GLOBAL compilations of tide records indicate that sea level has been rising throughout the twentieth century1,2, with potentially dangerous consequences for low-lying coastal regions. Thermal expansion of ocean water3 and melting of alpine glaciers4 in response to increased atmospheric temperatures may have been responsible for some of this change, but human activities can also influence sea level directly. For example, the rise in sea level would have been even larger5 if large quantities of water had not been stored in reservoirs, and channelled into aquifers by irrigation projects. Here we show, however, that these and other human activities have together caused a net increase in sea levels over the past century. We estimate that a combination of groundwater withdrawal, surface water diversion and land-use changes has caused at least a third of the observed rise, and suggest that the contributions of climate-related effects must therefore be smaller than has been previously supposed.