Eduardo Zorita sent me an interesting paper (Pollissar et al 2006, Solar modulation of Little Ice Age climate in the tropical Andes) hot off the press on June 1, 2006, co-authored by Bradley, which reported:
The intersection of the ELA [equilibrium line altitude] and pollen estimates indicate that during the LIA the Venezuelan Andes were both cooler (~3.2°C) and wetter (~208 mm/ yr, +22%) than present … Our data suggest considerable sensitivity of tropical climate to small changes in radiative forcing from solar irradiance variability.
They mention that "during most of the past 10,000 yr, glaciers were absent from all but the highest peaks in the Cordillera de Merida." Bradley has been one of the longest standing opponents of the LIA – has the leopard changed his spots?
You may recall that IPCC TAR notoriously concluded that the "conventional terms of “Little Ice Age” and “Medieval Warm Period” appear to have limited utility in describing trends in hemispheric or global mean temperature changes in past centuries. "
The terms “Little Ice Age” and “Medieval Warm Period” have been used to describe two past climate epochs in Europe and neighbouring regions during roughly the 17th to 19th and 11th to 14th centuries, respectively. The timing, however, of these cold and warm periods has recently been demonstrated to vary geographically over the globe in a considerable way (Bradley and Jones, 1993; Hughes and Diaz, 1994; Crowley and Lowery, 2000). … Thus current evidence does not support globally synchronous periods of anomalous cold or warmth over this timeframe, and the conventional terms of “Little Ice Age” and “Medieval Warm Period” appear to have limited utility in describing trends in hemispheric or global mean temperature changes in past centuries.
A quick browse through the IPCC references shows how reliant their viewpoint is on the corpus of Hockey Team articles that I’ve discussed here. A longer discussion by Mann is here.
The preamble to Pollissar et al is oddly different, now characterizing the LIA as a "significant global event" and even citing MBH99 as a source:
During the past millennium, significant climatic fluctuations have occurred. Prominent among these is the Little Ice Age (LIA), recognized in historical records (e.g., ref. 1 -Pfister) and documented in proxy climate records from many locations (2 – Jones and Mann 2004). Although the LIA was a significant global event (3- MBH99), its causes and regional differences in the timing and climatic response remain unclear (2, 4 – Jones and Mann 2004, Crowley 2000).
The research strategy of Pollissar is quite different from the twin Hockey Team strategies of heavy reliance on tree rings and equally heavy reliance on mining for statistical correlations between tree rings and gridcell temperatures. Pollissar et al use what I would call more old-fashioned methods used by Lamb, such as changes in glacier elevations or changes in vegetation altitudes, to deduce past climates. However, they link such changes here to changes in lake sediments, which provide a continuous record of change over the past century – as opposed to relying on moraine locations to measure glacier advance and retreat. Their strategy briefly:
The presence of glaciers in the L. Mucubaji watershed increased the flux of inorganic sediment to the lake, producing a continuous lake-sediment record of glacier activity. The Mucubaji record is corroborated by analysis of sediments from L. Blanca (Fig. 1), a small, closed-basin lake at 1,620 m in unglaciated terrain. At this site, the flux of inorganic sediment to the lake increases during wet periods, producing a distinctive signature in the sediments that contrasts with drier periods. Both records are subsequently compared with nearby pollen histories that chronicle vegetation change in response to climate during the LIA (13).
Their Figure 2 shows a variety of proxy records. The two sediment series are the top two series in the panel below. I like the sharpness of Series B (Lago Blanca), the expression of which they interpret as glacier presence/absence, with onset in the 13th century and ending in the 19th century. The non-existence of the glacier in the MWP is very distinct in this record. Series B shows its maximum extent in the late 17th century, the period of greatest North Atlantic chill in Lamb’s view of the world. One wonders whether the dating of Series A might have stretched out a bit relative to Series B – the changes would be sharper with some perhaps plausible dating differences. Series D shows the pollen biome.
Original Caption: Fig. 2. Lake-sediment records from the Venezuelan Andes compared with indices of solar activity and additional tropical paleoclimate proxies. (A-C) Glacial advances, indicated by increases of sediment MS in L. Mucubaji (A) (vertical gray shading), coincide with an increase in precipitation, shown by higherMSin L. Blanca (B) and higher abundances of Cyperaceae (sedge) pollen in the Piedras Blancas peat bog located near to L. Mucubaji (C) (13). (D) Lowering of ecological zones and colder-wetter climate during the LIA is indicated by the hbiome (equivalent to the minimum estimated departure in ref. 15) from the Piedras Blancas site. (E) Minima in reconstructed solar irradiance (black line) (16) using the scaling of ref. 17 or maxima in 14C (gray line, inverted scale) (18) are coeval with glacier advances. The 14C record reflects solar modulation of the 14C production rate and is scaled to the reconstructed irradiance curve of ref. 16. (F) Annual record of latitude-weighted volcanic aerosol forcing (gray bars and left axis) (4) and 50-yr averages (line and right axis, multiplied by 4 to scale with the reconstructed solar irradiance and plotted at youngest age of the 50-yr window). (G) Wetter conditions are supported by the Punta Laguna, Mexico, d18O record of higher P/E during Mucubaji glacial advances (19). (H) Abundances of the foraminifer Globigerina bulloides in Cariaco Basin sediments are higher during glacial advances indicating stronger trade winds (20).
From these results, they conclude a glacier ELA (equilibrium line altitude) lowering of ~300 to ~500 m , suggesting that a modern equilibrium has not been reached. They report a similar lowering in pollen biome, with a gradual decrease in hbiome during the LIA, with an average LIA value of approximately ~220 m and a minimum of ~460 m near the LIA termination (Fig. 2D).
Using physical lapse rates rather than mining for statistical correlations between annual series of ring widths, they conclude that this lowering:
is equivalent to a temperature depression of 2.6-4.3°C, using the modern annual precipitation of 950 mm (Fig. 5).
They note that declines of about 2 deg C had been inferred for the Caribbean and attributed the greater change in the Venezuelan Andes to changed lapse rate:
The reconstructed LIA temperature depression in the high altitudes of the Venezuelan Andes is greater than that inferred for Caribbean sea-surface temperatures (SSTs) [~2°C (34-36)]. This result is likely a consequence of changes in adiabatic lapse rates due to cooling. Cooler tropical SSTs would reduce the absolute humidity of the lower troposphere and steepen the slope of the moist adiabat above the condensation level. This effect would lead to cooling at 4,500 m above sea level, which was ~1.5 times that at sea-level (Fig. 6), in agreement with our glacier- and pollen-temperature estimates.
Big numbers for an event local to the North Atlantic. They attribute these changes to solar modulation, noting that this implies a high sensitivity to solar. A couple of other tangential notes: Christy, in his recent presentation at the Marshall Institute mentioned in passing that the Rocky Mountain glaciers were all Neoglacial i.e. post-Holocene Optimum. I posted up last October some questions about the dating of the Kilimanjaro glacier. In my opinion, Thompson’s evidence for the existence of Kilimanjaro glacier in the Holocene Optimum is extremely weak and his very limited "evidence" can be readily interpreted in a way consistent with its non-existence at that time. If the Venezuela glaciers are transient, it certainly adds weight to the possibility that the Kilimanjaro glacier is transient.
Reference:
P. J. Polissar, M. B. Abbott , A. P. Wolfe , M. Bezada, V. Rull and R. S. Bradley, 2006, Solar modulation of Little Ice Age climate in the tropical Andes Proc. Natl. Acad. Sci. USA, 10.1073/pnas.0603118103 http://www.pnas.org/cgi/content/short/0603118103v1?rss=1
34. Winter, A., Ishioroshi, H., Watanabe, T., Oba, T. & Christy, J. (2000) Geophys. Res. Lett. 27, 3365–3368.
35. Watanabe, T., Winter, A. & Oba, T. (2001) Marine Geol. 173, 21–35.
36. Nyberg, J., Malmgren, B. A., Kuijpers, A.
Climate Audit 
51 Comments
Does the paper make explicit reference to the MWP ?
“Bradley has been one of the longest standing opponents of the LIA – has the leopard changed his spots?”
Maybe Bradley has been visiting CO2 Science, where they have made a project of recording world-wide evidences for the MWP, and decided to cut his losses.
It’s interesting that the Andes appear wetter during the LIA. It suggests that El Nino predominates over La Nina during colder periods.
It’s also amazing that scientists are still making references to the global extent of the LIA to the Hockey Stick even though Mann, Bradley and Hughes went out of their way to deny such a conclusion.
The paper seems to show no “MWP” in the sense that we’ve usually understood it, but rather continuous warmth up until the LIA. So in that sense it’s not consistent with the older (1995 cartoon in IPCC) view of climate variability.
Perhaps some clues here on tree-line (or veg-zone) response time-lag. D (pollen) recovered quickly from 1850 to 1920. Comparing to the other proxies, it seems tho there may be some time-lag, the zones moved upward quite quickly after some warming threshold was reached.
But it hasn’t moved any since…
Steve, since I have to shell out to read the article (or I can wait until my school library gets the hard-copy), maybe you can clear something up for me. Do they go into any further detail about the material you block-quoted regarding adiabatic lapse rates? Without the full context, it looks like they made a boo-boo: you should not use adiabatic lapse rates to determine the difference between the temperatures at two different heights unless you are certain that you can explain the difference entirely through convective processes (which you cannot–I would love to see them try to demonstrate that). Also they say cooler SSTs would “steepen the slope of the moist adiabat…,” but that, too, is a little funky. The steepness of lapse rates is determined by the delta (T) with height; steep = rapid decrease. BUT, on thermodynamic diagrams and in the vernacular of operational atmospheric scientists, the “rate” is not the same as the “slope,” because pressure and height are the y-coordinates while temperature is the x-coordinate. So a “steep” lapse rate produces a shallow slope (and vice versa). It looks like they may have confused their terms, which is not big deal, but it makes it difficult to follow (and agree with) their explanation of the temperature differences.
I am assuming I am just missing some contextual piece. This is PNAS after all; I guess that they would have had at least one reviewer who knows what you can and cannot do with adiabats. If you posted up at least all of the figures that were referenced in your block quotes (figs 1, 5, 6–at least 6), that might help.
The denial of MWP and LIA are the things that made be doubt the Hockey Stick in the first place. I believe that such an extraordinary claim required extraordinary evidence. You have established the absence of even ordinary evidence.
I was also curious about the phase “extraordinary claims require extraordinary evidence.” It seems to have been popularized by Carl Sagan but has it source in the writings of David Hume in his essay An Enquiry Concerning Human Understanding in which he stated “That no testimony is sufficient to establish a miracle, unless the testimony be of such a kind, that its falsehood would be more miraculous, than the fact, which it endeavours to establish….'”
#3
This would agree with the Cane and Zebiak theory. If I am not wrong, they predict warmer Eastern Tropical Pacific in periods with weaker external forcing, contrary to the predictions of several coupled atmosphere-models. However, not all climate models agree in this point.
In any case, it seems interesting
eduardo
If you go back and read Lamb’s papers, they are thoughtful and interesting. Lamb had a relatively encyclopaedic knowledge and his view of the world was not trivially arrived at.
Your point about “extraordinary” is not unreasonable. However, I don’t count the HOckey Team corpus as evidence at all. For example, Hughes and Diaz 1994, still relied upon as a disproof of Lamb, is almost incoherent when you re-read it, displaying a series of tree ring chronologies with centennial variation removed and using that as “evidence” that there was no centennial variability.
Our position is that the issues are open, not that IPCC 1990 is restored. Having said that, if the Hockey Team corpus is invalid, then the views of their predecessors and opponents need to be re-assessed.
I’ll try to track down some Lamb. Any refs or pdfs to share?
I’ll temporarily post up a couple of pdfs that I have on hand. Remind me if I don’t do this in a couple of days.
re no 9. To explain why think it would require extraordinary evidence to deny the MWP and the LIA. Because there is plenty of historical evidence of both the MWP and the LIA. Then the Hockey Stick comes around the the graph doesn’t seem to reflect the historical evidence. Put that together with the simple fact the the Earth and its atmosphere are huge, people are small and CO2 is still measured in parts per million. So when something like the hockey stick come out I want something substantial before I will reject the historical evidence and my sense of geologic scale and physics. And you can add in the Coming Ice Age Headline of the 1970’s to the mix. As always it a matter of weighing the evidence and I see nothing in the Hockey stick that would explain away the history I learned. I’m willing to reject the history of the MWP and the LIA but the quantum of proof required to convince me would be substantial.
I am a lawyer by trade and I generally trust physical evidence over eyewitness testimony, so I would be willing to reject the history if given physical evidence, but I have learned a healty distrust of experts especialy when there seems to be a rush to judgment. I am inclined to say “That’s nice, now prove it.”
But there’s no mathematical compendium of statistically relevant evidence or even stories to support the LIA/MWP. It’s a frigging cartoon for gosh sakes.
Re #15, Then you should read Keigwin et al 1996
TCO, try reading about oxygen isotopes. Or just google geology 101 courses for that matter.
here’s an article from just last year where both LIA and MWP where mentioned in a study of a stalagmite (physical evidence in the layers that form it) the oxygen isotope ratios are the mathmatical evidence of ice or no ice on the planet :
http://news.bbc.co.uk/2/hi/science/nature/4636115.stm
I think the actual paper is downloadable online as well.
here’s a picture with a graph of the LIA and MWP layered over the actual stalagmite:
http://131.130.57.239/sina/Geology%20and%20Paleontology.htm
There is a bevy of anecdotal evidence to support the existence of the MWP and LIA. To “deny” them would require proving they didn’t exist, which would be just as difficult as proving the hockey stick. Of course, the hockey stick proclaims to be capable of doing both…
You don’t have to prove it definitely wrong, to prove that it is not conclusive. This is exactly what Steve does with Mann. I say again, the IPCC 1995 cartoon is a joke. It it telling that people trying to debunk Mann, do so by finding his study inconclusive rather then arguing the validity of the old alternate.
Hi TCO,
The thing is that the evidence in the geology that these events occurred is all in sync time wise , from different sources , research and locations.
This points to more conclusive then not. Something happened.
Mann et alls are trying to say this knowlege isn’t any good because it doesn’t fit in a computer model.
Think about it. Where do you think climate science (especially paleo climate science) originated from? What methods are used to get data and dates? Do these proven methods suddenly become no good only when the graph hits the eras of the MWP and LIA?
#20
I’m not sure that Steve actually does this.
Burger, et. al. 2006 makes a convincing case that Mann was on the right track, but may not have chosen an optimal method (his method may not have captured the proper amount of variablity). However, Burger did make a pretty good argument that S04 missed an important point (using undetrended data in the calibration period) which made their simulation much less accurate than the methods used by MBH98.
This paper also identified a method which may produce superior results to the MBH method (it did not address the Rutherford, et. al. 2005 analysis). I look forward to seeing the results from the method suggested in Burger, et. al. 2006. I do suspect that it will not provide much help for Steve and his minions.
TCO: I was amazed at the clarity and quality of the writing in Burger, et. al. 2006. It was really one of the most well written scientific papers I have ever seen. Clear, concise and understandable even to a layman. Being a good writer does pay off…
Steve has minions?
When did that happen/
Re #15 TCO
Try Weather in History: a site that attempts to provide a ‘one-stop’ source for major historical weather events across the British Isles, which serves to illustrate the nature of climate variation in the UK.
JMS, does it not strike you as sad that Mann, Bradley and Hughes were so evasive, in their *peer reviewed scientific study*, that Mr. McIntyre could not accurately reproduce their study? Because, I think it’s utterly ridiculous.
How can you criticise Mr. McIntyre for failing to reproduce MBH98 exactly, when he should have been able to do it easily from the methods described and from the data available, were this proper science? If it’s true he failed to reproduce it accuracy (and I do not believe it is, but assuming so), then the fault lies squarely on the shoulders of the MBH98 authors.
A study that can not reproduced nor falsified is not science. Full stop.
Re #25, Nicholas, what is ‘science’? Is it just reproducability?
Reproducability is necessary, but not sufficient.
This is pretty basic stuff, I’m surprised you’re not aware of the scientific method.
Here you go, here is an explanation of the scientific method. All I had to do was search for “scientific method” in google, it was the first result.
Re #28, exacty! How many times does the word ‘reproduce’ or reproducability’ appear in that definition? I can’t find one… Why? Because the strict requirement so many here have that something should be reproducable down to the last digit and last crossed T before it becomes ‘science’ just isn’t there.
I’d suggest willful blindness. The words may not be there, but the principle is right there. I’ll even bolden it up for you so you can see it:
Suggest what you want John, but I saw that. And that, the FOUR steps, are what many have done wrt past climate. What only this place does is demand that one study (or type of study) be replicated down to the last digit. CA is, essentially, just interested in step 4.
So, no step 1 one here, because that means getting out there and coring trees, or sampling ice/sediment or the rest. No step 2 because science (well, climate audit science) is just about replication. No step 3 because, again, science (CA style) is about replication – and just about replication.
No, John, science IS about the FOUR point you list not just point 4, so, perhaps you’re beginning to get it?
RE#20 –
I’d argue that to “deny the LIA and MWP” is not simply saying their existence “is not conclusive,” but saying they didn’t actually exist. Maybe we’re just caught up in semantics.
Re #31
Peter, you’re spluttering because the facts are clearly against you.
The scientific method, as Nicholas has clearly pointed out, makes independent reproduction of experimental results (such as the multiproxy studies) an essential part. You’ll notice that nowhere in that description are references to the importance of a "scientific consensus" or references to the qualifications of the experimenters, nor the legitimacy of the research based upon the sources of the financial backing.
In the multiproxy studies, as Steve has very carefully pointed out, there is no description of the relationship between the proxy response (tree ring width or maximum density) and temperature (so that counts out item (1) above). It also means that item (2), the mathematical description of the relation, is also missing and is assumed without proof. Item (3) is usually made as a plea for future funding to "see how bad the expected warming will get" and item (4) gets the lucky would-be reproducer or experimenter villified as "engaging in intimidation" ((c) Michael Mann, 2005) or as a shill of Big Oil or engaging in a witch-hunt ((c)John Hunter, U of Tasmania, 2006)
The entire point being that large parts of climate science pay lip service to the scientific method, while pretending to be its guardians against the barbarian hordes of Canadian statisticians and other malcontents.
Yes, clearly if the results are not reproducible then they can not be independently verified!
If you can think of a way that’s possible if they are not reproducible (e.g. if data is not available or methods are not properly described), I’m all ears…
Steve could go out and core some trees, and so could I, but that would be arguably a different experiment and therefore have no bearing on step 4 WRT MBH98/MBH99 and all our other favorite studies.
This particular discussion started when JMS criticized Mr. McIntyre for failing to properly replicate MBH98. But, being a “scientific” study, and bearing in mind that means that independent experimental tests should be carried out, the authors must make such tests possible in order to follow the scientific method. Has MBH98 or MBH99 been independently verified? And no, people who work with Mann, Bradley and Hughes are not independent. If it has, I would like to be pointed to where this has happened.
I feel myself coming back around to what Einstein said, approx. “if I am wrong it only takes one scientist to show that”. I’m thinking, it doesn’t matter how many seperate studies show a hockey stick if none of them are indepdently verified as being correct. One experiment which has been opened up to proper scrutiny and which nobody can find flaws in has a lot more explanatory power, scientifically speaking, than dozens seperate experiements which all come to a similar conclusion but none of which have been open to proper scrutiny. It strikes me that this is the situation in climate science today.
That’s a reference to a book published in Nazi Germany against Relativity called “One hundred scientists againsts Einstein” to which Einstein replied “If I were wrong, one would have been enough”
This is on a slight tangent, but I would like to suggest posting a map of the globe with indications (doubling as links) recording major studies with data supporting a MWP and/or LIA. A good visual representation of the global extent of these phenomena would go a long way toward undermining the argument that they were NAO or local phenomena as required by current climate models advanced by Mann et al..
Done properly, such a map would make immediately visually apparent the geographic breadth of the evidence and provide an easy way to access location specific results. Arguably, it would be less surprising if Venezuela exhibited MWP/LIA effects than it would if Peru, Chile, or Madagascar showed such evidence.
Re: #36
In which case, could I recommend Soon&Baliunas (2003), which does precisely that.
JMS, I agree that Burger is very clear in the writing. I think the clarity has benefits for the field and even for him as a researcher in his continued work. I don’t care about Steve or minions. I think Steve has actually done a lot to raise interesting issues to light, but that if he were to disaggregate issues and be more detached, it would benefit him, the reader, the field as well. But he is still additive. If we go through all the Steve work and counterwork and the lot and still end up with a hockey stick, that would not bother me. If we were to find that the hockey stick was more suitably constructed and proven. I am all about finding the truth and letting chips fall where they may.
#34. Oddly enough, Wahl and Ammann have confirmed several of our results, but have misrepresented this publicly. For example, as I’ve pointed out, under duress, they’ve confirmed our claim about the inisignificant MBH verification r2 statistic, one of the prominent claims in our GRL article. Statistical skill was one of the essential MBH “results”. In my opinion, for Wahl and Ammann and UCAR to then announce that “all” our claims were unfounded is a distortion of the research record. They’ve also confirmed our claim about non-robustness to bristlecones. However they say that bristlecones are “needed” to give statistical skill. That’s a different argument, maybe yes, maybe no (I think no), but it hardly disproves our point about non-robustness – which is against one of the other key MBH supposed result: robustness to presence/absence of all dendro indicators.
Wahl and Ammnn’s HS emulation accomplishes nothing that we had not already done and their code is comparable in many respects to our prior code. Of course they don’t acknowledge this. It’s not that we don’t know how Mann calculated a HS (although there are some puzzling still unexplained details in his particular HS) – we do. But it’s required virtual litigation to extract some of the details and many details were misrepresented in the original paper e.g. Mannian PC methodology was described as “conventional”, which it obviously isn’t.
One excellent point made by Burger and Cubasch that hasn’t been widely appreciated is that they point out that, if you’re using the RE statistic to verify models, you can’t use it to choose models. This is a subtle and clever point, that completely eludes Wahl and Ammann and the Mannians.
I thought they made the point fine. You’re never going to get satisfaction from advocates like Amman or Mann, so you are better off just taking the high road and seperating the debate on individual methods points (and just agreeing where there is a point being made that you agree with) and discussing overall issues or other methods that were not addressed seperately. I was really bugged that you think that Huybers needs to discuss CO2. I agree with him that it is a seperate issue. You are being a bit diverting and bit kitchen sink at times on this blog and even in comment replies.
Really, instead of trying to redebate each point with advocates who are never going to give you satisfaction, you should continue to publish new insights. This is both from the tactical point of oppenent advocacy (where I see you a bit) and from the view of someone who wants the field to move along (me).
But it’s your buggy and I’m just repeating myself. Have fun, but don’t cry to me when I’m proven right in the end (like when Nature rejected your paper that I said belonged in a specialty journal).
Re #40 Steve, MBH98 calls that “skillfully resolved”
I would reckon that tropical glaciers can form and decline very rapidly, as compared with higher latitude ones. Firstly, the sheer amount of precipitation available during times when they might form would tend to make them form very quickly. Secondly, the sheer power of the near overhead sun will turn everything into corn snow within a day. So, when things go into melt mode, they melt really quickly at those sorts of latitudes. Bottom line is, my own sense is that tropical glaciers respond to variations that are on a much higher frequency than do high latitude alpine and all continental glaciers.
#31 et al
Ok so tell us this, why is the method
used by scientists
(hundreds and hundreds of them, possibly more than that)
who do see the MWP and LIA
(and are published in peer reviewed papers btw)
suddenly no good when the graph of Mann comes to these two periods?
Re #44, hey, look, I don’t deny the reality of a ‘LIA’ or the ‘MWP’ OK? I doubt anyone does. What disagreement there is is about the magnitude of both events, whether they were globally synchronised. There WERE stark cold spells in Europe not so long ago, it was probably warmer prior to that. I just think (outragous of me I know) that it’s not done and dusted as to the magnitude of both globally. Is it OK to think that?
I’ve read the much maligned Michael Mann accept both the LIA and the MWP as reality actually…remember that even the HS shows a decline in temperatures from the warmer middle ages to somewhat cooler levels a few hundred years ago…
Re#45
These two links from RC suggest Mann accepts the LIA and MWP – but basically only as exaggerations and regional events.
http://www.realclimate.org/index.php?p=32
http://www.realclimate.org/index.php?p=33
I don’t think Mann’s hockey sticks show any significantly anomalous warm or cool periods longer than a few decades prior to the 20th century.
Fair enough #45.
Yes#46 surprise surprise..
Europe, Asia, The United States, Greenland, to name a few land masses are just “regions” and the signatures recorded in: lakes, tree rings, ice, caves, bogs, sea sediments, or critters combined with the hand written documentaion found in these places is just an exaggeration. exaggerated all in sync too! Amazing! LOL
I know. I am supposed to “move on” now. 😉
Cheers!
RE #43 It may be that most of the present tropical glaciers formed during the LIA & are the exception in the present Holocene period. Many long-term Holocene (last ~10000yrs) tropical proxies show the LIA as the coldest in the entire period. Remarkably, this coincides w/a known solar anomaly, the Maunder minimum. Fancy that.
If the system is capable of long periods (centenial) of different temperature across large regions (Europe), isn’t it possible that the system is capable of different temperatures over the overall surface? Expecially given that the departures from norm for the long periods are on the order of what happens in terms of year to year variation, but would just be sustained longer?
Re#49 TCO, certainly possible. But the hockey stick says it wasn’t so.
RE: #48. This would indeed be something to explore. Consider the case of glaciers in the Himalayas. Quite unlike the Alpine ones at higher latitudes and certainly unlike any Continental ones, they accumulate during the late astronomical Summer, due to Monsoon convection. During the astronomical winter, other than the odd Cold front that makes it that far south, its mostly bone dry in that part of the world. Therefore, logic would argue that Himalayan glaciers are precipitation proxies. It would be interesting to try and get a better handle on what tropical weather was doing during the LIA. Perhaps, it was not abnormal. Meanwhile, perhaps, there was more unseasonable winter precipitation in tropical wet – dry climates where the Summer is usually the precipitation maximum. Of course this is all speculation. The main theme for me is, when people try to claim that tropical glaciers are a proxy, my question would be, a proxy for what? Precipitation?
OT, TCO – Found one for you. Must have been frozen in a glacier for the last 500 years :).
This diagram was used in one of the AGU presentations that I attended – I think that Kurt Cuffey used it, but I don’t recall him mentioning that this was evidence of glacier absence in the MWP in this area.
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[…] et al 2006 studied sediments in a proglacial lake in the Venezuelan Andes (prior CA discussion here). I refer to it here because it also attempted to interpret glacier advance/recession. Polissar et […]
[…] be derived from them. For example, here is a graphic from Polissar et al on Venezuela (discussed here) showing a difference in magnetic susceptibility between MWP and LIA sediments in Venezuela, which […]