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Comment by yters | Hacker News Reader

> Scientists have assumed that ice cliffs taller than 90 meters (about the height of the Statue of Liberty) would rapidly collapse under their own weight, contributing to more than 6 feet of sea-level rise by the end of the century — enough to completely flood Boston and other coastal cities.

If I understand correctly, these are icebergs. I don't understand how melting icebergs should impact the sea level in the first place. If I melt an ice cube in a cup the water lever remains unchanged. Don't the icebergs already displace as much water as they would contribute? Is there something about icebergs that mean they displace less water, or am I somehow misunderstanding what is going on?

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> If I understand correctly, these are icebergs.

You do not understand correctly. From the article, italicized emphasis mine:

> In a warming climate, as Antarctica’s ice shelves collapse into the ocean, they expose towering cliffs of grounded ice, or ice over land. Without the buttressing support of ice shelves, scientists have assumed that the continent’s very tall ice cliffs would collapse, calving into the ocean, to expose even taller cliffs further inland, which would themselves fail and collapse, initiating a runaway ice-sheet retreat.

This is describing cliffs of ice on land, like the Antarctic and Greenland ice sheets. The worry this paper is investigating is whether those could rapidly collapse ones the sea ice that protects it calves off.

And in addition to the ice moving from land to water, the land will be less compressed by the weight of ice above it and will rise, displacing even more water.

Thanks I clearly misunderstood. So it seems the icebergs provided support that the grounded ice did not. How is not exactly clear. Why would icebergs be more supportive than solid ground?

> Don't the icebergs already displace as much water as they would contribute?

Yes, this is Archimedes' principle. However, large icebergs have a gravitational influence on nearby water. Effectively, large icebergs attract water meaning the local sea level rises and elsewhere on the planet the sea level becomes lower. When the iceberg melts, this gravitational pull disappears and the melt water is spread more evenly across the surface of the Earth. The sea level across the rest of the world thus slightly increases.

Modeling this effect is quite complex, but the above is a summary of the effect of melting (large) icebergs.

Huh, this idea is new to me. Could you help out with a link? I only found two articles that talk about icebergs and gravity, one from NASA that only mentions taking measurements with a gravimeter [0] and a popsci article from 2011 [1] that mentions the geoid and irregular gravitational field of earth, but only sourced its “sister site” which appears defunct and simply redirected to its own front page, which looks like a garbage fire of pseudoscientific advertisement. I also found a BI article (to which I link with great trepidation) that mentions I think the same iceberg as the NASA article but mentions it is not expected to contribute to sea level rise in any significant way [2].

[0] https://earthobservatory.nasa.gov/blogs/fromthefield/2017/11... [1] https://www.livescience.com/amp/15605-melting-glaciers-alter... [2] https://www.businessinsider.com/antarctica-larsen-c-iceberg-...

Yes, here's a link: http://sealevelstudy.org/sea-change-science/whats-in-a-numbe...

edit: the video in the link also provides an animation of the effect

Awesome, thank you! I’m now interested to try some back-of-the-envelope math to compare the mass of certain ice bodies to that of the moon, and the respective distances from moon to earth and, as the article mentions, Greenland to Scotland, and compare tidal differences induced by the moon, its squared inverse distance, and the gravitational effect of the ice on sea levels.

Can I get a source for this? It seems incorrect, given that water is more dense than ice, so the gravitational pull of melted water on nearby water should be stronger than the gravitational pull of ice on nearby water (due to the decrease in distance due to the shrinkage when melting).

See my sister post for link. The difference between ice and water is that water can freely move over the Earth's surface while massive icebergs cannot.

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