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This Plenary Paper on sea level is based on several Community White Papers submitted to OceanObs09. Considerable progress has been realized during the past decade in measuring sea level change globally and regionally, and in understanding the climate-related causes of observed changes. We first review current knowledge about sea level change, globally and regionally. We summarize recent results from the 2007 IPCC 4 th Assessment Report (AR4), as well as post-IPCC results relevant to sea level observations, causes and projections. New challenges are identified for the coming decade in terms of observations, modelling an