Sunday, December 27, 2009

Guest comment: Did the Hockeystick kill the medieval warm period?

by Gabi Hegerl

Over the coverage of 'climategate' and on lots of blogsites the view persists that Mike Mann's hockeystick is the one and only scientific obstactle to a warm (ie as warm as the late 20th century or warmer) medieval warm period.


Actually, there are scores of reconstructions of climate in the Northern Hemisphere over the last millennium, and none that I am aware of has the medieval warm period warmer than the late 20th century. Yet there are also lots of publications of individual records that show that the medieval warm period was very warm in places. This is no contradiction: the unusual thing about the recent warming is its widespread nature. While the medieval warm period was warm in places, it wasn't warm everywhere on the Northern Hemisphere, and it wasn't warm in different places at the same time. As the readers who are interested in climate point out, climate varies a lot. It is warm one place at one time and cold at another place at the same time (because the atmosphere shifts conditions around while it varies its circulation naturally, and the ocean does similar things). What the climate system cant usually do without external influences is create sustained, longterm and widespread warming trends - for that, you usually need external influences on climate.

So don't blame the hockeystick for getting rid of a warmer-than-today medieval warm period - climate science did that.

25 comments:

Anonymous said...

"What the climate system cant usually do without external influences is create sustained, longterm and widespread warming trends - for that, you usually need external influences on climate".

Or people prepared to manipulate the temperature record to make it appear that warming is happening!

Anonymous said...

If the global warming is what makes the current warming different, what if the current warming is not global? There are odd things being done to the global temperature, such as noticed at Darwin and Nashville.

MikeR said...

"Actually, there are scores of reconstructions of climate in the Northern Hemisphere over the last millennium, and none that I am aware of has the medieval warm period warmer than the late 20th century." This is pretty much the opposite of anything I've ever heard. The previous post had comments that talk about scores of papers pre-Mann that discussed the MWP. I certainly had the impression that Mann's group's work was the main force opposing the MWP. And you provided no reference at all. Would you care to explain what you're talking about? Are you claiming that all serious climate scientists know that there was no MWP? (I don't think Bray/von Storch asked this question.)
I don't mean to say that you're being disingenuous, but a blanket statement that "everyone knows" that what everyone else thinks isn't true - needs a pretty good justification.

Hans von Storch said...

@MikeR: GabiH said "none that I am aware of has the medieval warm period warmer than the late 20th century" - if you are aware of such reconstructions, with a MWP global or northern hemisphere temperature warmer than during the late 20th cnetury - why don't you just point out where to find it?

Cameron said...

Gabi

Thank you for being willing to join in this debate - post publication of the emails.

Reading IPCC Ch 6 on paleoclimate there is a selection of references to reconstructions of climaate in NH over the last millenium. A number of these are inevitably tainted as they are by Mann, Jones, Briffa and colleagues. It will be helpful if you have a link to a source where the ones to which you referred can be viewed. Or does your 2006 paper list the scores you were makinbg reference to?

MikeR said...

Dr. von Storch, you have me wrong. I'm not a climate scientist, and don't know any sources. However, I'm pointing out that everyone talks about the subject in a certain way. Your guest post is claiming that there are many sources for the opposite position, not just the Mann group. Isn't he the one who needs to bring sources? What are the non-CRU-type reconstructions, and are they indeed unanimous in their results? Seems to me that is something that the rest of us ought to be hearing about.

Anonymous said...

You asked for facts. Here are facts:

Sources confirming the existence of the world-wide Medieval Warm Period:

Harvard

CO2 Science

Michael Mann Debunked

Mann Debunked II

AGW Fraud Exposed

Worldwide Interactive Map debunking the lie that the MWP didn't exist [mouse over charts, or click on chart to expand]

Peter Dunford said...

Gabi,

"it wasn't warm everywhere on the Northern Hemisphere,"

Eminently reasonable, no arguments there.

"and it wasn't warm in different places at the same time."

This cannot possibly be true. It can be hot in Greece at the same time as in Morrocco. It can be hot in England at the same time as in Spain. It can be hot in California on the same day as in Italy, just a few hours difference for rotation.

"What the climate system cant usually do without external influences is create sustained, longterm and widespread warming trends - for that, you usually need external influences on climate."

There we need your definitions of "usually", "sustained", "longterm", "widespread" and "external influences".

The use of the word "usually" means you are uncertain, that there may well be /have been occasions when the climate system has created "sustained, longterm and widespread warming trends".

The reality here is that the situation is very uncertain and to pretend otherwise is disingenuous. "Hide the decline" exposes the obvious: if, for example, tree-ring proxies don't correlate since 1960, how on earth can we have any faith to use them to hind-cast over the past thousand or more years. People who make a living producing forestry management software will attest that it is very difficult to accurately forecast ring growth when temperature, sunlight, soil nutrients and rainfall are known.

Proxies someone's estimate, their interpretation. In doing so they select the "good" proxies to use and discard the bad ones. In no way are they the slightest bit definitive, we don't have a long enough temperature record to compare them against to to judge their efficacy (although there is no point in having a long record if the bits that don't correlate are thrown away instead of being explained).

Worse still, over the last 15 years many studies (as you indicate) have got funding to demonstrate that the MWP doesn't exist. Could you get funding if the stated aim was to prove that it DID exist? I don't think so, but I would be delighted to stand corrected.

Bishop Hill said...

There may well be many reconstructions that show current temperatures in excess of the medieval warm period but as the link to the CO2 Science site shows, there are many proxies that show the opposite.

This brings us neatly to the question of series selection in multiproxy studies. I wonder if Prof Hegerl would be willing to explain the proxy selection procedures used in Hegerl et al 2006 for example. There are many interesting questions here.

MikeR said...

Well, it's interesting to watch this. Presumably Prof. Hegerl will present his sources, but that's not enough. I, after all, don't have time (cannot be bothered? am not technically competent?) to read them all. So how am I supposed to make up my mind in any sensible way?
Remember, Prof. Hegerl, it's not enough to show me some sources for your point of view. You have to justify your brush-off: "none that I am aware of has the medieval warm period warmer than the late 20th century." Does that mean you're ignorant of these other studies, or that they are all demonstrably without merit?

Bishop Hill said...

MikeR

Prof Hegerl is a she.

Loehle's study had an MWP warmer than modern temps. It also does an admirable job of explaining its proxy selection methodology.

Steve Carson said...

I'm intrigued as well.

I've consulted a number of geology text books, mainly undergrad level - ok they are not "peer reviewed papers" but they draw on them - they all show, like the first IPCC report, a MWP warmer than today. And not a Greenland MWP - a global "average" MWP.

Then I've also read the abstracts of scores of papers showing an MWP in many places around the world. (This may be a cherry picked subset, I'm not sure, hence my continued interest).

Prior to Mann, Bradley & Hughes 98, was there a consensus that the MWP was warmer than today? I understood there was but I would like to know Gabi Hegerl's view.

Secondly, can you provide us amateurs a list of papers that don't rely on Mann or Briffa.

MBH98 was clearly compromised, and having seen Mann tell the world repeatedly that the NAS vindicated his paper it would be major leap to see his later papers as anything other than more self-justification.

Briffa's amazing Yamal reconstructions, heavily reused by everyone else, are clearly compromised. Almost 10 years to get the data and once it's shown to the world it's quite amazing. No wonder Briffa didn't want to release it. Briffa hasn't yet responded to McIntyre's points, but it's hard to imagine a resurrection. (Amusingly, Briffa said in the climategate emails, "..I believe that the recent warmth was probably matched about 1000 years ago..")

Both Mann and Briffa know a hundred times more about climate and stats than amateurs like me, but given that I can form my own opinion, I've formed it and I don't find believe their work is reliable.

But if there are other significant papers out there with a global temperature reconstruction that aren't based on, or by, Mann or Briffa - please let us know, it would be much appreciated.

Marcel Severijnen said...

One can read for instance a relatively unknown though peer-reviewed study of H.Grudd (http://people.su.se/~hgrud/documents/Grudd%202008.pdf) to find other conclusions about the NH-MWP. And if you want to dive in a bit deeper, do perform a search at McIntyre's site (http:\\realclimate.org) with Grudd as keyword.

All proxies however suffer from restrictions in available material, be it historical data, grape harvest data or tree rings. There is need of reconstructions where all possible proxies for a given region will be combined. The hockeystick negelected and contradicted solid historical information for the MWP and the little ice age, and such a contradiction should be a signal for the scientist that his approach is flawed. We have seen to much one-sided attempts to tackle the problem. Climate reconstruction is not a task for climatologists alone. A multidisciplinary approach is urgently needed to better understand past climate change and its effects.

Historians in such fields as social and cultural history and archeologists should cooperate with paleontologists and climatologists to attain a more acceptable and reliable reconstruction, taking account of all available knowledge.

MikeR said...

"The rhetorical impact of Team reconstructions largely derives from the modern-medieval differential: is it in the red or is it in the black?
Thus when one sees study after study which has modern-medieval differentials that are always just slightly in the black, any prudent analyst would arch his/her eyebrow slightly and examine any accounting policies that may have contributed to getting the result in the black. "
http://climateaudit.org/2009/09/18/is-kaufman-robust/
This kind of stuff makes me nervous!

Cameron Rose said...

Gabi wrote, "Actually, there are scores of reconstructions of climate in the Northern Hemisphere over the last millenium, and none that I am aware of has the medieval warm period warmer than the late 20th century."

The link below cites 32 referenced reconstructions, the majority of which show the medieval warm period warmer than the late 20th century. I wonder if Gabi could take a few minutes to comment on these.

http://pages.science-skeptical.de/MWP/MedievalWarmPeriod.html

eduardo said...

I am not really sure that the question of which period was warmer, the present or the MWP, is really that important. In the Northern Hemisphere the early Holocene, say 8000 y BP, was likely warmer than today. The forcings at that time were different, as they might have been also different during the MWP, for instance a more energetic sun.
If the MWP was warmer than today, does it invalidate future climate projections? how?

MikeR said...

"If the MWP was warmer than today, does it invalidate future climate projections? how?" Politically, I think it makes a big difference. It is much easier for a man in the street to understand that a hockey stick means: You must be right.
With the MWP, it comes across more like: This sure is complicated. That was a big bump over there too. Hadn't you guys better wait till we can really see what's going on?

Steve Carson said...

Eduardo has hit the nail on the head. What does it matter?

1. Regardless of whether or not the MWP was warmer than today, CO2 still has the same effect. The position of the temperature peak 1000 years ago doesn't impact the physics of climate science or the GCMs.
2. Earlier warming periods like the Eemian interglacial were warmer than today (currently believed).

Why does the first reconstruction to show the hockey stick become the poster child for the IPCC? After all, scores of previous papers - the consensus? - had a different result. MBH 98 didn't explain where everyone beforehand went wrong.

So why the massive effort to tell everyone that the modern warming is unprecedented in (drum roll) one thousand years!

It helps the sales process. And that, sadly, is where climate science is shooting itself in the foot, in my unimportant opinion. Simple stuff appears flawed in obvious ways and yet heavily defended/promoted by the IPCC, their marketing dept (realclimate.org) and their trade press (Nature).

So when it comes to complex matters like how reliable the GCMs are, many see the same snow job.

plazamoyua said...

If the MWP was warmer than today, does it invalidate future climate projections? how?

In my very layman point of view, it does not theoretically invalidate climate projectons, but it may affect their credibility. It may question the maturity (the reliability) of climate science. Unless there is a pretty good explanation of the difference in forcings, then and now. Or am I wrong, and I am no getting what you mean?

Svalgaard says, and says again, there is no solar forcing explanation for MWP.

eduardo said...

@ 17,19

I agree with you that if the MWP had been warmer the 'polar bear' story would not be credible, as they would have been already exposed to warmer temperatures naturally. This would apply to most present climate impacts.

Form a more scientific point of view the problem is that the solar forcing during the MWP is uncertain. Assuming the anthropogenic forcing since the 19th century is about 1.5 w/m2, the solar constant in the MWP should have been about 6-8 w/m2 larger than the preindustrial value to make up for the missing antropogenic forcing at that time (presently it is about 1367 w/m2. This is a highish change.
My point here is that the level of global temperature in the MWP will be relevant for climate model projections only when the solar forcing is better known. Only at that moment simulated and reconstructed temperatures can be compared and models can be tested.

MikeR said...

I guess I would add that for me, the fabled man on the street, Prof. Hegerl's defense of the hockey stick is a big part of the problem too. I don't know who to believe, but if she's running around saying that this is something that everyone agrees on, "none that I am aware of has the medieval warm period warmer than the late 20th century", and then I find out that there are at least as many reconstructions that say the opposite, and at the very least it's a very close call - well, I get the impression that someone is trying to "snow" me.

Not only that, but this is an important issue. For a decade there have been two narratives:
1) From the skeptics, that there's a team of climate scientists holding the field hostage, suppressing the opposite point of view, refusing to share their data and their methods, commandeering wikipedia and realclimate to create a pretended unanimity.
2) From the believers, that the skeptics (deniers) are all a big joke, clowns who don't know any science and don't trust science, shills of the oil companies. Who has the time to waste answering all their ridiculous requests, where the only point is to create doubt where there is none. Why would we send them our data - we are glad to send it to anyone real. We don't need to suppress them; they are self-suppressed, by the fact that no one who knows anything would consider publishing their nonsense. There is no such thing as a really competent climate scientist who doesn't know all this stuff beyond question.

Well, the big impact (to me) of ClimateGate is that it shows that narrative (b) is just not true, and that narrative (a) is. You can't brush me off by saying that scientists are human too - those emails are soaked in narrative (a). Things like Prof. Hegerl's comment (unless she has a convincing reply) tend to the same impression.
And it's a short step from there to doubting the conclusions in which the CRU team played such a prominent role. If the skeptics are right about the narrative, it's hard to say, "But maybe the results are right anyhow?"
Anyhow, it's hard for the Man on the Street.

Thras said...

I've yet to see a proxy record with a clear enough temperature signal to be able to tell us, to any degree of certainty, the magnitude of the MWP. Higher or lower than present.

Take a study. What is the resolution of the proxies used? Do they measure 1-year spikes? 10-year? What is the reliability?

The process, seemingly endemic in these reconstructions, of taking a load of proxies, running them through a regression meat-grinder algorithm, strikes me as little better than reading tea leaves.

Bad process, bad proxies, bad results. The archaeological data, at this point, is better than the temperature reconstructions.

The physical case for CO2 caused global warming is fairly good. But it's a sure sign of a lightweight when you see someone making a case from the temperature reconstructions. It's enough to make me disregard 99% of what they have to say.

Kim Dabelstein Petersen said...

@Marcel Severijnen - Re: Grudd:

Note that Hegerl specifically said "publications of individual records that show that the medieval warm period was very warm in places." - Tornaträsk is such an "individual record[s]", specifically for a location in Northern Sweden.

Grudd is not a reconstruction of the Northern Hemisphere, but of Fenno-Scandia.

klee12 said...

eduardo wrote I am not really sure that the question of which period was warmer, the present or the MWP, is really that important.

One of the claims of some believers of AGW is that we may be reaching a tipping point. A few degrees higher in temperature and all sorts of terrible things will happen because the temperature is higher. But if in the MWP the temperature was higher, and if in the MWP terrible things did not happen, why worry about AGW?

Was it very bad in Europe during MWP? In China?

klee12

eduardo said...

A tipping point is where you leave a tip, right?