I don’t have a problem ascribing changes in human fortune to changes in the climate. Certainly the times when the climate is warm fosters agriculture, prosperity, and therefore peace: people don’t go to war when they have full bellies and burgeoning farms. The problem arises when they confuse cause and effect, imagining that we humans are causing the climate change that affects our lives. Clearly humans did not cause any climate change that took place thousands of years ago. The climate changes, and will always do so, for reasons entirely unrelated to human activity.
It’s not like we don’t know Roman history, we have lots of reports from that time of weather, as well as everything else. Roman history is not like trying to figure out what. Happened to the Mayans or something.
If anyone would read some Roman history, they’d realize how sad it is trying explain a 400 year time period with one factor like lead in cups.
The Roman history, including another 1200 years of the Eastern Empire, is facinating stuff.