鈥淲别鈥檝别 for 20 years 鈥� and it鈥檚 higher than previously thought.鈥� Well, no, not really. This is just one of the misleading headlines about the first study to try to work out how much the ocean floor is sinking under the weight of all the extra water pouring into it.
What鈥檚 more, climate deniers twisted the findings beyond all recognition, claiming 鈥渃limate alarmists鈥� were using the sinking sea floor as an excuse for why sea level isn鈥檛 rising as fast as predicted (it is and they aren鈥檛).
So what is the actual story? Researchers have known for many decades that Earth鈥檚 crust is elastic and sinks in response to increased weight. 鈥淲e are really sure about this,鈥� says Thomas Frederikse of the Delft University of Technology in the Netherlands.
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We also know that a lot of water once locked away in mountain glaciers and the ice sheets of Greenland and Antarctica is now entering the oceans. The extra weight means the sea floor must be sinking, but the effect is so small that there is no way to measure this directly.
Instead, Frederikse and colleagues used indirect methods to estimate how much the sea floor has sunk, and published their findings in on 23 December. It was a technical paper, says Frederikse, so the team didn鈥檛 press release it or expect any press coverage.
Sinking sea floor
But the idea that the sea floor is sinking has astonished many people. After wrote about the study, it was recycled by many other websites after the New Year, becoming increasingly confused and distorted along the way.
One reason for the confusion is that there are several ways of measuring sea level.
Think of a bathtub. If you are worried about whether the bath will overflow, you need to measure the water level relative to the rim of the bath. Similarly, when it comes to coastal flooding, what matters is how sea level changes relative to the land.
But you could also measure water level relative to the bottom of the bath, or the bathroom floor. This latter approach is effectively what satellites do: they measure sea level relative to the centre of the planet, called absolute sea level.
Now suppose you work out what volume of water needs to be added to the bath to raise the water level by a centimetre. If it is a flexible plastic bath whose bottom sags under the extra weight, the water level relative to the floor will actually rise by less than a centimetre when this much water is added.
Small effect
This is just what Frederikse鈥檚 team thinks is happening on a planetary scale. Sea level rise as measured by satellites over the past 20 years is slightly less than expected given how much extra water is going into the oceans, the team says, and it has used that difference to estimate how much the ocean floor has sagged.
But is this something we should be concerned about in terms of future sea level rise? No.
Firstly, the subsidence effect is tiny. Satellite measurements suggest that over the past 20 years, sea level has risen between 2.6 and 3.4 millimetres per year. The ocean floor has subsided by just 0.1 millimetre per year, if Frederikse is right.
Secondly, what matters when it comes to flooding is sea level relative to coasts. All forecasts of future sea level rise are based on this measure. And sea level relative to the coast isn鈥檛 the same as absolute sea level as measured by satellites, because the coasts sink along with the ocean floor. To go back to our bath analogy, the inner rim of the bath also gets pulled down as the bath fills.
Still confused? Just forget about the whole sinking sea floor thing. The big picture is that sea level rise is being driven entirely by our greenhouse emissions.
If we don鈥檛 drastically cut emissions over the coming decades, , and by 20 metres or more over the following centuries, devastating coastal communities. The faster we cut CO2 emissions, the less sea level rise there will be.
