The Intriguing Problem Of The Younger Dryas—What Does It Mean And What Caused It?

This is a follow up posting to Younger Dryas -The Rest of the Story!

Guest post by Don J. Easterbrook

Dept. of Geology, Western Washington University.

The Younger Dryas was a period of rapid cooling in the late Pleistocene 12,800 to 11,500 calendar years ago. It followed closely on the heels of a dramatically abrupt warming that brought the last Ice Age to a close (17,500 calendar years ago), lasted for about 1,300 years, then ended as abruptly as it started. The cause of these remarkably sudden climate changes has puzzled geologists and climatologists for decades and despite much effort to find the answer, can still only be considered enigmatic.

The Younger Dryas interruption of the global warming that resulted in the abrupt, wholesale melting of the huge late Pleistocene ice sheets was first discovered in European pollen studies about 75 years ago. Terrestrial plants and pollen indicate that arboreal forests were replaced by tundra vegetation during a cool climate. This cool period was named after the pale yellow flower Dryas octopetella, an arctic wildflower typical of cold, open, Arctic environments. The Younger Dryas return to a cold, glacial climate was first considered to be a regional event restricted to Europe, but later studies have shown that it was a world-wide event. The problem became even more complicated when oxygen isotope data from ice cores in Antarctica and Greenland showed not only the Younger Dryas cooling, but several other shorter cooling/warming events, now known as Dansgaard-Oerscher events.

The Younger Dryas is the longest and coldest of several very abrupt climatic changes that took place near the end of the late Pleistocene. Among these abrupt changes in climate were: (1) sudden global warming 14,500 years ago (Fig. 1) that sent the immense Pleistocene ice sheets into rapid retreat, (2) several episodes of climatic warming and cooling between ~14,400 and 12,800 years ago, (3) sudden cooling 12,800 years ago at the beginning of the Younger Dryas, and (4) ~11,500 years ago, abrupt climatic warming of up to 10º C in just a few decades. Perhaps the most precise record of late Pleistocene climate changes is found in the ice core stratigraphy of the Greenland Ice Sheet Project (GISP) and the Greenland Ice Core Project (GRIP). The GRIP ice core is especially important because the ages of the ice at various levels in the core has been determined by the counting down of annual layers in the ice, giving a very accurate chronolgoy, and climatic fluctuations have been determined by measurement of oxygen isotope ratios. Isotope data from the GISP2 Greenland ice core suggests that Greenland was more than~10°C colder during the Younger Dryas and that the sudden warming of 10° ±4°C that ended the Younger Dryas occurred in only about 40 to 50. years.

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Figure 1. Temperature fluctuations over the past 17,000 years showing the abrupt cooling during the Younger Dryas. The late Pleistocene cold glacial climate that built immense ice sheets terminated suddenly about 14,500 years ago (1), causing glaciers to melt dramatically. About 12,800 years ago, after about 2000 years of fluctuating climate (2-4), temperatures plunged suddenly (5) and remained cool for 1300 years (6). About 11,500 years ago, the climate again warmed suddenly and the Younger Dryas ended (7).

Radiocarbon and cosmogenic dating of glacial moraines in regions all over the world and abrupt changes in oxygen isotope ratios in ice cores indicate that the Younger Dryas cooling was globally synchronous. Evidence of Younger Dryas advance of continental ice sheets is reported from the Scandinavian ice sheet, the Laurentide ice sheet in eastern North America, the Cordilleran ice sheet in western North America, and the Siberian ice sheet in Russia. Alpine and ice cap glaciers also responded to the abrupt Younger Dryas cooling in both the Northern and Southern hemispheres, e.g., many places in the Rocky Mts. of the U.S. and Canada, the Cascade Mts. of Washington, the European Alps, the Southern Alps of New Zealand, and the Andes Mts. in Patagonia of South America.

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Figure 2. Temperature fluctuations over the past 15,000 years showing the abrupt cooling during the Younger Dryas and other warming and cooling periods, the Oldest Dryas (cool), Bölllng (warm), Older Dryas (cool), Allerød (warm), InterAllerød (cool), and Younger Dryas (cool).

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Figure 3. Oxygen isotope record from the Greenland ice core showing an abrupt temperature drop 12,800 years ago, 1300 years of cool climate, and sudden warming 11,500 years ago.

The Younger Dryas had multiple glacial advances and retreats

The Younger Dryas was not just a single climatic event. Late Pleistocene climatic warming and cooling not only occurred before and after the YD, but also within it. All three major Pleistocene ice sheets, the Scandinavian, Laurentide, and Cordilleran, experienced double moraine-building episodes, as did a large number of alpine glaciers. Multiple YD moraines of the Scandinavian Ice Sheet have long been documented and a vast literature exists. The Scandinavian Ice Sheet readvanced during the YD and built two extensive end moraines across southern Finland, the central Swedish moraines, and the Ra moraines of southwestern Norway(Fig. 4). 14C dates indicate they were separated by about 500 years.

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Figure 4. Double Younger Dryas moraines of the Scandinavian Ice Sheet.

Among the first multiple YD moraines to be recognized were the Loch Lomond moraines of the Scotish Highlands. Alpine glaciers and icefields in Britain readvanced or re-formed during the YD and built extensive moraines at the glacier margins. The largest YD icefield at this time was the Scotish Highland glacier complex, but smaller alpine glaciers occurred in the Hebrides and Cairngorms of Scotland, in the English Lake District, and in Ireland. The Loch Lomond moraines consist of multiple moraines. Radiocarbon dates constrain the age of the Loch Lomond moraines between 12.9 and 11.5 calendar years ago.

Multiple Younger Dryas moraines of alpine glaciers also occur throughout the world, e.g., the European Alps, the Rocky Mts., Alaska, the Cascade Range, the Andes, the New Zealand Alps, and elsewhere.

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Figure 5. Double Younger Dryas moraines at Titcomb Lakes in the Wind River Range of Wyoming.

Implications

The multiple nature of YD moraines in widely separated areas of the world and in both hemispheres indicates that the YD consisted of more than a single climatic event and these occurred virtually simultaneously worldwide. Both ice sheets and alpine glaciers were sensitive to the multiple YD phases. The GISP2 ice core shows two peaks within the YD that match the glacial record. The absence of a time lag between the N and S Hemispheres glacial fluctuations precludes an ocean cause and is not consistent with the North Atlantic Deep Ocean Water hypothesis for the cause of the Younger Dryas, nor with a cosmic impact or volcanic origin.

Both 14C and 10Be production rates in the upper atmosphere changed during the YD. 14C and 10Be are isotopes produced by collision of incoming radiation with atoms in the upper atmosphere. The change in their production rates means that the Younger Dryas was associated with changes in the amount of radiation entering the Earth’s atmosphere, leading to the intriguing possibility that the YD was caused by solar fluctuations.

Why the Younger Dryas is important

What can we learn from all this? The ice core isotope data were hugely significant because they showed that the Younger Dryas, as well as the other late Pleistocene warming and cooling events could not possibly have been caused by slow, Croll-Milankovitch orbital forcing, which occurs over many tens of thousands of years. The ice core isotope data thus essentially killed the Croll-Milankovitch theory as the cause of the Ice Ages.

In an attempt to save the Croll-Milankovitch theory, Broecker and Dention (1990) published a paper postulating that large amounts of fresh water discharged into the north Atlantic about 12,800 years ago when retreat of the Laurentide ice sheet allowed drainage of glacial Lake Agassiz to spill eastward into the Atlantic Ocean. They proposed that this large influx of fresh water might have stopped the formation of descending, higher-density water in the North Atlantic, thereby interrupting deep-water currents that distribute large amounts of heat globally and initiating a short-term return to glacial conditions. If indeed that was the case, then the Younger Dryas would have been initiated in the North Atlantic and propagated from there to the Southern Hemisphere and the rest of the world. Since that would take time, it means that the YD should be 400-1000 years younger in the Southern Hemisphere and Pacific areas than in the Northern Hemisphere. However, numerous radiocarbon and cosmogenic dates of the Younger Dryas all over the world indicate the cooling was globally synchronous. Thus, the North Atlantic deep current theory is not consistent with the chronology of the Younger Dryas.

The climatic fluctuations before and after the Younger Dryas, as well as the fluctuations within it, and the duration of these changes are not consistent with a single event cause of the YD. Neither cosmic impact or volcanic eruptions could produce the abrupt, multiple climatic changes that occurred during the late Pleistocene.

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OK, I’ll bite.
If the Younger Dryas warming ended 11,500 years ago, how did it end the Ice Age 17,500 years ago?
Just asking.

Wasn’t there a big drop in dust starting about 20,000 years ago. As it dropped sunshine should have gone up.

Stephen Richards

I guess the lessons we still need to learn from these episodes of very rapid cooling and warming is that we still do not understand our climate. It is very possible that we are now entering an ice age or a period of rapid warming or that we are still plodding along the same line as that which began just after the end of the YD cooling. Worrisome really !

Ged

@Higley7,
Look at the graphs, especially figure 1. What is confusing you?
You have abrupt warming around 15,000 BC to levels comparible to today, and high above the ice age. Then Younger Dryas hits around 13,500 BC and temps plummet right back to ice age level. And then suddenly it rapidly warms right back up around 11,500 BC.
So the ice age ended around 17,500, and no later than 15,000 with rapid warming, then there was the strange sudden cold snap we call Younger Dryas, and then back to warming when that ended at 11,500 BC.

You will find under scrutiny that most of the glacial readvances happened just before or just after the Younger Dryas. Most notably the Two Creek glaciation event that turned out to be Allerod The only exception to that may be several Loch Lomond advances in Scotland.
You will also find that several biota proxies do show a clear change in conditions but not necesarily in lower temperatures.
For instance Bjorck et al 2002
http://www.geol.lu.se/personal/seb/Geology.pdf.pdf
or Lucke and Brauer 2004
http://www.sciencedirect.com/science/article/pii/S0031018204002597
who confirm the relatively warm and dry summers. The aridness is also confirmed in the Greenland ice core accumulation rates, which makes it extra difficult for glaciers to advance.
So it appears that the Younger Dryas is riddled with interpretation issues.

Lars P.

You miss the big picture: when I looked at the first figure I recognised it looks like a diagram of an engine where somebody turned first time the key and it rumbled a couple of times before it died.
That’s the Younger Dryas break.
Then at the second key turn it started rumbling happily. It is just the diagnose sheet that you look at.
Joking aside, thanks for the post, it is very interesting, I love these posts on climate history.

Stephen Wilde

I suggest that the 1500 year (approximately) Dansgaard-Oerscher events of which the YD was just a pronounced version would be a result of changing phasing between solar and oceanic cycles.
The solar periodicity seems to be around 500 years as per MWP to LIA to date and the thermohaline circulation is around 1000 years.
Over time they drift in and out of phase sometimes offsetting and sometimes supplementing one another.
During the glacial epochs they seem to largely supplement one another to cause violent climate swings whereas during interglacials they seem to offset one another leading to a more stable climate regime.
The Milankovitch cycles would still be the main driver of ice ages and interglacials but the sun / ocean interactions would affect the timing of the changes between ice ages and interglacials.

rbateman

Event windows during which bodies external to the Solar System entered, affecting all planets. One or more of the current planets may not have been here before the Events.
It would take some time for the Solar System to stabilize into what we know today.
We have evidence today of free-wandering planitessimals, so it’s possible.
Do we have legend or myth that speaks of great disaster?
All the earliest stories from antiquity say yes.
We just don’t know the details.

First, many thanks for your contribution and for stressing solar input as opposed to meteoric. But I am mystified as to how Milankovitch cycles can be discarded when we have such clear correlation as seen in this graph: http://en.wikipedia.org/wiki/File:Vostok_420ky_4curves_insolation.jpg
Another question, do not ocean currents have instantaneous effects as well as long term? That is, turn off Arctic downwelling, which can only exit through the North Atlantic, and replacement surface waters flowing northward must be immediately terminated. We don’t have to wait a thousand years for oceanic turnover. Thanks, –AGF

Don-It’s true that events like the younger dryas do not appear to be orbitally forced, but good agreement has been shown between the rate of change of global ice volume and the near Arctic circle summer insolation. It is certainly clear that some “spikes” of rapid change exceed what would be expected from Milakovitch alone, but I wouldn’t be so quick to say that the Milankovitch theory of the glaciations is “dead” at all. The long term correlation is quite good. We just need to invoke other factors for some episodes that deviate from the right Milankovitch model.

Carsten Arnholm, Norway

“…and the Ra moraines of southwestern Norway(Fig. 4).”

Fig. 4 shows Finland, not Norway.

About the sudden warming at the onset of the Bolling ~14500 years Cal BP, there are even more problems here, since the major glacial retreat started a few thousend years earlier. The problem was recognised by Denton et al 2006 and they dubbed it the mystery interval.
(Denton G.H., Broecker, W.S. and Alley, R.B., 2006: The mystery interval 17.5 to 14.5 kyrs ago, PAGES news, 2: 14-16.)
So if there are more not understood problems here, is it possible that one or more of our fundamental suppositions is fundamentally wrong?
Also that quote: ” Evidence of Younger Dryas advance of continental ice sheets is reported from the Scandinavian ice sheet, the Laurentide ice sheet in eastern North America, the Cordilleran ice sheet in western North America, and the Siberian ice sheet in Russia.
There was no Siberian ice sheet during the last glacial maximum, see for instance Hubberten et al 2004 http://epic.awi.de/9052/1/Hub2004a.pdf fig 1 on page 3. That was the maximum extent of the Eurasian ice sheet, way before the Younger Dryas. Let alone that there were glacial advances.
Really the interpretation of the last glacial transition and many others are troubled by interpretation issues of certain proxies. However my explaining post in an earlier thread was moderated away.

The article states:

The climatic fluctuations before and after the Younger Dryas, as well as the fluctuations within it, and the duration of these changes are not consistent with a single event cause of the YD. Neither cosmic impact or volcanic eruptions could produce the abrupt, multiple climatic changes that occurred during the late Pleistocene.

In point of fact, since the given astronomical model for the Younger Dryas Impact Hypothesis is the progressive disintegration of the progenitor of the Taurids, as described in The Structure, and Evolution of the Taurid Complex by D.I. Steel et al. and proposed in W.M. Napier’s Paleolithic Extinctions and the Taurid Complex, it should be noted that the YDIH as written does not propose a single event at all.
The progenitor of the Taurid family of objects is thought to have entered the inner solar system, and a very short period elliptical orbit that crossed the orbits of all the planets of the inner solar system sometime between 20,000 and 30,000 YA. The astronomical data on the Taurids is as good as anything you can dig up with a shovel, and trowel. And that evidence indicates the 50 to 100 km wide Taurid Progenitor object immediately began to breakup as soon as it entered the inner solar system.
The the Earth’s passage though the debris from the progressive breakup of the Taurid Progenitor would have resulted in devastating impact showers, and storms of varying intensity twice a year for thousands of years, both before, and after, the start of the Younger Dryas.
The evidence so far is implying that the event at the start of the YD that produced a global impact layer comparable only to the Cretaceous/Tertiary boundary layer that marks the extinctions of the dinosaurs 65 million YA was only the worst of many annual cluster airburst events of varying intensity over a period that lasted for many millennia.

crosspatch

This might be some interesting reading as well:
http://elpub.wdcb.ru/journals/rjes/v09/2007ES000250/2.shtml

Peridot

Very interesting and informative. Need more articles like this one.
(On an irrelevant note, the Dryas octapetala grows wild, naturally, in Co Clare, Ireland and other places at or near sea level – also in my garden though I planted it there. However it is not pale yellow…it is always white-petalled).

JohnH

To a layman the Vostock ice core temperature plot seems to show at least seven sudden temperature spikes within the last 120,000 year cycle between major interglacial temperature peaks; raising the possibility that a common cause might be found for at least some of them?

Jim Clarke

What I find most interesting is that, according to the Greenland ice core (figure 1) Greenland has been about 3-5 degrees C warmer than it is now for most of the Holocene! All those gloom and doom studies about the rapidly melting Greenland ice sheet are maybe just picking up on the very beginning of a return to Holocene average!
It is amazing to think that civilization actually took root and grew during a time when the Greenland ice sheet was trying to destroy the planet by being 3-5 degrees warmer. Good thing they didn’t know then what we think we know now. Would they have even bothered with Stonehenge, the pyramids and the Colosseum?

P Wilson

has no institution ever considered that there might be an irregular rotational hotspot in the earth’s core that gives more heat to the southern hemisphere during glaciations, and that it may be the cause of deglaciations? This is the case with some of Saturn’s moons Enceladus, where the southern tip is excessively warm, as a result of its core, and has more heat coming out of the southern cap, whilst the north is frozen. Geo-thermal events must have a considerable, if hitherto unstudied impact, particulary submarine.
Its one of climatology’s great weaknesses that the entire earth is not studied, and only the atmposphere

crosspatch

You miss the big picture: when I looked at the first figure I recognised it looks like a diagram of an engine where somebody turned first time the key and it rumbled a couple of times before it died.
That’s the Younger Dryas break.

If you look at the Pleistocene 10Be levels in Figure 2 of the link I provided in my previous comment (Russian Journal of Earth Sciences) you see a rapid decline of 10Be (denoting increased solar activity) as it warms and then a very sudden and deep increase in 10Be production during the YD which would normally indicate a sudden and deep reduction in solar magnetic field activity. There is also, of course, another possible cause. To assume that 10Be production is a good proxy for solar magnetic activity one assumes that GCRs are constant. Maybe they aren’t. Maybe we are buffeted from time to time by “gusts” of dense cosmic rays or maybe sometimes pass through a “stream” of them where there are just more of them. The point is that it could be a combination of both things working at the same time. Maybe we get more GCRs hitting the atmosphere because the solar system is passing through an area with more GCRs and not because the sun changed. The GCRs are subject to being channeled by magnetic phenomena in the Galaxy.
So while we do know that solar magnetic activity can modulate GCRs reaching Earth, to assume that is the ONLY thing that chances those numbers might be a bit naive. Maybe we are bombarded from time to time with what amounts to shock waves or gusts of these particles. Maybe we find ourselves in streams of them. But one things does seem clear: for most of the Pleistocene the 10Be production was much higher than it has been during the Holocene. Why? I guess we will find out “shortly”.

This graph must be wrong because it doesn’t show the hockey stick. Has anyone found out if Michael Mann agrees with this graph? After all he is the expert of record.

phlogiston

The YD can be looked at as a cool interval or two successive warm intervals, the first short, the second long (the Holocene). The 2 warm periods of different length paradigm makes more sense. Just before the YD were deep glacial conditions and the YD was just a drop back to current glacial “normality” after an abortive jump to interglacial conditions. The first spike was an abortive interglacial, the second a “successful” one.
Indeed, if we remind ourselves that these very short abortive interglacial spikes occurred regularly throughout the glacial period, then the YD cool interval disappears as an anomaly, it is just an abortive interglacial spike that just happened to occur shortly before the “successful” interglacial rize which – unlike the abortive spikes – held on stably to the interglacial attractor rather than falling away from it.
Further, if we consider glacial and interglacial being alternate attractors in a nonlinear/nonequilibrium climate system, then abortive interglacial spikes and the less frequent interglacial rises which “stick” are an expected and normal behaviour.
Thus the YD is not in any way a “problem” except a problem of imagination and paradigm of the observer. The need for every upward or downward wiggle of earths climate history to have some discreet and unique external forcing comes from ignorance of quasi-chaotic systems and a deficient paradigm. It is even slightly absurd to imagine the climate system to be so passive.
Earth’s climate can change BY ITSELF.
OK it might be entrained in a simple or complex way by external periodic forcing, but it is not slavishly forced. Hunting for the magic celestial rhythm is futile.

Werner Weber

In the articles, the sea level during the younger dryas is not mentioned (maybe I missed it). Looking at http://en.wikipedia.org/wiki/File:Post-Glacial_Sea_Level.png
during lower dryas, the sea level is approx. halfway between the level at last glacial maximum around 20 ky bp near -120 m and the present day level, or, more precisely, at around 7 ky bp (near -3 m) , see http://commons.wikimedia.org/wiki/File:Holocene_Sea_Level.png
There has been a rapid increase by 30 m between 15 ky bp and 14 ky bp, then a leveling off, another increase by 10 m before the younger dryas started, a second leveling off during younger dryas, and then the final increase by 65 m between 11 and 7 ky bp. This final and biggest increase of sea level occurred during the maximum of the Milancovitch cycle around 10 ky bp. Near the end of the sea level increase, there occurred the 8.2 ky bp event, a similar cold snap of about 50 years. Between 8 ky bp and 7 ky bp, there was a sea level increase by approx. 10 m – corresponding to twice the present content of the Greenland ice sheet.
I would argue (similar to Stephen Wilde), that there is no reason to abandon the Milankovitch cycle ideas. In addition to the melting caused by the Milancovitch maximum, there occurred perturbations, during dryas, and at 8.2 ky bp, which could have been caused by solar activity or better the lack of it.
But the period between 15 ky bp and 7 ky bp was a period of rapid changes, with many feed-back effects. Positive feed-back due to removal of reflecting ice shields, and negative feed-back possibly due to increase of water transport (due to lea level raise) in the great circulations such as the Antarctic Circumpolar Current (ACC). As the ACC splits off the Humboldt and Benguela currents, any increase of ACC would thus also affect, i.e. cool tropical climate which then feeds back into northern hemisphere climate.

timetochooseagain

Jim Clarke says: “What I find most interesting is that, according to the Greenland ice core (figure 1) Greenland has been about 3-5 degrees C warmer than it is now for most of the Holocene!”
Indeed. And all that Methane trapped in Siberian Permafrost didn’t get released when:
“Boreal forest development in this region commenced by 10,000 yr B.P. Over most of Russia, forest advanced to or near the current arctic coastline between 9000 and 7000 yr B.P. and retreated to its present position by between 4000 and 3000 yr B.P….During the period of maximum forest extension, the mean July temperatures along the northern coastline of Russia may have been 2.5º to 7.0ºC warmer than modern.”
MacDonald, G.M., et al., 2000. Holocene treeline history and climate change across northern Eurasia. Quaternary Research, 53, 302-311.
Amazing how climate disaster didn’t ensue due to the Holocene Optimum. Despite all the claims that big warming in the Arctic is the scariest thing ever.

LazyTeenager

The behavior looks like instability while on the cusp of a change from one state to another.
If its not instabilities that develop when changing from a frozen world to an unfrozen world then we need external things that by coincidence happen to match the end of the frozen state.
I am not sure the sun can show that kind of variability. Changes in the earths magnetic field perhaps?

Rob L

How far north did the antartic ice sheet extend? During ice age it seems likely that gap between Antartic and South America was frozen, clamping off ocean circulation. When it melted it would have a pretty massive global effect that may have overwhelmed warming trend for a while.

crosspatch

Amazing how climate disaster didn’t ensue due to the Holocene Optimum.

Or during the optimum of the last interglacial which was even warmer than it was in the Holocene. Virtually every plant species alive today was alive during that period and survived it with only few exceptions of species that might have appeared since that time.

Justthinkin

Don….I was with you until you stuck that “global warming” into the end of the graph. sorta throws some doubt on the other assumptions.Or it could just be me 🙂

I doubt very much that the Lake Agazziz related cooling would take 400+ years to spread from north to south.

Rosco

A super efficient stone age man’s carbon capture scheme ??

KenB

Excellent article, well presented, but I agree with Dennis Cox, no one can exclude impact or near miss from outer space. There is enough physical evidence to warrant consideration and modification as our knowledge grows. That is all part of the joy of science. Keep an open mind.

Robert of Ottawa

rbateman, I am still undecided on the flood myths. Is it due to the black sea flooding with the Dardenelles opening, or just due to glaial retreat? Or is it both. Interestingly, agriculture is though to have started in Asia Minor; maybe it started on farms 600 foot below current Black Sea levels, and tthe locals escaped the “deluge”. Of further note, the Biblical version involves Mount Arrarat, also in Asia Minor. I never did understand that as a kid; the Jews never lived in Asia Minor; now it makes more sense.

Gregory Ludvigsen

“Both 14C and 10Be production rates in the upper atmosphere changed during the YD. 14C and 10Be are isotopes produced by collision of incoming radiation with atoms in the upper atmosphere. The change in their production rates means that the Younger Dryas was associated with changes in the amount of radiation entering the Earth’s atmosphere, leading to the intriguing possibility that the YD was caused by solar fluctuations.”
At about this time there was a magnetic reveral (the Gothenberg). Changes in the magnetic field of the Earth could have changed the amount of radiation entering the Earth’s atmosphere.

beng

****
Stephen Wilde says:
June 19, 2012 at 1:40 pm
The solar periodicity seems to be around 500 years as per MWP to LIA to date and the thermohaline circulation is around 1000 years.
****
OK, but what could be the mechanism for a 500 yr solar periodicity? And is there any evidence?
The sun is an extremely well-mixed & uniform plasma ball (compared to differentiated planets & their oceans and/or atmospheres). Yes, there are the fairly well-understood magnetic cycles in the convection zone, but these cycle back to ~same conditions every cycle. I’m not sure the sun’s uniformity allows for longer “cycles”. Of course, there’s the gradual compression & heating of the core as it gets denser, but that’s not important for the timescales in question.

Neville

The abrupt warming after the YD at GISP2 was 10c in one decade according to this article from NOAA.
http://www.ncdc.noaa.gov/paleo/abrupt/data4.html
Clearly our present small warming of 0.7c over the last 100+ years is not unusual or unprecedented.
Also our present slight warming comes at the end of a minor ice age and after one of the coldest periods of the Holocene.
This recovery is normal and to be expected. It is not unusual or unprecedented at all.
But there are scores of these warmings and coolings throughout the glacials and interglacials, so what were the NATURAL causes during those times.,

John Game

Great article. Peridot has already commented that Dryas octopetala has white flowers (and it has eight petals, hence the name). But there is another Dryas species, Dryas dummondii, which grows for example in the Canadian Rockies and has yellow flowers that nod. Do we know which species the Younger Dryas is named for? I have always assumed its for D. octopetala, but I don’t really know. D. octopetala is a wonderful arctic-alpine plant in the rose family, occurring at or near sea level as one goes north, eg as Peridot mentions in western Ireland, but needing more elevation as one goes south, eg its in the Wallowas Mountains in Oregon and in the Rockies in Montana and elsewhere. It likes limestone, at least in nature, and I wonder if that impacted its distribution during the “Younger Dryas”.

Brian in Champaign-Urbana

Doesn’t Figure 1 have a typo error? It lists the Medieval Warm Period and Little Ice age incorrectly, those should be at 800 and 200 years BP marks. The larger warm period and cool period on #9 and #10 are the late Roman warm period and subsequent Dark Ages cool period that occur prior to the MWP and LIA.

mysteryseeker

Very good article Dr. Easterbrook. A couple of comments that in part are echoing Dennis Cook’s comments: 1) Why would an absence of a time lag negate the idea of a cosmic impact event? 2) Perhaps you might wish to check out a couple of references provided below that suggest the Younger Dryas may have taken hold in a year or even less. To me the most plausible way that such a rapid onset could takje place is through a significant cosmic event (likely a comet encounter). That said I ma becoming more and more confinced of the importance of solar variations to our climate, but not on the rapid scale that was displayed in the Younger Dryas Thank-you again Don, signed Rod Chilton.
78) J. P. Steffenson et al., “High-Resolution Greenland Ice Core Data Show Abrupt Climate Change Happens in a Few Years,” (2008): Science 321, 680-683.
79) K. Ravillious” Ice Age Took Hold in Less than a Year,” (2009): New Scientist, 10.

crosspatch

During ice age it seems likely that gap between Antartic and South America was frozen, clamping off ocean circulation.

Ice floats. It probably didn’t freeze all the way to the bottom. Could have certainly had an impact on currents, though, and there is evidence that it did. During the Pleistocene there was apparently a rather large area in the Southeastern Pacific that had no circulation for thousands of years during the glaciation. There was also a large increase in brine rejection from the increasing ice pack and this very dense, very cold water settled in that area and just sat. The restarting or speeding up or changing of ocean circulation could cause a lot of this really cold water to start moving around. What impact that might have will probably have to wait until 100K years or so for us to discover.

mysteryseeker

Sorry Dennis I of course mean Cox not Cook

Ian W

phlogiston says:
June 19, 2012 at 3:06 pm

I never thought I would agree with the Phlogiston theory 😉

The absence of a time lag between the N and S Hemispheres glacial fluctuations precludes an ocean cause and is not consistent with the North Atlantic Deep Ocean Water hypothesis for the cause of the Younger Dryas, nor with a cosmic impact or volcanic origin.

Don, this is asserted but not explained. Just how is this time lag not consistent with either the NADOW or an impact scenario? How precise are the time increments of time that this is based on? Wouldn’t an impact cause non-lagged effects?
Steve Garcia

@crosspatch June 19, 2012 at 2:50 pm:

If you look at the Pleistocene 10Be levels in Figure 2 of the link I provided in my previous comment (Russian Journal of Earth Sciences) you see a rapid decline of 10Be (denoting increased solar activity) as it warms and then a very sudden and deep increase in 10Be production during the YD which would normally indicate a sudden and deep reduction in solar magnetic field activity. There is also, of course, another possible cause. To assume that 10Be production is a good proxy for solar magnetic activity one assumes that GCRs are constant. Maybe they aren’t. Maybe we are buffeted from time to time by “gusts” of dense cosmic rays or maybe sometimes pass through a “stream” of them where there are just more of them.

Just thinking out loud, but couldn’t the impact of a very large (~1.0 to ~2.0 km) cometary impactor as suggested possibly screw up the atmospheric shield, mimicking a change in the solar magnetic field? This posits a mechanism that is not now on the radar, I know. But it might also provide a mechanism for the sudden warming at the end of the YD – that due to the atmosphere returning to its previous condition, the shield could be reintroduced on a short time scale.
IOW, might the impact, if big enough, with a plume/fireball large enough, suddenly change the chemistry of the atmosphere? And of so, what would it take to reverse that effect, and how short of time might it happen in? We do know the O18 changed. What else? Perhaps ozone? What else?
I would point out that any hypothesis that tries to explain the YD onset or its end should also be required to explain the sudden increase of global temps at the end of the Allerod. (…at least as shown in the O18 and Be10 data) We have two big rises and one big decline. The impact hypothesis is addressing the decline, but everyone throws up their hands about the two increases. Those might be the elephant in the room.
Steve Garcia

Alex Heyworth

J. Philip Peterson says:
June 19, 2012 at 2:56 pm
This graph must be wrong because it doesn’t show the hockey stick.

Turn your screen upside down. Problem fixed.

DAV

Interesting shoe print in figure 5.

Oh! My bad. LOL

crosspatch says:
June 19, 2012 at 2:50 pm
To assume that 10Be production is a good proxy for solar magnetic activity one assumes that GCRs are constant. Maybe they aren’t.
The 10Be we measure is determined both by production [about 2 ounces per year total over the globe] and by deposition. The latter depends strongly on climate.

commieBob

I think the graph is labelled wrong. The MWP and LIA should be moved one hump to the right. I think the arrow for the MWP is pointing instead at the Roman Warm Period.

The absence of a time lag between the N and S Hemispheres glacial fluctuations precludes an ocean cause and is not consistent with the North Atlantic Deep Ocean Water hypothesis for the cause of the Younger Dryas, nor with a cosmic impact or volcanic origin.

Hmmm,
On the contrary, I’m thinkin’ that an event that produced a global impact layer is perfectly consistent with “The absence of a time lag between the N and S Hemispheres glacial fluctuations”. So while that absence of a time lag between the two hemispheres is not consistent with the Deep Ocean Water hypothesis, it is consistent with a cosmic event of sufficient magnitude that it simultaneously emplaced high energy blast-effected materials into a global stratigraphic layer.

cotwome

Just for fun:
New Evidence Supports Cosmic Impact Theory
http://www.archaeorama.com/archaeology/cosmic-impact-theory/

dan johnston

Couldn’t the YD events be the result of a cosmic impact, as accumylating evidence supports, followed by a Maunder-type solar minimum which slows the recovery? Eliminating an impact as a part of the puzzle may be shortsighted as sh*t happens.