Germany, providing one of the best dated records of this climate transition. Our data indicate an abrupt increase in storminess during the autumn to spring seasons, occurring from one year to the next at 12,679 yr BP, broadly coincident with other changes in this region. We suggest that this shift in wind strength represents an abrupt change in the North Atlantic westerlies towards a stronger and more zonal jet. Changes in meridional overturning circulation alone cannot fully explain the changes in European climate. We suggest the observed wind shift provides the mechanism for the strong temporal link between North Atlantic Ocean overturning circulation and European climate during deglaciation.

[sokolow]

Some of the older papers from the 80s/90s especially could not find synchronicity or were skeptical of it..all this for various reasons. Our techniques have improved since then, and several miscalibrations have since been realized. The most recent literature essentially confirms the global nature of the YD...paleoclimate is the focus of my postgraduate studies..I wouldn't speak with such certainty if I did not believe it to be true.

Sure! I believe you -- my only angle is "global how" and "global how much" matter a lot, for land ice, especially given the potential for bizarre precipitation regimes. Conversely, with the potential for strong regional variability its worth caution trying to interpret glacial fluctuations, and also worth caution when (for example, not that we do this itf) taking climate into account in discussing human migration, changes in tool & resource use, &c.

As for the North Atlantic..it can't be ruled out 100% (and may have amplified the temperature swings in that domain). However, if it were the cause, then you'd expect a subsequent warming of the tropical Atlantic. A decrease in the efficiency of meridional overturning cell(s) requires a reduction in heat transport and exchange. We have several cases where the AMOC did slow down, and the proxies reflect the expected temperature profile.

I guess I was curious because none of the mainstream stuff I'd been reading made it sound that grim for North Atlantic crew; I had the impression the catch theory- obs- and modelwise wasn't tropical atlantic temps, but lack of a distinct outlet for the proposed Laurentide drainage reorganization. I'd come away with the idea that while the question is still open & intriguing, if researchers could give a plausible account for drainage its not just Broecker himself, but a large body of evidence & consensus that would end up, as per Richard Alley's bluntly titled 2007 paper, coming around to saying "Wally Was Right." As in the meltwater injection framework isn't by any means marginal -- it has a strong evidential & theoretical underpinning with useful explanatory & predictive power.

[sokolow]

At any rate if Broeckerland has been definitively discredited I'd be interested to read the gory details & postmortem just for the interest of it, but also b/c more than a few people I know in the paleoanth zone are still running around believing that it's credible and plausible, and how much fun is it to mess with your friends & colleagues

[StudentOfClimatology]

Sure! I believe you -- my only angle is "global how" and "global how much" matter a lot, for land ice, especially given the potential for bizarre precipitation regimes. Conversely, with the potential for strong regional variability its worth caution trying to interpret glacial fluctuations, and also worth caution when (for example, not that we do this itf) taking climate into account in discussing human migration, changes in tool & resource use, &c.

I agree 100% here. Hopefully we'll get more answers as we continue to educate ourselves

I guess I was curious because none of the mainstream stuff I'd been reading made it sound that grim for North Atlantic crew; I had the impression the catch theory- obs- and modelwise wasn't tropical atlantic temps, but lack of a distinct outlet for the proposed Laurentide drainage reorganization. I'd come away with the idea that while the question is still open & intriguing, if researchers could give a plausible account for drainage its not just Broecker himself, but a large body of evidence & consensus that would end up, as per Richard Alley's bluntly titled 2007 paper, coming around to saying "Wally Was Right." As in the meltwater injection framework isn't by any means marginal -- it has a strong evidential & theoretical underpinning with useful explanatory & predictive power.

The theory was very convincing to many, because there definitely was a large shift in circulation over & within the Atlantic during the YD. The Atlantic domain has been a climactic madhouse for millions of years, so many scientists initially assumed all of these changes were internal to the North Atlantic.

Now, many think that while the Atlantic system is obviously unstable and represents a regionally-based feedback amplification, the idea that it is responsible for the YD cooling across the globe (including the tropical Pacific a and Southern Hemisphere) does not seem plausible.

[StudentOfClimatology]

At any rate if Broeckerland has been definitively discredited I'd be interested to read the gory details & postmortem just for the interest of it, but also b/c more than a few people I know in the paleoanth zone are still running around believing that it's credible and plausible, and how much fun is it to mess with your friends & colleagues

Well nothing can be "definitively" discredited in this field. At least at this stage of the game. However, the likelyhoods of certain outcomes can be assessed scientifically.

[StudentOfClimatology]

This one of the many problems with the North Atlantic theory. A great read, actually:

Moraine Exposure Dates Imply Synchronous Younger Dryas Glacier Advances in the European Alps and in the Southern Alps of New Zealand

http://onlinelibrary.wiley.com/doi/10.1111/1468-0459.00060/abstract

Samples taken from the top surfaces of boulders on the Lake Misery moraines at Arthur's Pass, in the Southern Alps of New Zealand, were analysed for 10Be by accelerator mass spectrometry. Exposure ages calculated with the currently accepted production rate, along with scaling corrections for sample latitude and elevation (42°50’S, 960 m), are: 9300 ± 990, 11,000 ± 1360, 11,410 ± 1030, 12,050 ± 960, and 12,410 ± 1180 years. We consider the date of 9300 years to be an outlier, not included in our mean exposure age of 11,720 ± 320 years for the Lake Misery moraines. Based on exposure ages and geomorphologic similarities, we compare the Lake Misery moraines with an Egesen moraine complex at Julier Pass in the Swiss Alps (46°30’N, 2200 m). Based on the 10Be, 26Al, and 36Cl exposure ages of three boulders, we calculate a mean exposure age of 11,750 ± 140 years for the outer Egesen moraine at Julier Pass. Based solely on 10Be measurements, we obtain a mean exposure age of 11,860 ± 210 years for this outer moraine. Egesen moraines in the Swiss Alps represent glacier readvance during the Younger Dryas cold reversal, based on regional correlations and on basal radiocarbon dates from bogs located up-valley of Egesen moraines. The exposure dates from Arthur's Pass and Julier Pass show synchronous glacier advances both in the Southern Alps and in the European Alps during the European Younger Dryas chronozone of Mangerud et al.

[sokolow]

Well nothing can be "definitively" discredited in this field. At least at this stage of the game. However, the likelyhoods of certain outcomes can be assessed scientifically.

OK well I think your department might be on the cutting edge of this one or the news hasn't reached nonspecialists because near as I can tell it is an open, common statement that meltwater injection and hence north atlantic direction YD onset is a question up for debate and as yet unresolved. However I don't see and can't find any summary or overview field statement that its an outdated, obsolete, or widely rejected view. AFAICT the EGU and AGU were still taking papers on it as of the early 2010s, people at name brand programs in the USA (OSU!) UK CAN and DK were still publishing on it in meanstream journals, and have open research programs -- at least on the issue of Laurentide drainage.

It would be helpful to have a survey paper or chapter that traces the fall from "Wally Was Right" (2007) to "Wally's A Chump" (2014) before discarding it itt.

[sokolow]

This one of the many problems with the North Atlantic theory. A great read, actually:Moraine Exposure Dates Imply Synchronous Younger Dryas Glacier Advances in the European Alps and in the Southern Alps of New Zealandhttp://onlinelibrary.wiley.com/doi/10.1111/1468-0459.00060/abstract

Yah that's the one where they were having an argue over the 10Be dating & the terminal moraine:

Barrows, T. T., Lehman, S. J., Fifield, L. K., & De Deckker, P. (2007). Absence of cooling in New Zealand and the adjacent ocean during the Younger Dryas chronozone. Science, 318(5847), 86-89.

With replies comments & whatnot going back and forth

[chubbs]

A previous post in this thread showed a high correlation between GISS ST and the NOAA greenhouse gas index (AGGI). GISS has increased by 0.8C for every 1 unit increase of the NOAA GHG index. In the plot below this relationship was used to adjust the GISS series to  compensate for the increase in GHG since 1880.  Variation in the adjusted GISS series is presumably due to natural variation plus any man-made impacts not accounted for in AGGi. Note aerosal effects are correlated with GHG and therefore at least partially accounted for in AGGI

 

There is little long-term trend in  the adjusted GISS series indicating that natural forces have not played a large role in the increase in GISS temperatures since the 19'th century. There are longer-term natural fluctuations in the adjusted series with a period of around  60 years.  Most of the increase in global temperatures occurs during the warm phase of these 60-year cycles but warm and cool natural phases would tend to cancel out without increasing GHG.

 

[StudentOfClimatology]

OK well I think your department might be on the cutting edge of this one or the news hasn't reached nonspecialists because near as I can tell it is an open, common statement that meltwater injection and hence north atlantic direction YD onset is a question up for debate and as yet unresolved. However I don't see and can't find any summary or overview field statement that its an outdated, obsolete, or widely rejected view. AFAICT the EGU and AGU were still taking papers on it as of the early 2010s, people at name brand programs in the USA (OSU!) UK CAN and DK were still publishing on it in meanstream journals, and have open research programs -- at least on the issue of Laurentide drainage.

It would be helpful to have a survey paper or chapter that traces the fall from "Wally Was Right" (2007) to "Wally's A Chump" (2014) before discarding it itt.

There is definitely a very "vocal" group who cling tightly to the theory of meltwater injection as a trigger, despite the plentiful evidence against it. This is a great read...Dr. Saeger is one of the smartest PHDs I know:

http://www.ldeo.columbia.edu/res/div/ocp/pub/seager/Seager_Battisti_2007.pdf

http://www.ldeo.columbia.edu/res/div/ocp/gs/

The evidence against a North Atlantic trigger for the YD is now overwhelming. We now have:

- Recent data revealing a globally synchronous cooling and subsequent warming:

http://m.geology.gsapubs.org/content/early/2013/10/30/G34867.1.abstract

Knowledge of regional variations in response to abrupt climatic transitions is essential to understanding the climate system and anticipating future changes. Global climate models typically assume that major climatic changes occur synchronously over continental to hemispheric distances. The last major reorganization of the ocean-atmosphere system in the North Atlantic realm took place during the Younger Dryas (YD), an ~1100 yr cold period at the end of the last glaciation. Within this region, several terrestrial records of the YD show at least two phases, an initial cold phase followed by a second phase of climatic amelioration related to a resumption of North Atlantic overturning. We show that the onset of climatic amelioration during the YD cold period was locally abrupt, but time-transgressive across Europe. Atmospheric proxy signals record the resumption of thermohaline circulation midway through the Younger Dryas, occurring 100 yr before deposition of ash from the Icelandic Vedde eruption in a German varve lake record, and 20 yr after the same isochron in western Norway, 1350 km farther north. Synchronization of two high-resolution continental records, using the Vedde Ash layer (12,140 ± 40 varve yr B.P.), allows us to trace the shifting of the polar front as a major control of regional climate amelioration during the YD in the North Atlantic realm. It is critical that future climate models are able to resolve such small spatial and chronological differences in order to properly encapsulate complex regional responses to global climate change.

- The fact that the YD began before the hypothetical "draining" of Lake Agassiz, and new evidence that the lake actually evaporated as a result of the drier conditions in the YD:

http://www.uc.edu/news/nr.aspx?id=14355

https://gsa.confex.com/gsa/2011AM/finalprogram/session_28107.htm

The problem is, better dating of lake levels and associated organic materials do not support a rapid outflow at the right time.

“An alternative explanation is needed,” he said.

Lowell’s research shows that, although water levels did drop, the surface area of the lake increased more than seven-fold at the same time. His research suggests that the lower water levels were caused by increased evaporation, not outflow. While the melting glacier produced a lot of water, Lowell notes that the Moorhead Low was roughly contemporaneous with the Younger Dryas cold interval, when the atmosphere was drier and there was increased solar radiation.

“The dry air would reduce rainfall and enhance evaporation,” Lowell said. “The cold would reduce meltwater production, and shortwave radiation would enhance evaporation when the lake was not frozen and sublimation when the lake was ice-covered.”

- A cooling of the tropical Atlantic and Pacific, which would cannot occur under the scenario of meltwater injection:

The Cariaco Basin just north of Venezuela is an invaluable treasure trove of climate records. The waters above are anoxic, which prohibits mixing of the sediments by organisms living within. This, and very high sedimentation rates because of the proximity of the continent, has created a record with close to annual resolution extending back into the last ice age. Lea et al. (2003) used Mg/Ca ratios in Cariaco Basin sediments to reconstruct SSTs during the last deglaciation. They found an abrupt warming at the transition into the Bølling-Allerød warm period, an abrupt cooling of as much as 4◦C at the beginning of the Younger Dryas, and an abrupt warming at its termination. This result is consistent with Sachs and Lehman and Bond and, despite necessary concern about the proxy indicators, improves confidence that the entire North Atlantic surface ocean cooled dramatically, by a few to several degrees Celsius during Greenland stadials.

[chrisf97212]

This discussion has taken a fascinating turn in regards to YD causes.  I searched past topics and surprisingly, there has only been one other brief thread on the subject. 

[sokolow]

There is definitely a very "vocal" group who cling tightly to the theory of meltwater injection as a trigger, despite the plentiful evidence against it. This is a great read...Dr. Saeger is one of the smartest PHDs I know:

Sure, but bear in mind Seager is a vehement advocate for an opposing stance, as is plain in the second link you give, which is titled Climate Mythology in big bold letters.

Seager:

A few times a year the British media of all stripes goes into a tizzy of panic when one climate scientist or another states that there is a possibility that the North Atlantic ocean circulation, of which the Gulf Stream is a major part, will slow down in coming years or even stop. Whether the scientists statements are measured or inflammatory the media invariably warns that this will plunge Britain and Europe into a new ice age, pictures of the icy shores of Labrador are shown, created film of English Channel ferries making their way through sea ice are broadcast... And so the circus continues year after year. Here is one example.

...

North Atlantic Ocean circulation and abrupt climate change

The conflation of the Gulf Stream, ocean heat transport and Europe's climate has led to changes in ocean circulation being the reigning theory of the cause of glacial era abrupt climate change. These abrupt changes - the Dansgaard-Oeschger events of the last ice age and the Younger Dryas cold reversal of the last deglaciation - are well recorded in the Greenland ice core and Europe and involved changes in winter temperature of as much as thirty degrees C! For the Younger Dryas it has been proposed that the sudden release of glacial meltwater from ice dammed Lake Agassiz freshened the North Atlantic and shut down the overturning circulation causing dramatic regional coooling.

Seager:

Only through an inflated view of the impact of ocean circulation could it be thought that the enormous glacial era abrupt changes were caused by changes in ocean circulation. Instead, as we have argued, changes in atmospheric circulation regimes had to be the driver, see (Seager and Battisti,2007). Determining how this could happen has become more of a priority now that the geological evidence for the Lake Agassiz flood has not been found, see (Broecker,2006).

Moving beyond the myth

It is long time that the Gulf Stream-European climate myth was resigned to the graveyard of defunct misconceptions along with the Earth being flat and the sun going around the Earth. In its place we need serious assessments of how changes in ocean circulation will impact climate change and a new look at the problem of abrupt climate change that gives the tropical climate system and the atmosphere their due as the primary drivers of regional climates around the world.

... and that's fine, Seager-Battisti is a thorough analysis of the weaknesses for meltwater-AMOC theories. But he's not the final word, he's very much involved in an active conversation. So for instance Shakun and Carlson (2010) are defending & advancing an AMOC model over Seager's view, and chiang (2009, 2012) flat out writes off the tropical stance:

At this time, evidence firmly favors the North Atlantic as the origin of abrupt climate change; it is the simplest hypothesis that fits all the observed facts. However, there is also the strong realization that the Tropics has to be involved in some way in abrupt climate change—it is not merely a passive observer. We know that tropical convection reorganizes during AMOC slowdown; in principle, the altered convection could, in turn, affect global climate through teleconnections in a manner similar to the global impacts of El Niño. Thus, the Tropics could play the role of globalizing the climate changes initiated by the North Atlantic.

...

In the Wake of the Tropical Hypothesis

It is ironic that the original question leading into the tropical hypothesis—why CLIMAP did not show much mean tropical cooling—turned out to be a less interesting aspect of the hypothesis, at least from a dynamical point of view. What matters for the tropical climate is not so much the mean SST, but spatial gradients. SST gradients shift tropical convection, allowing the tropical convective climate to reorganize and, ultimately, to force extratropical Rossby waves. Other relevant paleoclimate scenarios on which I have not touched, including the dynamics of monsoons in paleoclimate and the tropics during LGM and deglaciation (an emerging topic that will be very interesting for the Tropics), only reinforce the view coming out of the tropical hypothesis that the tropical climate is highly susceptible to change, undergoes tropicswide reorganizations, and makes its changes felt globally. The tropical climate seems readily malleable to the prevailing climate forcings—an ominous portent of future climate change.

With that in mind,

The evidence against a North Atlantic trigger for the YD is now overwhelming. We now have:

- Recent data revealing a globally synchronous cooling and subsequent warming:http://m.geology.gsapubs.org/content/early/2013/10/30/G34867.1.abstract

What's the application to the N. Atl trigger? I don't doubt that it has potential implications, given the other authors (Brauer) but it doesn't seem to have disturbed subsequent researchers in starting off from a meltwater injection YD initiation and folding Lane into their analysis (Ana Moreno's 2014 paper in Nature Geoscience).

Also look at the modification of the synchroneity assertion in paper & in interview:

Lane:

An ideal test area is the North Atlantic realm due to the large amount of available background information. In particular, the Younger Dryas (YD), the last major climatic fluctuation of the last glaciation (Broecker et al., 1988; Alley et al., 1993;Bakke et al., 2009; Brauer et al., 2008), represents an ideal natural laboratory to study the complexity of abrupt shifts of the climate system. Past investigations concentrated on the large shifts and their potential trigger mechanisms at the onset and end of the YD (Broecker et al., 1988; Isarin et al., 1998; Brauer et al., 2008; Lohne et al., 2013), but changes within the YD have received less attention. There is common agreement that the YD is related to a weakening of the North Atlantic Meridional Overturning Circulation (AMOC) during the early YD (McManus et al., 2004; Jennings et al., 2006;Bakke et al., 2009) followed by a mid-YD recovery (Jennings et al., 2006; Elmore and Wright, 2011; Pearce et al., 2013).

...

Using this approach we have found convincing evidence that in the past, climatic changes have been reached diachronously, even within the same broad climate regime. This finding has implications for the anticipation of fine-scale regional responses to future global climate change.

Lane interview: "We can't assume climatic changes are synchronous worldwide, or even continent-wide. Some regions might feel changes in climate at different times," says Lane. "Climate models need to be able to handle subtle complexities in timing to give accurate future climate predictions."

- The fact that the YD began before the hypothetical "draining" of Lake Agassiz, and new evidence that the lake actually evaporated as a result of the drier conditions in the YD: http://www.uc.edu/news/nr.aspx?id=14355https://gsa.confex.com/gsa/2011AM/finalprogram/session_28107.htm

Plausible, sure, but also contested by Carlson in 2009, writing in QSR, and by Teller in 2013, writing in QR and challenging the feasibility of evaporation based off hydro budgets and insufficient evidence for appropriate changes in temp, runoff, and precip. Looks like Liu (2014) went out to buttress the evaporation hypothesis via sediment analysis and came back with ambiguous results.

- A cooling of the tropical Atlantic and Pacific, which would cannot occur under the scenario of meltwater injection:

Also presently contested, by Wan (2009) Geophysical Research Letters in and Schmidt & Stieglitz (2011) writing in Paleoceanography. The former on the basis of the Cariaco basin potentially reflecting localized dynamic effects appropriate to strong gradients, the latter applying evidence from the Florida Straits and concluding their findings are in line with a N.Atl AMOC initiation.

And the point of all that is not that I'm an ardent supporter of YD onset through meltwater injection or that I have an answer -- the point is that clearly, numerous qualified men and women are still arguing this in very recent high profile publications, that researchers still cite the "Wally hypothesis" in their introductions when giving context to their investigations and aren't laughed off the stage, and that this state of affairs does not support the view that meltwater injection is widely regarded within the relevant disciplines & editorial / referees bodies as dead, old, obsolete, outdated, discarded, marginalized, or overwhelmed by contrary evidence. Indeed it's still on the table, and frequently cited as the likely mechanism.

But the ongoing debage does speak to the importance of your friend's diss in giving an analysis of an open & intriguing question!

[StudentOfClimatology]

I don't have time to respond in depth now. I don't agree with Saeger's views on Anthropogenic forcing, but when it comes to geologic/paleoclimatological dilemmas, I'd turn to him in a heartbeat. You really need to know a guy, to know a guy.

Furthermore, yes, no one is ridiculed for supporting the North Atlantic theory and it is a very convenient solution for a number of reasons. However, I can say, firsthand, that scientists are being forced to look elsewhere for solutions. No one claims to have the answers...I know I certainly don't.

[sokolow]

I certainly have heard from the outside that Columbia's L-D crowd is home to strong and vibrant personalities with strong, vibrant opinions on this issue in particular