The seabed is sinking by about 0.1 millimetres a year due to the weight of the water from melting ice caps.
“Greenland and Antarctica are melting much faster [than they were previously], so we can expect much higher ocean deformation in the future,” says Bramha Dutt Vishwakarma at the University of Bristol in the UK.
He and his colleagues calculated how much the extra mass in the ocean is deforming the seabed, and what this means for sea levels. They found that it will only have a tiny effect on future sea level rise because the seas are rising about 3 millimetres per year – far faster than the seabed is sinking.
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Sea level rise is the result of climate change, which is in turn driven by our greenhouse gas emissions. Over the coming centuries, sea levels are expected to rise several metres. As the climate warms, so does seawater, and when water warms it expands – causing the sea to rise.
In the 20th century, this thermal expansion was the main driver of sea level rise, says Jonathan Bamber at the University of Bristol, who collaborated on the work. That meant the mass of the oceans stayed about the same, even as their volume grew.
Now, the warmer climate is melting ice in glaciers and the great ice caps, causing more water to flow into the sea. “Since the mid-1990s, both West Antarctica and Greenland have been contributing,” says Bamber.
That means the mass of the oceans has been increasing ever since, pushing down on the seabed, which is now being forced down like a sheet of Silly Putty with lead weights on top.
Both Vishwakarma and Bamber emphasise that the finding will make no meaningful difference to future sea level rise. While the sinking seabed will slow sea level rise, the effect is too small to be meaningful.
“Sea level is going up and the ocean bottom is going down, but it’s an order of magnitude difference in the numbers,” says Bamber. The sinking of the seabed is about 30 times slower than the rise in sea levels.
At the same time, there are large uncertainties about future sea level rise, partly because we don’t know exactly how fast the ice will melt. That means forecasts for 21st-century sea level rise vary between 1 and 2 metres.
Geophysical Research Letters
