We do not know exactly how many billions because China is not a transparent liberal democracy with independent budget accountability, but any number of Google Earth photographs reveal extensive work.
It stands to reason, that China is not going to let this go to waste. And as China goes about its business of spreading its influence in Pacific island nations through aid bribery, it is going to get a very clear message. That message is that climate-change induced rising sea levels pose an existential threat to these islands and of all issues, it is the most important for them.
China will then see. The very same rising sea levels that threaten Tuvalu, Kiribati and other island states will also threaten the billions of dollars’ worth of work they have done on the atolls in the South China Sea.
Moreover, under present UN Convention on the Law of Sea not only will the physical infrastructure be lost as the atolls go under, but also all the attendant fishing, mineral and sea and airspace rights that go with them.
The trouble is that the 1982 codification of the law of the sea, the maritime zones are measured from baselines. The baselines are straightened out where a coastline has a lot of indentations, bays and estuaries. But more importantly the baselines are “ambulatory”, that is like a walking human, capable of moving if the coastline moves for whatever reason.
It cuts both ways. For example, the creation of an island by volcanic activity in Iceland would create an extension to Iceland’s territorial, fishing, mining and economic zones. Similarly an Hawaiian lava flow would push out the maritime zones. On the other hand, if an island disappears the economic zone goes with it.