Out of pure interest...

[Mediolanicus]

I didn't know the Mediterranean could support so many people at that time period.

I don't know about the Mediterranean, but I do know that the argiculutrural production of the Belgae was about three times higher than during the Middle Ages. IIRC the production was only surpassed somewhere around 1800. (According to JANSSENS, "De Oude Belgen")

[antisocialmunky]

If anything, the land quality would have been much higher back then and more productive than it is now if you look at old cultural hearths like the Levant and Mesopotamia where highly advanced civilization has existed and intensive agriculture has been practiced since pre-history. The land has been fairly screwed up by war/overuse/salinization/deforestation/pollution in those places.

Granted yield wise we have post-industrial agriculture and Green Revolution super crops that suck up petroleum based fertilizer so that kinda borks the actual numbers so its hard to compare actual crop yields.

If anything those population numbers are fairly conservative since the Han Dynasty census is for taxable population and the Rome one is the most conservative estimant.

[WinsingtonIII]

I don't know about the Mediterranean, but I do know that the argiculutrural production of the Belgae was about three times higher than during the Middle Ages. IIRC the production was only surpassed somewhere around 1800. (According to JANSSENS, "De Oude Belgen")

Maybe this is true specifically for the Belgian lands for some strange reason, but overall it's well established that grain yields in Northern Europe were three times higher in the Central Middle Ages (10th/11th Century) than in Roman times. The Romans/their contemporaries still used the two field crop rotation, which is much less efficient than the three field crop rotation of the Middle Ages, and Medieval farmers also had access to the rigid horse collar (horses are much more efficient plow animals than oxen, and they were producing enough excess grain with the 3-field rotation to be able to feed horses), nailed horseshoes (it's hard to be efficient when your horse keeps breaking a hoof), and the iron plow. I can't find my notes so I cannot remember exactly what distinguished this plow from Roman-era plows, but I believe there was a design difference that made the plow more efficient.

I didn't know the Mediterranean could support so many people at that time period.

It's important to realize that at its High Medieval peak in the late 13th/early 14th century (before the disastrous population crash of the 1300's) the European population reached over 80 million (and this isn't even including the Middle East, which had an even higher population), so for the Roman Empire to have a population of 45 million at its height isn't that surprising; in fact, compared to Medieval Europe and the Middle East, it was a small population.