Mark Your Cosmic Calendar: 774/775
One wonders if anyone felt it. Did Charlemagne feel it as he led his forces across Pagan Saxon Westphalia, knocking down Irminsuls and making everyone pretend to be Christian or else? Did the people of Bagdad, just becoming the world’s largest city, notice anything aside from their own metro-bigness? Did the Abbasid Caliph Muhammad ibn Mansur al-Mahdi have the impression something cosmic was going on that year, other than his own ascendancy to power? Or was it mainly some of the Nitrogen molecules in the upper atmosphere that were changed, not forever but for an average of 5,730 years, by the event?
A long time ago, probably in our galaxy but kind of far away, a cosmic event happened that caused the Earth to be bathed in Gamma rays in AD 774 or 775. No one seems to have noticed. There is a mention, in 774, of an apparition in the sky that could be related, but talk of apparitions in the sky were more common back then, before they had certified astronomers to check things out. There is chemical and physical evidence, though, of the Gamma ray burst. The best evidence is the large scale conversion of stable Nitrogen isotopes into unstable Carbon–14 isotopes in the upper atmosphere. As you know, radioactive (meaning, unstable) Carbon–14 is created continuously but at a somewhat variable rate in the upper atmosphere. Some of that Carbon is incorporated, along with regular stable Carbon, into living tissues. After the living tissue is created and further biological activity that might retrofit some of the Carbon atoms ends (i.e., the thing dies) the ratio of radioactive Carbon to stable Carbon slowly changes as the radioactive Carbon changes back into Nitrogen. By measuring the ratio now, we can estimate how many years ago, plus or minus, the originally living thing lived and died.
But it does vary. Solar activity, nuclear testing, other things, can change the amount of Carbon–14 that gets produced. And, 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. For example:
Original Caption: High-resolution radiocarbon ages, superimposed on annually resolved radiocarbon measurements from Japan and Europe (grey lines and crosses) as well as the IntCal calibration curve based on decadal samples (blue shading), re-sampled at 5 year intervals (light blue crosses). Radiocarbon ages (that is, using 14C, 13C and 12C isotopes) were determined at ETHZ with the MICADAS system.
See the inverted spike there? That is, apparently, gamma rays messing up the Radiocarbon chronology. Hold that thought.
Climate Change Is Hard
When volcanoes erupt, they typically spew crap into the air. Some of this material stays in the atmosphere for a while (called aerosol, but not your underarm deodorant exactly) which will in turn reflect sunlight back out into space prematurely. This causes cooling. It is essential to know how much cooling of the atmosphere happens from aerosols because this is a potentially important factor in global warming. The effect of aerosols caused by volcanoes or industrial activity is an important term in the big giant equation that puts all the different factors together to produce global warming (or cooling). It is important that climate models be able to accurately and realistically incorporate the effects of aerosols. If the science isn’t right on aerosols, climate models may not run true when aerosols are included.
And indeed there is an apparent problem. When climate models are run and include aerosols, and the results are compared with real life data where we have good proxyindicators of past climate, the model predictions and the real life measurements don’t line up when aerosols are involved at any significant level. A big volcano goes off, but the proxy record consisting mainly of things like tree rings doesn’t show the level of cooling models predict. This has titillated denialists, as you might imagine, because it shows how the science has it all wrong and the only way to truly understand the climate change is to spend hours in the basement with your spreadsheet and a good internet connection, like Galileo would have done.
In fact, this was an interesting problem that needed to be addressed. The modeling methods had to be wrong, or the paleodata had to be wrong, or something had to be wrong.
In 2012 Michael Mann, Jose Fuentes and Scott Rutherford published a paper in Nature Geoscience proposing a hypothesis to explain this discrepancy. The problem was that when a known volcano went off, the tree ring record in particular tended to show only an anemic result. Volcanoes that were thought to totally mess up the weather seemed to have little effect on trees. This even applied to volcanoes which were very directly observed in recent times, when we know there was an effect because people were putting on sweaters and measuring things with actual thermometers.
Mann et al proposed that rather than having little effect on tree growth, the volcanoes had a huge effect on tree growth. What was being seen by the Dendrochronologists (tree daters, like tree huggers but more serious) as a normal, average growth ring at the time of a volcanic eruption was actually the ring for the next year in line; they were missing, understandably, one or more growth rings. The volcano goes off, the trees don’t grow at all. (The masquerading ring would typically be the year before the missing ring since dendrochronology is done backwards, since we know what year it is now.)
You don’t have to imagine a year in which no tree grows ever anywhere to accept this idea. The trees being used as temperature proxies are more the sensitive type. They respond to temperature changes by growing more or less (warmer vs. cooler). Trees that don’t do this are not chosen for study. This has to do with the species and the setting the tree grows in, combining to make temperature the key limiting factor most years, so that growth ring width reflects temperature more than any other factor. So yeah, when it gets very cool because of a big-ass volcanic eruption, one of those “year with out a summer” deals, the very sensitive trees respond by not growing at all that year. They may have a growth period of a few weeks, but trees don’t simply lay down wood every day they are biologically acvite. They usually start with leaves, then many move on to reproduction, and once they have finished reproducing, have a cigarette, wash up, whatever, they may lay down wood or roots. (Different species have different patterns). So a very short growing season can mean no ring at all. If a really bad nuclear-winter-esque volcano happens this may go on for a few years. This leads to the growth ring corresponding to the year of the volcano simply not being noticed by the dendrochronologists, with a different year standing in. Over time the record can be thrown off by several years, if there are a few volcanoes and one or more of them affects growth for more than one year.
So two things happen. Years with a very strong cold signal are lost entirely, and the record is quasi-randomly offset by a few years in some but not all tree records (because some will be thrown off while others are not) so the collective record gets out of alignment. A strong uptick in the signal (the zero growth year) does not contribute to the paleoclimate squiggle of temperature at all, and the other possibly contributing years (after the worst is over) are moved around in relation to each other and average in with less cold years. It’s a mess.
Consider the following made up numbers representing temperature over time. The top table is the hypothetical raw data of tree ring growth in relation to temperature across a very strong cold anomaly as might be caused by a massive volcanic eruption. Depending on the tree, there is one or more years of zero growth. The lower table is the same set of numbers but with the earlier years (top) shifted down to cover the zeros, because that is what would happen if a dendrochronologist was looking at the rings from more recent (bottom) to oldest; there would just be this void and it would be filled with the next data in line.
Here are the same data graphed showing a clear anomaly in the top chart, but the very clear anomaly utterly disappears because of missing rings and shifting sequences in the lower chart. This is an existential problem for ancient climate events. I squiggle therefore I am.
Mann Et Al proposed adjustments to the record of proxyindicators of temperature that accounted for missing tree rings at the time of major volcanic events. They made a good case, but it was a bit complicated and relied on some fairly complicated modeling.
Since the publication of Mann et al there has been quite a bit of back and forth between the climate modelers and the dendrochronologists. I’ve assembled a list of publications and blog posts below. I’ll only very briefly summarize here.
The dendrochronologists had a bit of an academic fit over the idea that they had missed rings. Understandably so. As an archaeologist, I’m partly trained in dendrochronology. There was actually a time when I considered making it my specialty, so I had read all the literature on the topic. I can tell you that missing rings was a serious concern, and taken seriously, and seriously addressed. Seriously, there’s no way modern dendrochronologists would totally miss an entire year’s growth rings. They had ways of dealing with missing rings.
The thing is, it is actually possible to miss rings. Here’s why. The assumption in Dendrochronology is that rings can be missed (or for that matter, added) for reasons that allow for correction by cross dating growth ring sequences with other trees or even other samples in a single tree. A particular part of a tree can be missing a ring while another is not (especially vertically; the lower part of a tree grows last in many species), or some trees in an area may be missing a ring, but others have that growth ring. This assumption is probably almost always valid; missing rings can ben adjusted for by cross checking across samples. But, if all of the trees of a given species and sampling area have one or more missing rings because of a major volcanic event, that won’t work. But this is not something Dendrochronologists are used to.
2 + 2 = 774/775
Eventually Mann and his Colleagues put two and two together and realize that the Dendrochronologists had a way to test the hypothesis that would not rely on fancy dancy climate modeling techniques, and that would potentially allow a better calibration of the tree growth ring record for certain time periods. It was that Gamma ray burst.
That moment in time is a clear marker. Any system involving Carbon–14 spanning this time interval should show the spike. Well, what about tree ring records that span both a major volcano and the 774/775 event? If Mann et all are right, an uncorrected tree ring record would show a lack of correspondence of any spike at 774/775. But, if missing rings are assumed for sensitive tree records at the time of the volcano, and the tree ring sequence for those trees shifted, perhaps the records will line up. That would be a test of the hypothesis.
And this is the gist of a letter to Nature from Scott Rutherford and Michael Mann. Very simply put, Mann and his colleagues took this graph, from an earlier paper:
And changed it to this graphic which shows mainly (see caption) the tree ring sequences that span both the 1258 volcanic eruption, which was a big one, and the 774/775 event.
This is a gauntlet, being respectfully thrown down. Mann et al erected a hypothesis, that missing tree rings are virtually universal in large parts of the dendrochronological sample for some events, were not accounted for in the tree ring chronology, and have thus messed up the tree rings as a proxyindicator for temperature. Various attempts to knock it down have not worked out. Now, Mann has himself provided an excellent way to assail his own idea. It is now up to the tree ring experts to try to knock this hypothesis down. I suspect Charlemagne might have had an easier time knocking down the Irminsul.
I asked Michael Mann how he felt about this latest development in the ongoing saga of the missing (probably) growth rings. He said, “I’m very pleased that we’ve reached some level of reconciliation with our dendroclimatology colleagues: there’s an objective test that is available to determine if there are indeed missing rings in some of the regional chronologies as we have speculated to be the case. I look forward to seeing the results of those tests. We proposed a hypothesis, other scientists were skeptical of the hypothesis, and now there is a way forward for testing the hypothesis. In the end, a fair amount of good science will have been done, and we will have learned something. This is the way science is supposed to work.”
This is going to make a great Master’s thesis for someone.
As promised, a list of writings on this topic, organized by date:
2012 Mann, M.E., Fuentes, J.D., Rutherford, S., Underestimation of volcanic cooling in tree-ring- based reconstructions of hemispheric temperatures, Nature Geoscience, doi:10.1038/NGEO1394, 2012. Press release here.
2012 Mann Et Al. Global Temperatures, Volcanic Eruptions, and Trees that Didn’t Bark. Real Climate.
2012 (November) Kevin J. Anchukaitis, Petra Breitenmoser, Keith R. Briffa, Agata Buchwal, Ulf Büntgen, Edward R. Cook, Rosanne D. D’Arrigo, Jan Esper, Michael N. Evans, David Frank, Håkan Grudd, Björn E. Gunnarson, Malcolm K. Hughes, Alexander V. Kirdyanov, Christian Körner, Paul J. Krusic, Brian Luckman, Thomas M. Melvin, Matthew W. Salzer, Alexander V. Shashkin, Claudia Timmreck, Eugene A. Vaganov & Rob J. S. Wilson. Tree rings and volcanic cooling. Nature Geoscience 5, 836–837 (2012) doi:10.1038/ngeo1645
2012 (November) Mann, Fuentes and Rutherford Reply to ‘Tree rings and volcanic cooling’. Nature Geoscience. 5, 837–838 (2012) doi:10.1038/ngeo1646
2012 Gavin at RealClimate Responses to volcanoes in tree rings and models
2012 Esper et al. Testing the hypothesis of post-volcanic missing rings in temperature sensitive dendrochronological data Dendrochronologia. Volume 31, Issue 3, 2013, Pages 216–222
2012 Esper et al. European summer temperature response to annually dated volcanic eruptions over the past nine centuries. Bulletin of Volcanology. June 2013, 75:736
2013 George et al. The rarity of absent growth rings in Northern Hemisphere forests outside the American Southwest. Geophysical Research Letters. 40(14) 3727-3731.
2013 D'Arrigo et al. Volcanic cooling signal in tree ring temperature records for the past millennium Journal of Geophysical Research: Atmospheres. Volume 118, Issue 16, pages 9000–9010, 27 August 2013
2014 Jull et al. Excursions in the 14C record at A.D. 774–775 in tree rings from Russia and America. Geophysical Research Letters. Volume 41, Issue 8, pages 3004–3010, 28 April 2014
2013 Mann, Michael, Scott Rutherford, Andrew Schurer, Simon Tett, Jose Fuentes. Discrepancies between the modeled and proxy-reconstructed response to volcanic forcing over the past millennium: Implications and possible mechanisms. J. of Geophysical Research: Atmospheres, Vol 118, 7617–7627.
2014 Büntgen. Et Al. Extraterrestrial confirmation of tree-ring dating. Nature Climate Change 4: 404-405.
2014 [Rutherford, Scott and Michael Mann. Missing tree rings and the AD 774–775 radiocarbon event](http://www.nature.com/nclimate/journal/v4/n8/full/nclimate2315.html?WT…]. Nature Climate Change. Vol 4, August 2014.
Superb piece Greg. I heard about the paper today, but have not had time to make sense of it (and I know nothing about dendro science). Great explanation and to Dr. Mann - great science!
Novel climate proxy reveals CO2 content of Earth’s atmosphere, of the past 400 million years http://climatestate.com/2014/07/30/co2-in-earth-history-the-past-400-mi…
Thank you for that explanation, Greg; I'd seen that paper mentioned a few times in my facebook feed, but hadn't been able to access it (and I suspect much of it would have been over my head if I had accessed it). I'll have to check your links now for more background.
Just out of curiosity, is this the "trick" that Mann was working on when he sent an email to Phil Jones?
Mike, no. It is my understanding that the "trick" was to use actual measurements of temperature where there were measurements and to use tree-ring data under CO2 conditions where tree ring data was reliable (i.e. had matched actual measurements of temperature), and to not use tree ring data at CO2 levels where tree ring data was unreliable (where it didn't match actual measurement).
Tree ring data becomes unreliable when CO2 levels are very high. We know the CO2 levels of the atmosphere quite well (from ice cores).
David, very close but not exactly. The sentence is
"I've just completed Mike's Nature trick of adding in the real temps to each series for the last 20 years (i.e. from 1981 onwards) and from 1961 for Keith's to hide the decline."
I will now rewrite that informally written sentence to make it more clear.
"I've just completed applying the technique Mann developed in his 1998 nature paper to concatenate recent instrumental data to earlier proxy data, the latter ending around 1980, so we can have a temperature series that runs up to nearer the present, in a way that retains a clear distinction between proxy data and instrumental data, all of which is available for anyone who wants to check.
Oh, and in a different matter, I extended the instrumental data back to 1961 for Keith's data because that tree ring record suffers from the divergence problem, which is a reduction, or decline but reduction is a better word, in the sensitivity of the trees to indicate temperature variation, making that data set inappropriate to use after 1961 in a scientifically valid study. So now those two legitimate scientifically valid and proven techniques are applied I just hope nobody finds this email!"
(Last part added for fun.)
Thanks! I remembered wrong as to who sent the email to whom.
Excellent post. I very much enjoyed it. I see I have some more stuff to chase down now.
In the photo caption you mention that Tambora erupted in 1816. The big eruption was actually in 1815 which then produced the "Year Without Summer" the following year in 1816.
Again, good piece.
The first graph doesn't match the first table very well, which might slow down some readers. Or maybe I'm missing something.
Thanks for the interesting article.
Alan, thanks, fixed.
Funny that your list of references is missing Büntgen et al (2014) from which you took your radiocarbon figure (and which is the 1st reference of Rutherford and Mann):
Büntgen U, Wacker L, Nicolussi K, Sigl M, Güttler D, Tegel W, Krusic PJ, Esper J (2014) Extraterrestrial confirmation of tree-ring dating. Nature Climate Change 4: 404-405.
From that article:
"All studies demonstrate the precision of tree-ring dating back to the year 775. No single volcanic eruption was strong enough to trigger summer cooling sufficient to cause trees to remain dormant throughout the growing season, thus forcing a dating error due to missing rings. This behaviour has now been observed for records from different tree species growing at temperate low-elevations
9,10 and near the upper treeline12, which is evidence that the records are correctly dated."
"Our study subscribes to the mounting evidence for the precise dating of tree rings."
Jean S, not so funny, just an oversight. I had linked to the paper in the text (but in a way that the link was invisible, as it turns out). I'll add it to the list of references.
"Carbon dioxide has increased about 40 percent in the atmosphere since the 1750s, due to pollution from dirty energy like coal, oil, and gas. The result is a warming climate." http://clmtr.lt/c/Klh0cd0cMJ
Steve McIntyre has no doubt audited this latest effort of Mann's and found all sorts of things wrong with it, after a few dozen FOI requests in order to get hold of Mann's data so that he doesn't have to put any effort into collecting his own.
Lars, maybe. But Mann's Data is all available publicly for this, I'm pretty sure.
What he really wants via FOI is an email that says:
"Hey, how's it going I hope everybody is doing well. We use that new company you suggested to make our annual christmas cards this year, and they do a great trick or treat card for October. Going to be a lot quieter around here this evening when the kids go to play hide and seek; I'll just take the evening off and recline in my new chair and and be glad they are not swarming around me for once"
Which can be modified to say, "Hey everybody use this trick to hide the recline in swarming"
Hey, wait a minute....
Wasn't serious, Greg. I was thinking about all of the fuss that McIntyre et al. made about dendrochronology in the past, without, apparently, having any experience whatsoever in field techniques, data collection methods, or analytical techniques.
I know, I was agreeing with your snark!
I dated a tree once...
I thought it was a sweet date. On a larch, I took her to the Grove (had to palm the mâitre d' a sawbuck, but it was worth it). Yet afterward this willowy blonde wood not see me again.
She said, "Look bud, yew leaf me alone or I'll call the copse." Not sure what the root of the problem was... Board with me so soon? And I thought she took a lichen to me.
I said, "Don't drupe this relationship", figuring I'd go out on a limb and try to ply her with log-ic... But she sawed through my thin veneer. Nuts!
Oak-ay, was it because I told her not to count on a ring from me? She said it stemmed from me knot being able to see the forest for the trees when it came to matters of the heartwood.
Fir sure, I felt like a sap after that, but I won't pine over this. Not mulch I could do... We were finished. I musta been barking up the wrong tree.
Ash I see it, only a light stain on my rep... I'm still poplar with the girls, but maybe I need to branch out some. The dating scene's a jungle, you know...
You probably got all spruced up for that date, too. It sounds like something you said or did just went against her grain. Holly cow! Did juniper? or just gum 'er? Perhaps she thought it was a beech of protocol.
Seriously, Greg, this is an excellent article. I look forward to seeing the followup. I have the feeling that some interesting experiments might be done with a few living trees and a particle accelerator, over a short span of years.
Its almost like we live in a culture based on wood!
Oh, the great ones I had to chop out of my first draft due to (what I assume are) joint interests of not furnishing something that, if prunish readers red,wood in-flame their sensibilities and look to take me down a peg.
Truss me, the ribald jokes suggested by this topic just dovetailed so nicely... I could have hammered them out at a rabbet pace -- butt I dado't go there and kept it plane & simple rather than risk screwing up & being thought a blockhead. (Sill, I hope I nailed it anyway.)
Okay, okay! I know the drill.. Don't want to be boring by going over-board; the jokes get wooden after a bit. I'll consider it a glue that perhaps I should phloem down -- before I end up in a xylem for inflicting this pun-ishing block of wood-be humor on everyone...
Thanks to Hank Roberts for linking to this post from RC today. It advanced my understanding of tree-ring dating by a great leap. Another triumph for your blog, Greg!
we use a tree-growth model driven by simulated temperature variations to show that the discrepancy between expected and reconstructed temperatures is probably an artefact caused by a reduced sensitivity to cooling in trees that grow near the treeline. This effect is compounded by the secondary effects of chronological errors due to missing growth rings and volcanically induced alterations of diffuse light. We support this conclusion with an assessment of synthetic proxy records created using the simulated temperature variations.