One of the main concerns of global climate change is its potential to raise sea levels, resulting in coastal cities and nations to be consumed by the ocean and requiring their denizens to grow gills a la Waterworld. In trying to to solve a longstanding discrepancy between melt water estimates and sea level measurements taken at harbors, they found some good news: Global sea levels did rise quite as much over the 20th century as previously believed.

The bad news? Since 1990, sea levels have risen much faster than anyone would have estimated.

The previous estimates suffered mainly due to inaccurate measuring techniques. Gauges used at harbors to measure sea levels, for instance, were prone to inaccuracies, as the very land they were attached to could often move or be changed. Scientists from Harvard and Rutgers have re-analyzed 20th century sea level data using what they believe are more precise statistical techniques, and found that the 1.6-1.9 millimeter annual sea level rise estimated between 1900 and 1990 is more like 1.2 millimeters per year.

Over the 90 years, that means levels rose about 4.2 inches verses the estimated 6.7 inches, a difference of 2.5 inches. Globally speaking, that’s a massive amount of water, adding up to about five quadrillion gallons, or 1.4 times the amount of water contained in Lake Superior.

The new estimates clear any discrepancies between melt water estimates and harbor readings, and will provide scientists with insight as to how and why sea levels rise if the numbers hold up to scientific scrutiny.

With the new-found accuracy comes some shocking discoveries: While scientists knew that sea level rise was accelerating, they found that levels rose 3.2 millimeters per year between 1993 and 2009. That’s nearly triple the rate of the preceding 90 years, and much faster than previously estimated.

Some scientists are concerned that the acceleration is just the beginning of a longer trend of increasing rises in sea level, one that could cost trillions as countries fight to keep coastal cities from going under. As the large ice sheets at Earth’s poles continue to break and split under the stress of global climate change, quadrillions more gallons of water could pour into the oceans.