Climate is a fickle mistress, so what if for some reason instead of the climate warming during 800-1300 AD, it kept getting colder. So, a Long Little Ice Age lasting for almost 1000 years from 400 AD, actually becoming its most severe around the time of OTL's Medieval Warm Period?
The retreat of agriculture and pasturalism and extent of reforestation in Scandinavia and Central Europe is likely to be even more extensive ITTL. Not enough Norsemen to go a'Viking, no settlement of Greenland and Iceland. Glaciers will spread out of the Swiss alps. Northern Europe will remain composed of densely populated settlements surrounded by wilderness, and the Latin West won't enjoy the recovery of OTL. The Polobian Slavs may survive in this TL.
Other regions will be affected differently. Equatorial East Africa may be wetter, for example. There may not be drought in the Yucatan, helping preserve Mayan civilization. The Niger river flooded in OTL Little Ice Age, perhaps West Africa would be wetter, and the Sahel wider in this period.
Say the Long Little Ice Age begins to end around the 19th century. What would the world look like then?