It was warmer actually if we compare e.g. 0 AD to the 19th and early 20th centuries. Climate change you’re talking about resulted in a decrease in temperatures when the “Roman warm period” ended.
> the characteristic warm, wet and stable weather was conducive to economic productivity in an agrarian society
> violent sequence of eruptions triggered what is now called the ‘Late Antique Little Ice Age,’ when much colder temperatures endured for at least 150 years.
> This phase of climate deterioration had decisive effects in Rome’s unravelling. It was also intimately linked to a catastrophe of even greater moment: the outbreak of the first pandemic of bubonic plague.
Perhaps the climates 2000 years ago were not as hot as we might think.