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Spectral biases in tree-ring climate proxies


External forcing and internal dynamics result in climate system variability ranging from sub-daily weather to multi-centennial trends and beyond1,2. State-of-the-art palaeoclimatic methods routinely use hydroclimatic proxies to reconstruct temperature (for example, refs 3, 4), possibly blurring differences in the variability continuum of temperature and precipitation before the instrumental period. Here, we assess the spectral characteristics of temperature and precipitation fluctuations in observations, model simulations and proxy records across the globe. We find that whereas an ensemble of different general circulation models represents patterns captured in instrumental measurements, such as land–ocean contrasts and enhanced low-frequency tropical variability, the tree-ring-dominated proxy collection does not. The observed dominance of inter-annual precipitation fluctuations is not reflected in the annually resolved hydroclimatic proxy records. Likewise, temperature-sensitive proxies overestimate, on average, the ratio of low- to high-frequency variability. These spectral biases in the proxy records seem to propagate into multi-proxy climate reconstructions for which we observe an overestimation of low-frequency signals. Thus, a proper representation of the high- to low-frequency spectrum in proxy records is needed to reduce uncertainties in climate reconstruction efforts.

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Figure 1: Spectral colour maps and distributions.
Figure 2: Long instrumental, GCM and proxy spectra.
Figure 3: Spectral colour of climate reconstructions.

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This study was financially supported by the EU project MILLENNIUM (#017008-GOCE) and by the Swiss National Science Foundation (SNSF) through its National Center of Competence in Research on Climate (NCCR Climate). C.R. and S.B. are also supported by the Synergia project FUPSOL, and C.R. additionally by the EU project Past4Future. We thank NOAA/DOE/OBER for providing 20CR and everyone else who contributed model or proxy data for this study. We thank E. Gleeson for her editing efforts.

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J.E., J.F. and D.F. designed the study. J.F. performed the analysis with input from D.F., C.C.R. and S.B. All authors contributed to the discussion and to writing the paper.

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Correspondence to Jörg Franke.

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The authors declare no competing financial interests.

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Franke, J., Frank, D., Raible, C. et al. Spectral biases in tree-ring climate proxies. Nature Clim Change 3, 360–364 (2013).

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