QotW: Is sea-level rise uniform worldwide?

26 November 2019

SEA LEVEL RISE

sea level rise

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Question

Geoff asked: “How is it that there are rising sea levels impacting some island nations such as the Maldives and Kiribati, yet 1000 kilometres in any direction there is no discernible sea level change at all?”

Answer

Nadeem Gabbani has been on the case...

Nadeem - Whilst this isn’t quite true for the islands mentioned, there is indeed a variation in sea level rise around the World, but the reasons behind this may not be what you expected. Dr. Rob Larter of the British Antarctic Survey breaks it down for us.

Rob -As the volume of water in the ocean increases, sea level does not rise uniformly everywhere, and in some places local sea level even goes down. There are three reasons for this. Firstly, some parts of Earth’s surface are being uplifted or subsiding due to geological processes.

Nadeem - This means that the height of ocean floors can change over large areas - causing the seawater above to either rise or sink with it.

Rob - Secondly, some areas that were beneath more extensive ice sheets during the last ice age, 20,000 years ago, were depressed under the great load of the ice and are still ‘rebounding’.

Nadeem - That is amazing. So as these massive amounts of ancient ice slowly disappear, the areas held beneath them begin to slowly move up again, and on top of that, you get more melted ice water returning to the oceans.

Rob - Finally, the ice sheets on Antarctica and Greenland are so massive, that they exert a significant gravitational attraction on the oceans around them. As they lose substantial amounts of ice, this gravitational force decreases so they become less effective in pulling ocean water towards them. This effect results in a smaller sea-level rise, or even a local sea-level fall, close to the ice sheets themselves and greater sea-level rise thousands of kilometres away.

Nadeem - So not only do the ice sheets contain a significant amount of water, their sheer size and mass actually helps keep global sea levels down. Although incredible, these processes highlight how global temperature change can affect sea levels, certainly something to think about.

 

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