[iofj]

The rest of Europe has more than enough farmland to provide for 7 billion people, calorie-wise. If we were to learn how to farm on the open sea, back of the envelope calculations tell me we should easily be able to multiply today's population by 10^4 or more.

If there is a problem, it's water. On the plus side, water is the easiest to recycle by far, the only limit we're close to hitting in some locations is the amount of water that's recycled for free by the weather.

Also, we have skyscrapers. The 11m2 area per person is ground-level area. If you have 10-storey skyscrapers on average, 120m2 for every family on average should be easy, and allow for population to increase 5 fold without exceeding available space in Austria.