2023 summer warmth unparalleled over the past 2,000 years

Including an exceptionally warm Northern Hemisphere summer(1,2), 2023 has been reported as the hottest year on record(3-5). However, contextualizing recent anthropogenic warming against past natural variability is challenging because the sparse meteorological records from the nineteenth century tend to overestimate temperatures(6). 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 degrees 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 degrees C. Although 2023 is consistent with a greenhouse-gases-induced warming trend(7) that is amplified by an unfolding El Nino event(8), this extreme emphasizes the urgency to implement international agreements for carbon emission reduction.