So far, we've mostly avoided a Malthusian catastrophe, but the human population is likely to grow to about 9 billion by 2050 (or somewhere between 7.5 and 10.5 billion, depending on your estimates). By that time, huge cities could house enormous populations, but the resources to feed all those people might need to be shipped in from vast farmlands. Who knows, maybe there will be some suburban middle ground where billions of people live near locally-produced agriculture. Vertical farming technologies could make it possible to grow food without acres and acres of land. Here are just a few links on some futuristic farming techniques.
If you'd like to read more awesome and interesting stuff, check out this unrelated (but not entirely random!) Techdirt post