1.) 20cm is huge. You cannot build a levee system around the whole Mississippi Delta. 2.) It is a combination of both. To put a number to it. 5–6 mm per year b

1.) 20cm is huge. You cannot build a levee system around the whole Mississippi Delta. 2.) It is a combination of both. To put a number to it. 5–6 mm per year because of land subsidence and about 3 mm per year because of rising sea levels [1]. So, about 30 % is due to rising sea levels, which is pretty significant in my opinion. Especially because this effect appears to be accelerating. These numbers seem small, but their effect is devastating. 3.) I have also lived in LA for a year :) [1] http://geology.gsapubs.org/content/43/6/519 http://geology.gsapubs.org/content/43/6/519