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China has a huge flat coastal river delta that's heavily populated. The US has that only at New Orleans, which is a much smaller city and has high ground nearby.

The US comes out fairly well on rising sea levels. Except for Florida. Florida has so much low-lying coast that it's hopeless. The US west coast is mountainous enough that high ground isn't far inland, and only short sections of the coast at SF and LA are vulnerable. The coastal cities of the east coast have reasonably short coastlines, enough money to build sea walls, and compact river mouths.

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Except for Florida. Florida has so much low-lying coast that it's hopeless

The New Yorker has an excellent article on precisely this topic: http://www.newyorker.com/magazine/2015/12/21/the-siege-of-mi... . There will be a weird sort of shadenfreud (or, more likely, whatever term is most appropriate for seeing someone's arguments utterly proven wrong) when FL floods, given the politicians FL has been electing.

The coastal areas of FL are blue, sadly.

I don't understand the relevance of nearby high ground. Assuming you can't build levees, doesn't the cost of losing a city come primarily from losing the buildings (which must be rebuilt), not the transport cost of moving items inside the building to a new location? In other words, if your house is in the path of a tornado, learning that there is a nearby patch of land guaranteed not to be hit is not useful.

The difference is that the sea level rise happens over decades, and the city will gradually move inland and upward as its (newly) beachfront property recedes. China's river delta cities can't.

Venice. Air, Sub and Boat-traffic.

It's not clear to me that this is less costly than moving across the country. You'd have to look carefully at the network effects, but from an infrastructure perspective you still end up building a whole new city.

In other words: what's the advantage of building San Diego II a few dozen miles from San Diego, even slowly, rather than just expanding Denver by moving people there.

(Note that I agree it's better than building a new city in the middle of nowhere; the question is whether the network benefits of San Diego II can be mimicked by Denver. In this sense, I agree that countries with cities that are almost all on the coast would benefit from having high ground nearby their doomed coastal cities.)

In the case of New Orleans, some of that high ground is within the city itself. If flooding eventually becomes frequent enough that the federal government decides to relocate flood insurance claimants and stop funding the Army Corps of Engineers' improvements to the levees, then the city can retreat to its pre-1930 borders. Most of those areas did not flood in Katrina and could get by with a much smaller flood protection system that might be justified by the city's economic productivity.

Have you ever noticed that the Sacramento river delta, while way inland, is actually quite near sea level?

True, but the problem with the Sacramento delta is keeping the fresh water flow high enough to keep salt water from going upstream, not getting rid of the river water. If sea levels rise, the delta becomes a salt flat and the fresh water is diverted to the California Water Project. See Peripheral Canal.[1] Maybe build a dam at Vallejo and stop salt water incursion from going further upstream.

[1] https://en.wikipedia.org/wiki/Peripheral_Canal

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