The ocean floor is slowly sinking under the weight of melting glaciers

Melting glaciers are causing sea levels to rise, but scientists have identified a factor that is usually not taken into consideration when calculating it. The enormous mass that gets added to the ocean is causing the sea floor to warp, sink, and deform. This can disguise the actual rise of sea levels in many parts of the planet and actually cover up the actual extent to which the seas are rising.

"The Earth itself is not a rigid sphere, it's a deforming ball," geoscientist Thomas Frederikse from the Delft University of Technology in the Netherlands told Earther, according to a report in Science Alert.

Barystatic sea level rise, the increasing volume of the ocean is masked from observation by satellites, said Frederikse, because of the way the ocean's floor subsides elastically. Satellites are good at reading sea levels from a geocentric point of view - from the surface side.

As a result, based purely on satellite imagery and study, researchers have estimated that true sea-level rise calculations could be off by about 8% when ocean floor deformation is not taken into account.

"Because satellite altimetry observes sea level in a geocentric reference frame, global mean sea-level estimates derived from altimetry will not observe the increase in ocean volume due to ocean-bottom subsidence, and hence, they may underestimate [global mean sea-level] rise," the original study observes.

The paper explains that global mean sea level (GMSL) changes are normally presented through two reference frames - relative to the local ocean floor, known as the "relative sea level change" or relative to the Earth's centre of mass, called "geocentric or absolute sea level change".

GMSL changes that are caused by mass redistribution are called barystatic changes. These are calculated by dividing the total volume change in the ocean with the total ocean's surface area. So when integrated over the entire ocean, barystatic changes are equal to relative sea level changes.

On the other hand, when the ocean floor deformation caused by changing load is taken into account, the study found that global mean geocentric sea level changes are not equal to the barystatic changes.

Deformation of the ocean floor is not uniform throughout the planet. "The regional or basin mean difference between relative and geocentric sea level change may deviate from the global mean difference," explains the study.

Quantifying just how much the seabed is warping from all this extra load from land ice melting, researchers have used data from the mass lost by glaciers and dumped into the sea. They have taken into account not just the polar ice caps melting, but also ground water depletion, dam retention, and land water storage.

The research has found that between 1993 and 2014 the sea floor has been pushed down by about 2.5mm which averages out to about 0.13mm per year. This is the worldwide average and might not seem like much, notes the report, but in some specific regions, this warping is a lot more significant. In the Arctic region, scientists found over 1mm per year of sinking; the south Pacific has recorded 0.4mm per year.

"To increase the accuracy of sea-level estimates, the effect of ocean-bottom deformation should be taken into account, either based on modelled estimates of ocean mass change, as was done in this study, or using more direct observations," the researchers said.

Calculating accurate sea-level rise has been a challenge as there are several variables that are constantly changing. Just as the Ozone layer is showing signs of definite recovery because of decisive action, rising temperatures could also be effectively controlled. The Paris Agreement is one such initiative that is working towards this goal.

Scientists believe that calculating the actual extent of sea level rise may not be possible until 2060.