Researchers have come up with a new and improved way of measuring the rise in the sea level, and the news is not good: The seas have risen dramatically faster over the last two decades than anyone had known.

For hundreds of years, the seas were measured by more or less the equivalent of plopping a yard stick into the ocean and seeing if the ocean went up or down. But now, that method looks to be outdated.

According to a new study published on Wednesday in Nature, the new method involves an advanced statistical model that analyzes all of the factors contributing to sea rise. It has yielded what appears to be a much more accurate picture of the oceans and suggests previous studies had severely underestimated the acceleration of recent sea rise.

“What this paper shows is that the sea-level acceleration over the past century has been greater than had been estimated by others,” lead writer Eric Morrow said in a statement. “It’s a larger problem than we initially thought.” Co-author Carling Hay added in an interview with BBC: “The acceleration into the last two decades is far worse than previously thought. This new acceleration is about 25 percent higher than previous estimates.”

Old sea measurements came to their conclusions by dividing the world’s oceans into sub regions. Then they determined the height of those regions with something called tide gauges — “essentially yard sticks used to measure ocean tides,” a release said. Then researchers would take those numbers, create estimates for each region, and average them out to come to a global estimate.

But that system was rife with problems. “These simple averages aren’t representative of a true global mean value,” Hay said in a statement. “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.” The tide gauges were unevenly distributed, or just skipped whole sections of the planet. Only the United States and Europe were measuring the oceans for decades. Then there was the issue of tectonic activity, which would suddenly cause a rise or fall in local sea levels, and skewed the numbers.

So the scientists realized they needed a whole new method. Soon, they arrived at what they call a “completely new perspective.”

“We know the sea level is changing for a variety of reasons,” Hay said. “There are ongoing effects due to the last ice age, heating and expansion of the ocean due to global warming, changes in ocean circulation, and present-day melting of land-ice, all of which result in unique patterns of sea-level change. These processes combine to produce the observed global mean sea-level rise.”

So they had all the puzzle pieces. Now they just needed to put them together.

To do so, the researchers looked for what they called “fingerprints” and “underlying patterns,” putting each factor through advanced statistical models to determine a more accurate sum figure. “What we were interested in — and remain interested in — was whether we can detect the sea-level fingerprints we predicted in our computer simulations in sea-level records,” Morrow said. “Using a global set of observations, our goal has been to infer how individual ice sheets are contributing to global sea-level rise.”

So they took the sum of those contributions from the ice sheets, and added in the rate for how much oceans are rising because of thermal expansion due to global warming to determine a “global mean of sea-level change.” And what that mean ultimately showed was that the rate of sea-level rise has increased much more dramatically than earlier estimates. “Unfortunately,” Hay said, it “is really much larger than anyone thought.”