Abstract
Including an exceptionally warm Northern Hemisphere summer1,2, 2023 has been reported as the hottest year on record3,4,5. However, contextualizing recent anthropogenic warming against past natural variability is challenging because the sparse meteorological records from the nineteenth century tend to overestimate temperatures6. Here we combine observed and reconstructed June–August surface air temperatures to show that 2023 was the warmest Northern Hemisphere extra-tropical summer over the past 2,000 years exceeding the 95% confidence range of natural climate variability by more than 0.5 °C. Comparison of the 2023 June–August warming against the coldest reconstructed summer in ce 536 shows a maximum range of pre-Anthropocene-to-2023 temperatures of 3.93 °C. Although 2023 is consistent with a greenhouse-gases-induced warming trend7 that is amplified by an unfolding El Niño event8, this extreme emphasizes the urgency to implement international agreements for carbon emission reduction.
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Data availability
The observational data, reconstruction and uncertainty estimates are available at https://doi.org/10.17605/OSF.IO/MDUVK.
Code availability
None of the statistical tests applied were performed with environment-specific code.
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Acknowledgements
This study was supported by the ERC Advanced projects MONOSTAR (AdG 882727), the ERC Synergy project SYNERGY-PLAGUE (101118880), the Czech Science Foundation grant HYDRO8 (23-08049 S), the co-funded EU project AdAgriF (CZ.02.01.01/00/22_008/0004635) and the Centre for Interdisciplinary Research (ZiF) in Bielefeld, Germany. We thank C. Oppenheimer, J. Quaas and M. Wild for their discussions of volcanic and solar radiation forcings.
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J.E., M.T. and U.B. designed the study. J.E. and M.T. conducted the analyses with support from U.B. The paper was written by J.E. together with M.T. and U.B.
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Extended data figures and tables
Extended Data Fig. 1 Instrumental temperature records.
a, Comparison of 30–90°N JJA land only temperatures from Berkeley Earth extending back to 1850 CE (red)20, CRUTEM5 back to 1878 CE (blue)21 and GISTEMP4 back to 1883 CE (grey)22. b, Frequency distributions of the three observational records. All data shown as anomalies with respect to 1883–1912 CE means.
Extended Data Fig. 2 Observational temperatures averaged over different spatiotemporal domains.
a, Northern Hemisphere JJA land only temperatures (red) shown together with global annual land and sea surface temperatures from 1850–2023 CE (black)20. The latter represents a combination of Berkeley Earth land and HadSST3 sea surface temperatures52. b, Frequency distributions. Data shown as anomalies with respect to their 1850–1900 CE means.
Extended Data Fig. 3 Early instrumental temperature offset.
Comparison of Berkeley Earth 30–90°N JJA land only observational temperatures20 with ensemble mean reconstructed JJA temperatures23 since 1850 CE. Bold horizontal lines emphasize the 1850–1900 CE offset of 0.24 °C between the two records. The reconstruction was scaled against the observations from 1901–2010 CE and both timeseries then displayed as anomalies with respect to the 1850–1900 CE reconstruction mean.
Extended Data Fig. 4 Ensemble reconstruction climate signals.
Field correlations of the ensemble mean23 against GISTEMP4 JJA land (a, b) and land and sea surface temperatures (c, d) from 1850–2010 CE. Maps produced using the KNMI Climate Explorer at https://climexp.knmi.nl/start.cgi.
Extended Data Fig. 5 Reconstruction verification.
Ensemble mean shown together with other NH extra-tropical summer temperature reconstructions (ref. 24. is Sch15, ref. 25. is Sto15, ref. 26. is Wil16, ref. 27. is Gui17, ref. 28. is Bün20) since 500 CE. All records scaled from 1901–2010 CE against 30–90°N JJA land temperatures (red) and shown as anomalies from 1850–1900 CE.
Extended Data Fig. 6 Reconstructed temperature extremes.
Comparison of temperature anomalies in the four warmest (246, 282, 1061, 986 CE) and four coldest summers (535, 627, 1601, 1642 CE) identified in ensemble mean reconstruction (see Table 1) with estimates from other NH extra-tropical reconstructions. “x” indicates if values are missing due to limited reconstruction lengths.
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Esper, J., Torbenson, M. & Büntgen, U. 2023 summer warmth unparalleled over the past 2,000 years. Nature 631, 94–97 (2024). https://doi.org/10.1038/s41586-024-07512-y
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DOI: https://doi.org/10.1038/s41586-024-07512-y
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