Ah, Paulo, did you really think you would get that sort of logical thinking past an AGW disciple like Andrew?

there is something odd happening to these trees

Even if that is true, how can we possibly be sure that they didn’t go through some odd phases of development in the previous 950 years?

The fact is that we can’t and, until we can transport thermometers in a time machine, we never will.

The reliability of tree rings as proxies can be judged by comparing their results to the instrumental records as far as we have them and to other proxies and on both counts they perform well enough apart from (in some not all cases) post-1960. What’s more, I believe that in the cases that do exhibit the post-1960 divergence it’s not just that the behaviour in terms of the width of the tree rings that changes but there are certain physiological changes as well which would suggest that there is something odd happening to these trees after that time.

Mmmm.

Surely the fact they are not reliable now must cast some smidgeon of doubt over whether they are much good at all?

Take a look at this

http://en.wikipedia.org/wiki/File:1000_Year_Temperature_Comparison.png

A rough glance tells me that leaving aside the one reconstruction which seems much cooler than the rest, the MWP temperatures are about 0.3° – 0.7° lower than at the end of the 20th century, which would be consistent with the comment at Realclimate.

There are a number of different reconstructions there, all of which, I believe, use tree rings to an extent but some also use other proxies, and they are all reasonably consistent apart from the one I mentioned above which is actually cooler than the rest. I don’t know why you would give Loehle’s paper more credence than all of these, and it is wrong to assume that the post-1960 divergence problem must neccessarily render reconstructions based on tree-ring data invalid.

And Loehee revised the paper in 2008 to take into account the more legitimate of the criticisms. There still shows a lot of earlier warming and it is still likely to be more reliable than the Briffa et al data given that we know that that data is unreliable because of the massive divergence post 1960.

Frome Realclimate, though, this para is interesting:

“All previous multiproxy reconstructions indicate a Northern Hemisphere mean temperature less than current levels, though possibly on a par with the mid- 20th century. But there are only a few tenths of a degree in it”

But you wouldn’t know from the hockey stick that all multiproxy reconstructions show temps as high as the modern era give or take a tenth of a degree, would you? Funny that.

If big oil would fund historians, my life would be much easier. My concern is a map which shows that all the areas where Anglophone historians are most likely to study had a medieval warm period, whilst less studied areas in the English-speaking world (such as the Baltic, which was undergoing a huge colonial expansion at this time which might suggest better climatic conditions, which might mean higher tempratures) have in general no such evidence.

At the least suspicious this would be selection bias – the evidence is sought where historians say we should find it (a common archaeological issue). At the worst this could be data manipulation to accord with certain views. As I say, the source of the map would be very nice (Unity – you are still reading this thread are you not?).

There’s also this commentary on Loehle’s paper which notes some or its limitations:

http://www.realclimate.org/index.php/archives/2007/12/past-reconstructions/

Wegman was criticising MBH98/99 and was, in some respects, badly out of date by the time it was presented to the Senate Commitee, as Thomas Crowley noted in evidence:

Bullet three – the researchers do not seem to be interacting with the statistical community. This statement is based on a small subsample of paleoclimate papers. Overall, there is increasingly strong incorporation of statistical methodologies in the climate sciences, including increased interactions with
statisticians. For example, the National Center for Atmospheric Research has had a postdoctoral program for statisticians for thirteen years. A key project jointly funded by DOE and NOAA for detection and attribution of climate change
involves not only several statistical climatologists but also explicitly seeks out input from statisticians.

The latter types of data require an intimate understanding of the raw data and simpler, more robust statistical methodologies that recognize the limitations of such data.

The polynomial fills are there simply to illustrate the direction of the trends, i.e. to make the divergence problem more readily visible as an adjunct to the explanation.

That graph is not intended to be an exact model/reconstruction. It does what it needs to do, just don’t try and analyse the detail or draw precise conclusions from it because that’s not what its for.

The stagnant conditions of peat bgs are ideal habitats for preserving wood; moist, aerated and warm environments are not.

One form of proxy should not be relied upon. Therefore tree ring, coral, ice core and sediments should all be used but with the input of statisticians to ensure the experiments have been properly designed, results analysed and peer review is fully independent.

That’s right. All the historians who noted the Warm Period (often decades ago) are tools of Big Oil. And so were the guys who wrote the Greenland Sagas in the 13th century.

As to the map of the Medieval Warm Period (now to be renamed the ‘where Anglophone Historians have noted better climatic conditions c. 1000 AD Period’?), I would love to know where that map comes from and on what data it is based (being a scholar of the period and all). The fact the Artic and Ireland are so well covered suggests tree ring data is partially responsible, and as Unity and I have for different reasons noted this is not the best way to analyse a Warm Period. More importantly, the British Isles, the Atlantic, the more habital bits of North America, west Africa and the Middle East, being the areas with mapped temprature rises, are wierdly the same places where Anglophone historians have undertaken the most study in the period – beyond British and Irish history and American archaeology, note the traditional Anglophone historical areas of Viking studies, African studies and Byzantine studies. The correlation is slightly concerning – in those areas where historians who primarily work in English are likely to comment, the warming they would expect is indeed found. I can’t think of an area where the same conditions would apply where it is not.

“It uses the non tree-ring datasets that are available and complete, ”

So there are only a handful of non tree ring proxies for reconstructing climate before the modern age ? Are you sure ? That sounds crazy.

http://www.timesonline.co.uk/tol/news/environment/copenhagen/article6956783.ece

following on from the “million degrees” brilliance

http://www.powerlineblog.com/archives/2009/11/024973.php

Loehle has also offered strong theoretical grounds for believing that tree-ring data will always be unreliable during warming periods.

That, of course, would explain almost everything- divergence, MWP etc.

The only remaining mystery would be why scientists continue to believe that tree ring data might be a good proxy for temperature measurement.

It uses the non tree-ring datasets that are available and complete, I think. The advantage of this is that it does not use data that we know (because of the post 1960s divergence) to be unreliable. Loehle has also offered strong theoretical grounds for believing that tree-ring data will always be unreliable during warming periods. It may be that Loehle’s reconstruction is faulty, of course, but we have no better reason for dismissing it than we have for dismissing other reconstructions, which means we cannot know that current warming is unprecedented or even unusual.

It is true that, in Watson’s phrase, in the autumn of 1999 Jones and his colleagues were trying to ‘tweak’ a diagram. But it wasn’t just any old diagram.

It was the chart displayed on the first page of the ‘Summary for Policymakers’ of the 2001 IPCC report – the famous ‘hockey stick’ graph that has been endlessly reproduced in everything from newspapers to primary-school textbooks ever since, showing centuries of level or declining temperatures until a dizzying, almost vertical rise in the late 20th Century.

But the “hide the decline” email doesn’t refer to the graph in the IPCC report, it refers to a similar one in the “WMO Statement on the Status of the Global Climate in 1999″.

For his 2008 paper, in which the reconstruction was based on over 1,000 datasets, Michael Mann did publish both his data and program code in full. Haven’t got the link to hand but its all on the web.

The Loehle study, which Bishop Hill mentions and which is often put forward as debunking Mann’s work, is based on a mere 18 datasets only five of which are suitable for an MWP reconstruction.

Much of the obsession with gaining access to raw, unadjusted, data comes from asshats like Watts and Eschenbach who simply want to spit out graphs based on raw data and claim, without any credible supporting evidence or analysis, that the adjustments that climate scientists have to use to iron out confounding factors, such as the re-siting of weather stations, are part of a scam to artificially inflate temperature records in support of AGW.

What neither have done, nor shown themselves capable of doing, is challenging the scientific basis on which these adjustments are made. There are at least four summary papers that cover the process used to make adjustments to the US surface record, none of which Watts has ever attempted to critique beyond whining about suppose urban heat islands, even though many of the adjustments to the NOAA dataset relate specifically to dealing with siting artefacts in the data.

That’s one of the key reasons why Watts ended up having his arse handed to him by Peter Sinclair when it was found that the data from the weather stations that Watts identified as being free from confounding factors showed the same temperature trend as the full data set.

Fuck me, you’ve obviously never read any of the research published by big pharma, have you?