Sea-level rise is accelerating, not declining as some have hoped, scientists said today citing melt water from Earth's ice sheets as the likely cause.
In 2013, the UN's Intergovernmental Panel on Climate Change (IPCC) said the global mean sea level rose by 19 centimetres (7.6 inches) from 1901-2010, an average 1.7 mm (0.06 of an inch) per year.
This accelerated to 3.2 mm per year between 1993 and 2010, the IPCC said in its landmark Fifth Assessment Report.
But in 2014, another study raised a big question.
In the past decade, it said, sea-level rise had been much lower than the previous decade.
That raised hopes in some quarters that, far from being an inexorably rising threat, sea levels could fluctuate in response to some hidden but natural variability.
The new study deals a blow to this scenario.
Both the IPCC estimate and the 2014 paper were based on satellite observations of sea levels.
But they were unable to take an important variable into account: something called vertical land motion.
This is natural movement in the height of the Earth's land surface, which can happen through subsidence, earthquakes or uplift.
For instance, parts of the northern hemisphere are still rising after the end of the last Ice Age, the land was crushed by glacial weight and even today is slowly "rebounding," thousands of years after the ice melted.
The new study, published in the journal Nature Climate Change, takes land movement into account, along with an important statistical tweak, hourly data from a network of tide gauges deployed around the world's oceans.
It finds that the overall rate of sea level rise between 1993 and mid-2014 is between 2.6 and 2.9 mm per year, with a margin of error of plus or minus 0.4 mm.
The bad news is that the first six years of the satellite data -- 1993 to 1999 -- is the period that is most affected by these corrections.
For those six years, estimates have to be scaled down by 0.9-1.5 mm a year.
That mean in more recent years the rate of sea-level rise has actually increased rather than declined, according to the paper, led by Christopher Watson of the University of Tasmania, Australia.
The acceleration "is higher than the observed twentieth-century acceleration but in reasonable agreement with an accelerating contribution from the Greenland and West Antarctic ice sheets over this period," the team said.
