Rate of rising sea levels trebles

A NEW scientific analysis has found that the Earth’s oceans are rising nearly three times as rapidly as they were throughout most of the 20th century, one of the strongest indications yet that a much feared trend of not just sea level rise, but its acceleration, is under way.

“We have a much stronger acceleration in sea level rise than formerly thought,” said Sönke Dangendorf, a researcher with the University of Siegen in Germany who led the study along with scientists at institutions in Spain, France, Norway and the Netherlands.

Their paper, published in the Proceedings of the National Academy of Sciences, finds a bigger rate of increase than in past studies. It says before 1990, oceans were rising at about 1.1mm a year, or just 1cm a decade. From 1993 through 2012, though, it finds that they rose at 3.1mm a year, or 3cm a decade.

The cause, said Dangendorf, was that sea level rise throughout much of the 20th century was driven by the melting of land-based glaciers and the expansion of seawater as it warmed, but sea level rise in the 21st century had added in major contributions from the ice sheets of Greenland and Antarctica.

“The sea level rise is now three times as fast as before 1990,” Dangendorf said.

Studying the changing rate is complicated by the fact that scientists have only a precise satellite record of its rate going back to the early 1990s. Before that, the records rely on tide gauges spread around the world. But sea level rise varies in different places, due to the rising and sinking of land, large-scale gravitational effects on the water.

The study tries to piece together a sea level record by adjusting the results of local tide gauges based on an understanding of the factors affecting sea level rise in a given region, and weighting different regions differently.

“Sea levels will further accelerate, but how much is related to how we act,” said Dangendorf.