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The assassination of Julius Caesar in 44 BCE triggered a power struggle that ultimately ended the Roman Republic and, eventually, the Ptolemaic Kingdom, leading to the rise of the Roman Empire. Climate proxies and written documents indicate that this struggle occurred during a period of unusually inclement weather, famine, and disease in the Mediterranean region; historians have previously speculated that a large volcanic eruption of unknown origin was the most likely cause. Here we show using well-dated volcanic fallout records in six Arctic ice cores that one of the largest volcanic eruptions of the past 2,500 y occurred in early 43 BCE, with distinct geochemistry of tephra deposited during the event identifying the Okmok volcano in Alaska as the source. Climate proxy records show that 43 and 42 BCE were among the coldest years of recent millennia in the Northern Hemisphere at the start of one of the coldest decades. Earth system modeling suggests that radiative forcing from this massive, high-latitude eruption led to pronounced changes in hydroclimate, including seasonal temperatures in specific Mediterranean regions as much as 7 °C below normal during the 2 y period following the eruption and unusually wet conditions. While it is difficult to establish direct causal linkages to thinly documented historical events, the wet and very cold conditions from this massive eruption on the opposite side of Earth probably resulted in crop failures, famine, and disease, exacerbating social unrest and contributing to political realignments throughout the Mediterranean region at this critical juncture of Western civilization. 

The SUMup database is a compilation of surface mass balance (SMB), subsurface temperature and density measurements from the Greenland and Antarctic ice sheets. This 2023 release contains 4 490 442 data points: 1 778 540 SMB measurements, 2 706 413 density measurements and 5 489 subsurface temperature measurements. This is respectively 1 477 132, 420 825 and 4 715 additional observations of SMB, density and temperature compared to the 2022 release. This new release provides not only snow accumulation on ice sheets, like its predecessors, but all types of SMB measurements, including from ablation areas. On the other hand, snow depth on sea ice is discontinued, but can still be found in the previous releases. The data files are provided in both CSV and NetCDF format and contain, for each measurement, the following metadata: latitude, longitude, elevation, timestamp, method, reference of the data source and, when applicable, the name of the measurement group it belongs to (core name for SMB, profile name for density, station name for temperature). Data users are encouraged to cite all the original data sources that are being used. Issues about this release as well as suggestions of datasets to be added in next releases can be done on a dedicated user forum: https://github.com/SUMup-database/SUMup-data-suggestion/issues. Example scripts to use the SUMup 2023 files are made available on our script repository: https://github.com/SUMup-database/SUMup-example-scripts. 

Abstract. The last glacial period is characterized by a number of millennial climateevents that have been identified in both Greenland and Antarctic ice coresand that are abrupt in Greenland climate records. The mechanisms governingthis climate variability remain a puzzle that requires a precisesynchronization of ice cores from the two hemispheres to be resolved.Previously, Greenland and Antarctic ice cores have been synchronizedprimarily via their common records of gas concentrations or isotopes fromthe trapped air and via cosmogenic isotopes measured on the ice. In thiswork, we apply ice core volcanic proxies and annual layer counting toidentify large volcanic eruptions that have left a signature in bothGreenland and Antarctica. Generally, no tephra is associated with thoseeruptions in the ice cores, so the source of the eruptions cannot beidentified. Instead, we identify and match sequences of volcanic eruptionswith bipolar distribution of sulfate, i.e. unique patterns of volcanicevents separated by the same number of years at the two poles. Using thisapproach, we pinpoint 82 large bipolar volcanic eruptions throughout thesecond half of the last glacial period (12–60 ka). Thisimproved ice core synchronization is applied to determine the bipolarphasing of abrupt climate change events at decadal-scale precision. Inresponse to Greenland abrupt climatic transitions, we find a response in theAntarctic water isotope signals (δ18O and deuterium excess)that is both more immediate and more abrupt than that found with previousgas-based interpolar synchronizations, providing additional support for ourvolcanic framework. On average, the Antarctic bipolar seesaw climateresponse lags the midpoint of Greenland abrupt δ18O transitionsby 122±24 years. The time difference between Antarctic signals indeuterium excess and δ18O, which likewise informs the timeneeded to propagate the signal as described by the theory of the bipolarseesaw but is less sensitive to synchronization errors, suggests anAntarctic δ18O lag behind Greenland of 152±37 years.These estimates are shorter than the 200 years suggested by earliergas-based synchronizations. As before, we find variations in the timing andduration between the response at different sites and for different eventssuggesting an interaction of oceanic and atmospheric teleconnection patternsas well as internal climate variability.