We gotta stop downplaying the effects here. Land mass is NOT the concern. 94% of China lives on the right side of the red line here. 94% of 1.4 billion is 1.26 billion people. That 94% lives across the red line because the western provinces of China are largely unsustainable for the level of population China has.
Climate change isn't a gradual process. The weather cataclysms just happen, and suddenly the land isn't inhabitantable anymore. We don't get time to adjust.
Cities that contain upwards of 30 million people in a single metro area in a footprint of a couple hundred square kilometers would have to relocate. The second map shows the hardest hit areas if global temperature rises 1.5C and 2.0C. That kind of shifting in population would upset the balance in a country and could potentially collapse it.
You add in the fact that Bangladesh will sink into the ocean, climate refugees REGARDLESS of nationality will come barreling over into China and there will be 2 options for China; either let them come in and overwhelm China's limited resources or to start hostilities with neighboring countries not to let them in.
This is going to be a fukking mess, I hope you realize that.