Sea levels have been rising at an accelerating rate over the last 20 years — not slowing down, as some previous studies have suggested.

Rather than depending on tide gauges that are spread thinly around the world's oceans, scientists have been increasingly relying on satellite data to determine sea levels, a method which came into use in 1993. A new study suggests their measurements – which yielded a slowing in sea level rise -– weren't as accurate as previously believed.

Scientists have been suspicious of the apparent slowdown, because the melting of ice sheets in Greenland and Antarctica has been demonstrably increasing, adding massive amounts of water to the world's oceans.

"The thing that was really puzzling us was that the last decade of sea level rise was marginally slower, ever so subtly slower, than the decade before it," said study leader Christopher Watson, a geodesist from the University of Tasmania.

So Watson and his colleagues set out to more closely examine available data from satellites and tide gauges data, taking account of factors that might distort sea level measurements, such as small changes in coastal elevation.

Their corrected data showed the record of sea level rise chronicled during the early 1990s was too high, creating an illusion that the rate of that rise in the ensuing decades was decreasing. It was in fact increasing — between 2.6 and 2.9 millimeters a year, in contrast with prior estimates of 3.2 millimeters a year.

"While you might only be talking about a few millimeters per year, a few millimeters add up," said Watson. "That few millimeters is a massive amount of water and [of] huge importance to society."

The newly confirmed acceleration, by an additional 0.04 millimeters a year, is in line with predictions made by the Intergovernmental Panel on Climate Change (IPCC). Watson noted that sea level rise, in the first two decades of this century, is happening faster than at any time in the 20th century.

"I have no doubt there are members of the community who may wish to re-evaluate [the predictions for sea level rise]. But as a scientist I come back to the data," Watson added.

The IPCC predictions hold that unless greenhouse gas emissions can be significantly reduced, sea levels could rise by a full meter, or 3.3 feet, by 2100.

Some experts say that whereas the recent study is valuable, it's uncertain whether it confirms the IPCC's 2100 prediction.

"The IPCC is looking way out in time," said geodesist Steve Nerem of the University of Colorado, Boulder, who was not involved in the study. "This is only 20 years of data."

Still, Watson said, the certitude of an increase in the rate of rise is clear from the data — and it should be a worry to those who would be most affected by rising sea levels.

"That rate of change is actually increasing," he says. "For everyone that lives around the coastal margin, that's a really concerning fact."

This study was published in the journal Nature Climate Change.