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Abstract Global mean temperature rapidly warmed during 2023, making 2023 the second warmest year on record at 1.45°C above pre‐industrial climate, and 2024 became the first year on record to surpass 1.5°C. Here we explore the likelihood, mechanisms, and predictability of the rapid warming during 2023 with CMIP simulations and a fully‐coupled forecast ensemble initialized on 1 November 2022. The year‐to‐year (Y2Y) warming for the second half of 2023 of 0.49°C equaled the largest on record since 1850, and is simulated as a 1 in 6,000 years event. The forecast ensemble‐mean predicts about 75% of the observed warming during 2023. The remaining 25% of the warming lies within the forecast spread, with members that forecast a strong 2023 El Niño and positive absorbed shortwave anomalies more likely to forecast the entirety of the observed warming. The forecast ensemble succesfully predicts 2024 to be the first year on record above 1.5°C. 

Antarctic sea ice prediction has garnered increasing attention in recent years, particularly in the context of the recent record lows of February 2022 and 2023. As Antarctica becomes a climate change hotspot, as polar tourism booms, and as scientific expeditions continue to explore this remote continent, the capacity to anticipate sea ice conditions weeks to months in advance is in increasing demand. Spurred by recent studies that uncovered physical mechanisms of Antarctic sea ice predictability and by the intriguing large variations of the observed sea ice extent in recent years, the Sea Ice Prediction Network South (SIPN South) project was initiated in 2017, building upon the Arctic Sea Ice Prediction Network. The SIPN South project annually coordinates spring-to-summer predictions of Antarctic sea ice conditions, to allow robust evaluation and intercomparison, and to guide future development in polar prediction systems. In this paper, we present and discuss the initial SIPN South results collected over six summer seasons (December-February 2017-2018 to 2022-2023). We use data from 22 unique contributors spanning five continents that have together delivered more than 3000 individual forecasts of sea ice area and concentration. The SIPN South median forecast of the circumpolar sea ice area captures the sign of the recent negative anomalies, and the verifying observations are systematically included in the 10-90% range of the forecast distribution. These statements also hold at the regional level except in the Ross Sea where the systematic biases and the ensemble spread are the largest. A notable finding is that the group forecast, constructed by aggregating the data provided by each contributor, outperforms most of the individual forecasts, both at the circumpolar and regional levels. This indicates the value of combining predictions to average out model-specific errors. Finally, we find that dynamical model predictions (i.e., based on process-based general circulation models) generally perform worse than statistical model predictions (i.e., data-driven empirical models including machine learning) in representing the regional variability of sea ice concentration in summer. SIPN South is a collaborative community project that is hosted on a shared public repository. The forecast and verification data used in SIPN South are publicly available in near-real time for further use by the polar research community, and eventually, policymakers.