The rate of global sea level rise is greater than previously estimated, a new study has found.

Previous calculations indicated that sea level rise from 1900-1990 was between 1.5 and 1.8 millimeters annually, but Harvard researchers Carling Hay and Eric Morrow found that the rate was closer to 1.2 millimeters a year, meaning the previous rate could be over-estimated by as much as 30 percent. Their analysis found that estimates for the rise between 1993 and 2010 were consistent were with previous estimates for that time range.

But that doesn’t give an optimistic scenario. Rather, it points to a faster rise in sea level.

“If we’ve been over-estimating the sea-level change during that period,” Morrow, a recent PhD graduate of the Department of Earth and Planetary Sciences (EPS), said, “it means that these models are not calibrated appropriately, and that calls into question the accuracy of projections out to the end of the 21st century.”

The problem, said Hay, a post-doctoral fellow in the EPS, and Morrow, is that the previous calculations were made using tide gauges, and those provided inadequate data.

Explaining the inadequacies, Hay stated, “Tide gauges are located along coasts; therefore, large areas of the ocean aren’t being included in these estimates. And the records that do exist commonly have large gaps.”