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CrustalTrudger t1_iue9g0y wrote

Reply to comment by ReddFro in Why does the elevation of Greenland sink to near or below sea level in the center? by ILikePenguinss

They're definitely included in terms of detailed projections for local relative sea level rise in certain areas. Relative sea level change is the rate of sea level change relative to a local datum, which differs from global eustatic sea level change, which is the change relative to a fixed global datum (e.g., the center of the Earth). For example, in a hypothetical scenario where global eustatic sea level rise is 3 mm/yr, but the local rate of surface uplift at the coast in a particular area is 5 mm/yr from isostatic or tectonic forces, the rate of relative sea level change would actually be a 2 mm/yr apparent sea level fall in that location.

With respect to projections of global eustatic sea level rise over time frames like 50-100 years, most won't necessarily include projections of isostatic responses to recent (i.e., anthropogenically related) ice mass redistribution and resultant changes to ocean basin volume, because the effects will be relatively small given the time frame of responses (see correction by u/agate_ below) and the pretty large uncertainties in other aspects like the "right" concentration pathways and associated ice sheet responses (e.g., Horton et al., 2020) or steric components of projected eustatic sea level change (e.g., Camargo et al., 2020).

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agate_ t1_iuetd4b wrote

> With respect to projections of global eustatic sea level rise over time frames like 50-100 years, most won't necessarily include projections of isostatic responses to recent (i.e., anthropogenically related) ice mass redistribution and resultant changes to ocean basin volume, because the effects will be relatively small

It's rare I get to correct /u/CrustalTrudger ! Modern sea level rise predictions do include the effects of vertical land motion, because that effect is significant over 50-100 year time frames.

The IPCC has released a sea level rise interactive map that shows its projections for the rest of the century, and lets you compare the various terms.

In New York City, for example, sea level is projected to rise about 40% more than the global average. Much of this extra sea level rise is because the crust beneath New York is currently moving downward due to the glacial isostatic adjustment process Crustal described. The rest of the extra sea level rise in New York is due to the changing gravitational pull of Greenland as it melts. (!)

In a few areas, such as Hudson Bay, the crust is moving upward fast enough to completely cancel out the effects of sea level rise caused by global warming. But that's pretty rare.

Anyway, point being that modern sea level forecasts do include isostatic response, and while it's not a dominant effect, it is big enough to make a difference.

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agate_ t1_iueuo0w wrote

Oh, and to follow up on my followup: the case of New York shows one important subtlety, namely that glacial loads can cause both downward and upward motions at the same time.

/u/CrustalTrudger described the earth as a viscous trampoline. I'd like to suggest you think of it as a viscous air mattress. The overall volume of the mantle remains unchanged, so if the weight of glaciers pushes one area down, nearby areas must move up, as the mantle displaced under the glacier has to go somewhere. When the glacial load is removed, the opposite effect occurs.

This means that while most of Canada is currently rising as it recovers from the weight of the Laurentide ice sheet being removed, much of the United States is currently sinking by the same effect.

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ReddFro t1_iuecypi wrote

Huh, so no help on sea level rise expected from this. I wondered.

Well here’s hoping scientists underestimated the short term impact an unprecedented amount of ice sliding into the ocean over a short period has on crust deflection and we get a fast enough rebound to reduce the impact of sea level rise (without, you know, massive earthquakes and the like)

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Ma1eficent t1_iueoty0 wrote

Depends where you live. Overall, no. If you live in the PNW, then the land is rising enough to offset a significant portion of sea level rise.

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CrustalTrudger t1_iuerkvf wrote

With respect to GIA in the areas with extant ice sheets, there have been arguments that it could slow ice mass loss in certain areas (e.g., Vaughah et al., 2006, Zeitz et al., 2022), but broadly, these are pretty complicated dynamics with a lot of uncertainty in terms of how they'll actually play out.

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