The Migration Period Pessimum (also referred to as Dark Ages Cold Period) was a period of unusually cold climate in the North Atlantic region, lasting from about 450 to about 900 AD.^[1] It succeeded the Roman Age Optimum and was followed by the Medieval Warm Period.

This Migration Period Pessimum saw the retreat of agriculture, including pasturing as well as cultivation of crops, leading to reforestation in large areas of central Europe and Scandinavia.^[2] This period corresponds to the time following the Decline of the Roman Empire around 480 and the Plague of Justinian (541-542).^[3] Climatically this period was one of rapid cooling indicated from tree-ring data^[4] as well as sea surface temperatures based on diatom stratigraphy in Norwegian Sea^[5], which can be correlated with Bond event 1 in the North Atlantic sediments.^[6] It was also a period of rising lake levels, increased bog growth and a peak in lake catchment erosion.

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