greenland ice sheet
Sea levels are rising faster than previously though, experts say. Getty

Global sea levels are rising "significantly" faster than previously believed, calling into question the accuracy of projections for the rest of the 21<sup>st century.

Scientists at Harvard University have found that while there was an overestimation of sea level rises between 1900 and 1990, the acceleration from this point has been vastly misjudged.

Published in the journal Nature, authors found pre-1990 sea levels were over-estimated by as much as 30%, with experts predicting a rise of between 1.5mm and 1.8mm annually – the true figure was about 1.2mm.

Since then, levels have risen by around 3mm per year. Eric Morrow, from the Department of Earth and Planetary Sciences, said: "What this paper shows is that sea-level acceleration over the past century has been greater than had been estimated by others. It's a larger problem than we initially thought.

"Scientists now believe that most of the world's ice sheets and mountain glaciers are melting in response to rising temperatures. Melting ice sheets cause global mean sea level to rise. Understanding this contribution is critical in a warming world."

Normally, scientists divide the world's oceans into sub-regions and gathered records from tide gauges for each area. They then average them together to estimate seal levels for each region to create a global estimate.

Co-author Carling Hay said: "But these simple averages aren't representative of a true global mean value. 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.

"It wasn't until the 1950s that there began to be more global coverage of these observations, and earlier estimates of global mean sea-level change across the 20th century were biased by that sparsity."

A number of variables affect sea level, including effects of the last ice age, global warming, changes in circulation and melting ice caps.

To get a more accurate picture, the researchers looked at sea level "fingerprints" that aims to model the physics involved in the changes.

They looked at all the available sea-level records and then added in the rate that oceans are changing because of thermal expansion to get a global mean sea-level change.

"Unfortunately, our new lower rate of sea-level rise prior to 1990 means that the sea-level acceleration that resulted in higher rates over the last 20 years is really much larger than anyone thought."

Morrow added: "If we've been over-estimating the sea-level change during that period, 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."