The rule discovered by the Russian scientists to. S. Berg and A. and. Shnitnikov: The higher the level of the Caspian Sea, the higher the Arctic ice cover, which means the climate in the Northern Hemisphere (NH) is colder.
This graph shows that the level of the Caspian Sea has now dropped slightly, meaning that a slight warming is observed in the north. But it is still far from the early medieval optimum of 900-1200. In the graph on the right, this is clearly visible around the "1,000" date. When those Slavs inhabited even Northern Rus' (in today's Vologda-Kostroma-Yaroslavl territories, the climate was like now in Voronezh). And in the south of Sweden grapes grew.
You can see the dry cooling around 400 AD, which transported huge masses of nomads to Europe from Europe (the "Great Migration"), when the steppe, which was dry in summer and frozen in winter, could not feed them.
This graph of Caspian sea level fluctuations also shows the peak of 1500-1750 - the Little Ice Age.
Before us is the climate of ancient Russia, which is good.