Sea levels haven’t been rising as quickly during the 20th century as scientists believed — but the rate has really sped up in the last couple decades, according to a new study.

Estimates overstated the sea level rise by about half a millimeter, states a Harvard study published in the journal Nature on Wednesday. However, it does confirm that the annual rate has accelerated since the 1990s, according to a Los Angeles Times report.

Scientists had been using tide gauges to measure the sea levels over the past century. The Harvard study hadn’t set out to disprove this method, but rather attempted to use the method to figure out how the sea level changed from glaciers on land that were melting and running off into the sea.

However, as the team progressed in its study, they found that they were recording lower sea levels than what had been reported, which was a “surprise” to them, said the study’s lead author, Carling Hay, who is a geophysicist at Harvard.

The reason that tidal gauges aren’t completely accurate is that they are widely spaced, with time gaps that can be influenced by man-made things like dikes and dams. Also, many of these gauges are located at mid-latitude Northern Hemisphere ports and don’t take all tidal areas into consideration.

Also, Earth’s crust is shifting partly due to the loss of some of the ice that was weighing it down, which also affects sea levels.

To deal with these issues, researchers are attempting to eliminate the statistical outliers and rely more on regional averages, and supplement them with modern satellite measurements to account for those shifts in the Earth’s crust. The old model caused them to come to a result of 1.6-1.9 millimeters of sea level rise per year for every decade except the 1990s.

The new study is based on 622 tide gauges and pegs the historical rise at closer to 1.2 millimeters per year, which increased to 3 millimeters per year in the 1990s and 2000s.