One of the books I just finished reading is The Fate of Rome: Climate, Disease, and the End of an Empire. The thesis of the book is fascinating to those of us interested in the rise and fall of empires: Rome expanded for many reasons, but one that is overlooked was the good fortune of an era of moderate weather from around 200 BC to 150 AD: rain was relatively plentiful/ regular and temperatures were relatively warm.
Then one of Earth’s numerous periods of cooling–a mini ice age–replaced the moderate weather, pressuring agricultural production.
Roman technology and security greatly expanded trade, opening routes to China, India and Africa that supplied much of Roman Europe with luxury goods. The Mediterranean acted as a cost-effective inland sea for transporting enormous quantities of grain, wine, etc. around the empire.
These trade routes acted as vectors for diseases from afar that swept through the Roman world, decimating the empire’s hundreds of densely populated cities whose residents had little resistance to the unfamiliar microbes.
Rome collapsed not just from civil strife and mismanagement, but from environmental and infectious disease pressures that did not exist in its heyday.