At first, the results of a just-published study (paywall) from Harvard University sound like good news. The research found that we’ve been overestimating how much the planet’s seas have risen during the first 90 years of the 20th century—by as much as 30%.

The catch, though, is this: If the seas weren’t rising as fast as we thought between 1900 and 1990, then they must have been rising even faster than we thought since then to land us where we are now.

“What this paper shows is that sea-level acceleration over the past century has been greater than had been estimated by others,” says Eric Morrow, a recent PhD graduate of Harvard’s department of Earth and Planetary Sciences. “It’s a larger problem than we initially thought.”

"Probabilistic Reanalysis of 20th Century Sea-Level Rise," Hay et al. 2015 Time series of global mean sea-level for the period 1900-2010. Blue=Hay et al. model 1 (KS); black=Hay et al. model 2 (GPR); magenta=Church and White 2011 estimate; red=Jevrejeva et al. estimate. Shaded regions show uncertainty. In inset: Jevrejeva ends in 2002, so the rate quoted for 1993-2010 is actually for 1993-2002. Since the GPr methodology outputs decadal sea level, no trend is estimated fro 1993-2010. Accelerations in inset are estimated from 1901 to end of the reconstructions.

Previous calculations put sea-level rise somewhere between 1.5mm and 1.8mm for each year, on average, throughout the 20th century. Those estimates were based mainly on tide gauge records. Since tide gauges are generally placed around coasts and in the northern hemisphere, they’re not necessarily all that representative of the overall ocean.

In order to create a more holistic picture of sea-level rise, the researchers build models that included not just tide-gauge data, but factors like melting of ice sheets and glaciers, thermal expansion, and ocean circulation. In doing so, they found average annual sea-level increases that were much smaller than those calculated in the previous models.