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The influence of millennium reconstructions on climate projections

by FerdiEgb Sun Feb 12th, 2006 at 07:04:44 PM EST

As promised, with some delay, the diary about climate reconstructions, solar changes and climate models, "projecting" the future.

To introduce myself: retired engineer process automation of a large chemical factory. Because of that, I have some knowledge of (and defined and used) models, be it for multivariate physico-chemical processes, not for climate.

Interested in (and activist for) the environment since 1968 and specifically interested in climate from some 30 years ago, after reading a book about sun-climate connections (like rain and drought, lake levels, ocean/land temperatures, even number of wars...).

I do believe that anthropogenic greenhouse gas emissions have an influence on climate, be it far less than the alarming stories that reach the general public. In my opinion, after reading a lot of literature and following the discussions of both sides of the debate, the influence of anthropogenic greenhouse gases and especially aerosols are overestimated, and solar influences are by far underestimated by current climate models.

That doesn't imply that we shouldn't do anything to reduce the use of fossil fuels. To the contrary, as we need to reduce pollution and our oil dependence of not so stable countries...

The hockeystick graph
I was rather shocked by the acception of the MBH98 "hockeystick" millennium temperature reconstruction, without much debate, which overturned the scientific understanding of that time with a warm NWP (medieval warm period) and a much cooler LIA (little ice age). That triggered for me a deeper search for what is known in climate...

Other reconstructions, Huang ea.
After MBH98, the first publication which differs in amplitude vs. the other temperature reconstructions was for borehole temperatures by Huang ea., 2000. This revealed a much cooler LIA of app. -1 degr.C, vs. the -0.2 degr.C in the shaft of the hockeystick of the MBH98 reconstruction. This is important, as that has consequences for the impact of natural changes (especially solar) on current and future temperature changes. Mann ea., 2003 optimised the Huang data (but corrected this partly in 2004), but Pollack and Smerdon protested against this optimisation.

Esper ea.
The second reconstruction with a larger variability (and a higher MWP) was from Esper ea., 2002. He used a different method for (regional) calibration of tree rings (see Cook ea., 2004), which retains long-term trends better than the method used by MBH98, but also a different calibration period. The effect of this can be read in Esper, Wilson and Briffa, 2005. Mann ea. reacted on the Esper publication, and Cook and Esper responded to this critique.

Moberg ea.
While Esper's reconstruction is only based on extratropical NH tree ring sites, Moberg ea., 2005 used different non-tree ring proxies for the long-term temperature trend and tree rings only for superposed short term temperature variations, effectively downplaying the influence of tree rings on the total reconstruction.

The differences
The reconstructions of Esper, Moberg and Oerlemans, 2005 (the latter based on glacier length records), all show a difference of app. 0.8 degr.C between the LIA and the mid-20th century temperatures (and a MWP app. at the same height of around 1950) vs. a difference of only 0.2 degr. C in MBH98 and other reconstructions. This is discussed by Esper, Moberg, Luterbacher and others, 2005 for its importance towards future climate expectations. The variability in the pre-industrial period was mainly from two sources: volcanic and solar. Volcanic gives a long-term average variation of maximum 0.1 degr.C over the past 600 years (see Fig. 6 from Briffa ea., 1998), that means that solar changes had an influence between 0.1-0.9 degr.C (the latter for the borehole reconstruction). Further, several of the reconstructions don't show a hockeystick shape, but more sinusoidal variations.

The consequences
The amplitude of the temperature variation is a very important difference. If there was a small solar influence in the past (with a 0.1 degr.C solar induced variation in climate), then the recent upswing in temperatures since 1850 is in large part GHG driven. If there was a large solar influence in the past (with a 0.7-0.9 degr.C solar induced change in climate), then the upswing was largely solar driven. In both cases, the instrumental temperature variation must be explained with a different ratio between GHG/aerosols at one side and (enhanced) solar at the other side (of course besides the accuracy of the instrumental temperature record). That means that climate models must be adjusted, depending of what is known of the pre-industrial (and pre-instrumental) climate era.

Thus we still are in need of (far) more accurate reconstructions of the past millennium to have a better view on what can be expected in the future...

The latest reports
Last week two new reports were published on millennium reconstructions. The first was by Osborne and Briffa (OB) (one need subscription for the full text), the other is from D'Arrigo, Wilson and Jacoby (DWJ), using a partly different dataset. This is discussed by both opponents in the reconstruction arena: ClimateAudit (CA) and RealClimate (RC).
RC sees OB as a confirmation of the MBH98 hockeystick graph, while the main critique by CA is that OB used the same disputable bristlecone pine data (where the 1850-1980 upswing probably has nothing to do with temperature) and the same disputable PC method, which mines for "hockeystick" forms, as was used by MBH98.

Interesting is that DWJ didn't use the bristlecone pine data (and other questionable series), as these have a bad correlation with local/regional temperatures, while OB use these series, as they show a good correlation with local temperature trends... Quite contradictory!


Thank you FerdiEgB, for this diary...and welcome! Look forward to reading more of your thoughts!!

"Once in awhile we get shown the light, in the strangest of places, if we look at it right" - Hunter/Garcia
by whataboutbob on Mon Feb 13th, 2006 at 04:40:05 AM EST
This diary was initially a comment to the Hockeystick diary I posted about a week ago. I asked Ferdinand to work his comment into a diary, as he was exposing an enormous range of new insights into this subject. Thanks very much for that, it's appreciated.

Although I've had little spare time to read further into the issue, but most of what I've read was about alternative (non-tree ring) methods to establish the millennium scale temperature records. One of these methods was borehole measurements, the other historic literature. I find the borehole data pretty interesting, although they only seem to track back the temperature differences accurately for a certain depth (hence, time). A few literature studies, however, seem to report a larger MWP and LIA.

Other diaries I wanted to write were exactly about the two contented points you list: solar variation and the aerosol global dimming effect. With this diary you're handing me part of my homework, so also in that regard, your diary appreciated.

In the meantime, I completely concur with your statement that there's a long way to go before there is  a really accurate reconstruction of historic temperature. There isn't one (yet) - or so I feel - and there are numerous of methods that have not been exhausted. And this is something that the science reporting in the daily press often does not seem to understand. Yesterday I read a lovely little rant (of Jerome, I thought) on how reporters get so bad in math and science and although generalised, I completely share the sentiments. Actually, it describes to me the root of the problem in reporting science.

by Nomad on Mon Feb 13th, 2006 at 04:49:06 AM EST
OB used the same disputable bristlecone pine data (where the 1850-1980 upswing probably has nothing to do with temperature) and the same disputable PC method

OB also did tests by taking out one, two, three data series (see page 9/pdf page 10 here), so the first criticism is not significant. As for the second, I thought they did NOT use PC analysis or any other interpolation.

*Lunatic*, n.
One whose delusions are out of fashion.

by DoDo on Mon Feb 13th, 2006 at 10:18:10 AM EST
Checking ClimateAudit, I find the PC use refers to one of the proxies.

*Lunatic*, n.
One whose delusions are out of fashion.
by DoDo on Mon Feb 13th, 2006 at 10:35:26 AM EST
[ Parent ]

I have always had suspicions about the validity of any tree ring data for climate reconstructions. The first problem is that tree ring width and temperature are not linear, but forms a upside down U-shaped curve. Both higher and lower temperatures than the "optimum" temperature reduce tree growth.
Further, at many places, tree growth is more related to precipitation than to temperature and for several series, CO2 and/or nitrogen or (the lack of) minerals are limiting or fertilising factors. And sunlight (direct, indirect, diffuse, aerosols, clouds,...) also plays a huge role...
And last but not least, a large majority (over 300 of near 400) of tree ring series show a reduced growth since 1960, while temperatures are increasing (see e.g. one of the longest: North Finland (Lapland), pages 14/15).
IMHO, this may have been a result of "global dimming", the reduction of sunlight reaching the surface (more water vapour related than aerosol related, I suppose), in the last decades.

If we may believe Stephen McIntyre at ClimateAudit (CA), the choice of the series by OB was not by chance, as they used several series (of the many long ones available) which have a (spurious?) growth spurt in the last century/centuries. The bristlecone pines are not mentioned directly, but compose over 80% of the variance in PC1 of West USA (regional), thanks to the specific PC method used by Mann. Other debatable series are W USA (boreal/upperwright) and Yamal (Russia), and there are several problems with other series too. Further, several series are too short to see the MWP.

Thus while the proxies seems to be robust for any three omissions, the choice itself of only proxies which have a growth spurt in the last century/centuries or proxies with no trend at all, makes the choice itself not robust...

On CA, several comments were made about the OB study. One which surprised me (in addition to my doubts for tree rings as proxy), was that one part of the trees on the same site shows growth with higher temperatures, others show negative growth and some others show no correlation at all...

by FerdiEgb on Mon Feb 13th, 2006 at 04:08:03 PM EST
[ Parent ]

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