Including an exceptionally warm Northern Hemisphere summer1,2, 2023 has been reported as the hottest year on record3-5. 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.

Department of Geography Johannes Gutenberg University Mainz Germany

Department of Geography Masaryk University Brno Czech Republic

Department of Geography University of Cambridge Cambridge UK

Global Change Research Institute of the Czech Academy of Sciences Brno Czech Republic

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McKie, R. World experiences hottest week ever recorded and more is forecast to come. The Guardian https://www.theguardian.com/world/2023/jul/16/red-alert-the-worlds-hottest-week-ever-and-more-is-to-forecast-to-come (16 July 2023).

Sands, L. This July 4 was hot. Earth’s hottest day on record, in fact. The Washington Post (5 July 2023).

Poynting, M. & Rivault, E. 2023 confirmed as world’s hottest year on record. BBC (9 January 2024).

Copernicus. 2023 is the hottest year on record, with global temperatures close to the 1.5 °C limit. Copernicus (9 January 2024).

Bardan, R. NASA analysis confirms 2023 as warmest year on record. NASA https://www.nasa.gov/news-release/nasa-analysis-confirms-2023-as-warmest-year-on-record (12 January 2024).

Schneider, L., Konter, O., Esper, J. & Anchukaitis, K. J. Constraining the nineteenth-century temperature baseline for global warming. J. Climate 36, 6261–6272 (2023). DOI

Friedlingstein, P. et al. Global carbon budget 2023. Earth Syst. Sci. Data 15, 5301–5369 (2023). DOI

van Oldenborgh, G. J. et al. Defining El Niño indices in a warming climate. Environ. Res. Lett. 16, 044003 (2021). DOI

Rohde, R. Global Temperature Report for 2023 (Berkeley Earth, 2024).

Zachariah, M. et al. Extreme heat in North America, Europe and China in July 2023 Made Much More Likely by Climate Change https://www.worldweatherattribution.org/extreme-heat-in-north-america-europe-and-china-in-july-2023-made-much-more-likely-by-climate-change (World Weather Attribution, 2023).

NOAA Climate Prediction Center. El Niño/La Niña Home (NOAA Climate Prediction Center, 2024).

NOAA Global Monitoring Laboratory. Trends in Atmospheric Carbon Dioxide (Global Monitoring Laboratory, 2024).

United Nations. 7. d Paris Agreement, Treaty Series, Vol. 3156, 79 (United Nations, 2015).

Jones, P. The reliability of global and hemispheric surface temperature records. Adv. Atmos. Sci. 33, 269–282 (2016). DOI

Frank, D., Büntgen, U., Böhm, R., Maugeri, M. & Esper, J. Warmer early instrumental measurements versus colder reconstructed temperatures: shooting at a moving target. Quat. Sci. Rev. 26, 3298–3310 (2007). DOI

Parker, D. E. Effects of changing exposure of thermometers at land stations. Int. J. Climatol. 14, 1–31 (1994). DOI

Trewin, B. Exposure, instrumentation, and observing practice effects on land temperature measurements. Wiley Interdiscip. Rev. Clim. Change 1, 490–506 (2010). DOI

Masson-Delmotte, V. P. et al. (eds) Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge Univ. Press, 2021).

Menne, M. J., Williams, C. N., Gleason, B. E., Rennie, J. J. & Lawrimore, J. H. The Global Historical Climatology Network monthly temperature dataset, version 4. J. Clim. 31, 9835–9854 (2018). DOI

Rohde, R. et al. A new estimate of the average Earth surface land temperature spanning 1753 to 2011. Geoinform. Geostat. 1, 1000101 (2013).

Osborn, T. J. et al. Land surface air temperature variations across the globe updated to 2019: The CRUTEM5 data set. J. Geophys. Res. Atmos. 126, e2019JD032352 (2021). DOI

Lenssen, N. J. L. et al. Improvements in the GISTEMP uncertainty model. J. Geophys. Res. Atmos. 124, 6307–6326 (2019). DOI

Büntgen, U. et al. The influence of decision-making in tree ring-based climate reconstructions. Nat. Commun. 12, 3411 (2021). PubMed DOI PMC

Schneider, L. et al. Revising midlatitude summer temperatures back to A.D. 600 based on a wood density network. Geophys. Res. Lett. 42, 4556–4562 (2015). DOI

Stoffel, M. et al. Estimates of volcanic-induced cooling in the Northern Hemisphere over the past 1,500 years. Nat. Geosci. 8, 784–788 (2015). DOI

Wilson, R. et al. Last millennium Northern Hemisphere summer temperatures from tree rings: Part I: the long term context. Quat. Sci. Rev. 1–18 (2016).

Guillet, S. et al. Climate response to the Samalas volcanic eruption in 1257 revealed by proxy records. Nat. Geosci. 10, 123–128 (2017). DOI

Büntgen, U. et al. Prominent role of volcanism in Common Era climate variability and human history. Dendrochronologia 64, 125757 (2020). DOI

Büntgen, U. et al. Cooling and societal change during the Late Antique Little Ice Age from 536 to around 660 AD. Nat. Geosci. 9, 231–236 (2016). DOI

Esper, J. et al. Large-scale, millennial-length temperature reconstructions from tree-rings. Dendrochronologia 50, 81–90 (2018). DOI

Esper, J. et al. European summer temperature response to annually dated volcanic eruptions over the past nine centuries. Bull. Volcanol. 75, 736 (2013). DOI

Esper, J., Büntgen, U., Hartl-Meier, C., Oppenheimer, C. & Schneider, L. Northern Hemisphere temperature anomalies during the 1450s period of ambiguous volcanic forcing. Bull. Volcanol. 79, 41 (2017). DOI

Esper, J. et al. Orbital forcing of tree-ring data. Nat. Clim. Change 2, 862–866 (2012). DOI

Cai, W. et al. ENSO and greenhouse warming. Nat. Clim. Change 5, 849–859 (2015). DOI

Huang, B. et al. Extended Reconstructed Sea Surface Temperature, version 5 (ERSSTv5): upgrades, validations, and intercomparisons. J. Clim. 30, 8179–8205 (2017). DOI

Columbia Climate School. ENSO forecast. Columbia Climate School (20 May 2024).

Kumar, A. & Hoerling, M. P. The nature and causes for the delayed atmospheric response to El Niño. J. Clim. 16, 1391–1403 (2003). DOI

Bluth, G. J., Doiron, S. D., Schnetzler, C. C., Krueger, A. J. & Walter, L. S. Global tracking of the SO DOI

Parker, D. E., Wilson, H., Jones, P. D., Christy, J. R. & Folland, C. K. The impact of Mount Pinatubo on world‐wide temperatures. Int. J. Climatol. 16, 487–497 (1996). DOI

Self, S. & Rampino, M. R. The 1963–1964 eruption of Agung volcano (Bali, Indonesia). Bull. Volcanol. 74, 1521–1536 (2012). DOI

Francis, P. & Oppenheimer, C. Volcanoes 2nd edn (Oxford Univ. Press, 2004).

Medhaug, I., Stolpe, M. B., Fischer, E. M. & Knutti, R. Reconciling controversies about the ‘global warming hiatus’. Nature 545, 41–47 (2017). PubMed DOI

Karl, T. R. et al. Possible artifacts of data biases in the recent global surface warming hiatus. Science 348, 1469–1472 (2015). PubMed DOI

Wild, M. et al. Global dimming and brightening: a review. J. Geophys. Res. Atmos. 114, D00D16 (2009). DOI

Stern, D. I. Reversal of the trend in global anthropogenic sulfur emissions. Glob. Environ. Change 16, 207–220 (2006). DOI

Allan, R. P. in Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (eds Masson-Delmotte, V. et al.) (Cambridge Univ. Press, 2021).

Anchukaitis, K. J. & Smerdon, J. E. Progress and uncertainties in global and hemispheric temperature reconstructions of the Common Era. Quat. Sci. Rev. 286, 107537 (2022). DOI

Morice, C. P. et al. An updated assessment of near‐surface temperature change from 1850: the HadCRUT5 data set. J. Geophys. Res. 126, e2019JD032361 (2021). DOI

Esper, J., Frank, D. C., Wilson, R. J. S. & Briffa, K. R. Effect of scaling and regression on reconstructed temperature amplitude for the past millennium. Geophys. Res. Lett. 32, L07711 (2005). DOI

Ohmura, A. Observed decadal variations in surface solar radiation and their causes. J. Geophys. Res. Atmos. 114, D00D05 (2009). DOI

Wild, M. Decadal changes in radiative fluxes at land and ocean surfaces and their relevance for global warming. Wiley Interdiscip. Rev. Clim. Change 7, 91–107 (2016). DOI

Rohde, R. A. & Hausfather, Z. The Berkeley Earth land/ocean temperature record. Earth Syst. Sci. Data 12, 3469–3479 (2020). DOI

Tree-ring stable isotopes from the European Alps reveal long-term summer drying over the Holocene

Highest Occurring Vascular Plants from Ladakh Provide Wood Anatomical Evidence for a Thermal Limitation of Cell Wall Lignification

Taxus tree-ring chronologies from southern England reveal western European hydroclimate changes over the past three centuries

Climate signal age effects in Pinus uncinata tree-ring density data from the Spanish Pyrenees

Physiological meaning of bimodal tree growth-climate response patterns