[adrianN]

Japan is way overpopulated. In the case that agriculture stops producing as much as it does now and food imports also dry up, Japan will be a very bad place to be. The country is mostly forest and mountain, arable land is rather scarce.

[m_mueller]

There's two main reasons way this doesn't scare me that much:

1) Japan's population is falling and I expect it to have fallen further, maybe to half or two thirds of current, until a global economical collapse.

2) There are ways to heavily increase the land use intensity with modern technology, which I'd expect to happen when the situation gets dicey. The Netherlands are a good example of how this can be done, essentially green houses everywhere. The actual bottleneck is how much energy you have, not the amount of land - Japan has lots of space in the sea for wind and solar energy, plus lots of unused geothermal.