Data from the 1990s showed an accelerated pace of sea-level rise, but new analysis shows that the speed-up started far earlier.

The rate of sea-level rise has accelerated year after year since the mid-1960s, when parts of the world’s oceans began to expand as seawater absorbed more heat.

Thanks to satellite observations that began in 1993, scientists know that Earth’s average sea-level rise has quickened in recent decades. But the onset of that acceleration — caused in the last two decades mainly by ever-faster ice loss in Greenland and Antarctica — has been unclear.

Sönke Dangendorf at the University of Siegen, Germany, and his colleagues reconstructed global and regional sea levels from 1900 to 2015, using statistical techniques to combine satellite measurements and tide-gauge records. They found that twentieth-century sea-level rise began to speed up around 1968 and has continuously accelerated since.

Seas probably began to climb in the 1960s because of ocean warming in the Southern Hemisphere. Accelerating ice loss and continued temperature-driven expansion of sea water in a warming climate are likely to further steepen the rate of sea-level rise over the next few decades, the authors say.