The Migration Period Pessimum (also referred to as Dark Ages Cold Period), a period of unusually cold climate in the North Atlantic region, lasts from about 450 to about 750.

This period, which corresponds to the time following the decline of the Roman Empire around 480 and the Plague of Justinian (541-542), sees the retreat of agriculture, including pasturing as well as cultivation of crops, leading to reforestation in large areas of central Europe and Scandinavia.

Climatically this period is one of rapid cooling indicated from tree-ring data as well as sea surface temperatures based on diatom stratigraphy in the Norwegian Sea, which can be correlated with Bond event 1 in the North Atlantic sediments.

It is also a period of rising lake levels, increased bog growth and a peak in lake catchment erosion.