Bishop Hill A bogging temperature reconstruction
Oct 5, 2011 Climate: MWP Actually it's not that bad, but the headline was too good to resist. I'm grateful to a reader for sending me a copy of the new temperature reconstruction from Moschen et al, recently published in Climate of the Past. The authors reconstruct temperatures from carbon isotope analysis of Sphagnum moss.
The temperature reconstruction is based on the Sphagnum δ13C cellulose/temperature dependency observed in calibration studies. Reconstructed GST anomalies show considerable centennial and decadal scale variability. A cold and presumably wet phase with below-average temperature is reconstructed between the 4th and 7th century AD which is in accordance with the so called European Migration Period, marking the transition from the Late Roman Period to the Early Middle Ages. At High Medieval Times, the amplitude in the reconstructed temperature variability is most likely overestimated; nevertheless, above-average temperatures are obvious during this time span, which are followed by a temperature decrease.
I did like the bit about the MWP being overestimated - the journal editor will have breathed a sigh of relief at that comment.
;)
The authors seem keen to downplay the MWP, emphasising the similarities to tree ring studies (they cite a northern hemisphere temperature reconstruction by Büntgen et al., which is the green line in the chart above).
The local [growing season temperature] anomalies show a remarkable agreement to northern hemispheric temperature reconstructions based on tree-ring datasets and are also in accordance with climate reconstructions on the basis of lake sediments, glacier advances and retreats, and historical datasets. Most notably, e.g., during the Early Middle Ages and at High Medieval Times, temperatures were neither low nor high in general. Rather high frequency temperature variability with multiple narrow intervals of below- and aboveaverage temperatures at maximum lasting a few decades are reconstructed.
I'm not sure about this - that sure looks like a medieval warm period to me. (They find no evidence of the Roman Warm Period and peat cutting has destroyed any record of the Little Ice Age.)
That said, I think I would also need some convincing about the validity of the proxy - some of it sounds a bit iffy to me.
Calibration studies have systematically investigated the relationship between climate parameters and the stable carbon isotope composition of cellulose (δ13C cellulose) from modern Sphagnum plants. Ménot and Burns (2001) found that in addition to atmospheric CO2 partial pressure, temperature and water availability play significant roles for their δ13C cellulose values. Regarding potential relationships between the δ13C cellulose of Sphagnum plant material and micro-climatological parameters, Skrzypek et al. (2007) report strong correlations between the δ13C cellulose values of Sphagnum and air temperature during the growing season. The major problem in the application of the Sphagnum δ13C cellulose to peat records in order to derive climatic signals arises from the finding that a significant offset exists between the stable carbon isotope composition of cellulose from different Sphagnum plant components (Loader et al., 2007; Moschen et al., 2009). Thus, physical separation of individual plant parts prior to isotope analyses is a necessity to avoid misinterpretations of stable isotope time series.
And you go on to read things like:
Due to the closed coupling of several environmental factors to air temperature, a (presumably indirect) dependency of the stable carbon isotope composition of Sphagnum cellulose on local air temperature has been assumed. To date there is no laboratory study on this relationship, however, the Sphagnum δ13C cellulose/temperature dependency has been proven in field studies (Ménot and Burns, 2001; Skrzypek et al., 2007).
I assume this means that they found a correlation and are assuming causation.
So, not entirely convinced, but it's interesting nevertheless.
The paper is: R. Moschen, N. Kühl, S. Peters, H. Vos, and A. Lücke. Temperature variability at Dürres Maar, Germany during the Migration Period and at High Medieval Times, inferred from stable carbon isotopes of Sphagnum cellulose, Clim. Past, 7, 1011–1026, 2011
Reader Comments (72)
"You are not in a position to tell me ....
(a field about which you are clearly not well informed)"
If that is the case I will refrain myself from wasting your magnanimous patience, now and as long as I remember your tactics
fun..I usually have.
My tactics are to answer questions and provide supported argument.
For example, when you asked me a question:
I went to the trouble of tracking down and linking to a relevant paper.
Then I as you a question and am contemptuously told to 'do my own homework'.
Faced with this sort of thing I am free to draw my own inferences about your level of knowledge and degree of actual scepticism.
BBD, I'm often really puzzled by your level of confidence about some of these issues. Two of the biggest messages I got from the Bishop's excellent book were: (a) reconstructions of details of past climate are hard due to the low signal-to-noise ratio of available proxies, and (b) some people in the field can not be considered to be honest brokers on such topics. I don't, by the way, take (b) to mean that these people are wrong - merely that their pronouncements must be dealt with using more caution than might otherwise be the case. The same rule applies to some enthusiastic sceptics also, such as Spencer and even more so Monckton. Our host made his name in the world of blogging with his famous and excellent post about the shenanigans that some climate scientists went through to assist inclusion of a particular paper purporting to debunk McIntyre & McKitrick's work in AR4. You may remember that post... The paper, whose value is somewhat debatable, was by a certain Caspar Ammann as one of two authors. The paper you refer to above, at 3:38 pm, 'Gao et al. (2008)', is in fact by Gao, Robock, and Ammann. Doesn't prove it is wrong, I'm sure Ammann is a good scientist, but interesting observation nevertheless. Do you have no qualms at all about citing papers supporting your argument that are written by people like that?
Concerning (a), what is true for temperatures will surely be true for other things like past forcings as well. And indeed, looking at the paper by Gao et al. (to be frank, I did not fight my way through all its "mind-numbing detail"), I don't think it is possible that it provides a robust and exact historical record of the radiative forcing caused by aerosols. As discussed in their section 6, on "Uncertainties in the Reconstruction", there are two factors that will lead to some errors. First, converting data on the amount of dust found in ice cores into an amount of dust in the atmosphere at the time is fraught with statistical and physical difficulties - that the authors discuss at length. Next, converting the modelled amount of dust into a forcing may not be an exact operation either - I do not think that climate models have a good track record of predicting (as opposed to retrodicting) the magnitude of temperature effects of volcanic eruptions of a given magnitude.
In conclusion: your statement at 1:30 pm that "The MWP appears to have been caused by increased TSI and low volcanic activity (reduced sulphate aerosols)" is certainly plausible - but I think that the odds that this is the full and correct interpretation are probably little better than 50:50. If that is the case, then maybe such statements should be made in a more tentative way?
Jeremy Harvey
I knew that Ammann was a co-author of the Gao paper, and to be brutally honest, did not wish to draw attention to the fact. I don't think his presence as a co-author in detracts from Gao 2008, but in the light of the 'Jesus Paper' nonsense, his is not a name to wave about on this blog.
Yes, which is why is said:
So I'd assign better than 50:50 odds to a TSI-wot-dunnit explanation for the MWP, and happily leave the aerosols if you are unhappy even considering them. My point in referencing Gao (2008) was specfically that Patagon said:
I was attempting to show that the parameterisation for volcanic aerosols wasn't just made up on the spot to achieve a desired result.
This is an even sticker wicket than it might look, as the other paper dealing with the causes of the MWP which begs to be referenced here is of course Mann (2009). And I didn't have the chutzpah :-(
Finally, there comes a point at which the reflexive hyper-focus on 'uncertainties' degenerates into a simple rhetorical trick to undermine any and every study which sceptics don't like.
Especially as it very much cuts both ways. There is great uncertainty about how rapidly mass loss will accelerate from the WAIS as warmer sub-surface waters destabilise the ice shelves and glacer footings along the W Antarctic seaboard.
Uncertainty isn't really your friend.
And I'm really puzzled by the level of confidence of most here that CO2 isn't a problem.
Which do you think is the more dangerous area of over-confidence?
I searched "delta 13 C Sphagnum" - the top google hit was this paper.
To quote the authors: "Results show that 13C-depleted values are related to low water table depths (WTD), while 13C-enriched values correspond to a water table that is close to the peat surface."
The authors also note a variety of other factors leading to variation in 13C incorporation:
position in water table - seems discrimination against 13C happens if too dry or too wet
Sphagnum species
light
temperature
atmospheric co2 concentration
carbon sources
to these I would add
1) because there is inter-species variation in 13C incorporation, you have to be able to identify the Sphagnum remains in the peat. This becomes increasingly difficult as the peat gets older. Contrary to popular belief, the waterlogged peat does decay, which lead me on to:
2) Anaerobic decay in the catotelm (the layer of peat constantly below the water table, i.e. all but the top 50cm or so) gives rise to CH4. This rises through the peat, and is captured by methanotrophs as it goes. These can be symbionts of the living Sphagna, providing C for photosynthesis - so deep C is cycled up to higher layers.
Overall though I would say the biggest problem in using peat remains to guess ancient temperatures is that the 13C ratio is affected by water availability.
So Y (13C) is a function of X1 (temp) and X2 (precipitation), both of which are non-linear relationships. Now from Y, without knowing what X2 was, you're going to guess what X1 was. Good luck...
"Uncertainty isn't really your friend."
BBD,
You are in no position sir, to tell anyone who their friends are. ;)
Andrew
BBD, thanks for both of your responses. I agree, uncertainty cuts both ways and I have trouble understanding highly confident sceptics such as Monckton. My point was rather that there is so much uncertainty in this field that over-confidence about any position, such as you often seem to have, looks unjustified. E.g. I doubt the TSI argument for the MWP is fantastically good either. And even the existence of a global MWP seems worthy of doubt.
Jeremy Harvey
Of course you are right to caution against unblinking certainty.
We should all take note.
BBD & OTHERS
have you read "heaven & earth" by Ian Plimer?
if so, chapter 3 deals with the sun & TSI, any comments on his viewpoint on the peer reviewed lit?
dougieh
doughieh
Yes, I read H&E.
This is the most comprehensive critique I could find. Chapter 3 starts at p.100.
Patagon, check out the Pacific thermostat theory by Mark Cane. If the medieval warm period was not global, and this theory is the explanation, then models are overstating warming.
MikeN,
I will, any particular paper that you recommend?
What seems to be clear after this discussion is that we need to pay a bit more attention to oceanic amplification mechanisms.
As for the MWP not being global, the example I mentioned before about the buried Patagonian forest is in the Southern Hemisphere, and there is quite a lot of other evidence for a warming period in both hemispheres.
P.S. The Bond TSI chart in the suplemmentary material seems to be from Lean see here.
ftp://ftp.ncdc.noaa.gov/pub/data/paleo/climate_forcing/solar_variability/bard_irradiance.txt
http://www.leif.org/research/
Patagon
Indeed it is Lean. Also obsolete, like H&S. The current understanding is that TSI shows far less variability over centennial timescales than once thought. See the reconstructions from Wang, Krivova, Preminger and Svalgaard in the xls I linked upthread.
Well I'm going to wade in here.
I started off believing in DAGW (dangerous AGW) like BBD and got my views reinforced by Skeptical Science's treatment of many of the climate skeptics' issues. When I finally started to have doubts I realized I needed to undo every single one of Skeptical Science's articles, with better evidence, if I was to change sides.
A lot of work. And I'm not a degree-toting scientist. But at that point, I had time on my hands. And what I found devastated me so much that I decided to write it up to help those with less time on their hands, not to handwave but to address the science. That was three years back but I have not changed my conclusions since then, despite being challenged many many times by "warmists" and each time I checked to see if their challenges had any merit, or if I needed to reword my "primer" to take out the emotion I felt when I realized the depth of corruption in the scientific process. Always I answered.
In time, I started to find I was simply repeating myself, and that my arguments were simply being ignored - "peer review" being considered more authoritative to my challengers than my own honest pursuit of Scientific Method. So I regret having no time to answer the likes of BBD myself, any more. But my piece might give Bad Andrew the material he needs. I have had many notes of thanks, from both scientists and non-scientists. It's a long web pabe but still a lot shorter than Plimer, has lots of references, is much better illustrated, and misses (I hope) Plimer's own bias. Having said which, all Plimer's challengers are lilliputian in comparison to Plimer. Yes BBD I have examined your ref and found I had already dealt with it to my satisfaction.
http://www.greenworldturst.org.uk/Science/Curious.htm
"So I regret having no time to answer the likes of BBD myself, any more. But my piece might give Bad Andrew the material he needs."
Thanks Lucy. I think I read your piece once before. My personal opinion is that BBD is in "Baghdad" mode and has been aware of the answers for quite some time. But I will bookmark your link (I had to fix the typo in the URL) for referential purposes.
Andrew
Lucy and BA
You've got it backwards. I was a lukewarmer for years until I became better informed on the matter of climate sensitivity.
Plimer's book is absolutely riddled with errors and cannot be taken seriously as a scientific rebuttal of the AGW hypothesis.
Lucy Skywalker
Your first graph CO2 rising & temp falling since 2002 is out of date.
See Lucia's take at the Blackboard.
Or:
UAH 2002 - present
UAH 2000 - present
UAH 1998 - present
UAH 1995 - present
Plimer's book is absolutely riddled with errors and cannot be taken seriously as a scientific rebuttal of the AGW hypothesis.
Been a long time since I visited Realclimate but I just had a DejaVu moment reading the last post.
BOFA
Well I'm very happy for you.
Now read this.
Your Grace
One often hears the argument that while the Mann and other dendro Hockey Sticks are indeed Illusive, there are Hockey Sticks using other proxies that have much the same conclusions. Might it be worth your while to evaluate this in a future, perhaps smaller, publication ?
BBD
Having read and checked the paper, the only thing one can be sure about is that the sphagnum varieties are unreliable proxies. They point in certain directions, then in others. There are several variables other than temperature that will drive their indications. They correlate to some other proxies, some of the time, and not to others at other times.
In essence, this is not much use at all. And that's fine, because it might eventually prove useful.
I remain interested why you should suggest this as “More evidence for an NH MWP (ca 800 - 1200 CE) that was neither consistently warm nor warmer than the C20th?
If Moschen et al’s results correspond to anything at all, it is to a significant MWP. So my question remains. Let me rephrase it simply:
The raw findings show a significant MWP
The proxy’s accuracy is acknowledged as questionable, including the affecting factors’ variability. The authors also assume, without verification, a closed coupling between the factors.
The authors’ acceptance of the proxies’ shortcomings is slanted only one way - that the ‘variability’ results in a probable overstatement of the MWP (no evidence is supplied.)
You endorse this interpretation “because they calibrated the sphagnum d13C proxy and best understand how much confidence to place in it.”
There is no evidence stated or referred to in Moschen et al that the proxies overstate the MWP - the admission is only of its weaknesses in general. To be blunt, the proxies could also underestimate the MWP signals or get them significantly wrong.
So why do you see this as “ More evidence for a [MWP] that was neither consistently warm nor warmer than the C20th?”
If you are going to reply to this (anyone) can you please keep the discussion on-thread? No tracking off to Arrenhuis, HADCRUT or even Svensmark, puh-lease.