Rebuttal to the Skeptical Science "Crux of a Core"

Guest post by Dr. J Storrs Hall

A bit over a year ago, in the wake of Climategate, I put up a blog post over at the Foresight Institute which got picked up and run here at WUWT.  The essence of the post was that there was lots of natural variation in the ice core record of climate, so that it was reasonable to be skeptical of scientists who claimed that recent CO2 variations were “the only thing that could account for the recent warming trend” (quoting myself).

Apparently that got enough exposure — and was persuasive enough — that over a year later the alarmists still feel the urge to “debunk” it.  Most recently, Rob Honeycutt at the “SkepticalScience” alarmist fanboi blog weighed in with this: Crux of a Core, Part 1 – addressing J Storrs Hall. Now the thing about this particular piece that jumped out at me at first was the fact that he associated me with a graph I never used, and he calls me “Mr. Hall” to make me sound less qualified than other sources such as “Dr. Alley” he refers to.  It’s Dr. Hall (and yes, I am a scientist, not a nanotech engineer as he claims), a fact that he could have discovered in 3 seconds with Google. That told me about all I needed to know about Honeycutt’s bona fides (in the original Latin sense of acting in good faith).

The only substantive point in the post is that GISP2 (or any specific ice core) is a local as opposed to global temperature record.  Is it misrepresentation to use it as a proxy for global climate?  Well, the inconvenient truth is that I’m hardly the first person to use ice cores as climate proxies in popular presentations:

Al Gore in AIT

… but, on the other hand, it’s actually an interesting question and one worth looking at.

How Ice Cores Record a History of Climate

That’s not my title, it’s from this page at the GISP2 site. Not “a history of local temperature,” — of climate. Here are some quotes from the abstracts of papers by GISP2 authors:

“Ice cores provide high-resolution, multi-parameter records of changes in climate and environmental conditions spanning two or more full glacial- interglacial cycles. …”

“Polar ice contains a unique record of past climate variations; …”

“One of the most dramatic climate events observed in marine and ice core records is the Younger Dryas (YD), … High resolution, continuous glaciochemical records, newly retrieved from central Greenland, record the chemical composition of the Arctic atmosphere at this time. This record shows that both onset and termination of the YD occurred within 10-20 years …”

“The Greenland Ice Sheet Project 2 (GISP2) core can enhance our understanding of the relationship between parameters measured in the ice in central Greenland and variability in the ocean, atmosphere, and cryosphere of the North Atlantic Ocean and adjacent land masses. …”

“High-resolution, continuous multivariate chemical records from a central Greenland ice core provide a sensitive measure of climate change…”

“The accumulation record from the GISP2 core as an indicator of climate change throughout the Holocene” (paper title)

So, sure, a single ice core is not a global average temperature record; but it is quite a bit more than one thermometer. It’s just mud-slinging to claim that using it for a climate proxy is “misinformation”.

… especially when I didn’t just use one ice core in my post but two, and the other one was from Antarctica.  One way to cut past the verbiage is simply to look at a comparison of the Greenland and Antarctic data and see how well they correlate:

(This is GISP2 in green, NGRIP, another Greenland core, in cyan, and the Vostok Antarctic core in blue. The Vostok has been scaled and shifted for a best match with the others; the temperature in Antarctica is colder, with smaller variations, than in Greenland. Furthermore, there are some time-scaling issues — note the temporal divergence of the two Greenland records before about 40 kya. It’s possible that NH/SH actually match better than this plot indicates.  Look here for data.)

Nowhere near a perfect match, but it’s pretty clear that these are all from the same planet. Even Vostok shows the Younger Dryas, which is generally believed to be a mostly northern-hemisphere event. The NH has more variability in ice ages, notably the Dansgaard-Oeschger events, but the SH more, on a relative scale, in the Holocene.

The GISP2 people also compared their core’s record with Antarctic ones; on this page they say that it “shows close correlation between GISP2 and Vostok in the delta 18O of air in these ice cores.” (That’s a key temperature proxy.) On this page they say “Holocene climate is characterized by rapid climate change events and considerable complexity. GISP2 Holocene ¶18O (proxy for temperature) (Grootes, et al., 1993) and EOF1 (composite measure of major chemistry representing atmospheric circulation) show parallel behavior for the Early Holocene but not for the Late Holocene (O’Brien, et al., 1995).”

Note that bit about “rapid climate change events.” In the words of Jeffrey Masters here, “The historical records shows us that abrupt climate change is not only possible–it is the normal state of affairs. The present warm, stable climate is a rare anomaly.” (And he’s talking specifically about the lessons of GISP2 — although alas he takes home the wrong lesson from it.) See also this recent post here by Don Easterbrook.

Does GISP2 — or any other paleoclimate record — show us that climate change isn’t happening?  No, of course not.  It shows us that climate change always happens.  The 20th-century warming was hardly unprecedented, and doesn’t call for unusual explanations.



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Brian H

Maybe there’s an existential explanation. Every age and era has prominent factions who want urgently to believe that they live in the End Times. A bunch of little Louis XVs: “Après moi nous, le déluge!” Can’t bear the thought of life going on after their unremarked passing, I guess.


Oh Dr. Hall, the warmistas are going to be very angry that you are using their own information against them! Good show!


I wonder why it seems that the NH and the SH were “out of phase” between 50-60kya. I also notice little blips of the same thing at other times, SH will go cold, NH will shoot warm or vice versa. Look at around -40k. SH blips warm, NH goes cold. A similar event happens at about -65k.

Seems that if they do not like your comments on Real Climate a new impossible capcha is presented.


Dr. Hall said: “I am a scientist, not a nanotech engineer.” Let’s see.
Here on WUWT you were only identified as J. Scott Hall, not Dr. Hall. Did you complain?
According to wikipedia: “John Storrs Hall is involved in the field of molecular nanotechnology. He founded the sci.nanotech Usenet newsgroup and moderated it for ten years, and served as the founding chief scientist of Nanorex Inc. for two years. He has written several papers on nanotechnology and developed several ideas such as the Utility fog, the space pier, a weather control system called The Weather Machine and a novel flying car.”
Is this you: ?
If so your CV lists nothing about climate science.
The Foresight Institute’s mission is to ensure the beneficial implementation of nanotechnology.
You are entitled to your views but one or two ice cores do not provide enough information to reconstruct global climate.

James Sexton

This is an interesting discussion. The graph presented, in my view, looks a bit noisy, but I’d like to see more and how they’d line up. It would be nice if there were a consistent message about ice-cores and their representation of the globe or not. Let me see if I get this right…… terms of historic CO2 levels, yes ….very valid and speaks to the entire globe……in terms of temps, no way! Its just local! 😐 :-0 😐 :-0 😐
Dr. Hall, thanks for the money quote and a nice summary statement. “It shows us that climate change always happens. The 20th-century warming was hardly unprecedented, and doesn’t call for unusual explanations.” This can’t get repeated often enough.


Excellent post.
Don’t mess with Dr Hall!
Another takeaway for me from the chart is that while we are enjoying the warmest climate in the past 80+K years, the recent trend is down.

The new tactics are really quite pathetic. They are not doing themselves any favours. Childish really.


Looking at these long-term ice cores always reminds me just how ironic the term “climate change” is. The climate has always been changing and hopefully always will. It would be amusing to start a parody ‘counter-scare’ by announcing that it appears earth’s climate has *stopped* changing. Climate data from the last 13 years certainly supports that conclusion. Perhaps the underlying magnetic, geothermal, etc forces that have driven our planetary climate variation for billions of years are finally “running out”. Such a trend of climate stagnation would be unprecedented and if it continues the consequences are likely to be catastrophic! Is the “engine of the earth” dying? The alarming press release practically writes itself.
Of course the cause of climate non-change is obviously man-made. As our society has industrialized more people and their machines have been put in motion. When in contact with the earth, this motion causes forces which have a measurable impact on the planet. This can easily be shown with Newton’s Laws so the science is settled. Clearly, we need to strive for a motion-neutral society. Fortunately, those who need to move mass at high speeds, such as drag racers and fire trucks, can buy motion offsets. 🙂


If the data supports the AGW alarmist orthodoxy it’s teleconnected data that is a global measurement, if not it’s obviously a local anomaly.


> The Vostok has been scaled and shifted for a best match with the others…
Could you be more specific about this? I have no issues about scaling or shifting the Y-axis (temperature) — it’s shifts of the X-axis (time) that I’m asking about.
What do these three plots look like, without X-axis shifting? Do you think shifting is informative because it corrects for dating errors? Or because there is a temporal lag between Arctic and Antarctic events, as shown in these records? (Or something else, of course.)


You’re looking at the finger!


Mark says:
March 1, 2011 at 9:54 am
It would be amusing to start a parody ‘counter-scare’ by announcing that it appears earth’s climate has *stopped* changing.

Great idea Mark, but I hate to tell you that the folks pushing the Maya calendar 2012 apocalypse have beaten you to it.
Give it another shot after December 21, 2012 when the end of the world prognostications start fresh.

James Sexton

Mike says:
March 1, 2011 at 9:33 am
Dr. Hall said: “I am a scientist, not a nanotech engineer.” Let’s see.
Here on WUWT you were only identified as J. Scott Hall, not Dr. Hall. Did you complain? ……
Are you intentionally being obtuse? Try applying a concept called “context”.
“…..and he calls me “Mr. Hall” to make me sound less qualified than other sources such as “Dr. Alley….”
It seems, all Dr. Hall is asking for is to be afforded the same respect and treatment given to others. Is that really too much to ask? Do you think it likely he didn’t complain about WUWT because he was afforded fair treatment?

Ged Darkstorm

@ Mike
“served as the founding chief scientist of Nanorex Inc.”
Did you miss that part, that you yourself read? An engineer is different from a scientist, though a person can swing between professions, it’s a completely different sort of skill set and role. He’s a scientist in the nanotech world, not an engineer.
And when ice cores from completely different sides of the planet are in general agreement, then the local climate of two different sides of a planet are generally the same… then how is that not global climate? I don’t think you understand what global is if you honestly believe that.

Richard M

Clearly Rob Honeycutt has made a fool of himself. Those on Skeptical Science that couldn’t take the time to validate Honeycutt’s statements also look like fools. Now, we see Mike (March 1, 2011 at 9:33 am) adding his name to the list of fools.
Tell me Mike, if you were going to claim someone is passing misinformation don’t you think one should at least figure out who the person is? If Honeycutt can’t get that right why would you expect anything else he says to be right?

Series of geomagnetic storms in last 2-3 hours shifted Earth’s magnetic meridian by 1.5 degrees.

jonny old boy

@ Mike
WOW , don’t ever participate in archery ! If you miss the target with as bigger margin as you missed the point with your post then no one would want to be in the same square mile !! LOL !! did you not read the article properly ?? No wonder people fall for AGW, are there many more who scan, misread, and then accept ??

Jack Greer

Dr Hall, you did “it” again. Look at the last sentence of you post … is there a “global” inference again?! If you compare the GISP2 data with the Vostok data on a more revealing time scale (aside from well understood glacial/interglacial events) you’ll note that the abrupt climate events between the poles is typically anti-phase. Again, indicating local swings, not global in scope.


@Ged Darkstorm
Hall has zero background in climate science. The difference between an applied scientist in industry and an engineer is nil. Further, Honeycutt intended no degradation in identifying Hall as an engineer. And two points simply do not cover the global.

I’ve analyzed as much ice core data as I could find on the web and came to the same conclusions. We must recognize that data are bi-variant proxies. Both time and temperature are calculate values, their values and accuracy changing with depth and latitude value. Calibrating time scales to the Younger- Dryas and adjusting latitude effects to a common latitude produces a time temperature relationship that reveals many climate cycles of different frequencies that have continued into the present so that our present changes are not unique.


The article would appear to speak for itself when the only rebuttal is a rather poorly researched ad hominem, re: @Mike says:
March 1, 2011 at 9:33 am


stupidboy says:
March 1, 2011 at 10:37 am
You’re looking at the finger!

Jack Greer

Jack Greer said:
“a more revealing time scale”
Correction: “a more revealing time scale with smoothing representation x-noise.”


@Bushy –

The new tactics are really quite pathetic. They are not doing themselves any favours. Childish really.

I am not an Al Gore basher, but since they point out so often anyone who isn’t exactly one of the Hockey Team, it should be pointed out that Al Gore is about as far from a climatologist as can be. The double standard is pretty despicable.
Many of the scientists who have AGW papers are physicists or other scientists, but if they are AGWers, fine and dandy. Yet perhaps the greatest scientist of the last 40 years, Freeman Dyson, is frowned upon as “just a physicist,” when he voices skeptical points.

Crispin in Ulaanbaatar

Right on Severian!
We have see it claimed so many times before: MWP is European weather when it shows it is not as hot today as it was then. If the numbers can be ‘stepped on’, it is climate because is it supports escatology. Dr becomes Mr in order to emphasize that the priestly class of self-appointed alarmist oracles are demanding real money to stop the metaphorical eclipse that is eating the metaphorical sun.
Is anyone interested in the actual science? You know, the kind with numbers and relationships, where effects have causes? Where non-effects don’t need a cause? Must be a slow news week if we have to stoop to mentioning RC and science in the same breath.
Mr, Dr? Scientist, schmientist? These are manifestations of the World of Names. It is polite and correct to refer to people by their legitimate titles as part of our social convention. It is one of the foundations of civil discourse. If you can’t at a minimum research someone’s title(s) correctly, what does that indicate about the quality of rest of your work?

Excellent article. Michael Mann’s acolytes are the ones who don’t believe in climate change prior to the mid-1800’s. Skeptical Science, a blog run by a cartoonist, has a large number of Mann followers.
Mann’s Hokey Stick chart showed almost no change until the beginning of the industrial revolution, when suddenly temperatures shot straight up. But Mann’s chart has been completely debunked by McIntyre & McKittrick, and the IPCC dares not use it any more, much as they would love to; it was visually spectacular. It was also bogus science fiction.
We know from both Northern Hemisphere and Southern Hemisphere ice cores [and numerous other proxies] that the Medieval Warm Period was warmer than today, and the Roman warm period was warmer than the MWP, and the Minoan Optimum and Holocene Optimum were much warmer than the RWP.
What we observe currently is indistinguishable from previous warming cycles. While there may be a minuscule amount of warming due to the increase in CO2, it is insignificant and can be disregarded. Most of the *mild* 0.7°C warming over the past century and a half is natural variability.
It appears that the current warming cycle may be topping out. If so, we are cutting back on CO2 at exactly the wrong time; warm interglacials are rare.


Well, I am confused as to the purpose of this post titled “Rebuttal to the Skeptical Science ‘Crux of a Core,’” I thought the rebuttal was to Honeycutt’s point.
You wrote “The only substantive point in [Honeycutt’s] post is that GISP2 (or any specific ice core) is a local as opposed to global temperature record. Is it misrepresentation to use it as a proxy for global climate? ”
That’s all Honeycutt is addressing. I don’t know if your answer to your question is Yes or No.
Or are you saying that Honeycutt has the purpose of your original post wrong?


Mike says:
March 1, 2011 at 9:33 am
“You are entitled to your views but one or two ice cores do not provide enough information to reconstruct global climate.”
Tell that to James Hansen. In this paper:
…he simply divides the Vostok temperatures by two and calls it “calculated global temperature” (and he has used the same stunt in several papers).

Stephen Richards

Mike says:
March 1, 2011 at 9:33 am
So blinded by your faith that you are reading words that are simply not there.

This NSF overlay may show the correlation a little better.


Skepticalscience is a fundamentalist propaganda site. I have had my fair share of online discussions where fundamentalist warmists fling links to Skepticalscience, presented as the ultimate “truth”. Afterall, they have been following the home-cooked recipe for “how to argue against skeptics”.
When the posts at Skepticalscience are examined more closely, most of them present only a carefully selected review of the scientific basis, and then goes on to misrepresent even this selection in a warmist direction.
Of course when these facts are brought to the table, the warmists have a way of disappearing from the discussion..
I have found only one redeeming example of somewhat balanced reporting from Skepticalscience, and this was actually acknowledging Lindzen’s estimate for forcing, making the span 0,5 to 1,0 degrees for a doubling of CO2. (then of course going on to make 3-4 degrees the most probable from a consensus sort of view).
It says a lot about their credibility that one is surprised to find that they don’t totally misrepresent the facts in ALL their posts, as they have in the case of the main post here, and in many others.

Ged Darkstorm

I’m a scientist, so I know how the system works. Therefore I can say you honestly have no idea if you truly believe what you say. You don’t know what an “applied” scientist is, and you don’t know what an “engineer” is when in comparison to; if you honestly think there’s little difference.
Him not being a “climate scientist” is also irrelevant. Science is about data and data only, it cares not about titles or professions, only data. The title someone has simply says they are the ones working in that field to generate new data which EVERYONE can then evaluate. And indeed, the point of science is to make data which EVERYONE then evaluates and judges, not just oneself. An hypothesis must be challenged by many. If others challenge my hypothesis, even those outside my field, if their analysis is sound, then it’s completely valid. Nor is anything I would say about my own work more or less valid than what someone else says if they have EVIDENCE to back it up.
Finally, two points on completely other sides of the planet when in agreement, how would you explain that agreement if it isn’t a general trend across the globe? Some areas are more responsive to the “global” average anomaly and some less, but ice should be the most responsive of all. Or did you forget that warming is supposed to be “amplified” at the poles? If anything, ice core data should be an exaggeration of the planet’s complex climate trend, and both ice cores generally agreeing from two different sides of the world means we can be confident in that trend. Nor are they the only evidence. Read some of the papers on the ice core data, and you’ll see what I’m talking about.
Or, do you disagree with all paleoclimatology?

James Sexton

Mike says:
March 1, 2011 at 11:13 am
@Ged Darkstorm
Hall has zero background in climate science. The difference between…..
Meaning he’s not adept at making stuff up.
Coincidentally, I agree, one can’t infer much from 2 points on the globe. However, as pointed out up the thread, this is exactly what many alarmists do. The problem, Mike, is when you attack messengers such as Dr. Hall, any valid thoughts you may have to this particular issue is muted by your violation of societal mores.

vukcevic says:
March 1, 2011 at 11:00 am
Series of geomagnetic storms in last 2-3 hours shifted Earth’s magnetic meridian by 1.5 degrees.
Misrepresentation or misunderstanding. The Declination varied over a few hours [and very different amount at different locations. E.g. at Thule 4 degrees, at other stations but a fraction of a degree], as it usually does during a geomagnetic storm and is back again as it always is. Nothing ‘shifted’.

Mike said:
You are entitled to your views but one or two ice cores do not provide enough information to reconstruct global climate.
OK… What else do you have that goes back that far, and do they disagree with the ice cores??????

Stephen Richards

Fred H. Haynie says:
March 1, 2011 at 11:17 am
Thanks for this. I’ve read a lot of your work and it find it thought provoking and interesting. Many thanks

AMac: The graphs as presented are without any x-axis (time) shifting from the original data. I noted the issues because the estimated time scales that come with them are uncorrected for things like different ice flow rates a mile down over umpteen thousand years. You can see the effects clearly in the two Greenland records, which match very nicely ca. 30-40 kya and then diverge for the earlier 10 ky — though the peaks would seem to match if some time-warping were done. The scientists who do core studies are well aware of this phenomenon and indeed there is a substantial literature on the whole subject.
In general, folks, scientists who study a subject are both very bright and have dug a lot deeper than casual onlookers, or even well-read amateurs. You just don’t catch them on the details very often. Saltative paradigm shifts are rare and generally involve very deep assumptions that are inaccessible to empirical test. One instigates them at one’s peril — the rebels are more often wrong than the old guard.
The main reason we can have any confidence in questioning the current CO2-phobia orthodoxy in climate science is that it is reasonably cast as the hobby-horse of rebel paradigm shifters who took over the reins by political means rather than a fair scientific fight on the basis of evidence and experiment. The status quo ante orthodoxy was natural variation. One doesn’t have to question the entire edifice of climate science to favor it, but simply take part in a debate which should have happened but was played out on a very, very tilted playing field.


Climate changes. Its something it does naturally. Always has and always will.
The pseudoscience of AGW should be recognised as such.
Here is an excerpt from an article in the Science Daily:
“In a letter published recently in the journal Nature, Los Alamos National Laboratory researchers and an international team of scientists report that the Southwest region of the United States undergoes “megadroughts” — warmer, more arid periods lasting hundreds of years or longer. MORE SIGNIFICANTLY, A PORTION OF THE RESEARCH INDICATES THAT AN ANCIENT PERIOD OF WARMING MAY BE ANALOGOUS TO NATURAL PRESENT-DAY CLIMATE CONDITIONS. [Amazing what?]
But then it goes onto say:
“If so, a cooler, wetter period may be in store for the region, unless it is thwarted by increased concentrations of greenhouse gasses in the atmosphere that could warm the planet.” – No signs of that so far


Mike says:
March 1, 2011 at 9:33 am
You are entitled to your views but one or two ice cores do not provide enough information to reconstruct global climate.
But one tree does?

There’s a difference between a climate scientist and an engineer.

If Greenland ice core shows business as usual, it is not valid, because it is just one core. If Greenland temp. station shows warm year because of positive AMO, it is a sure sign of the end of the world.
Mere GISP2 and CET record efficiently demolish the whole AGW scam.


Mike says:
March 1, 2011 at 11:13 am
“Hall has zero background in climate science.”
“The difference between an applied scientist in industry and an engineer is nil.”
Not so sure about that one. But heck, what do I know, I’m just an engineer working in industry alongside other engineers and scientists. And don’t forget, engineers are the ones who keep notes.
“Further, Honeycutt intended no degradation in identifying Hall as an engineer.”
You know that because…?
“And two points simply do not cover the global.”
Dr Hall doesn’t claim they do. He does say ‘Nowhere near a perfect match, but it’s pretty clear that these are all from the same planet.’ Do you disagree with that?


Thanks for the graphic, Smokey. I always knew us engineers had a particularly interesting view of the world.

It seems to me that this “rebuttal” can be boiled down to 3 points:
1) Hall has a PhD
2) Al Gore did it too
3) The Vostok temperature proxy looks kinda sorta like the GISP2
So #1 – okay you’re “Dr.” not “Mr.”
#2 – Gore was simply showing the correlation between CO2 and temperature. It makes sense to use the CO2 and temperature data from the same source (Vostok, I believe) for that purpose.
#3 – In his article, Rob provided a plot which shows GISP2, Vostok, 6 other Holocene temperature proxies, and the average. If we’re keeping score, he just beat you 9 to 2, being generous since you didn’t actually do any real analysis of the Vostok data in your “rebuttal”.
Not much of a rebuttal.


Mike says:
March 1, 2011 at 9:33 am
“You are entitled to your views but one or two ice cores do not provide enough information to reconstruct global climate.”
Ok, how about one tree? Remember YAD061, the magical enchanted Larch of Yamal? We do.

Geoff Sherrington

Sorry to differ, Dr Hall, but the correlation in the rescaled graphs of the 3 ice cores you show is quite poor. There is no way I’d invest money in a scheme that relied on these, as in a prospectus.
For a start, you would need to put error bars on them. Then you’d need to show audit evidence, say 1 in 10 samples replicated. Then you’d need to explain the various known influences on the data, including the proposition that the Greenland pattern is partly dependent on the direction of prevailing winds at the time. Then you’d need to explain how a qualitative mechanism like fractionation of isotopes on evaporation and condensation, at distances from the core holes that are uncertain, with uncertain mixing processes, becomes a quantitative relationship with temperature. (It becomes quantitative only when the researcher uses circular logic).
Just as dendrothermometry has shown failure, I expect that oxygen isotopes in ice cores will also fail. There is altogether too much wishful thinking. I too, wish that we could find a truly reliable proxy that would end the volumes of speculation that now exist. We are not there yet.


The heart of Mr. Hall’s post seems to be tossing an insult at Skeptical Science, and mentioning that Mr. Hall has a doctorate. Just like Richard Alley, except Alley is world-class paleoclimatologist who helped drill and analyze the Greenland ice core.
Greenland and other northern-hemisphere reconstructions sometimes show opposite phases to Antarctica. Climatologists understand that neither by itself is a proxy for global climate. Hall’s graph hides this substantive point, but Honeycutt’s Skeptical Science post stated it clearly.
So, what’s the take away?
•GISP2 is clearly a local record of temperature for the summit of the Greenland ice sheet, not a proxy for global temperature.
•It requires looking at multiple lines of evidence to piece together a complete picture of the Holocene.
•The Holocene shows a very slow, gradual cooling trend over the past 6,000 years but the mechanisms behind the cooling are well understood.
•The cooling during the past 6,000 years globally is on the scale of 1-2C and we have abruptly altered the trend and are now pushing the planet toward warming.

Al Gored

“It shows us that climate change always happens. The 20th-century warming was hardly unprecedented, and doesn’t call for unusual explanations.”

Rob R

A couple of points of disagreement from someone who regularly uses the published ice core data:
1. Prior to approx 20,000 years ago there was an element of anti-correlation between Antarctic and Greenland temperatures as measured from ice cores. This has been confirmed through the analysis of temperatures and currents in oceanic bottom waters. So when the Greenland and Antarctic temperature graphs are overlaid, one on the other, the position of the temperature peaks actually should be displaced by around 800 to 1500 years. There is plenty of published research on this in the various Geophysical and Quaternary Geology/Climate Journals. Please note that the relative timing is usually determined via dating of the ice using a synchronous tracer, typically variations in the atmospheric methane content of each core.
2. It is becoming increasingly clear that the Younger Dryas is not represented in Antarctic Ice Cores. The cooling event one sees in the Vostok core dates from around 15,000 years ago. It is called the “Antarctic Cold Reversal (ACR)” and can also be found in climate records from New Zealand, Tasmania and Chile. This event predates the Younger Dryas by at least 1000 years. It is another manifestation of anticorrelation or at least “phase shift” between events at the two ends of the world.
The causes/ forcing mechanisms for the offset in climate trends recovered from the Northern and Southern Hemisphere ice-cores are sill being debated.
Other than this I see no problem with the concept that temperature trends identified from ice cores can be used to identify temperature trends that are at least Hemispheric in extent. The simultaneous trends identified at a host of Greenland ice core sites have been replicated from 100’s of Northern Hemisphere sea floor cores (sea surface and ocean floor temp), from dozens of pollen records, from records of Loess deposition and from numerous speleothem records. These temperature fluctuations have also been tied in with changes in sea level (measured for instance at coral reefs) that appear to relate to melt-recharge and ice-flow cycles on the Northern Hemisphere ice sheets.
I agree with Dr Hall that temperature trends identified in Greenland ice are more than just local. This applies not just over the last ice-age cycle shown in the graph above, but also through the Holocene warm period that we now live in. Numerous Hemispheric scale temperature trends identified in Greenland for the last 10,000 year period are as large or larger than the modern trend coming out of the Little Ice Age.
This last point is significant. In the modern period the supposed AGW trend is mainly a Northern Hemisphere event. It is not present in the Southern Ocean sea surface temperature record and is barely discernable in the South Pacific (refer to numerous postings by Bob Tisdale). SO a change in the average Northern Hemispere temperature can result in a change in the global average without the event being truely global. There is no doubt that such circumstances have applied frequently in the past (i.e. throughout the Quaternary period and probably longer still).