RealClimate logo

Note 3/23/2021: we had a few hiccups with comments after moving the site to https/SSL. Hopefully they're fixed now. Please let us know if there are remaining issues.

The mystery of the offset chronologies: Tree rings and the volcanic record of the 1st millennium

Filed under: — group @ 19 February 2015

Guest commentary by Jonny McAneney

Volcanism can have an important impact on climate. When a large volcano erupts it can inject vast amounts of dust and sulphur compounds into the stratosphere, where they alter the radiation balance. While the suspended dust can temporarily block sunlight, the dominant effect in volcanic forcing is the sulphur, which combines with water to form sulphuric acid droplets. These stratospheric aerosols dramatically change the reflectivity, and absorption profile of the upper atmosphere, causing the stratosphere to heat, and the surface to cool; resulting in climatic changes on hemispheric and global scales.

Interrogating tree rings and ice cores

Annually-resolved ice core and tree-ring chronologies provide opportunities for understanding past volcanic forcing and the consequent climatic effects and impacts on human populations. It is common knowledge that you can tell the age of a tree by counting its rings, but it is also interesting to note that the size and physiology of each ring provides information on growing conditions when the ring formed. By constructing long tree ring chronologies, using suitable species of trees, it is possible to reconstruct a precisely-dated annual record of climatic conditions.

Ice cores can provide a similar annual record of the chemical and isotopic composition of the atmosphere, in particular volcanic markers such as layers of volcanic acid and tephra. However, ice cores can suffer from ambiguous layers that introduce errors into the dating of these layers of volcanic acid. To short-circuit this, attempts have been made to identify know historical eruptions within the ice records, such as Öraefajökull (1362) and Vesuvius (AD 79). This can become difficult since the ice chronologies can only be checked by finding and definitively identifying tephra (volcanic glass shards) that can be attributed to these key eruptions; sulphate peaks in the ice are not volcano specific.

Thus, it is fundamentally important to have chronological agreement between historical, tree-ring and ice core chronologies: The ice cores record the magnitude and frequency of volcanic eruptions, with the trees recording the climatic response, and historical records evidencing human responses to these events.

But they don’t quite line up…

The importance of frost rings

An additional piece of evidence is the existence and dating of “frost ring” phenomena observed in bristlecone pines growing at high altitude in Western North America. These rings form when temperatures drop well below freezing for extended periods during the tree’s growing season, and are evidenced by physical scarring of that year’s growth ring that can be caused by volcanically induced climate dislocation – an idea first suggested by LaMarche and Hirschboeck in 1984.

Figure 1: Dates of large historical eruptions (Siebert et al., 2011) within the last 600 years that likely caused significant Northern Hemisphere negative temperature anomalies as noted and ranked by Briffa et al. (1998). Bristlecone pine frost rings as recorded by LaMarche and Hirschboeck (1984)(†) and Salzer and Hughes (2007)(‡), are also listed, as are the start years of large ice acid signals (deposition SO42- in units of Kg/km2 are given in parentheses) observed in the NEEM S1 and WDC06A ice cores Sigl et al. (2013). Space analysis between successive phenomena (highlighted in bold) shows high consistency, and that large volcanoes can induce frost damage in bristlecone pines.

If we take some notable volcanoes in the past 600 years (Figure 1), we can confirm that frost rings in bristlecone pines are good indicators of large explosive volcanic eruptions, similar to the known coincidence of hemispheric cooling evidenced in growth rings of European trees in the years around historically dated eruptions. For this period, dates for large volcanic acid signals in ice cores are consistent with both historical and tree ring observations. However, this consistent linkage breaks down in the 1st millennium when frost ring dates and ice core acidity dates are compared. So if frost rings are indicative of large explosive volcanism in recent centuries, is it that the ice cores may be misdated in the 1st millennium? This is the question we explore in a recent paper (Baillie and McAneney, 2015).

Re-dating the ice cores?

Figure 2: Temperature sensitive Swedish pine chronology showing synchronisms with bristlecone pine frost rings (Baillie, 2010). Ice acid dates (o) original (Clausen et al. 1997); (m) moved 2.5 years (Larsen et al. 2008); (B) moved 7 years (Baillie 2008).

In 2008 and 2010, Baillie used bristlecone pine frost rings as well as temperature sensitive Swedish pine tree ring chronologies to propose that the dating of the Greenland ice cores was too old by approximately 7 years in the period before the 7th century AD, see Figure (2). In our current paper, we reinforce this proposed re-dating by looking at ice core data published since 2010, from both Greenland (NEEM S1 (Sigl et al., 2013)) and Antarctica (WDC06A (Sigl et al., 2013), Law Dome (Plummer et al., 2012)) as well as the Antarctic DML core (Traufetter et al., 2004). We consider the space intervals between frost rings, and compare them to space intervals between ice acid layers in each core during the 6th and 7th centuries, noting that there is a similarity in the event intervals. The similar space intervals in each data set would appear to suggest that trees and ice cores are recording the same volcanic events, but that the ice cores are offset from tree rings. (See tables 1-3 in our paper for more details). All cores except DML appear to be too old with respect to tree-ring dates, with the latter being slightly too young.

Why should we listen to the trees?

At this point one might quite rightly stop and ask whether it is realistic that a large set of independently replicated ice cores could be misdated? Might the problem instead lie with the dendrochronology? Fortunately, dendrochronological replication is often provided by comparing chronologies produced by independent workers in independent laboratories. In this respect, it is extremely unlikely that any of the tree ring data used in this exercise is incorrectly dated. [Though see this post discussing this exact issue – Ed.]

Figure 3: Historical dust veil events as noted by Stothers and Rampino (1983) in the period from 100 BC to AD 700, compared with bristlecone pine frost rings as recorded by LaMarche and Hirschboeck (1984)(†) and Salzer and Hughes (2007)(‡) and observation of ice acid in the GICC05 (Vinther et al. 2006) and NEEM S1 timescales (Sigl et al., 2013). Space analysis demonstrates that while there is a high consistency between phenomena spacing (highlighted in bold), ice core dates are offset by approximately 7 years.

One could also re-examine the hypothesis that frost rings are useful volcanic markers. In fact, it is possible to have some frost rings without any evidence of explosive volcanism, presumably occurring due to extremes in local weather, but, as shown above, they do seem to record large volcanic eruptions in recent times very well indeed. Why would that change in the 1st millennium? Furthermore, when a space analysis is performed between historically-dated severe dust veils between the 1st century BC and the 7th century AD, these compare well with frost rings, but again volcanic horizons in Greenland ice appear to be too old by 7-years (see Figure 3). Accepting, of course, that historical documentation is correct, the dated bristlecone pine frost rings agree perfectly as a response to the volcanically induced climatic events. We conclude then that these early ice core chronologies are at odds with both history and dendrochronology.


The implications of an ice core misdating are important. Global tree rings show that there was a major climatic event beginning in AD 536 lasting to at least AD 545, which may have been a catalyst for the Justinian plague in the 540s [This was discussed in a previous post in 2008 – Ed.]. Tree-rings suggest that this was a two stage event, with an initial abrupt reduction of tree growth at AD 536, a recovery over the next couple of years, followed by another abrupt reduction in tree growth following AD 540. This agrees extremely well with the frost rings at AD 536 and AD 541, and with the widely documented prolonged dust veil in Europe of AD 536. Current ice core chronologies conversely suggest that there was only one massive volcano in AD 536 which was responsible for the decade long climatic event (Larsen et al. 2008). They showed large volcanic signals in the ice at AD 529 and AD 533.5 and proposed that the latter related to AD 536 (see Figure 4). With the proposed re-dating of the ice cores, it now appears that the two massive volcanoes occurred in AD 536 and 540.5, which fits better with a two-stage event.

Figure 4: Greenland ice cores suggesting two large eruptions in AD 529 +/- 2 and AD 533.5 +/- 2. Moving these acid dates by approximately 7 years would explain the two stage environmental event recorded in trees in AD 536 and AD 540 (Larsen et al., 2008).

The suggested ice dating offset would have other implications. It would imply that the identification of Vesuvius tephra in Greenland ice dating to AD 79 (Barbante et al, 2013) is in error, and it would also bring to prominence a major eruption in 44 BC, the year of Caesar’s death; the acidity associated with this eruption currently having an ice-acid date around 50 BC.

Figure 5: Swedish pine temperature reconstruction showing sudden cooling at AD 800, which may be volcanically induced. The nearest acid layer in NEEM is dated to AD 793 (Grudd, 2010).

With the proposed dating revision, it is possible to tease out other potential candidates for unrecognised historical eruptions. Writing around AD 810-15 the chronicler Theophanes the Confessor records that after the capture and blinding of Byzantine emperor Constantine VI in August AD 797:

“The sun was darkened for seventeen days and did not emit its rays so that ships
lost course and drifted about. Everyone acknowledged that the sun withheld its rays
because the emperor had been blinded”

(Mango and Scott, 1997).

Such an event is suggestive of a volcanic dust veil or ash cloud observed from Constantinople, possibly from a Mediterranean eruption. The nearest acid signal in the NEEM S1 ice core occurs at 793.0 (Sigl et al., 2013), and is the only acid within +/-14 years of AD 797. Note that we are told Constantine’s blinding occurred after August 797, but we cannot know how long after. It is conceivable that the obscuration event could have been as much as a few years after the Constantine’s capture and blinding, but that the two events were associated as direct cause and effect by the popular psyche at the time, and recorded as such. It is thus tempting to link this event with the sudden cooling observed in Swedish pine that occurred in AD 800 (see Figure 5).

Another consequence might be that the current ice dating of the Icelandic Eldgjá eruption may need to be moved from around AD 933 to around AD 939/940. Sun et al. (2014) have identified stratigraphical evidence of Changbaishan eruption tephra in NEEM S1 lying 7 annual layers above Eldgjá tephra. Changbaishan can be dated to AD 946 by historical documents of local ash fall and unseasonably cold weather. This implies any ice dating offset occurs above this horizon. How far above is uncertain, though it is likely below the mid 13th century.


Tree rings cannot prove that ice cores are misdated; but the offset identified here should prompt a re-examination of the ice core chronology. If the dating revision proposed here is correct, it would rewrite the volcanic history of the 1st millennium. In particular, it would definitively show that the mid-6th century was not dominated by a single massive volcanic event in AD 536, but was also influenced by an important eruption in AD 540-541 – an event currently missing from all ice core and historical literature. The mystery deepens…


Mango, C. and Scott, R., eds.: The Chronicle of Theophanes Confessor: Byzantine and Near Eastern History AD 284-813, Oxford University Press, New York, 1997.

Siebert, L., Simkin, T., and Kimberly, P.: Volcanoes of the World, University of California Press and Smithsonian Institution, Berkley, California, 3rd ed., 2011.


  1. V.C. LaMarche, and K.K. Hirschboeck, "Frost rings in trees as records of major volcanic eruptions", Nature, vol. 307, pp. 121-126, 1984.
  2. K.R. Briffa, P.D. Jones, F.H. Schweingruber, and T.J. Osborn, "Influence of volcanic eruptions on Northern Hemisphere summer temperature over the past 600 years", Nature, vol. 393, pp. 450-455, 1998.
  3. M.G.L. Baillie, and J. McAneney, "Tree ring effects and ice core acidities clarify the volcanic record of the first millennium", Climate of the Past, vol. 11, pp. 105-114, 2015.
  4. H.B. Clausen, C.U. Hammer, C.S. Hvidberg, D. Dahl-Jensen, J.P. Steffensen, J. Kipfstuhl, and M. Legrand, "A comparison of the volcanic records over the past 4000 years from the Greenland Ice Core Project and Dye 3 Greenland ice cores", Journal of Geophysical Research: Oceans, vol. 102, pp. 26707-26723, 1997.
  5. M.G.L. Baillie, "Proposed re-dating of the European ice core chronology by seven years prior to the 7th century AD", Geophysical Research Letters, vol. 35, 2008.
  6. M. Baillie, "Volcanoes, ice-cores and tree-rings: one story or two?", Antiquity, vol. 84, pp. 202-215, 2010.
  7. C.T. Plummer, M.A.J. Curran, T.D. van Ommen, S.O. Rasmussen, A.D. Moy, T.R. Vance, H.B. Clausen, B.M. Vinther, and P.A. Mayewski, "An independently dated 2000-yr volcanic record from Law Dome, East Antarctica, including a new perspective on the dating of the 1450s CE eruption of Kuwae, Vanuatu", Climate of the Past, vol. 8, pp. 1929-1940, 2012.
  8. F. Traufetter, H. Oerter, H. Fischer, R. Weller, and H. Miller, "Spatio-temporal variability in volcanic sulphate deposition over the past 2 kyr in snow pits and firn cores from Amundsenisen, Antarctica", Journal of Glaciology, vol. 50, pp. 137-146, 2004.
  9. R.B. Stothers, and M.R. Rampino, "Volcanic eruptions in the Mediterranean before A.D. 630 from written and archaeological sources", Journal of Geophysical Research, vol. 88, pp. 6357, 1983.
  10. M.W. Salzer, and M.K. Hughes, "Bristlecone pine tree rings and volcanic eruptions over the last 5000 yr", Quaternary Research, vol. 67, pp. 57-68, 2007.
  11. B.M. Vinther, H.B. Clausen, S.J. Johnsen, S.O. Rasmussen, K.K. Andersen, S.L. Buchardt, D. Dahl-Jensen, I.K. Seierstad, M. Siggaard-Andersen, J.P. Steffensen, A. Svensson, J. Olsen, and J. Heinemeier, "A synchronized dating of three Greenland ice cores throughout the Holocene", Journal of Geophysical Research, vol. 111, 2006.
  12. L.B. Larsen, B.M. Vinther, K.R. Briffa, T.M. Melvin, H.B. Clausen, P.D. Jones, M. Siggaard-Andersen, C.U. Hammer, M. Eronen, H. Grudd, B.E. Gunnarson, R.M. Hantemirov, M.M. Naurzbaev, and K. Nicolussi, "New ice core evidence for a volcanic cause of the A.D. 536 dust veil", Geophysical Research Letters, vol. 35, 2008.
  13. C. Barbante, N.M. Kehrwald, P. Marianelli, B.M. Vinther, J.P. Steffensen, G. Cozzi, C.U. Hammer, H.B. Clausen, and M. Siggaard-Andersen, "Greenland ice core evidence of the 79 AD Vesuvius eruption", Climate of the Past, vol. 9, pp. 1221-1232, 2013.
  14. H. Grudd, K.R. Briffa, W. Karlén, T.S. Bartholin, P.D. Jones, and B. Kromer, "A 7400-year tree-ring chronology in northern Swedish Lapland: natural climatic variability expressed on annual to millennial timescales", The Holocene, vol. 12, pp. 657-665, 2002.
  15. M. Sigl, J.R. McConnell, L. Layman, O. Maselli, K. McGwire, D. Pasteris, D. Dahl-Jensen, J.P. Steffensen, B. Vinther, R. Edwards, R. Mulvaney, and S. Kipfstuhl, "A new bipolar ice core record of volcanism from WAIS Divide and NEEM and implications for climate forcing of the last 2000 years", Journal of Geophysical Research: Atmospheres, vol. 118, pp. 1151-1169, 2013.
  16. C. Sun, G. Plunkett, J. Liu, H. Zhao, M. Sigl, J.R. McConnell, J.R. Pilcher, B. Vinther, J.P. Steffensen, and V. Hall, "Ash from Changbaishan Millennium eruption recorded in Greenland ice: Implications for determining the eruption's timing and impact", Geophysical Research Letters, vol. 41, pp. 694-701, 2014.

44 Responses to “The mystery of the offset chronologies: Tree rings and the volcanic record of the 1st millennium”

  1. 1
    wili says:

    If I recall correctly, methane can migrate between ice layers yielding a somewhat false idea about length and intensity of methane release events (sorry, no links on hand; I’d be happy if someone could find them, or correct me if I’m off here).

    Could it be that the acid associated with these volcanic events can also migrate?

  2. 2
    Alan Robock says:

    This agrees with our paper in 2006, which already showed that Eldgja was in 939 A.D., and it should be acknowledged in your work:

    Oman, Luke, Alan Robock, Georgiy L. Stenchikov, and Thorvaldur Thordarson, 2006: High-latitude eruptions cast shadow over the African monsoon and the flow of the Nile. Geophys. Res. Lett., 33, L18711, doi:10.1029/2006GL027665.

  3. 3

    Interesting and provocative post. I come from an ice core lab and am provoked by this post. I am not directly involved in any ice core dating work.

    Here’s what provokes me: zero uncertainty:
    “In this respect, it is extremely unlikely that any of the tree ring data used in this exercise is incorrectly dated.”
    The argument is based on independent datings. But how independent can they be if they are all based on the same type of data? I am convinced that the tree ring dating is really good, but no uncertainty whatsoever sounds too good to be true.

  4. 4
    Russell says:

    Among the sick elephants in the room is the palaeoepidemiology of epidemic plant diseases.

    Tree growth rings may also record the evolving virulence of blights and pests, from plant viruses to wind born insects, rendering dendrochronology even more difficult to deconvolute.

  5. 5
    Jonny McAneney says:

    This is why independent replication is so important. for example, the Irish Oak, English oak, and German oak chronologies, were all constructed independently of each other, using trees from many different locations, and they are found to correlate extremely well over many millennia. This minimises any local effect from disease or predation. One can also correlate trees globally, through specific events such as the AD536/540 signatures, or indeed their radiocarbon signature. For example the recent discovery of excess radiocarbon at AD 774 (Miyake et al 2012) appears in the correct place as a hemispheric signal from Japan, to Siberia, Germany, Ireland, and Western USA (see Jull et al. 2014 and reference within

    This is the power of dendrochronology, and replication and global correlation allows us to ignore any sick pachyderms.

    see also Buntgen et al 2014 (

    [Response: Since you mention Buntgen et al 2014 and the AD 774 event, I would also refer readers to our piece last year in Nature Climate Change, “Missing tree rings and the AD 774–775 radiocarbon event”. It is a challenge to the dendrochronological community, and we look forward to any future related developments. -Mike]

  6. 6
    Jonny McAneney says:

    Alan, you will be pleased to know that we recognised your work and did indeed cite your publication regarding your AD 939 date of Eldgja in our paper.


  7. 7
    Jonny McAneney says:

    Aslak, I can understand why you as an ice core worker would be provoked by the paper.

    As scientists, it is in our nature to bristle at the thought of a zero uncertainty measurement, but as I have mentioned above though, the agreement in tree ring chronologies are really very good, and do lead to precisely dated events. One can check any chronology through comparisons of independently replicated. By this I mean that (for example), dendrochronologists in Belfast construct a chronology, using statistical methods and interpretation to match ring patterns within their own lab. In Germany dendrochronologists will do their own work on their own trees, with their own analysis and interpretation. If one then compares the two chronologies, one will find that key signatures are dated to the same date, with zero uncertainty. Further more, all these chronologies are anchored to the present by living trees. So even though they are effectively measuring the same data, they do so independently of each other. So you can guarantee that if a ring is dated to AD 536, then it is AD 536.

    Uncertainties can come about in the felling date of trees, where there is incomplete sapwood, but when constructing long chronologies, absence of sapwood is not important if you have enough tree rings to form a statistically significant overlap with the rest of your chronology. If I have a tree sample, and it has 200 rings on it, but incomplete sap wood, then while there is uncertainty in the year the tree was felled, there would be no uncertainty in the age of each existing ring, when cross dated with a master chronology.

  8. 8
    Jonny McAneney says:

    Wili, I certainly do not have the experience or knowledge to answer your question, but perhaps an ice core expert would be able to help.

  9. 9
    Hank Roberts says:

    In hindsight, this also, tho’ the authors presume the 934 date (but what physics would connect that date for an eruption with _hot_ summers in China in the following four years?) But in 939, sudden cold:

    … Eldgjá eruption starting around 934 AD was investigated. An extremely hot summer was reported in 934 AD; hundreds of people died of the intense heat …. Snowless (and possibly also mild) winters probably occurred successively following the Eldgjá eruption until 938 AD. In 939 AD, cold weather set in abruptly and lasted for about 3 years; whereas peak cooling occurred in 939 AD….

    Climatic Change, June 2006, Volume 76, Issue 3-4, pp 443-457, 10 Jun 2006
    The Possible Climatic Impact in China of Iceland’s Eldgjá Eruption Inferred from Historical Sources
    Jie Fei, Jie Zhou

    Not saying there’s anything unique about that study, just noticed while poking around. Can other work be read as supporting the 939 date even where the authors didn’t say so at the time, in hindsight? Little surprises …

  10. 10
    Hank Roberts says:

    … a cosmic event that happened in 774/775 caused the production of enough Carbon–14 to throw off the chronology by hundreds of years. This is seen in the close examination of Carbon in the tissue of trees placed in a tree ring chronology.

    Greg Laden’s blog, 7/29/2014

  11. 11
    Hank Roberts says:

    Fei and Zhou are well discussed in
    and I’ll tiptoe back to the peanut gallery where I belong.
    Fascinating. I’ll learn more if I write less.

  12. 12
    jyyh says:

    (delete) As a lighthearted note I’ll ask the obvious, that is, why haven’t we heard about this through WUWT before?? Seven years is a hugely long time period and this gamma-ray burst you’re talking about might be the final proof of the influence of ancient aliens on the psyche of medieval man! A lot of history can be rewritten in the lost seven years! Have we finally traced the origins of climate conspiracy to correct time in history? Did Charlemange and Pope Adrian I wipe out the memories of 7 years of all the people by using alien tech?? This revelation could lead to a whole new chronology of mankind all together! I mean, what do we know about the effects of gamma ray bursts of cosmic origin to the psyche of contemporary man, let alone the medievalists? (/delete)

    Interesting article, thanks. I’ve sometimes thought that global cataclysms like the largest volcanic eruptions would disrupt the glacial records by many years, like Oruanui eruption c.26500bp, as these would induce unrecorded behavior in weather and other things, f.e. the huge ash deposits might decrease the albedo so much a local melting event happens. In addition there’s of course the possibility of no snow on the drilling site (losing a layer here and there), and on some sites there could be additional layers if there’s a normal melting event before the winter snowfalls, I guess. I’d imagine these would be much more common during the HCO (Holocene Climate Optimum), and thus I’ve been assuming these ice core chronologies are just advisory – what would be the proper period for the N-S seesaw is to me very much an open question. But its useful to have a reference chronology in science (and in history) to make it easier for making cross discipline research.

  13. 13
    jyyh says:

    the 14C spike in 773+-14 has been attributed to a comet collision,
    this would connect to the later date increases quite well if the numbers on amounts match, part of the comet could have splintered high in the atmosphere producing a fallout of the said radioisotope during next few years.

  14. 14
    Thomas P says:

    How do varve data (lake sediments) fit into this?

  15. 15
    Jonny McAneney says:

    jyyh, regarding the comet impact hypothesis. This was a nice idea, but it doesnt hold up to scrutiny.

    If one reads the paper, ( – See more at: can see the authors datingis not very tight, which is crucial. They date their coral using thorium radiometric dating, and get a date of AD 783 +/- 14. They then note that it is the layer 7 years below this that has the 14C anomaly, making the date 776 +/- 14 years, then they arbitrarily move it to 773 to coincide with the beginning of the 14C excess in Japanese cedar.

    This date of AD 773 then seems to coincide with an entry in Chinese records of the “comet impact”, with it stating that the impact occurred in the 7th year of Dai Zong. As best that I can make out, the accepted date for Dai Zong beginning his reign is 762 or 763, meaning that the 7th year of his reign is between 768-770 depending upon whether the 7th year is inclusive. So unless the authors of the paper are privy to other information, then I cant see how the two events coincide.

    Furthermore this paper The Solar Cosmic-Ray Origin for the Rapid 14C Increase in AD775 states the following with respect to the chinese chronicle

    “In the evening on the Chinese lunar calendar day of 11
    Dec. 774, i.e., 17 Jan. AD775, in the east and above Moon,
    there were more than ten bands of white lights like the
    spread silk, penetrating and covering eight grand constellations
    named in Chinese, corresponding to the sky composed
    of Taurus-Auriga, m Gemini, q Cancer, l Orion,
    V Orion, e Taurus, d Hydra and a Leo, and the lights
    were ceased gradually after middle night, as recorded in
    the Old Tang Book – a Chinese Chronicle [7]. The auroras
    described are actually located in a wide region from east
    to west and from north to south in Earth’s northern hemisphere.
    The records indicated clearly that time to see auroras
    is most probably from 5:00-6:00 PM to 1:00-2:00 AM,
    therefore, the event lasted about 8 hours. In Tang dynasty,
    the natural abnormal events were observed and recorded by
    qualified officers, so the records are highly trustable and,
    the work was conducted in the Tang capital Xian, which
    is located in a geomagnetic latitude at lower twenties. The
    low geomagnetic latitude indicates that the solar particles
    which caused very strong auroras are very intense and with
    higher energies.”

    We also know that there was another large 14C excess again in 993/994, but less intense (about 0.6 the energy flux of the 774/775 event) but that the flux was similar to the 774/755 event

    So if we look to “historical records”, we see the following events suggesting aurora displays worthy of note.

    Annals of the Four masters M992.19 The colour of fire was in the heavens till morning.
    Annals of Ulster U992.4 A remarkable manifestation on St. Stephen’s night, the sky appearing blood-red.

    Thomas Short

    992 In November, for three Nights successfully, the heavens seemed bloody.
    993 On the 7th of the Calends of January, at one a Clock in the Night, suddenly Light Shined out of the N. like mid-day; it lasted an Hour, but the Sky turning red, the Night returned.

    Usoskin and Kovaltsov 2014 ( have also shown that the comet impact hypothesis is untenable. In a nut shell, to generate the amount of excess radiocarbon observed, the comet would have to be massive (at least tens of kilometres, if not 100 km in size). If such an impact occurred, we would not be here to discuss it.

    So I dont think that 773 is the date of a comet impact, and that the Sun is the likely culprit for the 14C excess (Usoskin et al. 2013

  16. 16
    Jonny McAneney says:

    Thomas P, I havent been privy to any varve data, but it is an interesting question. Someone here might have an answer though.

  17. 17
    Hank Roberts says:

    Aside: et al. and Usokin explain much by “It’s The Sun!” — but I note they’re said to have discovered a new kind of sunspot (according to “Watts Up With That? Feb 22, 2014 – Guest Post by Willis Eschenbach) and a ‘hockey stick’ rapid recent increase of solar influence on climate (…19U).

    Looking for ways to “prove” rather than disprove a hypothesis leads one into strange places.

  18. 18
    jyyh says:

    thank you for the very detailed response. yes, agree that auroras visible during the early evening would be a result of hugely energetic particles.

  19. 19
    Hank Roberts says:

    Broken link above; can’t make a working link to Usoskin’s hockey stick paper.
    It is SAO/NASA Astrophysics Data System (ADS)
    Title: Solar activity over the last 1150 years: does it correlate with climate?
    Authors: Usoskin, I. G., Schüssler, M., Solanki, S. K., & Mursula, K.
    Journal: Proceedings of the 13th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun, held 5-9 July, 2004 in Hamburg, Germany. Edited by F. Favata, G.A.J. Hussain, and B. Battrick. ESA SP-560, European Space Agency, 2005., p.19 Bibliographic Code: 2005ESASP.560…19U

  20. 20
  21. 21
    Hank Roberts says:

    > Solar activity over the last 1150 years: does it correlate with climate?
    Shorter: it did, until the past 30 years, when climate got warmer faster while the solar activity diminished.

    For JJ et al., that’s that global warming activity (always in the same direction) emerging in the past several decades from the background variation (“noise” that goes up and down).

  22. 22
    Thomas says:

    So let us assume for the purposes of discussion that the ice core dating is seven years too long. Doesn’t this imply that seven years were somehow double counted? Could this happen as a result of some extreme weather event, creating an extra ice horizon which is mistakenly taken as an annual layer? It would seem odd if geographically separated ice cores contained the same skips, wouldn’t it.
    Anyone care to comment?

  23. 23
    C Lott says:

    The dating problem may relate to the changing ratios of Carbon isotopes.
    Atmospheric bomb tests created a “spike” (max at 1963.) of Carbon-14.
    Fossil fuels make things date “older” and bombs make things date “younger”. Are the trees Radiocarbon Dated?

    [Response: Other way around. Tree ring chronologies are used to calibrate the radiocarbon time scale. – gavin]

  24. 24
    Jonny McAneney says:

    Thomas, if we are correct in our hypothesis, then only the ice core workers would be able to assess why independent ice cores exhibit similar offsets in their dating. One can always speculate many different scenarios to explain such an offset, but we suspect that it may be something to do with a methodological issue of replication, rather than a global weather issue, though the latter would be really interesting if this were the case!

  25. 25
    Jonny McAneney says:

    Dendrochronologically derived dates do not rely on radiometric dating. In fact, it is actually the other way around. When we make a tree-ring chronology, we begin with measuring the tree ring pattern of living, growing trees. That way we have a solid, correct anchor date, i.e. the year that living tree was sampled. So, we map out the tree-ring pattern of those living trees, then we look to historical timbers, such as those used in construction of buildings. It doesn’t matter if we do not know the date of construction of the building, since we need only look at the ring patterns in the timbers, and then cross match it to our living tree patterns. Then we try to extend the chronology backwards in time by trying to find older and older timbers, moving from historical timbers, to archaeological and otherwise naturally preserved timbers (e.g. bog timbers).

    The above describes the process for the likes of European oak, but when it comes to some trees we find that they are sufficiently long lived to provide quite extended chronologies from living specimens. Such a tree is the bristlecone pine, which can live at least a few thousand years, and furthermore, its habitat is at high altitude and arid so any dead samples tend to be preserved.

    This all leads to chronologies that extend many thousands of years into the past, with ring patterns that are precisely dated to annular resolution. As a result, we can select any ring on any sample and can radiocarbon date it, and compare the radiocarbon age to the calendar date of that ring. If we do this for a series of rings through any given chronology, we can then establish a radiocarbon calibration curve, which allows us to translate any radiocarbon age into a true calendar date. Indeed, many tree ring chronologies were first constructed for this very purpose. This then overcomes the natural fluctuations of radiocarbon concentrations in the atmosphere and carbon reservoirs.

  26. 26

    Being a microscopist, I would like to see some good photomicrographs of these frost rings. I have some methods of polishing cross sections of wood that might be helpful for tree ring analysis. Very interesting, these tree rings. I want to learn.

  27. 27
    Jim Coyle says:

    Jonny; Just how far back have the tree rings taken you in dating? I was thinking about the tree specimens that have been found in the far north( above the arctic circle). If dating has progressed back far enough maybe some of the impact dates could be confirmed through ring data. Specifically the YDB. If dating could confirm an impact at that time frame it may go a long way in firming the impact hypothesis. Same would apply to the Carolina Bays theory.

  28. 28
    Hank Roberts says:

    > photomicrographs of these frost rings

    Google image search is productive, e.g.

    Photo caption (the original has links to sources):

    This image of Mongolian frost rings was taken by Lamont-Doherty Earth Observatory’s Dee Breger and won First Prize in the first annual NSF/Science Magazine Award For Visualization. (Breger also took second prize with her breathtaking magnification of Black Sea pyrite.) These Mongolian frost rings are from a sample of Siberian pine tree collected by Lamont-Doherty’s dendrochronologist Gordon Jacoby, and records the years 534-539 C.E. (left to right). The narrow, distorted rings for 536 and 537 indicate a drastic cooling in the northern hemisphere that froze sap in the cells during the growing season. Evidence for this abrupt climate change points to a massive eruption of the volcanic precursor of Krakatoa. Another theory invokes a cosmic impact that possibly triggered the eruption. Magnification ~ 19.32 X.
    view more images by Dee Breger

  29. 29
  30. 30
    Jonny McAneney says:

    Jim Coyle, there are various tree-ring chronologies around the world, and each of different lengths. As far as I am aware, the longest is the German chronology that extends back 12,410 years in total. This chronology is composed primarily of oaks, but far back it also contains Preboreal pines. The pine part of the chronology has had to be inserted into the master chronology by cross matching with the oak chronology. As such the entire chronology covers 820 years of the Younger Dryas (see Friedrich et al. Radiocarbon 46, 1111-1122, 2004

    The problem is though, while trees can identify sudden climatic deterioration, usually are unfortunately blind as to what caused it. I say usually, since there are cases such as the trees flattened by the Tunguska impact, or buried by volcanic ash, that quite obviously recorded direct effects these event. However, a tree some distance from the event, would likely respond the same way to a volcanic dust veil as an impact dust veil, all other things being equal. In this respect, while having a tree ring chronology that extends across the entire YD would be interesting, I would have reservations regarding whether it could shed any light on the contentious YD impact hypothesis, or the origin of the Carolina Bays.

  31. 31
    Jonny McAneney says:

    Hank Roberts, just a note regarding the figure caption of that lovely image. It states that the eruption that caused led to the frost ring formation was Krakatoa (or a precursor), but in actual fact, there is no positive identification of the volcano (or indeed volcanoes if there were more than one culprit), that caused the climatic dislocation. The Krakatoa hypothesis was, if I remember correctly, originally postulated by David Keys. Other candidates for the 536 event have also been put forth, such as Rabaul, or Ilopango.

    Also, the caption also points out that it could have been an impact induced eruption. This to me pushes Occam’s razor a bit. We could have a large impact, or a large volcano, but why do we need one to cause the other to explain the AD 536 event.

    Much of the impact hypothesis stems from my co-author, Mike Baillie’s work, who (to my knowledge) was the first to propose that the AD 536 and/or AD 540 events may have had an impact origin. Much evidence pointed to a large volcanic eruption around AD 536, with no evidence of one in in AD 540, and there was much debate over its climatic effects. Could a single large eruption really have caused a decade of climatic disturbance beginning in AD 536, with a brief recovery, and then a dramatic downturn at AD 540? So without any ice core evidence of a volcanic event in AD 540, a cosmic impact could be seen as the next best cause for a two stage event.

    Now, if our hypothesis is correct, and the ice cores are mis-dated in the 6th century, we find that there are indeed volcanic eruptions in AD 536 AND AD 540, which means that a cosmic impact is no longer required a la Occam’s razor. That being said, a cosmic element cannot be totally ruled out, since there are independent lines of evidence from history and astronomy that suggest that the skies may have been much busier than normal during this time. Furthermore, and this we feel is important, the AD 540 eruption is missing from the various records.

  32. 32
    JBL says:

    Just wanted to say that this thread, and in particular the engagement of the author with commenters, is everything that makes RealClimate such a wonderful place. So, many thanks to the group for hosting and to Jonny McAneney for the post and comments!

  33. 33
    Arthur Smith says:

    If I understand this correctly, the argument is that from the present back to around 1000 AD the tree rings and ice core dates agree, but before that there is a 7-year discrepancy. There should be some notable event associated with the cause of the discrepancy itself, I would have thought – can that date when the discrepancy began (/ended) be narrowed down at all?

  34. 34
    Hank Roberts says:

    Re that caption from 2003, definitely the attributions need to be reviewed — I pointed to that just as one handy example of the availability of photomicrographs, of which there are many available.

    There are several large archives of tree ring material going back many years — and I would guess they’ll all be annotated at some point to make clear whether they’re captioned using the “old date” or a “revised as of… date”

    The potential for confusion on any large science archive like this is always important to watch for — especially here where several different dating systems (tree rings, ice cores, lake sediments, ocean sediments) are being correlated.

    I recall years ago Gavin came back from a China meeting and wrote here that while there were many detailed studies at single geographic locations, there were not yet enough studies trying to cross-compare them. Good to see it happening.

  35. 35
    Jonny McAneney says:

    Arthur Smith, again, if we are correct, then all we can ascertain is that the misdating appears to occur above circa AD 950, and below circa 1250, that it is difficult to determine where in the ice cores the error occurs from frost rings etc. The seven year offset may not happen all at once, and could be an accumulation of misinterpreted ambiguous layers in that approximate three century interval. Originally we had thought that Eldgja might have been correctly dated to AD 934, since it was thought to be large, and occurs, as chance would have it, at a frost ring date. But when we considered other factors such as the arguments of. Oman et al. 2006, Sun et al 2014 etc, which forms the growing contention of it having a later date, that this frost ring date may be a red herring.

    Hopefully the ice core workers will reexamine their chronologies to try to falsify our hypothesis, since ultimately it will only be the ice cores that can prove us correct or incorrect, and if we are correct, identify where the offset occurs.

  36. 36
    Jonny McAneney says:

    JBL, thank you for the kind words. I didnt want to exploit this blogs kindness in giving us a platform to promote our work by leaving a commentary orphaned of any further input. Besides, I do enjoy engaging in scientific discussions online (when i have time to do so).

  37. 37
    andy says:

    Is it possible to estimate the probability of a ‘lost tree ring’ level volcano event for eg 21st century, based on these older records?

  38. 38
    Jonny McAneney says:

    Andy, in order to have a “lost ring event”, the volcanic eruption would likely need to be quite large (I would imagine being 6 or seven on the Volcanic explosive index VEI). So I tried digging around to see what I could find, and I came across Global Catastrophes and Trends: The Next 50 Years by Vaclav Smil (

    As you would read there, it states that one could expect a VEI 7 eruption to occur once every 1700-10,0000 years, which translates to a 0.5-2.9% probability of such an eruption in any 50 year period, or 1.0-5.8% probability in the next century.

    This is outside of my experience, and I am sure a good google search might reveal much more of the probability of such hazards.

  39. 39

    Does anyone have a volcanic activity time series that goes later than 1995? That’s as far as the Lamb DVI extends (it went to 1983, but Dr. Mann extended it to 1995, as a I remember). Still, there’ve been 20 more years since then, and I’d like to have more data.

  40. 40
    Celestial Elf says:

    In support of raising awareness about Global Warming and Climate Change, my animation – The World Has Got A Problem;

  41. 41
    Halldór says:

    Regarding Eldgjá could you please explain what you mean with the single year you keep quoting. This eruption produced about 18km3 of lava. The recent Holuhraun eruption ran from Sep 1st 2014 to Feb 27 2015,- or 6 months. During this time it produced about 2km3 of lava which is an exceptionally high effusion rate. In Iceland the highest since Laki 1783-84 which produced about 14 km3 in 9 months.
    Now, if Eldgja had the same effusive rate as Laki it could have produced 18km3 in year, but sources in Iceland usually talk about 3 to 8 years for Eldgjá (which actually makes the effusion rate similar as to that of Holuhraun). With an eruption lasting a few years it could have gone on between 934 and 942. So I am confused about this 934 vs 939 redating. Are you talking about the initial phase of the eruption, the end, the middle or you just don’t know?
    (For details see )

  42. 42
    Jonny McAneney says:

    Halldor, when we are talking about the dates of eruptions, such as the contention of the Eldgja eruption date, either 934 or 939, we are typically talking about the start date for the eruption, as evidenced through historical documentation, or physical evidence such as volcanic horizons in ice cores.

  43. 43
    15120092 says:

    If I recall correctly, methane can migrate between ice layers yielding a somewhat false idea about length and intensity of methane release events.
    Could it be that the acid associated with these volcanic events can also migrate? (15120092)

  44. 44
    Hank Roberts says:

    for ‘15120092’
    I pasted your question into Google Scholar and found you’re not the first to ask that question. Among the highly cited papers, I found:

    … Migration appears to be limited to either the preceding or following seasonal layer for each species, suggesting that paleoclimatic interpretations based on data with lower than annual resolution are not likely to be affected.

    Repository Citation
    Kreutz, Karl J.; Mayewski, Paul Andrew; Whitlow, Sallie I.; and Twickler, Mark S., “Limited Migration of Soluble Ionic Species in a Siple Dome, Antarctica, Ice Core” (1998). Earth Science Faculty Scholarship. Paper 235.

    You can find much more on the subject; that’s one quick example of what’s out there to be found on the question.