Tricky Questions About Sea Level Rise

Last week in our Coastal Dynamics course, we discussed the topic of sea level rise (SLR) and its impact on coastlines around the world.  A rather perceptive student asked me about an assignment question about the subtleties of regional sea level rise:

Where in the world does the melting of the Antarctic Ice Sheet have the greatest impact?

This is a good question, because it shines a light on the complex reality of sea level rise: it is not the same everywhere.  Local factors like isostatic adjustment and regional subsidence mean that certain locations can deviate significantly from the global average rate of sea level change.

Isostatic adjustment is the flexing of the earth’s crust as it rebounds from the heavy weight of ice sheets that once pressed down on it (much like the behaviour of your couch cushions after you stand up). This means that some formerly-glaciated areas like Northern Canada and Norway actually experience a local sea level fall, not a rise.  Further away from those glaciated areas, you can have the opposite effect and experience increased sea level rise due to a sort of levering effect.

However, another important principle (and a mind-blowing one to me, when I first heard it), is that of gravitational attraction: ice sheets are so massive that they actually exert a gravitational pull on the oceans, pulling water levels up towards them.  The flipside of this is that when those ice caps melt, the water near them redistributes to the other parts of the oceans further away.  This leads to the remarkable notion that us in the northern hemisphere will suffer most from the melting of Antarctica, while our southern cousins will get their feet wet primarily from the deglaciation of Greenland and mountainous ice caps in the northern hemisphere.

Contribution to sea level rise from Greenland (top panel) and the West Antarctic Ice Sheet (bottom panel).  In general, the further away from the ice sheet, the greater the increase in sea level (orange colours).  Source: Milne, et al. (2009). Identifying the causes of sea-level changeNature Geoscience2(7), 471-478.

There has recently been some interesting research on the idea of “fingerprinting” the impact of a particular glacier on sea level rise at a given location through exploratory modelling:

“The loss of mass changes Earth’s gravitational field causing the fresh meltwater and ocean water to move away towards faraway coastlines; the resulting pattern of sea-level rise is the fingerprint of melting from that particular ice sheet or glacier. For example, the latest study found that ice melt in Antarctica causes sea level to rise 52% faster in California and Florida than it does in other parts of the world, Velicogna says. Much of Earth’s middle and lower latitudes bear the brunt of rising sea levels because they’re sandwiched between Antarctica and Greenland, which are home to massive ice sheets that are shedding mass as meltwater or icebergs.” Source.

This article in the Guardian nicely illustrates the relevant concepts.  See also here and here for more details.  From these sources (and the references therein) we can see that the mechanisms contributing to sea level rise are highly location-dependent, and I barely skim the surface here.  Although these questions of “which glacier is submerging my house?” are scientifically fascinating, it shines some light on just how complicated our planet’s climate system really is.  It is scary to wonder about all of the future impacts like these that we can’t foresee or don’t yet understand…

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