Do European tree ring analyses indicate unusual recent hydroclimate?

From Dr. Judith Curry’s Climate Etc.

by Frank Bosse and Nic Lewis

Not really.

A recent paper (M. B. Freund et al 2023, MBF23 thereafter) in “Nature communication earth and environment” investigates the variability of the summer drought events since 1600. It uses the method of “stable isotope analyses C13/O18” to extend the “Standardised Precipitation-Evapotranspiration Index (SPEI) from 1950 to now back to 1600.

The paper describes and uses a multi proxy network over large parts of Europe (see Fig. 1 of MBF23) to reconstruct the history of summer droughts for a longer historic period. It finds interesting results about the dependency of those events on volcanos and solar forcing. It’s a worthwhile read and we were interested in whether the headline title is justified and likewise this claim in the Abstract:

“We show that the recent European summer drought (2015–2018) is highly unusual in a multi-century context…”

Thanks to the authors the used SPEI reconstruction annual data are available, so we were able to perform calculations to check these assertions.

An apparent first “confirmation” of the headline title of the paper appears in Figure 3a in MBH23:

Fig.1: A reproduction of Fig. 3a of MBH23. Annual European mean SPEI-data in blue/red, the low pass filter output is shown in black.

The black line in this figure shows the effect of applying a 13-year low-pass smooth, so it relates to the recent past. Indeed, after 2010 the used 13-year Chebyshev filter shows a “dramatic” downward dip to a far lower precipitation index than at any other time during the 1600-2018 reconstruction period. However, when eyeballing one finds also dry periods, before 1950, the onset of the classical SPEI dataset marked with “SPEI”, or before 1880 marked with dark grey in Fig.1, and the low pass filter didn’t react in the way it did after 2010.

The reason for this behaviour is quite simple: All smoothing filters struggle with the beginning and the end of a filtered dataset. They estimate the output because there are no precursors/ successors in the raw data. To test the impact of this properties we used the same data with a similar filter (Loess) and made a comparison with Fig. 1 but stopped the filtering in 1949:

Fig. 2: Fig.1, but with the smoothed SPEI-Index ending in 1949.

If the paper was written in 1950 it would find “unusual recent hydroclimate”, in 2023 it finds the same for the recent conditions due to a filter issue. The beginning after 1600 is also very unusually wet in the filter output for the same reason.

The dip in the end in Fig.3a of MBH23 is not real, it’s an artefact of the used filter.

A simple running mean filter which while it has no output in the early years, is unartefacted, gives a fairer smoothing of fluctuations over 1600-2018:

Fig. 3: Summer SPEI-Data (black) filtered with a trailing running mean (red). The historical minimum of this filter is shown as a broken red line. Clear to see minima in the 1870s and 1680s in addition to at the end of the 1600-2018 period.

Fig. 3 gives the contrary result of the headline title of MBF23: to 2018 (the last datapoint in the set in MBF23) it indicates that the recent European summer hydroclimate was NOT unusual, the SPEI index was in the ballpark of natural variability.

To show that also the claim in the Abstract (“2015-2018 highly unusual”) is not true we had a deeper look in the data and calculated those 4 years averages over the whole timespan.

It turned out that during many periods the average of 4 years in the SPEI data was more negative than during 2015-2018, for which this average is -0.273:

Since 1900 there have been four such periods, all in the years leading up to 1950: 1947-1950; 1946-1949; 1945-1948; 1944-1947. The period before 1950 (not strongly influenced from anthropogenic forcing) was indeed marked by very dry summers, not mentioned in one word in MBF23.

Before 1900 there are also some periods:

1892-1895; 1760-1763; 1759-1762; 1738-1741; 1688-1691.

The “European summer drought 2015-2018” was NOT highly unusual in a multi-century context”, as falsely claimed in the abstract.

To further bolster this point we looked also if longer period averages were “highly unusual”.

It turned out that a trailing average of 5 years produces 10 periods during 1600-1950, a time span predominantly affected by natural variability, with more negative SPEI-values than the most recent period to 2018; a 10-years average gives 9 such pre-1951 periods.  And a 3-year trailing average produces no less than 57 pre-1951 periods with more negative SPEI values than the most recent period.

Furthermore we had a look at the variability of the annual data after 1950 (the time span of the “native SPEI”) and before this year, the time span of the reconstruction of the “European hydroclimate based on a network of tree-ring stable isotopes of oxygen and carbon ratios” in MBF23. We calculated running 21 years standard deviations (sigma) of the annual data (Fig.4):

Fig. 4: The variability of the annual SPEI data. The averages before 1950 and after this year are marked with a dotted line. Note the jump.

The lower time variability of the reconstruction cast some doubts, as to whether the reconstruction of the SPEI 1600…1950 is useful to compare 1:1 the newer native SPEI data with the historical reconstruction data pre- 1950. It looks as if the reconstruction, even if otherwise valid, significantly understates natural variability. This is a common problem with proxy-based reconstructions. It results in the extent of fluctuations during the post-1950 instrumental SPEI era being an exaggerated relative to natural variability, so that normal fluctuations can appear to be unusual.


MBF23 is a very valuable paper when it comes to the description of the variability of European summer droughts since 1600. However, neither its title “European tree-ring isotopes indicate unusual recent hydroclimate” nor the claim in its Abstract that “recent European summer drought (2015–2018) is highly unusual in a multi-century context” are justified by the data used in the paper.

The lower resolution in time and spatially of the reconstruction before 1950 in relation to the determined SPEI after 1950 casts some doubts if the comparison of some years after 1950 to the historical reconstructed values is appropriate.

MBF23 should be corrected and retitled because some key conclusions, including the headline claim in its title, are not supported by proper statistical analysis of the SPEI values that their reconstruction method produces. The recent European drought to 2018 remained within the range of natural variability.

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Tom Halla
February 23, 2023 2:26 pm

Combining proxy estimates of anything and instrumental readings is always fraught. I think Bosse and Lewis are being a bit too kind.

February 23, 2023 2:56 pm

Wiki does not specify the proxies used in SPEI. But you cannot be using tree rings as temperature sensors AND precipitation meters. It is one or the other. And I think they reflect precipitation more than temperature (unless they ever bothered using willows).

But this implies that both dendrothermology and dendrochronology are pseudo-science.

a. Tree-rings are hopeless as temperature sensors. Tree-ring thickness is determined by moisture, nutrients, canopy cover, disease, pests – and lastly by temperature. To say that tree-ring thickness is directly proportional to temperature is a complete nonsense. A very hot but very dry summer will produce thin rings (ie: cold dendrothermology temperatures). So the best tree for dendrothermology would be the willow.

b. Furthermore, you can find thick and thin rings within different radii ON THE SAME TREE. The 4 o’clock ‘temperature’ data may well be completely different to the 12 o’clock temperature data. The core-borers that provide the ring-data are only 1 cm in diameter, so they only provide a small snap-shot of the tree circumference, and cannot see nor evaluate the fat and thin ring segments within the same tree. The methodology is so unreliable, that any climate research including dendro-temperatures should be thrown out.  

c. However, this unreliability also calls into question dendrochronology. If tree-growth is effected more by local conditions – moisture, nutrients, canopy cover, pests, and individual tree ring variability – then you cannot compare an ancient ship’s timber to a reference tree that may have grown many hundreds or thousands of miles away (ie: the Californian bristle-cone pine or Irish bog-oak dendro-data). 
     There can be no comparison, because you cannot even compare two cores from the same tree!  Take a look at the full circumference of a tree, and you will find rings of all shapes and sizes, around the full circumference. And all kinds of ring widths even in adjacent trees.  So how can you compare a ship’s timber with a bristle-cone pine in California – when you cannot even compare that timber with a timber taken from the very same tree or the very same forest?

Both dendrothermology and dendrochronology are snake-oil science.


Last edited 1 month ago by ralfellis
Reply to  ralfellis
February 23, 2023 3:14 pm

Spot on Ralph. There are also other considerations that affect ring thickness such as altitude, aspect to the sun, fire history and competition from other trees.
Temperature in my opinion is mainly related to where a particular species grows best in what climatic zone around the globe.

Frank from NoVA
Reply to  ralfellis
February 23, 2023 3:36 pm

‘b. Furthermore, you can find thick and thin rings within different radii ON THE SAME TREE. The 4 o’clock ‘temperature’ data may well be completely different to the 12 o’clock temperature data.’

Spot on! The local utility has been doing a lot of veggie management lately. A cursory inspection of the resultant stumps indicates that the above is the rule, not the exception – – the use of dendro-tree-mometers in climate studies is absolute junk science.

Reply to  ralfellis
February 23, 2023 6:31 pm

Tree ring data isn’t even junk science. It’s as far from science as one can get. Tree ring data – brought to you by the same people who adjust the temperature record. It’s hard to not laugh at what is passed off as science.

Last edited 1 month ago by iflyjetzzz
Rod Evans
Reply to  iflyjetzzz
February 23, 2023 10:48 pm

You think its junk science now, just wait until some loon starts using Bristle Cone Pines for their proxy….oh.

Reply to  Rod Evans
February 24, 2023 1:42 pm

LOL! The planet’s been in a cooling cycle for several years now, even with ‘adjusted’ temperature data. I expect to see more outlandish junk science as the world continues to cool.
I find it both extremely arrogant and completely laughable that some believe that human activity has a major impact on the earth’s temperature.
This is a huge Ponzi scheme where many of the ‘believers’ know they’re scamming a very gullible public at large.
At what point does the average rube wake up and realize that the earth isn’t warming anymore? And that the world’s temperature rises and falls in multidecade cycles?

Last edited 1 month ago by iflyjetzzz
Joseph Zorzin
Reply to  ralfellis
February 24, 2023 5:07 am

As a forester for exactly 50 years, I agree with everything you say- but apparently the analysis was done by studying isotopes not tree ring widths. But, first of all, how good is that type of analysis? How good is the science? Even if it’s pretty good- that doesn’t prove what the cause of the unusual droughts is. It suggests the usual CO2 emissions by humans since that seems to be “des rigueur“, but doesn’t prove it or even attempt to. So, all we know is that it’s been some dry years recently.

Jim Gorman
February 23, 2023 3:11 pm

Willis E. just recently posted an article by W.M. Briggs that is very pertinent.

Do not smooth times series, you hockey puck! – William M. Briggs (

“If, in a moment of insanity, you do smooth time series data and you do use it as input to other analyses, you dramatically increase the probability of fooling yourself!”

Frank from NoVA
Reply to  Jim Gorman
February 23, 2023 3:40 pm

Luckily, this doesn’t apply to the infilling of missing station data! (/sarc)

John Hultquist
Reply to  Jim Gorman
February 23, 2023 7:05 pm

By Briggs on September 6, 2008

Frank from NoVA
February 23, 2023 3:29 pm

‘MBF23 should be corrected and retitled because some key conclusions, including the headline claim in its title, are not supported by proper statistical analysis of the SPEI values that their reconstruction method produces.’

I’m sure M.B. Freund et al will get right on it (making the corrections)! (/sarc) 

February 23, 2023 3:50 pm

Maybe I’m missing something but how any serious scientist can smooth that data and claim there’s anything of remote statistical inference is beyond me. Interesting, sure, but the headline is ridiculous without a big NO after it.

Izaak Walton
February 23, 2023 5:02 pm

The low pass filter is a red herring. Nowhere in the paper itself do they appear to use it to make any conclusions. It appears to be added to Fig. 3a just as a visual guide. As for the rest the authors state in the conclusions that:
“Our reconstruction also provides additional evidence of longer and possibly more severe
droughts. We find that during the Maunder Minimum and at the end of the Little Ice Age, summer conditions across Europe were drier and multi-year drought episodes occurred frequently.”
which again appears to be what Frank Bosse and Nic Lewis are stating as well. So I am not sure that there is any disagreement here at all.

Reply to  Izaak Walton
February 24, 2023 12:41 am

Did you read the Post below Fig.3? There is deteiled analysis to which degree the title and some Paragraphs in the Abstract is justified by the results from the data they use. .

February 23, 2023 5:47 pm

A recent paper (M. B. Freund et al 2023, MBF23 thereafter)

We show that the recent European summer drought (2015–2018) is highly unusual in a multi-century context.


Who is peer reviewing this paper? Who the hell are these people? What the hell is going on around here!? (H/T Close Encounters of the Third Kind)

February 23, 2023 6:01 pm

No. Next stupid question, please.

John Hultquist
February 23, 2023 7:12 pm

Dendrochronology: dating tree rings to the exact year they were formed starting with living trees and matching rings backward, say with beams in a building and old buried logs —

Dendrochronology is interesting research.
Trying to tease “climate”™ from this is funny stuff.

Martin Brumby
Reply to  John Hultquist
February 23, 2023 7:43 pm

Of course, all this stuff is the old GangGreen Zombie Psyence.

I note that they dug up the old Mosquito & Malaria threats Zombie again recently. Once more shuffling along, thirsting for brains.

A few weeks back they even dusted off the Snows of Kilimanjaro Zombie, thanks to the Fat Albert Zombie.

February 23, 2023 8:38 pm

Tree ring analysis is all too often having results being ones that get paid for, similarly to polygraph “lie detector” tests. And, the actual problem with the “Hockey Stick” is in straightness of the “handle”, not in the “blade” part.

Ben Vorlich
February 24, 2023 4:37 am

The LIA lasted from early 14th century to the end of the 19th, with three particularly cold intervals. One began about 1650, another about 1770, and the last in 1850.
Is it any surprise that a reconstruction that only goes back to just before the first of those shows something different in more recent warmer times. More importantly will worrying about it help?

Tom Johnson
February 24, 2023 7:14 am

There’s a myriad of ways of smoothing ‘noisy’ data, but a running mean is among the worst. It also happens to be about the easiest to perform. It shifts the time that events occur, it alters the amplitude of events depending on the frequency of occurrence, and it ends before the data ends (if properly used). There is simply no way to include anything that hasn’t happened into the analysis. These all add together to make the result barely useful, if that. If you ever see a ‘smoothed’ curve that ends at the same point as the data, BEWARE.

February 24, 2023 9:00 am

All too often, tree ring studies say what their authors want them to say, similarly to polygraph lie detector usage. For example, the “Hockey Stick” that’s significantly by Michael Mann, who gets many things wrong. Mann’s Hockey Stick is Half True. However, the false half of Mann’s Hockey Stick is the straightness of its handle; the sharp uprise of its blade (based on an older version of HadCRUT at least as old as HadCRUT2) is true. Even though HadCRUT got revisions that report more warming than older versions of HadCRUT did, even Steve Milloy (junkscience) and Ryan Maue favorably cite global temperature datasets (such as JRA55) that indicate HadCRUT of version as recent as any subversion of HadCRUT4 hasn’t been overstating global warming.

February 24, 2023 1:35 pm

“by Frank Bosse and Nic Lewis

Not really.”


Looking at the graphs, why list the volcano eruptions?

Willis already showed us detailed information regarding volcanic eruptions and their minimal effects on climate.

Coeur de Lion
February 25, 2023 2:09 am

I’m glad the ‘M’ is not ‘Mann’ for once

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