[Fuhrer01]

I'm a bit stupid, but I understand if we are dropping 6.4M cubic miles from space, there will be an increase in water level, but if the 6.4M cubic miles of water was already present in the system, just frozen, how does it increase the total sea level? There is no net change in volume right?

[exitb]

Antarctic glacier mostly rests on the continent, rather than float in the ocean.

[dredmorbius]

Land ice vs. sea ice.

Sea ice floats in the sea. When melted ... it's still floating in the sea, as water. There's no net sea-level change, at least not from contributed ice.[1]

Land ice sits on land. When it melts, most of it flows to the sea.[2] That increases the total amount of water in the oceans, and hence, sea level. The total rise if all Antarctic and Greenland ice melted would be about 60-70m (200-230ft).[3]

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Notes:

1. There are other effects, including thermal expansion (water expands slightly as it warms), and centripetal effects (water can flow more than continental crust does, and would spread out slightly more at the equator than the poles with more liquid water in the oceans). Those effects are comparatively small, though not fully negligible.

2. The exception would be enteric basins which have no outlet to the oceans, in which case melted ice flowing into these would form lakes. Examples of enteric basins include the Great Salt Lake and surrounding former Lake Bonneville, the Dead Sea, and Death Valley. Note that as glaciers melt, there's a rebound effect in continental crust, and regions presently below sea level or which would otherwise form enteric basins might not after that rebound effect is taken into account.

3. See: https://science.howstuffworks.com/environmental/earth/geophy... https://www.usgs.gov/faqs/how-would-sea-level-change-if-all-...

[dredmorbius]

Erm, that should be endorheic basins, not enteric, which has a different set of guts.

https://en.wikipedia.org/wiki/Endorheic_basin

https://en.wikipedia.org/wiki/Enteric