26 Although recent work has highlighted a host of significant late 20 th century 27 environmental changes across the mid to high latitudes of the Southern Hemisphere, the 28 sparse nature of observational records limits our ability to place these changes in the 29 context of long-term (multi-decadal and centennial) variability. As a result, investigating 30 the impact of anthropogenic forcing on climate modes of variability and ecosystems is 31 particularly challenging, though historical

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... from sub-Antarctic islands offer the 32 potential to develop highly resolved records of change. In 1905, a whaling and 33 meteorological station was established at Grytviken on Sub-Antarctic South Georgia in 34 the South Atlantic (54°S, 36°W) providing near-continuous daily observations through 35 to present day. Here we report this new, previously unpublished, daily observational 36 record from Grytviken for temperature and precipitation, which we compare to different 37 datasets (including Twentieth Century Reanalysis; 20CR version 2c). We find a 38 significant trend towards increasingly warmer daytime extremes commencing from the 39 mid-20 th century accompanied by warmer night-time temperatures, with an average 40 rate of temperature rise of 0.13°C per decade over the period 1907-2016 (p<0.0001). 41 Analysis of these data, and reanalysis products, suggest a realignment of synoptic 42 conditions across the mid to high-latitudes since the mid-20 th century, characterised by 43 stronger westerly airflow linked to warm foehn winds across South Georgia. These rapid 44 rates of warming have negative implications for biodiversity levels and the continued 45 survival of some marine biota across the region. 46 47 429