Cited research: Geophys. Res. Lett. doi:10.1029/2010GL043462 (2010)

The coastal tide gauges that we rely on for information about sea-level rise may not be giving an accurate picture of the effects of climate change. This is because many of these instruments are located close to dams, which impound huge amounts of water on land. Dams depress sea levels globally, by taking some of the water out of the equation. However, the impounded water also depresses Earth's surface, gravitationally attracting more sea water and resulting in local sea-level increases.

Julia Fiedler and Clinton Conrad at the University of Hawaii in Honolulu analysed data from 674 of the world's largest reservoirs collected between 1900 and 2009. After accounting for the reservoir water, the location of tide gauges and variations in regional rise rates, the researchers estimate that global sea levels have increased by about 2.3 millimetres per year over the past century — around 20% faster than indicated by the tide gauges. C.L.