The Southern Ocean provides the principal connection between the Earth's ocean basins and between the upper and lower layers of the global ocean circulation. As a result, the Southern Ocean strongly influences climate patterns and the cycling of carbon and nutrients. Changes in the Southern Ocean would therefore have global ramifications. However, the short and incomplete nature of existing time series makes the causes and consequences of observed changes difficult to assess. Sustained, multi-disciplinary observations are required to detect, interpret and respond to change.