While sea-level rise will generate major reshaping of coasts in the next decades , severe or catastrophic coastal erosion is commonly generated by local to regional factors among which are variations in sediment supply , natural or human-induced subsidence, especially in deltaic areas subject to cyclones , and tsunami . Here, we confirm the hypothesis of and show from satellite imagery that the fluctuations of the 1500 km-long muddy coast of South America between the Amazon and the Orinoco rivers have been governed primarily by the lunar 18.6 year nodal cycle over the last twenty years, with sea-level fluctuations from global warming or Niño Niña events being of secondary importance. From now to 2015, the predictable 18.6 cycle will lead to an approximate mean high water sea-level rise of 6 cm on the Amazon-Orinoco coast, compared to a more than 2 cm rise due to global warming, and will generate sixty percent of a projected 150 m shoreline retreat in French Guiana. Many of the world’s coasts will experience a tide constituent-induced rise in sea-level exceeding ten centimetres over the next decade.