I've posted this under climate change but it is as much history as anything else, or even geology, as the processes involved are not understood. At https://notrickszone.com/2021/05/26/scientists-particularly-intense-warm… … which in part is true, and in another part, untrue. The Roman Warm Period did not encompass the whole period of the Roman Empire. It was a backdrop to the rise of Rome but the climate was already deteriorating by AD230 – but the really cool weather did not set in until the 6th century AD. That is the history lesson, courtesy of Kyle Harper in his big tome on the Romans, 'The Fate of Rome', where he quite clearly shows that a series of epidemics, and not just the climate, brought Rome to the brink. The Roman Warm Period is roughly dated from the second century BC to the 3rd century AD, although Harper would contract that a bit at the edges. The warmth certainly had an effect on Britannia as new kinds of crops introduced by the Romans flourished – but went out of use as the climate reverted to wet and cool.
The Romans created their empire on the back of the Greeks, using offensive military tactics to overcome a host of other peoples. It is also quite true that Rome fell as a result of a cooling climate which created political unrest in various corners of the empire. This led to the military being overstretched. The same thing happened in the 3rd century AD but the climate settled back down, although not as warm as before. This had the effect of reducing rebellions and unrest enabling the ruling elite to restore order and move forwards. The 4th century was successful in many ways but only because the barbarians on the other side of the borders were not under threat of starvation, and settled back down – until the climate really did a nose dive.
The study is in Scientific Reports, 'Persistent Warm Mediterranean surface waters during the Roman period' analyses the climate during the Roman Warm Period, and the aftermath. The study claims to show it was the warmest era of the last 2000 years – which is already an established fact, long known. The specific claim is that the Mediterranean waters were some 2 degrees warmer than average, in Sicily and the western Mediterranean at large. Two degrees warmer than the average temperature of the last 200 years. They appear to have picked up some tricks from the global warming people in picking on when and where to date the average to and from. It is not saying it was 2 degrees warmer than it has been over the last 40 years, for example, but over a period of 200 years – which encompassed some critical cooling phases, in mid 19th and late 19th century. Going back that far relies on proxy data, in any case. I would have preferred some probing into why the Roman Warm Period was as warm as it was, and why rainfall in the Mediterranean regions was bountiful – quite unlike the modern world where it hardly rains in mid summer. In other words, in the Mediterranean it was warm, and moist. Harper is quite clear on this but as an historian he doesn't venture a reason – as that is the reserve of scientists.
The study is based around sediment cores prised from the Sicily channel. They provide a scientific temperature record, we are told. However, they combined their own research with that of earlier studies to create a wider temperature reconstruction for a large part of the northern hemisphere. They found a broad pattern of warmth that affected not just the Romans but the barbarians that had not been subdued. The actual term, Roman Warm Period, goes back many years and is mentioned liberally by HH Lamb [as an example] and Basil Cracknell [in the UK].