Statistics gone wrong in a climate change paper

 Frank Bosse and Nic Lewis point out that the paper "European tree-ring isotopes indicate unusual recent hydroclimate" by Freund, et al is fatally flawed. An apparent indication of significant recent climate change is due to misuse of statistical filtering.

Here are some excerpts from Bosse and Lewis.

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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…”

Conclusion

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.