Finally got around to watching the ep, when I happened to spot a 720p release. A jolly good episode. It has the same atmosphere like David mentioned, and served the larger story really well, far better than most extras ever do.

Regarding the dream to find new, living land, logic dictates it should be easy enough unless there's some otherworldly killer virus going around killing everything with DNA, but that seems unlikely since these folks have lived for a good long while in their shrinking sphere. Thus it seems more like a combination of two things: Larger climate change drying East Asia and all the people living there in their ignorance believing their little corner is the last viable place in the world.

We can already deduce from the fact the quite southern land of former Japan seems to be relatively friendly and moderate that the temperature of Earth hasn't drastically risen turning the whole planet into a hellish Venus v2. So, it's a simple climate change. Which ultimately and absolutely means that there must still be vast lands receiving sufficiently rain to support lush life.