How Much Sea Level Rise Makes A Sea Port Extinct?

That is enough (~1.25%)

I. Not Much

What percent of the Cryosphere has to melt in order to endanger world sea ports?

It varies with the age of the sea port and the location.

In other words, some sea ports are more vulnerable than others.

But a very general rule of thumb is that about one meter of global mean average sea level rise will cause serious problems at some seaports.

II. Cryosphere Locations

Many sea ports have been discussed in principle in this light in many previous Dredd Blog posts (e.g. Proxymetry³).

The table below indicates how much melt, percentage wise, of the various Cryosphere locations would add up to one meter of mean sea level rise.

That table shows location, volume, maximum sea level rise, and the percentage of melt it takes to add up to one meter of global mean average sea level rise.

Isn't it surprising that only about 1.25% of the Cryposphere needs to melt in order to threaten civilization's seaports with a meter of sea level rise?

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Location	Volume (km)	Max SLR (m)	SLR @ ~1.25% (m)
East Antarctic ice sheet	26,039,200	64.8	0.806773
West Antarctic ice sheet	3,262,000	8.06	0.100349
Antarctic Peninsula	227,100	0.46	0.00572709
Greenland	2,620,000	6.55	0.0815488
All Other	180,000	0.45	0.00560259
Totals	32,328,300	80.32	1

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The added impact of ghost water (click to enlarge table below):

See USGS Source and The Ghost-Water Constant

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III. Closing Comments

For individual sea port characteristics, see Seaports With Sea Level Change - 6 and appendices.

Note also that "ghost water" can hasten a sea port's demise (The Ghost-Water Constant, 2, 3, 4, 5, 6, 7, 8, 9; The Gravity of Sea Level Change, 2, 3, 4, 5; NASA Busts The Ghost).