Decades of measurements and predictions of sea level rise could have underestimated the scale of the problem, experts warn, due to scientists not accounting for the weighty, warping effects of our ever burgeoning oceans.

Existing assessments of sea level rise haven't factored in that as the total ocean mass increases due to melting glaciers and ice sheets, the weight of all that extra water pushes down on the sinking ocean floor, deforming the seabed – and disguising just how much the oceans are truly swelling.

"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.

"With climate change, we do not only change temperature."

The implications, according to Frederikse and his team, is that as the ocean bottom subsides elastically, the actual increasing volume of the ocean – called barystatic sea level rise – is masked from measurements based on satellite observations.

That's because satellite readings only tell us one side of the story: geocentric sea level rise, as seen from the surface side.

"[B]ecause 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 researchers explain in a new paper.

To quantify how much the bottom of the ocean is deforming under the extra load of meltwater, the researchers used various estimates of mass loss from glaciers, the Greenland and Antarctic ice sheets, and land water storage, including groundwater depletion and dam retention.

For roughly the last two decades (the period 1993–2014), the team calculates the increase of the total ocean load has pushed the seabed down by about 0.13 mm (0.005 inches) per year, or around 2.5 mm (almost 1/10th of an inch) in total for the period.

That might not sound like much, but in some regions the deformation is significantly greater – up to 1 mm (0.04 inches) per year over the Arctic Ocean, and 0.4 mm (0.016 inches) per year in the South Pacific.

Overall, the researchers say purely satellite-derived assessments of sea level rise for the period could have underestimated barystatic sea level rise by as much as 8 percent – which is definitely something we need to think about in the future, especially this hidden variable will only become more significant as the world gets hotter and sea level rise accelerates.

"In a future warming climate, the sea-level rise induced by ice sheets will increase, and therefore, the magnitude of the bias due to elastic ocean-bottom deformation will grow," the team writes.

"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 findings are reported in Geophysical Research Letters.