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Climate reconstructions by data assimilation need to accommodate for sensitivities to proxies and prior estimates because models are uncertain and proxies are spatiotemporally limited. This study examines these sensitivities using multiple climate model simulations and different combinations of proxies (i.e., corals, ice cores, and tree‐ring cellulose). Experiments were conducted using an offline data assimilation approach; the results showed annual variations in the global distribution of surface air temperature and precipitation amount from 850 to 2000. Standard deviations of surface air temperature and precipitation amount during the entire period differed by up to 36% due to prior estimates. Experiments with different types of proxies showed that the El Niño‐like distribution of positive anomalies in the central to eastern tropical Pacific may only be adequately reproduced in experiments with corals, and not experiments without corals. The correlation coefficient of the NINO.3 index from 1971 to 2000 between experiments with corals and the Japanese 55‐year Reanalysis (JRA‐55) was 0.81 at maximum. By contrast, the correlation coefficient between experiments without corals and JRA‐55 was a maximum of 0.19. Key Points: Climate reconstruction by data assimilation illustrates annual variations in surface air temperature and precipitation from 850 to 2,000Spatiotemporal differences in climate reconstruction were compared based on prior estimatesThe impact of proxies on the results of past reproduced El Niño cases from 1971 to 2000 was validated [ABSTRACT FROM AUTHOR]

Arctic sea‐ice extent (SIE) has declined drastically in recent decades, yet its evolution prior to the satellite era is highly uncertain. Studies using SIE observations find little variability prior to the 1970s; however, these reconstructions are based on limited data, especially prior to the 1950s. We use ensemble Kalman filter data assimilation of surface air temperature observations with Last Millennium climate model simulations to create a fully gridded Arctic sea‐ice concentration reconstruction from 1850 to 2018 and investigate the evolution of Arctic SIE during this period. We find a decline of ∼1.25×106 km2 during the early 20th‐century warming (1910–1940). The 25‐year trends during this period are ∼33–38% smaller than the satellite era (1979–2018) but almost twice as large as previous estimates. Additionally, we find that variability of SIE on decadal timescales prior to the satellite era is ∼40% greater than previously estimated. Plain Language Summary: Arctic sea ice is an important part of the climate system, serving as the interface between the ocean–atmosphere system. Arctic sea ice has undergone a rapid decline in recent decades, prompting the question of whether there have been changes of similar magnitude in the past. To answer such questions, a long record of sea ice is necessary, but spatially and temporally complete satellite observations are only available starting in 1979. Previous studies combining sea‐ice observations from various sources during the Instrumental Era (1850–2014) found little variability in sea‐ice extent prior to the satellite era, but data availability is limited prior to the 1950s. Here, we create an independent estimate of Arctic sea ice from 1850 to 2018 using a data assimilation approach that blends more abundant temperature observations with data from climate models. Our results show substantial loss of sea ice between 1910 and 1940, with a trend that is about ∼33–38% less than what has been observed in satellite observations. These results reinforce previous findings not only that the current trend is unprecedented in duration since 1850 but also that sea‐ice variability prior to 1979 is ∼40% larger than previously estimated. Key Points: Data assimilation is a skillful technique for reconstructing Arctic sea‐ice extent during the satellite eraReconstructed sea ice shows large declines in total Arctic sea‐ice extent during the early 20th‐century warming (1910–1940)Trends in total Arctic sea‐ice extent during the satellite era are ∼33–38% greater than during the early 20th‐century warming [ABSTRACT FROM AUTHOR]