What is the "Netherworld" in the novel, Termination Shock?

The Netherworld is a new area of interest as rising sea levels caused by climate change bring attention to the low-lying areas of the world. The conference in Vadal is simply called the Netherworld. The signage features a map of the world that includes only these low-lying areas. Saskia is at first disturbed when a welcome sign when she arrives in Venice acknowledges her as Queen of the Netherworld. She later decides that these parts of the world do need an ambassador and that she will take on this title.