Well, fairly recently. At https://phys.org/news/2025-04-ancient-greenland-iceland-reveal-effects.html … the discovery of rocks with an origin in Greenland, is linked, to the 5th and 6th centuries AD cooler phase of climate. In this piece it is confusing as it calls it the Late Antique Little Ice Age, as opposed to the Little Ice Age of the 16h and 17th centuries AD. Whether you could really call it the Antique Little Ice Age when it was just a few very cold summers here and there is debatable. They say it lasted 200 to 300 years. If so, it was very spasmodic. See Trevor Palmer’s SIS article on the 8th and 9th centuries AD, for example. Why limit it to 300 years when you could expand it to include the two very cold spells in the 10th century AD. The Medieval warm period began around 950AD, lasting until at least 1250AD – when cool and wet weather kicked in during the 13th century. However, why rocks from Greenland were embedded in cliffs in Iceland around the 540s AD is a mystery as the cold spell is usually blamed on two faraway volcanoes. Could the rocks have arrived by icebergs. If so how did they become embedded. It sounds like a more catastrophic event, possibly involved a tsunami wave. What else might explain the arrival of the rocks from across a stretch of sea. See also https://doi.org/10.1130/G53168.1 …
At https://notrickszone.com/2025/04/21/new-study-plant-remains-embedded-in-a-modern-glacier-evidence-a-warmer-antarctica-1000-years-ago/ … moss plants, dated to the Medieval Warm Period, has been found embedded in an Antarctic glacier that is today permanently snow covered – with no evidence of meltwater. This appears to imply that the summer melt during the MWP was greater than it is in the modern world. In other words, it was warmer back then. Not just in Europe and North America, but everywhere. That is why civilisations flourished around the globe a thousand years ago.
Over at https://www.sciencedaily.com/releases/2025/04/250417145258.htm … we are told extreme drought contributed to the barbarian invasions of Europe, which included Roman Britain. However, this was long before the Late Antique Little Ice Age of the first story. It was between AD364-66 when invasions of the Roman world in 367AD included Saxons, Picts, and the Irish. Roman commanders were captured or killed in fighting. Soldiers, it is alleged, joined in with the invaders to pillage Britain south of Hadrian’s Wall. It toook 2 years to quell. Basically, they are saying that summer droughts was the spur to the invasion, rather than the melee in continental Europe.
The claim arises from tree rings. However, they are not thermometers but merely pinpoint years when there was a crisis in the environment. Low growth can be caused by cool summers – restricting tree growth. Or the chosen trees were shaded in a woodland situation and there was a lot of competition for moisture. Another problem that occurs to me is that the drought was largely confined to southern Britain. Why would Picts and the Irish decide to up sticks and settle in Britain – north and south. The Roman Warm Period caused a spike in sea levels and it was this that may have led to restless movements in Scotland and Ireland, and in the lowlands across the North Sea.