I can't specifically name my source, but I believe it was a Human Geography textbook. (From the late de Blij, perhaps?)

One of the reasons why human prosperity correlates with relatively warm periods in the average global temperature is the fact that warm temperatures (Or in this case ideal temperatures) facilitates agricultural production in many places around the world.

This is because:

Average agricultural production SUSTAINS populations.

BUT

SURPLUSES in agricultural production, facilitated by warmer or more ideal temperatures, promotes and allows population growth and trade (and thus prosperity) as you have more produce to work with, produce that doesn't necessarily have to go into sustaining human populations.

It should be noted that this a general, broad answer to your question, and I of course could be wrong.

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