Antarctic sea ice has swung from a slow, decades-long expansion into a sudden, record-shattering collapse in 2016, and scientists do not fully understand why. This matters because Antarctic sea ice is not a passive bystander in the climate system. It reflects sunlight back into space, insulates the ocean from the atmosphere, and controls how the Southern Ocean absorbs heat and CO₂. The current generation of climate models failed to predict the 2016 crash, revealing a fundamental gap in our understanding of the processes that drive sea ice behaviour from weeks to decades. Without that knowledge, projections of future sea level rise and global warming remain uncertain. The DEFIANT project will embed year-round instruments—on ships, aircraft, satellites, underwater vehicles like Boaty McBoatface, and drifting buoys—into the Antarctic system, including through the harsh winter when data is scarcest. The resulting observations will feed directly into improved climate models. If successful, the research will deliver more reliable forecasts of how Antarctic sea ice loss affects global ocean circulation, heat distribution, and CO₂ uptake—systems that quietly regulate the climate everyone depends on. This is fundamental science with direct consequences for the accuracy of the climate projections that inform national infrastructure planning, coastal defence, and international emissions policy.
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Since the start of the industrial revolution the CO2 concentration in the atmosphere has steadily risen. Scientists have confirmed that the recent loss of Arctic sea ice in summer directly follows this rise in human-induced CO2 emissions, reducing from about 7 million km2 of Arctic sea ice in the late 1970s to around 3.5 million km2 in the 2010s. While climate models suggest Antarctic sea ice extent should also reduce in response to rising CO2, satellite observations reveal that during 1979-2015 the opposite was in fact true. The trend in Antarctic sea ice extent has been a small increase of approximately 1.5% per decade. In 2016, however, this increase was abruptly interrupted by a dramatic reduction in sea ice extent that was far outside the previously observed range. Since the extreme event in 2016, Antarctic sea ice extent has almost returned to its pre-2016 values, highlighting the significant variability in Antarctic sea ice conditions that can occur from one year to the next. These variations in sea ice are important to the whole Earth's climate, because they affect the melting of the glacial Antarctic Ice Sheet, and the capture of atmospheric heat and CO2 by the Southern Ocean. The recent extreme swings in Antarctic sea ice extent, and the challenge of accurately predicting, understanding and modelling them, emphasise the need to: (i) increase our knowledge of the processes that drive Antarctic sea ice variations, including extreme events, and (ii) understand the drivers and climate implications of Antarctic sea ice loss over different time-scales, from weeks to decades. To address this knowledge gap requires a significant research programme, one that takes year-round observations, including throughout the harsh Antarctic winter, and is effective in improving the underlying processes in the latest computer climate models. Our project, known as DEFIANT (Drivers and Effects of Fluctuations in sea Ice in the ANTarctic), will embark on one of the most ambitious observational campaigns aimed at understanding Antarctic sea ice variability. Scientific measurements from the German research ship Polarstern, the UK's new polar research ship Sir David Attenborough, the British Antarctic Survey's Rothera research station, aircraft overflights and satellites will work seamlessly together with cutting-edge robotic technologies (including the underwater vehicle Boaty McBoatface and a suite of on-ice buoys) to provide us with comprehensive, year-round measurements of atmosphere, sea ice and ocean. The knowledge gained from these observations will enable our team to develop new ocean and climate models in order to more accurately represent Antarctic sea ice processes. The analysis of these improved models will allow us to better understand the underlying drivers of the sudden decrease in Antarctic sea ice, determine the impact of these extreme events on the global ocean circulation, and forecast the implications for the movements of heat and CO2 through the climate system. By developing new observations, new satellite records, and new models, DEFIANT will deliver a major advance in our understanding of the Antarctic sea ice system and its wider impacts on global climate.
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