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The sea floor is sinking under the weight of climate change

The sea floor is sinking under the weight of climate change
15 Jan
2018
The ocean floor is being deformed under the weight of ever-increasing water from melting ice and redistribution of land water. This unexpected consequence of climate change also appears to be skewing global sea level data making it appear less severe

The consequences of the alarming deformation of the Earth has been highlighted by researchers from Delft University of Technology in the Netherlands.

Combining data on mass loss from the Greenland and Antarctica ice sheets, and changes in water storage on land surfaces (due to dam, reservoirs and irrigation), they were able to feed this information into mathematical equations that could calculate sea level estimates around the globe.

‘I was quite surprised by the outcomes,’ says Thomas Frederikse who authored the study. ‘Over the last 20 years the oceans have become ~2.5 mm deeper. It was already assumed that bottom deformation was small, compared to sea-level rise on a global scale. However, we show that for some regions, especially the Arctic and Southern oceans, its size is considerable.’

grl56685 fig 0002Changes in a) the seafloor, b) relative sea level taking into account sea floor deformation, and c) ‘geocentric’ sea level as measured by satellites (not taking into account sea floor deformation). Note in the second graph how sea level change relative to the seafloor is negative for the Arctic, as the sea floor has risen due to mass loss from melting ice. (Image: Frederikse et al. 2017)

Sea level rise is caused primarily by two factors related to global warming: the added water from melting ice sheets and glaciers, and the expansion of sea water as it warms. This rise has long been documented, but what these new findings mean is that the global sea level rise is actually higher than previously thought.

The existing numbers were based on satellite data which measured surface level relative to the centre of the Earth, assuming the ocean floor as basically a fixed constant depth. Now this assumption has been proved to be wrong, the significance being that even more water from melting ice has reentered the oceans than was thought, thanks to climate change and human activity.

Perhaps almost as telling of the state of wilful ignorance to climate change and its consequences, is that the unexpected phenomena of our swelling oceans actually contracting the Earth has been known by scientists for some time, it is simply a lesser documented and less visible symptom of climate change.

The team of researchers from Delft have managed to quantify this extreme effect, finding that the increase in weight of the oceans has caused the sea floor to sink by about 0.1 mm/year between 1993-2014, or 2.5 mm over the entire period, and this is a trend that will only worsen with time.

‘It is widely accepted that, when greenhouse gas emissions won’t be cut, the ice sheets will retreat at a much faster pace than today, and then, due to the massive increase in ocean mass, seafloor deformation will become significant,’ says Frederikse.

grl56685 fig 0004Red lines show the ‘relative’ sea level change taking into account changes in the seafloor, blue lines show previous ‘geocentric’ estimates at sea level change. Solid lines are average recordings and the dashed lines are tide gauge recordings. The ‘altimetry’ area consists of the global oceans, bounded by ±66º latitude. Note the red lines appear higher in all regions except the Arctic and North Atlantic, which suffer the most ice mass loss, and thus where the sea floor has risen rather than sunk. (Image: Frederikse et al. 2017)

The research also revealed some other unexpected consequences, as some areas of the sea floor are forced down, others rise. ‘On regional scales, the effect was certainly larger than expected: in the Arctic, which becomes less and less heavy due to mass loss in Greenland and many glaciers, the ocean floor rises at about 1 mm/y,’ explains Frederikse.

These incremental seafloor changes might not seem all that problematic from land, but for scientists documenting the rate of melting ice and sea level change they are more than simply numbers and maths equations, they signal once again the need for action on climate change.

For Frederikse, this is one of the most important implication of the study. ‘I’m fascinated by the fact that due to our behaviour, we do not only change the Earth’s thermostat, we’re literally deforming our own planet. Fascinating, but very worrisome,’ he says.

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