Global warming is changing Earth’s SHAPE: Melting glaciers are causing the bottom of the ocean to sink

Melting glaciers have become the poster child of the symptoms of global warming. As temperatures rise, large bodies of ice around the poles are losing mass at an unprecedented rate, driving up sea levels and reducing feeding and breeding grounds for animals.

Now, a new study has revealed these events are having another, significant impact. The weight of these glaciers is causing the bottom of the ocean to sink.

A team at Delft University of Technology in the Netherlands has found that the net increase in ocean load has caused the seabed to sink by 0.13mm per year for the past two decades. The finding puts the total depression at 2.5mm.

The findings are significant as they not only highlight an oversight in the ways in which global warming is physically changing our planet (warping the seabeds under swelling oceans), they show how much we may have underestimated the oceans’ expansion.

“With climate change, we do not only change temperature,” said geoscientist Thomas Frederikse of the team of researchers. “The Earth itself,” he continued “is not a rigid sphere, it’s a deforming ball.” The team’s findings go a long way in unveiling the extent of that deformity.

Researchers were thorough in methodology, estimating the mass loss from glaciers, ice sheets in Greenland and Antarctica, and land water storage (including groundwater and dam retention). They then quantified how much the ocean’s bed is deforming under the excess weight.

One prominent oversight the study exposes regards the actual increasing volume of the ocean, known as barystatic sea level rise. This is measured via satellite observations, a method which fails to take into account the incrementally sinking seabed levels.

“[S]atellite altimetry,” the paper explains, “observes sea level in a geocentric reference frame, [so] 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”.

With a net depression level of 2.5mm spanning the years 1993 to 2014, this might sound negligible, but the truth is far from it; the satelline-gleaned assessments may have underestimated barystatic sea level rise by as much as eight percent.

That’s a hell of an oversight, particularly given that this hidden variable is likely to snowball in its physical influence over the coming years. What’s more, some regions are more vulnerable to sinking than others, with up to 0.4mm annual depression in the South Pacific and 1mm in the Arctic Ocean.

The team warned that, “[I]n 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.”

“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.”

Depressing indeed.

Header image: Kimberly Vardeman, used under Creative Commons

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