The global mean temperature has risen by 1°C relative to pre-industrial times, with half of the rise occurring during the last 30 years. The temperature rise is almost entirely due to human-made greenhouse gas emissions, particularly CO2 emissions. The ocean has taken up 20-30% of total anthropogenic CO2 since the 1980s and more than 90% of the excess heat in the climate system. Ocean warming is not only changing the habitats of many marine species but also causes changes in, e.g., the ocean circulation. At the Alfred Wegener Institute, Helmholtz Centre of Marine and Polar Research, we run long-term ocean time series and carry out process studies in the polar oceans. We aim to improve our understanding of the ocean variability on various time and spatial scales. Furthermore, observational data are used to improve climate-ocean model projections.