Quote Originally Posted by Azelor View Post
It is an average and you need rather large area to use it. Looking at a valley with a city in it, you will have a large variation of population so you need to take something larger.

But, even in ancient history and medieval times, countries could trade large quantities of food?
Oh yes !
Even Thucidides (contemporary of Périclès) describes in his "Peloponesian war" published some 4 centuries BC that the main target of Athenes was to conquer Sicily because Sparta couldn't survive without the grain deliveries from there.
The expedition was a disaster and Sparta won.
So yes already almost 3 000 years ago the food trade flows were important enough to decide of the destiny of empires.

However the point is that a surface is not and never was correlated to population because of trade and agriculture. Of course if one takes the whole Earth for surface then one gets a tautology that there was globally always enough food for the global world's population. The problem being that what is true globally is not true locally. That's why the trade was always Paramount to equalise the playing field by transferring food from surplus regions to deficit regions.