Fennoscandian tree

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Earth system models and various climate proxy sources indicate global warming is unprecedented during at least the Common Era1. However, tree-ring proxies often estimate temperatures during the Medieval Climate Anomaly (950–1250 ce) that are similar to, or exceed, those recorded for the past century2,3, in contrast to simulation experiments at regional scales4. This not only calls into question the reliability of models and proxies but also contributes to uncertainty in future climate projections5. Here we show that the current climate of the Fennoscandian Peninsula is substantially warmer than that of the medieval period. This highlights the dominant role of anthropogenic forcing in climate warming even at the regional scale, thereby reconciling inconsistencies between reconstructions and model simulations. We used an annually resolved 1,170-year-long tree-ring record that relies exclusively on tracheid anatomical measurements from Pinus sylvestris trees, providing high-fidelity measurements of instrumental temperature variability during the warm season. We therefore call for the construction of more such millennia-long records to further improve our understanding and reduce uncertainties around historical and future climate change at inter-regional and eventually global scales.

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The A-FEN reconstruction is available at the National Centers for Environmental Information on the NOAA homepage (https://www.ncei.noaa.gov/access/paleo-search/?dataTypeId=3). The data used in the reconstruction are available at the NOAA International Tree Ring Data Bank. The data used to perform our analysis as well as our results are uploaded to Zenodo and are freely accessible using the following link: https://doi.org/10.5281/zenodo.7993298. Source data are provided with this paper.

The code that supports the findings of this study is available alongside source data on the Zenodo repository and can be accessed using the following link: https://doi.org/10.5281/zenodo.7993298.

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This work has been funded by a grant from the Swiss National Science Foundation awarded to G.v.A., supporting J.B., K.S., M.V.F. and S.K. (Project XELLCLIM no. 200021_182398). M.S., P.F. and J.B. received funding from the SNF Sinergia project CALDERA (no. 183571). K.S. received funding from Formas grant no. 2019-01482. J.E. received funding from the ERC Advanced grant MONOSTAR (AdG 882727). We thank E. Rocha for assistance with Torneträsk samples from the Dendrolab at Stockholm University; S. Helama for making us aware of the early twentieth-century study55 describing the pine-tree canopy damage; and M. Timmonen and U. Büntgen for their assistance in sampling the dead-wood material at the N-scan site.

These authors contributed equally: Marco Carrer, Georg von Arx

Swiss Federal Institute for Forest Snow and Landscape Research WSL, Birmensdorf, Switzerland

Jesper Björklund, Kristina Seftigen, Marina V. Fonti, Sven Kottlow, Patrick Fonti, Daniel Nievergelt & Georg von Arx

Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland

Jesper Björklund, Marina V. Fonti, Patrick Fonti, Daniel Nievergelt & Georg von Arx

Regional Climate Group, Department of Earth Sciences, University of Gothenburg, Gothenburg, Sweden

Kristina Seftigen

Climate Change Impacts and Risks in the Anthropocene (C-CIA), University of Geneva, Geneva, Switzerland

Markus Stoffel

Dendrolab.ch, Department of Earth Sciences, University of Geneva, Geneva, Switzerland

Markus Stoffel

Department F.-A. Forel for Environmental and Aquatic Sciences, University of Geneva, Geneva, Switzerland

Markus Stoffel

Laboratory of Tree-Ring Research, University of Arizona, Tucson, AZ, USA

David C. Frank

Department of Geography, Johannes Gutenberg University, Mainz, Germany

Jan Esper

Global Change Research Institute of the Czech Academy of Sciences (CzechGlobe), Brno, Czech Republic

Jan Esper

Earth and Life Institute, Université Catholique de Louvain (UCLouvain), Louvain-la-Neuve, Belgium

Hugues Goosse

Swedish Polar Research Secretariat, Abisko Scientific Research Station, Abisko, Sweden

Håkan Grudd

Department of Physical Geography, Stockholm University, Stockholm, Sweden

Björn E. Gunnarson

Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden

Björn E. Gunnarson

Department of Land, Environment, Agriculture and Forestry (TeSAF), University of Padua, Padua, Italy

Elena Pellizzari & Marco Carrer

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J.B., G.v.A., K.S. and M.C. conceptualized the study. H. Grudd, B.E.G., J.E., M.C., E.P. and D.N. conducted fieldwork and provided physical samples for the wood anatomical analyses. G.v.A., M.C., M.V.F., S.K. and E.P. coordinated, processed and measured the wood anatomical data. J.B. and K.S. performed the analyses with input from H. Goosse, G.v.A., P.F., D.C.F. and M.S. J.B. wrote the paper and all authors have reviewed and helped to revise the paper.

Correspondence to Jesper Björklund.

The authors declare no competing interests.

Nature thanks the anonymous reviewers for their contribution to the peer review of this work. Peer reviewer reports are available.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

The instrumental temperature data was sorted from coldest to warmest and plotted together with the reconstruction values of the corresponding years. The grey boxes are bound by the 10% coldest and warmest years, and the 10th and 90th percentile of the zscore temperatures, respectively. If an extreme reconstruction value is found within the grey box, the extreme is defined as “captured”. The sum of the captured values divided by the potential sum of values, is calculated and presented as a percentage of extreme value capturing (EVC). In McCarroll, et al.21, a significance testing was implemented, and for 160–170 years of climate data, p < 0.001 is achieved if more than 40% of values are captured. a) A-FEN’s ability to capture JJA temperature extremes. b) X-FEN’s ability to capture JJA temperature extremes. c) and d) show same analysis as a) and b) but using the target MJJA. Both datasets thus display significant amounts of extremes captured, but the A-FEN captures significantly more than the X-FEN for the MJJA target season. The X-FEN captures a higher percentage of cold extremes if the MJJA target season is used but the same percentage of warm extremes regardless of target season. The rationale for using MJJA as the target season for X-FEN is thus less clearcut than for the A-FEN. JJA is the target season used in the publications originally presenting the MXD data2,37 and is thus used in the main text for the other comparisons.

Source data

a) high-pass filtered data (cubic smoothing splines with 50 % frequency response cut-off at 40 years (HP40yrs)) correlated with identically treated temperature data. b) RCS detrended data correlated with untreated temperature data. c) low-pass filtered data (LP5yrs), and d) (LP10yrs), correlated with identically treated temperature data, respectively. The monthly temperature data were retrieved from HadCRUT568 (5° gridded monthly dataset, Lat. 65–70° N, Lon. 15–30° E). Correlation coefficients in white are significant at p < 0.01, and black coefficients are insignificant. When 10-year low-pass filtered data are used, the autocorrelation is so high that it is impossible to detect significance after adjusting for loss of degrees of freedom79, why it is meaningless to continue the analysis over even lower frequencies. The parameters or reconstructions reside on the y-axis, and each monthly temperature or monthly target season on the x-axis. First order autocorrelation, AR(1), of the JJA and MJJA temperatures are given on top of each panel as a reference, and the tree-ring parameter AR(1) can be found in the right margin of each panel. The period of analysis covers the full length-overlap between all datasets (1850–2019 for anatomical parameters and 1850–2010 for the X-FEN). The results are very similar if the 1850–2010 period is used for the QWA data. The delta radial cell wall thickness (DeltaCWTRAD) parameter was established as predictor for the A-FEN reconstruction due to overall performance in the analysis.

Source data

The non-QWA datatypes are identical to Fig. 2 of the Main manuscript and the vertical arrows have the exact positions and dimensions as in Fig. 2 for reference. a) A-FEN (produced in this study) calibrated using regional mean air MJJA temperatures68 (R2 ensemble range within brackets (a = 0.05)), and results for the X-FEN (from Wilson, et al.9) using corresponding JJA temperatures. The irregular winter/spring of 1902/1903, led to a massive dieback of yearly branch-shoots in the region55, highlighted by the yellow area. In these years with extremely narrow rings, the X-ray technique struggles to measure high MXD values due to its comparatively lower effective measurement resolution29 (see Extended Data Fig. 4). b) Replication and pairwise inter-series correlation ($\bar{R}$) of A-FEN in blue and the X-FEN in red. c) Centennial-scale variations (see Methods) compared between A-FEN, X-FEN, climate model simulations, and NH and global temperature reconstructions. The five large-scale reconstructions1,9,10,38,39, as well as the eleven regionally extracted climate-model simulations40,41,42,43,44,45,46,47,48,49,50 are represented by probabilistic percentile ranges. The vertical arrows highlight the overall discrepancies of the X-FEN compared to the other data.

Source data

a) X-ray image with analysis track path indicated within the solid white rectangle, and examples of the effect of different effective measurement resolutions. b) The photosensors in a) build up measurement profiles, where the blue sensor builds the blue profile corresponding to a 20-micron effective measurement resolution, and the orange sensor builds up the orange profile corresponding to a 60 micron effective measurement resolution, approximating the effective measurement resolution of the X-ray methodology29. Note how the time series of MXD reflect inverse variations if developed using high-resolution or low-resolution equipment, i.e., the middle ring exhibits the lowest or highest value depending on resolution. The explanation for this is that very narrow latewood widths are associated with comparatively lower MXD values even though the “true” MXD value may be high. c) Relationships between TRW and A-FEN and d) LWW and A-FEN. e) Relationships between TRW and anatomical MXD (MXDCWT) and f) LWW and MXDCWT. g) Relationships between TRW and X-FEN and h) LWW and X-FEN. All datasets display correlations and using datapoints covering 850–2005 CE. Note how the X-FEN always is stronger correlated with TRW and LWW than the MXDCWT. A higher correlation is expected if TRW or LWW is affecting the measurement. Spearman rank correlation coefficients were used due to the possibly non-linear relationships between width and density. Rraw and rdiff. refers to untreated and first differenced data prior to correlations, respectively.

Source data

Ring width (TRW) versus anatomical MXD (MXDCWT) and X-FEN, as well as latewood width (LWW) versus MXDCWT and X-FEN. Spearman rank correlations were used on RCS-detrended chronologies with a 100-year base-lengths and 10-year overlaps. For the anatomical MXDCWT data, 100 sub-sampled chronologies with 15 trees/year were used to create ensemble ranges represented in blue shades. Deviations from these blue shaded areas represent significant differences (p < 0.05) from the TRW and LWW correlations with MXDCWT, respectively. The X-FEN correlations often reside outside the blue areas, and at higher correlations with TRW and LWW respectively, indicating occasionally stronger dependence of X-FEN on these parameters.

Source data

RCS-detrended A-FEN (data from this study) versus the X-FEN (data from Wilson, et al.9), smoothed using cubic smoothing splines with 50% frequency response cut-off at 100 years. Note that no A-FEN ensemble member exhibit the protracted warmth during the MCA and the relatively low temperatures during the CWP, as does the X-FEN.

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The MXD based temperature reconstructions from Fennoscandia are retrieved from Wilson, et al.9, Schneider, et al.10 and McCarroll, et al.61 represented by a probabilistic percentile range.

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a) Spectral properties of the A-FEN ensemble and X-FEN on the backdrop of the model ensemble range, as well as the range of a 1000 timeseries, of equal length to the A-FEN, of colored noise with a beta coefficient of 0.5. (Beta coefficient for White noise = 0, Pink noise = 1). b) Running autocorrelations AR(1) calculated for 100-year window lengths, shifted by 10-year lags.

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SEA’s using Gao, et al.86 event lists of the 10 a) and 30 b) of the largest (based on sulfate aerosol injection) northern Hemisphere events. The model simulations were all extracted from the corresponding grid cells Lat 65–70° N, Lon 15–30° E. We used only Gao et al as basis for the event lists because most models in our ensemble were forced with Gao et al, but note that this list may not be optimal for some models and the tree-ring data. We employed a model ensemble mean in the SEA, to explore the degree of volcanic cooling the models express. c-h) Proxy vs model response to some specific major volcanic events dated according to Toohey and Sigl87. The responses to U.E. 1453 CE, Huaynaputina and Eldgjá are pronounced in the proxy data but not in the models. The responses to Samalas and Tambora are pronounced in the models but not in the proxy data. The response to Parker is present in both models and proxy.

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Björklund, J., Seftigen, K., Stoffel, M. et al. Fennoscandian tree-ring anatomy shows a warmer modern than medieval climate. Nature 620, 97–103 (2023). https://doi.org/10.1038/s41586-023-06176-4

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Received: 11 February 2023

Accepted: 05 May 2023

Published: 02 August 2023

Issue Date: 03 August 2023

DOI: https://doi.org/10.1038/s41586-023-06176-4

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