they can be convert to rice farmland by using the seawater rice specie. China is planning to unlock 8 to 10 million hectacres for saltwater rice farming in the next 10 years. Probably 60 million hectacr to 80 million hectacres in the next couple decades. The rice yield per hectacre on average is more than 1,000 KG per year (2 tons).  I dont' think China has any choice. The climate change and the sea level will force China venture into seawater rice farming.  The agricultral department of China said the fertile lands can repurpose to farm other crops like soybean.